Making reliable
distributed systems
in the presence of
sodware errors
Final version (with corrections) — last update 20 November 2003

Joe Armstrong

A Dissertation submitted to
the Royal Institute of Technology
in partial fulﬁlment of the requirements for
the degree of Doctor of Technology
The Royal Institute of Technology
Stockholm, Sweden

December 2003

Department of Microelectronics and Information Technology

ii

TRITA–IMIT–LECS AVH 03:09
ISSN 1651–4076
ISRN KTH/IMIT/LECS/AVH-03/09–SE

and

SICS Dissertation Series 34
ISSN 1101–1335
ISRN SICS–D–34–SE

c(cid:13)Joe Armstrong, 2003
Printed by Universitetsservice US-AB 2003

iii

To Helen, Thomas and Claire

iv

Abstract

T he work described in this thesis is the result of a research program

started in 1981 to ﬁnd better ways of programming Telecom applica-
tions. These applications are large programs which despite careful
testing will probably contain many errors when the program is put into
service. We assume that such programs do contain errors, and investigate
methods for building reliable systems despite such errors.

The research has resulted in the development of a new programming
language (called Erlang), together with a design methodology, and set of
libraries for building robust systems (called OTP). At the time of writing
the technology described here is used in a number of major Ericsson, and
Nortel products. A number of small companies have also been formed
which exploit the technology.

The central problem addressed by this thesis is the problem of con-
structing reliable systems from programs which may themselves contain
errors. Constructing such systems imposes a number of requirements on
any programming language that is to be used for the construction. I discuss
these language requirements, and show how they are satisﬁed by Erlang.

Problems can be solved in a programming language, or in the stan-
dard libraries which accompany the language. I argue how certain of the
requirements necessary to build a fault-tolerant system are solved in the
language, and others are solved in the standard libraries. Together these
form a basis for building fault-tolerant sodware systems.

No theory is complete without proof that the ideas work in practice. To
demonstrate that these ideas work in practice I present a number of case
studies of large commercially successful products which use this technol-
ogy. At the time of writing the largest of these projects is a major Ericsson

v

vi

ABSTRACT

product, having over a million lines of Erlang code. This product (the
AXD301) is thought to be one of the most reliable products ever made by
Ericsson.

Finally, I ask if the goal of ﬁnding better ways to program Telecom
applications was fulﬁlled—I also point to areas where I think the system
could be improved.

Contents

Abstract

1 Introduction

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1.1 Background .

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Ericsson background .
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Chronology .
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3 Erlang

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5 Programming Fault-tolerant Systems
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ABSTRACT

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6.5

6.6 Application principles

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Supervisor principles .
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6.7
6.8 Discussion .

7 OTP
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8 Case Studies

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8.1 Methodology .
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8.3 Quantitative properties of the sodware .
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System Structure .
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Evidence for fault recovery .
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Trouble report HD90439 .
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9 APIs and Protocols
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A Acknowledgments

B Programming Rules and Conventions

C UBF

D Colophon

References

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xii

ABSTRACT

1 Introduction

H ow can we program systems which behave in a reasonable manner

in the presence of sodware errors? This is the central question
that I hope to answer in this thesis. Large systems will probably
always be delivered containing a number of errors in the sodware,
nevertheless such systems are expected to behave in a reasonable manner.
To make a reliable system from faulty components places certain re-
quirements on the system. The requirements can be satisﬁed, either in
the programming language which is used to solve the problem, or in the
standard libraries which are called by the application programs to solve
the problem.

In this thesis I identify the essential characteristics which I believe are
necessary to build fault-tolerant sodware systems. I also show how these
characteristics are satisﬁed in our system.

Some of the essential characteristics are satisﬁed in our programming
language (Erlang), others are satisﬁed in library modules written in Erlang.
Together the language and libraries form a basis for building reliable sod-
ware systems which function in an adequate manner even in the presence
of programming errors.

Having said what my thesis is about, I should also say what it is not
about. The thesis does not cover in detail many of the algorithms used
as building blocks for construction fault-tolerant systems—it is not the al-
gorithms themselves which are the concern of this thesis, but rather the
programming language in which such algorithms are expressed. I am also

1

2

CHAPTER 1.

INTRODUCTION

not concerned with hardware aspects of building fault-tolerant systems, nor
with the sodware engineering aspects of fault-tolerance.

The concern is with the language, libraries and operating system re-
quirements for sodware fault-tolerance. Erlang belongs to the family of
pure message passing languages—it is a concurrent process-based language
having strong isolation between concurrent processes. Our programming
model makes extensive use of fail-fast processes. Such techniques are com-
mon in hardware platforms for building fault-tolerant systems but are not
commonly used in sodware solutions. This is mainly because conven-
tional languages do not permit dicerent sodware modules to co-exist in
such a way that there is no interference between modules. The commonly
used threads model of programming, where resources are shared, makes
it extremely diecult to isolate components from each other—errors in one
component can propagate to another component and damage the internal
consistency of the system.

1.1 Background

The work reported in this thesis started at the Ericsson Computer Science
Laborator (CSLab) in 1981. My personal involvement started in 1985 when
I joined the CSLab. The work reported here was performed in the period
1981-2003. During this time the Erlang programming language and OTP
was developed by the author and his colleagues and a number of large
applications were written in Erlang.

The system as we know it today is the result of the collective ecort of
a large number of people. Without their talents and the feedback from
our users Erlang would not be what it is today. For many parts of the
system it is diecult to say with precision exactly who did what and when,
In
and exactly who had the original ideas for a particular innovation.
the acknowledgements I have tried to credit everybody as accurately as
possible.

The chronology of this work is as follows:

• 1981 — The Ericsson CSLab was formed. One goal of this labora-
tory was “to suggest new architectures, concepts and structures for future

1.1. BACKGROUND

3

processing systems developments” [29].

• 1986 — I start work on the language that was to become Erlang,
though at the time the language had no name. The language started
as an experiment to add concurrent processes to Prolog—this work
is described in [10]. At this stage I did not intend to design a new
programming language, I was interested in how to program POTS
(Plain Old Telephony Service)—at the time the best method for pro-
gramming POTS appeared to be a variant of Prolog augmented with
parallel processes.

• 1987 — First mention of Erlang. By 1987 the term Erlang had been
coined (probably by the head of the CSLab Bjarne D¨acker). By the
end of the year a Prolog implementation of Erlang was available.
This version of Erlang was embedded in Prolog using Prolog inﬁx
operators and did not yet have its own syntax.

Towards the end of 1987 the ﬁrst major experiment with Erlang
started—a group of Ericsson engineers at Bollmora, led by Kerstin
¨Odling, started work on a prototyping project. They chose Erlang to
prototype something called “ACS/Dunder.” ACS was an architecture
which was designed for implementing the Ericsson MD110 private
automatic branch exchange (PABX).

The project was to implement a number of typical PABX features in
Erlang using the ACS architecture and compare the programming
ecort with the time it was estimated that the same job would have
taken in PLEX.1

Many of the ideas found in the current Erlang/OTP system can be
traced back to this project.

• 1988 — Some time during 1988 it became clear that Erlang was
suitable for programming Telecoms systems—so while the Bollmora
group wrote applications in Erlang the CSLab group now augmented

1PLEX was the programming language used to program the MD110.

4

CHAPTER 1.

INTRODUCTION

by Robert Virding and Mike Williams worked on improving the Er-
lang system.
An attempt was made to improve the eeciency of Erlang by cross
compilation to the parallel logic programming language Strand. The
compilation of Erlang to Strand is described in chapter 13 of [37].
Cross compilation to Strand improved the performance of Erlang by
a factor of six and the project was viewed as a dismal failure.2

• 1989 — The ACS/Dunder project began to produce results. The
Bollmora group showed that using ACS/Dunder with Erlang lead
to an improvement in design eeciency of a factor of somewhere
between 9 and 22 times less than the corresponding ecort in PLEX.
This was result was based on the experience of prototyping some
10% of the functionality of the Ericsson MD 110—these ﬁgures were
hotly debated (depending on whether you believed in Erlang or not).

The Bollmora group estimated that they would need a factor seventy
in performance improvement (which we had rashly promised) in
order to turn the ACS/Dunder prototype into a commercial product.

To improve the performance of Erlang I designed the JAM machine
(Joe’s abstract machine). The design of the JAM machine was loosely
based on the Warren abstract machine [68]. Since Erlang started as
an extension to Prolog it seemed reasonable that the techniques used
to eeciently implement Prolog could also be applicable to Erlang.
This intuition proved correct. The JAM machine was similar to the
WAM with the addition of parallel processes, message passing and
failure detection and with the omission of backtracking. Compilation
of pattern matching was essentially the same as in the WAM. The
original JAM instruction set and details of the compilation process
were published in [9].

The design of the JAM was completed in 1989. The ﬁrst imple-
mentation was an instruction set emulator written in Prolog which
emulated the JAM machine. This was very ineecient and could

2A fact not recorded in the Strand book!.

1.1. BACKGROUND

5

evaluate approximately 4 reductions per second, but it was suecient
to evaluate and test the virtual machine and to allow the Erlang
compiler to be written in Erlang itself.

Once the design of the JAM was complete I started a C implemen-
tation of the virtual machine—which was soon abandoned ader Mike
Williams read some of my C code—ader which I worked on the com-
piler. Mike Williams wrote the virtual machine emulator and Robert
Virding worked on the Erlang libraries.

• 1990 — By 1990 the JAM machine was working well and had sur-
passed the original goal of being seventy times faster than the origi-
nal Prolog interpreter. Erlang now had its own syntax (up to now it
could be regarded as a dialect of Prolog) and could be regarded as
a language in its own right, rather than as a dialect of Prolog.

• 1991 — Claes Wikstr¨om added distribution to Erlang. The JAM ma-
chine was now stable and had replaced the Prolog implementation
of Erlang.

• 1992 — A decision was made at Ericsson Business Systems (EBC)
to develop a product based on ACS/Dunder. This product was
called the Mobility Server—Ericsson presented Erlang developments
[1, 33], at The XIV International Switching Symposium in Yoko-
hama, Japan.

• 1993 — Ericsson starts a wholly owned subsidiary company called
Erlang Systems AB. The purpose of this company was to market
and sell Erlang to external customers and to provide training and
consulting services to both internal and external customers. Support
for Erlang itself was performed by the Ericsson Computer Science
Laboratory. The ﬁrst commercial version of Erlang was released.

• 1995 — The Ericsson AXE-N project collapsed [55]. The AXE-N
project was a project to build a “next generation switch” to replace
the Ericsson AXE-10. This extremely large project ran from 1987-95.

6

CHAPTER 1.

INTRODUCTION

Ader the AXE-N project collapsed a decision was made to “restart”
the project using Erlang. This project eventually resulted in the
development of the AXD301 switch.

This project was on a much larger scale than any previous Erlang
project. For this reason a new group was started to provide support
to the AXD project. The Erlang libraries were renamed OTP (The
Open Telecom Platform) and a new group was created.

• 1996 — In order to provide Erlang users with a stable sodware base
a project called OTP (The Open Telecom Platform) was started.
OTP was to be used primarily in the newly started AXD project; all
existing projects were to migrate to OTP. The OTP project consol-
idated a number of ideas derived from experience with Erlang and
in particular from a earlier set of libraries developed for use in the
Mobility Server.

• 1997 — The OTP project turned into the OTP product unit which
was started in order to take over formal responsibility for Erlang.
Prior to that, the CSLab had formally been responsible for Erlang.
I moved from the CSLab to the OTP group where I worked as
the chief technical co-ordinator. During the period 1996–1997 a
three-person group (myself, Magnus Fr¨oberg and Martin Bj¨orklund)
redesigned and implemented the OTP core libraries.

• 1998 — Ericsson delivered the ﬁrst AXD301. The AXD301 is the
subject of one of our case studies in Chapter 8. At the time of
writing (2003) the AXD301 has over 1.7 million lines of Erlang code
which probably makes it the largest system ever to be written in a
functional style of programming.

In February 1998 Erlang was banned for new product develop-
ment within Ericsson—the main reason for the ban was that Erics-
son wanted to be a consumer of sodware technologies rather than a
producer.

In December 1998 Erlang and the OTP libraries were released sub-
ject to an Open Source License. Since that date it has been freely

1.2. THESIS OUTLINE

7

available for download from http://www.erlang.org/
In 1998 I led Ericsson together with a number of the original Erlang
group to found a new company Bluetail AB—in all 15 people led
Ericsson. The idea behind Bluetail was to use the Erlang technology
to program products which make Internet services more reliable.

• 1999 — Bluetail produced two products written in Erlang. The Mail
Robustiﬁer [11] and the Web Prioritizer. Ericsson produced a number
of Erlang products (including the AXD301 and GPRS systems).

• 2000 — Bluetail is acquired by Alteon Web Systems [3] and subse-

quently Alteon is acquired by Nortel Networks.

• > 2001 — The Erlang/OTP technology is well established. By now
there are so many projects that nobody knows the exact number.
Erlang products developed by Nortel are selling for “Hundreds of
millions of kronor per year” [51]—The Ericsson AXD301 is one of
Ericsson’s most successful new products and there are a dozen or so
small companies using Erlang for product development.

1.2 Thesis outline

This thesis is organized into the following chapters:

• Chapter 1 introduces the main problem area that the thesis ad-
dresses, gives a background to the work in the thesis and a chronol-
ogy of the work performed in the thesis together with a detailed
chapter plan.

• Chapter 2 introduces an architectural model that is the basis for the
later chapters in the thesis. I deﬁne what is meant by an architecture,
and specify which components must be present in an architecture.
I talk about the problem domain that my architecture is designed
for. I talk about the underlying philosophy behind the architecture
and I introduce the idea of “Concurrency Oriented Programming”
(COP).

8

CHAPTER 1.

INTRODUCTION

I develop the idea of COP and state the desirable properties that a
programming language and system must have in order to support a
concurrency oriented style of programming.

I review some previous related work, showing the similarities and
dicerences between this prior work and the material presented in
this thesis.

• Chapter 3 describes the programming language Erlang. I describe
a reasonably large sub-set of the Erlang programming language, and
motivate some of the design decisions made in Erlang.

• Chapter 4 gives some examples of Erlang programming techniques.

I show how to “factor” a design into its functional and non-functional
components. I show how to factor out notions of concurrency and
fault-tolerance, and how to program a generic client–server model.

I describe a technique for maintaining the illusion that “everything
is an Erlang process,” and give examples of how to write code which
handles errors.

• Chapter 5 gets to the central question of the thesis. It is concerned
with how to program systems which behave in a reasonable manner
even in the presence of errors. Central to the idea of fault tolerance
is the notion of an error—I describe what is meant by an error and
what I mean when I talk about a “fault-tolerant” system. I describe a
strategy based on the idea of “Supervision trees” which can be used
for writing fault-tolerant sodware.

• Chapter 6 links the general principles of programming a fault-tolerant
system, developed in the previous chapter, to a number of speciﬁc
programming patterns developed for programming fault-tolerant sys-
tems. These programming patterns are central to the understanding
of the OTP system, and of how to build fault-tolerant sodware in
Erlang.

I give a complete example,
involving the use of a client–server
model, an event-handler and a ﬁnite-state machine. These three

1.2. THESIS OUTLINE

9

components are added to a supervision tree, which will monitor
their progress and restart them in the event of an error.

The entire program, is packaged into a single OTP “application.”

• Chapter 7 describes the OTP system. OTP stands for “Open Tele-
coms Platform” and is an application operating system (AOS) for
programming fault-tolerant applications together with the delivery
platform for the Erlang programming language. It includes a large
set of libraries for implementing fault-tolerant systems, together with
documentation and guides etc for understanding the system.

In this chapter I brieﬂy describe the OTP architecture and give
details of the main components in the system.

• Chapter 8 is the acid-test of our technology. Did the ideas work in
practice? In this chapter I analyse a number of large commercially
successful products that make use of OTP. The intention of this
chapter is to see if we have achieved our goals of programming a
system which functions reliably in the presence of sodware errors.
One of the projects studied in this chapter is the Ericsson AXD301,
a high-performance highly-reliable ATM switch. This project is in-
teresting in its own right, since it is one of the largest programs ever
written in a functional style.

• Chapter 9 is concerned with APIs and protocols. I ask how we can
specify the interfaces to modules or the interfaces between commu-
nicating components.

• In Chapter 10 I ask broader questions. Did the ideas work? Did
they work well or badly? Where can things be improved? What can
we look for in the future and how are we going to get there?

10

CHAPTER 1.

INTRODUCTION

2 The Architectural

Model

There is no standard, universally-accepted deﬁnition of the term, for software
architecture is a ﬁeld in its infancy, ... While there is no standard deﬁnition,
there is also no shortage of them ...

The Carnegie Mellon Institute of Software Engineers

T his chapter presents an architecture for building fault-tolerant sys-

tems. While everybody has a vague idea of what the word architec-
ture means, there are few widely accepted deﬁnitions, which leads
to many misunderstandings. The following deﬁnition captures the general
idea of what is meant by a sodware architecture:

An architecture is the set of signiﬁcant decisions about the organi-
zation of a sodware system, the selection of the structural elements
and their interfaces by which the system is composed, together with
their behaviour as speciﬁed in the collaborations among those ele-
ments, the composition of these structural and behavioural elements
into progressively larger subsystems, and the architectural style that
guides this organization—these elements and their interfaces, their
collaborations, and their composition.

Booch, Rumbaugh, and Jacobson [19]

11

12

CHAPTER 2. THE ARCHITECTURAL MODEL

2.1 Deﬁnition of an architecture

At the highest level of abstraction an architecture is “a way of thinking
about the world.” To be useful, however, we have to turn our way of
thinking about the world into a practical rulebook, and a set of procedures,
that tells us how to construct a particular system using our particular way
of looking at the world.

Our sodware architecture is characterised by descriptions of the follow-

ing things:

1. A problem domain — What type of problem is the architecture de-
signed to solve? Sodware architectures are not general purpose but
are designed for solving speciﬁc problems. No description of an ar-
chitecture is complete without describing the type of problem that is
supposed to be solved using the architecture.

2. A philosophy — What is the reasoning behind the method of sodware

construction? What are the central ideas in the architecture?

3. A set of construction guidelines — How do we program a system?
We need an explicit set of construction guidelines. Our systems
will be written and maintained by teams of programmers—it is im-
portant that all the programmers, and system designers, understand
the system architecture, and its underlying philosophy. For practical
reasons this knowledge is conveniently maintained in the form of
construction guidelines. The full set of guidelines, includes sets of
programming rules, and examples, course material etc.

4. A set of pre-deﬁned components — Design by “choosing from a set
of pre-deﬁned components” is far easier than “design from scratch.”
The Erlang OTP libraries contain a complete set of pre-deﬁned com-
ponents (called behaviours) with which commonly used system com-
ponents can be built. Some examples of these are the gen_server
behaviour which can be used to build client-server systems, or the
gen_event behaviour which can be used to build event-based pro-

2.2. PROBLEM DOMAIN

13

grams. The pre-deﬁned components will be discussed more fully in
section 6.1.

Section 6.2.2 gives a simple example of how to program a server
using the gen_sever behaviour.

5. A way of describing things — How can we describe the interfaces
to a component? How can we describe a communication protocol
between two components in our system? How can we describe
the static and dynamic structure of our system? To answer these
questions we will introduce a number of dicerent, and specialised
notations. Some for describing program APIs, and other notations
for describing protocols, and system structure.

6. A way of conﬁguring things — How can we start, stop, and conﬁg-
ure, the system. How can we re-conﬁgure the system while it is in
operation?

2.2 Problem domain

Our system was originally designed for building telecoms switching sys-
tems. Telecoms switching systems have demanding requirements in terms
of reliability, fault-tolerance etc. Telecoms systems are expected to operate
“forever,” they should exhibit sod real-time behaviour, and they should be-
have reasonably in the presence of sodware and hardware errors. D¨acker
[30], gave ten requirements for the properties of a telecoms system:

1. The system must be able to handle very large numbers of concurrent

activities.

2. Actions must be performed at a certain point in time or within a

certain time.

3. Systems may be distributed over several computers.

4. The system is used to control hardware.

14

CHAPTER 2. THE ARCHITECTURAL MODEL

5. The sodware systems are very large.

6. The system exhibits complex functionality such as, feature interac-

tion.

7. The systems should be in continuous operation for many years.

8. Sodware maintenance (reconﬁguration, etc) should be performed

without stopping the system.

9. There are stringent quality, and reliability requirements.

10. Fault tolerance both to hardware failures, and sodware errors, must

be provided.

We can motivate these requirements as follows:

• Concurrency — switching systems are inherently concurrent since in
a typical switch many tens of thousands of people may simultane-
ously interact with the switch. This implies that the system should
be able to eeciently handle many tens of thousands of concurrent
activities.

• Sod real-time — in a telecommunications system many operations
have to be performed within a speciﬁc time. Some of these timed
operations are strictly enforced, in the sense that if a given operation
does not succeed within a given time interval then the entire oper-
ation will be aborted. Other operations are merely monitored with
some form of timer, the operation being repeated if the timer event
triggers before the operation has completed.

Programming such systems requires manipulating many tens of thou-
sands of timers in an eecient manner.

• Distributed — switching systems are inherently distributed, so our
system should structured in such a way that it is easy to go from a
single-node system to a multi-node distributed system.

2.2. PROBLEM DOMAIN

15

• Hardware interaction — Switching systems have large amounts of
peripheral hardware which must be controlled and monitored. This
implies that it should be possible to write eecient device drivers,
and that context switching between dicerent device drivers should
be eecient.

• Large sodware systems — switching systems are large, for example,
the Ericsson AXE10, and the AT&T 5ESS switch, have several mil-
lion lines of program code [71]. This means that our sodware systems
must work with millions of lines of source code.

• Complex functionality — switching systems have complex functional-
ity. Market pressure encourages the development, and deployment
of systems with large numbers of complex features. Oden systems
are deployed before the interaction between such features is well
understood. During the lifetime of a system the feature set will prob-
ably be changed and extended in many ways. Feature, and sodware
upgrade must be performed “in place,” that is, without stopping the
system.

• Continuous operation — telecommunications systems are designed
for many years of continuous operation. This implies that opera-
tions like sodware, and hardware maintenance, must be performed
without stopping the system.

• Quality requirements — switching systems should run with an ac-
ceptable level of service even in the presence of errors. Telephone
exchanges are expected to be extremely reliable.1

• Fault tolerance — switching systems should be “fault tolerant.” This
means that from the outset we know that faults will occur, and that
we must design a sodware and hardware infrastructure that can deal
with these faults, and provide an acceptable level of service even in
the presence of faults.

1Typically having less than two hours of down-time in 40 years [48].

16

CHAPTER 2. THE ARCHITECTURAL MODEL

While these requirements came originally from the telecoms world they
are by no means exclusive to that particular problem domain. Many mod-
ern Internet services (for example, web servers) would have a strikingly
similar list of requirements.

2.3 Philosophy

How can we make a fault-tolerant sodware system which behaves in a
reasonable manner in the presence of sodware errors? Answering this will
take the rest of this thesis. I will start by giving a short answer which will
be reﬁned in the remainder of the thesis.

To make a fault-tolerant sodware system which behaves reasonably in

the presence of sodware errors we proceed as follows:

1. We organise the sodware into a hierarchy of tasks that the system has
to perform. Each task corresponds to the achievement of a number
of goals. The sodware for a given task has to try and achieve the
goals associated with the task.

Tasks are ordered by complexity. The top level task is the most
complex, when all the goals in the top level task can be achieved
then the system should function perfectly. Lower level tasks should
still allow the system to function in an acceptable manner, though it
may ocer a reduced level of service.

The goals of a lower level task should be easier to achieve than the
goals of a higher level task in the system.

2. We try to perform the top level task.

3. If an error is detected when trying to achieve a goal, we make an
attempt to correct the error. If we cannot correct the error we imme-
diately abort the current task and start performing a simpler task.

Programming a hierarchy of tasks needs a strong encapsulation method.
We need strong encapsulation for error isolation. We want to stop pro-

2.3. PHILOSOPHY

17

gramming errors in one part of the system adversely acecting sodware in
other parts of the system.

We need to isolate all the code that runs in order to achieve a goal in
such a way that we can detect if any errors occurred when trying to achieve
a goal. Also, when we are trying to simultaneously achieve multiple goals
we do not want a sodware error occurring in one part of the system to
propagate to another part of the system.

The essential problem that must be solved in making a fault-tolerant
sodware system is therefore that of fault-isolation. Dicerent programmers
will write dicerent modules, some modules will be correct, others will have
errors. We do not want the errors in one module to adversely acect the
behaviour of a module which does not have any errors.

To provide fault-isolation we use the traditional operating system no-
tion of a process. Processes provide protection domains, so that an error in
one process cannot acect the operation of other processes. Dicerent pro-
grammers write dicerent applications which are run in dicerent processes;
errors in one application should not have a negative inﬂuence on the other
applications running in the system.

This is, of course, only true to a ﬁrst approximation. Since all processes
use the same CPU, and memory, processes which try to hog the CPU or
which try to use excessive memory can negatively acect other processes in
the system. The extent to which processes can interfere with each other
depends upon the design characteristics of the operating system.

In our system processes, and concurrency, are part of the programming
language and are not provided by the host operating system. This has a
number of advantages over using operating system processes:

• Concurrent programs run identically on dicerent OSs—we are not
limited by how processes are implemented on any particular operat-
ing system. The only observable dicerence when moving between
OS’s, and processors should be due to dicerent CPU speeds and
memory sizes etc. All issues of synchronization, and inter-process
interaction should be the same irrespective of the properties of the
host operating system.

18

CHAPTER 2. THE ARCHITECTURAL MODEL

• Our language based processes are much lighter-weight than conven-
tional OS processes. Creating a new process in our language is a
highly eecient operation, some orders of magnitude faster than pro-
cess creation in most operating systems[12, 14], and orders of mag-
nitude faster than thread creation in most programming languages.

• Our system has very little need of an operating system. We make
use of very few operating system services, thus it is relatively easy
to port our system to specialised environments such as embedded
systems.

Our applications are structured using large numbers of communicating

parallel processes. We take this approach because:

1. It provides an architectural infrastructure — we can organize our sys-
tem as a set of communicating processes. By enumerating all the
processes in our system, and deﬁning the message passing channels
between the processes we can conveniently partition the system into
a number of well-deﬁned sub-components which can be indepen-
dently implemented, and tested. This is the methodology implied
by the top level of the SDL [45] system design methodology.

2. Potential eeciency — a system which is designed to be implemented
as a number of independent concurrent processes can be imple-
mented on a multi-processor or run on a distributed network of
processors. Note that the eeciency is only potential, and works best
when the application can be partitioned into a number of truly in-
dependent tasks. If there are strong data dependencies between the
tasks this might not always be possible.

3. Fault isolation — concurrent processes with no data sharing provide
a strong measure of fault isolation. A sodware error in a concurrent
process should not inﬂuence processing in the other processes in the
system.

2.4. CONCURRENCY ORIENTED PROGRAMMING

19

Of these three uses of concurrency, the ﬁrst two are non-essential char-
acteristics and can be provided by some kind of in-built scheduler which
provides various forms of pseudo-parallel time sharing between processes.
The third characteristic is essential for programming fault-tolerant sys-
tems. Each independent activity should be performed in a completely
isolated process. Such processes should share no data, and only commu-
nicate by message passing. This is to limit the consequences of a sodware
error.

As soon as two processes share any common resource, for example,
memory or a pointer to memory, or a mutex etc the possibility exists that
a sodware error in one of the processes will corrupt the shared resource.
Since eliminating all such sodware errors for large sodware systems is an
unsolved problem I think that the only realistic way to build large reliable
systems is by partitioning the system into independent parallel processes,
and by providing mechanisms for monitoring and restarting these pro-
cesses.

2.4 Concurrency oriented programming

In our system concurrency plays a central role, so much so that I have
coined the term Concurrency Oriented Programming to distinguish this style
of programming from other programming styles.2

In Concurrency Oriented Programming the concurrent structure of
the program should follow the concurrent structure of the application. It
is particularly suited to programming applications which model or interact
with the real world.

Concurrency Oriented Programming also provides the two major ad-
vantages commonly associated with object-oriented programming. These
are polymorphism and the use of deﬁned protocols having the same mes-
sage passing interface between instances of dicerent process types.

When we partition a problem into a number of concurrent processes
we can arrange that all the processes respond to the same messages (ie

2Such as Object Oriented programming which models the world in terms of Objects, Functional Pro-

gramming which uses functions, or Logic Programming with uses relations.

20

CHAPTER 2. THE ARCHITECTURAL MODEL

they are polymorphic,) and that they all follow the same message passing
interface.

The word concurrency refers to sets of events which happen simulta-
neously. The real world is concurrent, and consists of a large number
of events many of which happen simultaneously. At an atomic level our
bodies are made up of atoms, and molecules, in simultaneous motion.
At a macroscopic level the universe is populated with galaxies of stars in
simultaneous motion.

When we perform a simple action, like driving a car along a freeway,
we are aware of the fact that there may be several hundreds of cars within
our immediate environment, yet we are able to perform the complex task
of driving a car, and avoiding all these potential hazards without even
thinking about it.

In the real world sequential activities are a rarity. As we walk down the
street we would be very surprised to ﬁnd only one thing happening, we
expect to encounter many simultaneous events.

If we did not have the ability to analyze and predict the outcome of
many simultaneous events we would live in great danger, and tasks like
driving a car would be impossible. The fact that we can do things which
require processing massive amounts of parallel information suggests that
we are equipped with perceptual mechanisms which allow us to intuitively
understand concurrency without consciously thinking about it.

When it comes to computer programming things suddenly become
inverted. Programming a sequential chain of activities is viewed the norm
, and in some sense is thought of as being easy, whereas programming
collections of concurrent activities is avoided as much as possible, and is
generally perceived as being diecult.

I believe that this is due to the poor support which is provided for con-
currency in virtually all conventional programming languages. The vast
majority of programming languages are essentially sequential; any concur-
rency in the language is provided by the underlying operating system, and
not by the programming language.

In this thesis I present a view of the world where concurrency is pro-
vided by the programming language, and not by the underlying operating
system. Languages which have good support for concurrency I call Concur-

2.4. CONCURRENCY ORIENTED PROGRAMMING

21

rency Oriented Languages, or COPLs for short.

2.4.1 Programming by observing the real world
We oden want to write programs that model the world or interact with the
world. Writing such a program in a COPL is easy. Firstly, we perform an
analysis which is a three-step process:

1. We identify all the truly concurrent activities in our real world activ-

ity.

2. We identify all message channels between the concurrent activities.

3. We write down all the messages which can ﬂow on the dicerent

message channels.

Now we write the program. The structure of the program should
exactly follow the structure of the problem. Each real world concurrent
activity should be mapped onto exactly one concurrent process in our
programming language. If there is a 1:1 mapping of the problem onto the
program we say that the program is isomorphic to the problem.

It is extremely important that the mapping is exactly 1:1. The reason
for this is that it minimizes the conceptual gap between the problem and
the solution. If this mapping is not 1:1 the program will quickly degenerate,
and become diecult to understand. This degeneration is oden observed
when non-CO languages are used to solve concurrent problems. Oden
the only way to get the program to work is to force several independent
activities to be controlled by the same language thread or process. This
leads to a inevitable loss of clarity, and makes the programs subject to
complex and irreproducible interference errors.

In performing our analysis of the problem we must choose an appro-
priate granularity for our model. For example, if we were writing an instant
messaging system, we might choose to use one process per user and not
one process for every atom in the user’s body.

22

CHAPTER 2. THE ARCHITECTURAL MODEL

2.4.2 Characteristics of a COPL

COPLs are characterised by the following six properties:

1. COPLs must support processes. A process can be thought of as a

self-contained virtual machine.

2. Several processes operating on the same machine must be strongly
isolated. A fault in one processe should not adversely ecect another
process, unless such interaction is explicitly programmed.

3. Each process must be identiﬁed by a unique unforgeable identiﬁer.

We will call this the Pid of the process.

4. There should be no shared state between processes. Processes inter-
act by sending messages. If you know the Pid of a process then you
can send a message to the process.

5. Message passing is assumed to be unreliable with no guarantee of

delivery.

6. It should be possible for one process to detect failure in another

process. We should also know the reason for failure.

Note that COPLs must provide true concurrency, thus objects repre-
sented as processes are truly concurrent, and messages between processes
are true asynchronous messages, unlike the disguised remote procedure
calls found in many object-oriented languages.

Note also that the reason for failure may not always be correct. For
example, in a distributed system, we might receive a message informing
us that a process has died, when in fact a network error has occurred.

2.4.3 Process isolation

The notion of isolation is central to understanding COP, and to the con-
struction of fault-tolerant sodware. Two processes operating on the same

2.4. CONCURRENCY ORIENTED PROGRAMMING

23

machine must be as independent as if they ran on physically separated
machines.

Indeed the ideal architecture to run a CO program on would be a
machine which assigned one new physical processor per sodware process.
Until this ideal is reached we will have to live with the fact that multiple
processes will run on the same machine. We should still think of them as
if they ran on physically separated machine.
Isolation has several consequences:

1. Processes have “share nothing” semantics. This is obvious since they

are imagined to run on physically separated machines.

2. Message passing is the only way to pass data between processes.
Again since nothing is shared this is the only means possible to
exchange data.

3. Isolation implies that message passing is asynchronous.

If process
communication is synchronous then a sodware error in the receiver
of a message could indeﬁnitely block the sender of the message
destroying the property of isolation.

4. Since nothing is shared, everything necessary to perform a dis-
tributed computation must be copied. Since nothing is shared, and
the only way to communicate between processes is by message pass-
ing, then we will never know if our messages arrive (remember we
said that message passing is inherently unreliable.) The only way to
know if a message has been correctly sent is to send a conﬁrmation
message back.

Programming a system of processes subject to the above rules may
appear at ﬁrst sight to be diecult—ader all most concurrency extensions to
sequential programming languages provide facilities for almost exactly the
opposite providing things like locks, and semaphores, and provision for
shared data, and reliable message passing. Fortunately, the opposite turns
out to be true—programming such a system turns out to be surprisingly

24

CHAPTER 2. THE ARCHITECTURAL MODEL

easy, and the programs you write can be made scalable, and fault-tolerant,
with very little ecort.

Because all our processes are required to be complete isolated adding
more processes cannot acect the original system. The sodware must have
been written so as to handle collections of isolated processes, so adding a
few more processors is usually accomplished without any major changes
to the application sodware.

Since we made no assumptions about reliable message passing, and
must write our application so that it works in the presence of unreliable
message passing it should indeed work in the presence of message passing
errors. The initial ecort involved will reward us when we try to scale up
our systems.

2.4.4 Names of processes

We require that the names of processes are unforgeable. This means that it
should be impossible to guess the name of a process, and thereby interact
with that process. We will assume that processes know their own names,
and that processes which create other processes know the names of the
processes which they have created. In other words, a parent process knows
the names of its children.

In order to write COPLs we will need mechanisms for ﬁnding out the
names of the processes involved. Remember, if we know the name of a
process, we can send a message to that process.

System security is intimately connected with the idea of knowing the
name of a process. If we do not know the name of a process we cannot
interact with it in any way, thus the system is secure. Once the names of
processes become widely know the system becomes less secure. We call
the process of revealing names to other processes in a controlled manner
the name distribution problem— the key to security lies in the name distribu-
tion problem. When we reveal a Pid to another process we will say that
we have published the name of the process. If a name is never published
there are no security problems.

Thus knowing the name of a process is the key element of security.
Since names are unforgeable the system is secure only if we can limit the

2.4. CONCURRENCY ORIENTED PROGRAMMING

25

knowledge of the names of the processes to trusted processes.

In many primitive religions it was believed that humans had powers
over spirits if they could command them by their real names. Knowing
the real name of a spirit gave you power over the spirit, and using this
name you could command the spirit to do various things for you. COPLs
use the same idea.

2.4.5 Message passing

Message passing obeys the following rules:

1. Message passing is assumed to be atomic which means that a mes-

sage is either delivered in its entirety or not at all.

2. Message passing between a pair of processes is assumed to be or-
dered meaning that if a sequence of messages is sent and received
between any pair of processes then the messages will be received in
the same order they were sent.

3. Messages should not contain pointers to data structures contained
within processes—they should only contain constants and/or Pids.

Note that point two is a design decision, and does not reﬂect any under-
lying semantics in the network used to transmit messages. The underlying
network might reorder the messages, but between any pair of processes
these messages can be bucered, and re-assembled into the correct order
before delivery. This assumption makes programming message passing
applications much easier than if we had to always allow for out of order
messages.

We say that such message passing has send and pray semantics. We
send the message and pray that it arrives. Conﬁrmation that a message has
arrived can be achieved by returning a conﬁrmation message (sometimes
called round-trip conﬁrmation.) Interestingly many programmers only be-
lieve in round-trip conﬁrmation, and use it even if the underlying transport
layers are supposed to provide reliable data transport, and even if such
checks are supposedly irrelevant.

26

CHAPTER 2. THE ARCHITECTURAL MODEL

Message passing is also used for synchronisation. Suppose we wish to
synchronise two processes A, and B. If A sends a message to B then B
can only receive this message at some point in time ader A has sent the
message. This is known as casual ordering in distributed systems theory.
In COPLs all interprocess synchronisation is based on this simple idea.

2.4.6 Protocols

is
Isolation of components, and message passing between components,
architecturally suecient for protecting a system from the consequences of
a sodware error, but it is not suecient to specify the behaviour of a system,
nor, in the event of some kind of failure to determine which component
has failed.

Up to now we have assumed that failure is a property of a single
component, a single component will either do what it is supposed to do or
fail as soon as possible. It might happen, however, that no components are
observed to fail, and yet the system still does not work as expected.

To complete our programming model, we add therefore one more
thing. Not only do we need completely isolated components that com-
municate only by message passing, but also we need to specify the com-
munication protocols that are used between each pair of components that
communicate with each other.

By specifying the communication protocol that should be obeyed be-
tween two components we can easily ﬁnd out if either of the components
involved has violated the protocol. Guaranteeing that the protocol is en-
forced should be done by static analysis,
if possible, or failing this by
compiling run-time checks into the code.

2.4.7 COP and programmer teams

Building a large system involves the work of many programmers, some-
times many hundreds of programmers are involved. To organise their
work these programmers are organised into smaller groups or teams. Each
group is responsible for one or more logical component in the system. On
a day-to-day basis, the groups communicate by message-passing (e-mail

2.5. SYSTEM REQUIREMENTS

27

or phone) but do not regularly meet. In some cases the groups work in
dicerent countries, and never meet.
It is amusing to note that not only
is the organisation of a sodware system into isolated components which
communicate by pure message passing desirable for a number of reasons,
but also that it is the way that large programming groups are organised.

2.5 System requirements

To support a CO style of programming, and to make a system that can
satisfy the requirements of a telecoms system we arrive at a set of require-
ments for the essential characteristics of a system. These requirements are
for the system as a whole—here I am not interested in whether these re-
quirements are satisﬁed in a programming language or in the libraries, and
construction methods, which accompany the language.

There are six essential requirements on the underlying operating sys-

tem, and programming languages.

R1. Concurrency — Our system must support concurrency. The
computational ecort needed to create or destroy a concurrent
process should be very small, and there should be no penalty
for creating large numbers of concurrent processes.

R2. Error encapsulation — Errors occurring in one process must

not be able to damage other processes in the system.

R3. Fault detection — It must be possible to detect exceptions both
locally (in the processes where the exception occurred,) and
remotely (we should be able to detect that an exception has
occurred in a non-local process).

R4. Fault identiﬁcation — We should be able to identify why an

exception occurred.

R5. Code upgrade — there should exist mechanisms to change
code as it is executing, and without stopping the system.

28

CHAPTER 2. THE ARCHITECTURAL MODEL

R6. Stable storage — we need to store data in a manner which

survives a system crash.

It is also important that systems satisfying the above requirements are
eeciently implemented—concurrency is not much use if we cannot reliably
create many tens of thousands of processes. Fault identiﬁcation is not much
use if it does not contain enough information to allow us to correct the
error at a later date.

Satisfying the above requirements can be done in a number of dif-
ferent ways. Concurrency, for example, can be provided as a language
primitive (as, for example, in Erlang), or in the operating system (for ex-
ample, Unix). Languages like C or Java which are not concurrent can
make use of operating system primitives which gives the user the illusion
that they are concurrent; indeed concurrent programs can be written in
languages which are not themselves concurrent.

2.6 Language requirements

The programming language which we use to program the system must
have:

• Encapsulation primitives — there must be a number of mechanisms
for limiting the consequences of an error. It should be possible to
isolate processes so that they cannot damage each other.

• Concurrency — the language must support a lightweight mechanism
to create parallel process, and to send messages between the pro-
cesses. Context switching between process, and message passing,
should be eecient. Concurrent processes must also time-share the
CPU in some reasonable manner, so that CPU bound processes do
not monopolise the CPU, and prevent progress of other processes
which are “ready to run.”

• Fault detection primitives — which allow one process to observe an-
other process, and to detect if the observed process has terminated
for any reason.

2.7. LIBRARY REQUIREMENTS

29

• Location transparency — If we know the Pid of a process then we

should be able to send a message to the process.

• Dynamic code upgrade — It should be possible to dynamically
change code in a running system. Note that since many processes
will be running the same code, we need a mechanism to allow exist-
ing processes to run “old” code, and for “new” processes to run the
modiﬁed code at the same time.

Not only should the language satisfy these requirements, but it should
also satisfy them in a reasonably eecient manner. When we program we
do not want to limit our freedom of expression by “counting processes” etc,
nor do we want to worry about what will happen if a process inadvertently
tries to monopolise the CPU.

The maximum number of processes in the system should be sueciently
large that for programming purposes we do not have to consider this max-
imum number a limiting factor. We might need, for example, to create of
the order of one hundred thousand processes in order to make a switching
system which maintains ten thousand simultaneous user sessions.3

This mix of features is needed to simplify applications programming.
Mapping the semantics of a distributed set of communicating components
onto an Erlang program is greatly simpliﬁed if we can map the concurrent
structure of the problem in a 1:1 manner onto the process structure of the
application program which solves the problem.

2.7 Library requirements

Language is not everything—a number of things are provided in the ac-
companying system libraries. The essential set of libraries routines must
provide:

• Stable storage — this is storage which survives a crash.

3Assuming 10 processes per session.

30

CHAPTER 2. THE ARCHITECTURAL MODEL

• Device drivers — these must provide a mechanism for communica-

tion with the outside world.

• Code upgrade — this allows us to upgrade code in a running system.

• Infrastructure — for starting, and stopping the system, logging errors

etc.

Observe that our library routines, which are mostly written in Erlang,
provide most of the services which are conventionally provided by an
operating system.

Since Erlang process are isolated from each other, and communicate
only by message passing, they behave very much like operating system
processes which communicate through pipes or sockets.

Many of the features which are conventionally provided by an operat-
ing system have moved from the operating system into the programming
language. The remaining operating system only provides a primitive set of
device drivers.

2.8 Application libraries

Stable storage etc is not provided as a language primitive in Erlang, but
is provided in the basic Erlang libraries. Having such libraries is a pre-
condition for writing any complex application sodware. Complex applica-
tions need much higher-level abstractions than storage etc. To build such
applications we need pre-packaged sodware entities to help us program
things like client-server solutions etc.

The OTP libraries provide us with a complete set of design patterns
(called behaviours) for building fault-tolerant applications. In this thesis I
will talk about a minimal set of behaviours, which can be used for building
fault-tolerant applications. These are:

• supervisor — a supervision model.

• gen_server — a behaviour for implementing client-server applica-

tions.

2.9. CONSTRUCTION GUIDELINES

31

• gen_event — a behaviour used for implementing event handling

sodware.

• gen_fsm — a behaviour used for implementing ﬁnite state machines.

Of these, the central component that is used for programming fault-

tolerant applications is the supervision model.

2.9 Construction guidelines

In addition to explaining a general philosophy of programming fault-
tolerant applications, we need more speciﬁc guidelines that apply to the
programming languages that we wish to use to program our applications.
We also need example programs, and examples of how to use the library
routines.

The open source Erlang release contains such guidelines which have
been used as the basis for systems with millions of lines of Erlang code.
Appendix B reproduces the programming guidelines, which can be found
in the Erlang open source release. This thesis contains additional guide-
lines, which are organized as follows:

• The overall philosophy of our architecture is described in this chap-

ter.

• The notion of an error is discussed in several places. Sections 5.3,
and 4.3 describe what is meant by an error; section 4.4 gives advice
on the correct programing style to use when programming for errors
in Erlang.

• Examples of how to program simple components can be found in
Chapter 4, and examples of how to use the OTP behaviours in
chapter 6.

32

CHAPTER 2. THE ARCHITECTURAL MODEL

2.10 Related work

The inability to isolate sodware components from each other is the main
reason why many popular programming languages cannot be used for
making robust system sodware.

It is essential for security to be able to isolate mistrusting pro-
grams from one another, and to protect the host platform from
such programs. Isolation is diecult in object-oriented systems
because objects can easily become aliased.4—Bryce [21]

Bryce goes on to say that object aliasing is diecult if not impossible to
detect in practice, and recommends the use of protection domains (akin to
OS processes) to solve this problem.

In a paper on Java Czajkowski, and Dayn`es, from Sun Microsystems,

write:

The only safe way to execute multiple applications, written
in the Java programming language, on the same computer
is to use a separate JVM for each of them, and to execute
each JVM in a separate OS process. This introduces various
ineeciencies in resource utilization, which downgrades perfor-
mance, scalability, and application startup time. The beneﬁts
the language can ocer are thus reduced mainly to portability
and improved programmer productivity. Granted these are
important, but the full potential of language-provided safety is
not realized. Instead there exists a curious distinction between
“language safety,” and “real safety”. — [28]

In this paper they introduce the MVM (an extension to the JVM) where

their goal is:

to turn the JVM into an execution environment akin to
...
an OS. In particular, the abstraction of a process, ocered by

4An aliased object is one where at least two other objects hold a reference to it.

2.10. RELATED WORK

33

modern OSes, is the role model in terms of features; isolation
from other computations, resources accountability and control,
and ease of termination and resource reclamation.

To achieve this they conclude that:

...
tasks cannot directly share objects, and that the only way
for tasks to communicate is to use standard, copying commu-
nication mechanisms, ...

These conclusions are not new. Very similar conclusions were arrived
at some two decades earlier by Jim Gray who described the architecture
of the Tandem Computer in his highly readable paper Why do computers
stop and what can be done about it. He says:

As with hardware, the key to sodware fault-tolerance is to hier-
archically decompose large systems into modules, each mod-
ule being a unit of service and a unit of failure. A failure of a
module does not propagate beyond the module.

...

The process achieves fault containment by sharing no state
with other processes; its only contact with other processes is
via messages carried by a kernel message system. — [38]

Language which support this style of programming (parallel processes,
no shared data, pure message passing) are what Andrews and Schneider
[4] refer to as a “Message oriented languages.” The language with the
delightful name PLITS5 (1978) [35] is probably the ﬁrst example of such a
programming language:

The fundamental design decision in the implementation of
RIG6 was to allow a strict message discipline with no shared

5Programming language in the sky.
6RIG was a small system written in PLITS.

34

CHAPTER 2. THE ARCHITECTURAL MODEL

data structures. All communication between user and server
messages is through messages which are routed by the Aleph
kernel. This message discipline has proved to be very ﬂexible
and reliable. — [35]

Turning away from language for a while, we ask what properties should

an individual process have?

Schneider [60, 59] answered this question by giving three properties
that he thought a hardware system should have in order to be suitable for
programming a fault-tolerant system. These properties Schneider called:

1. Halt on failure — in the event of an error a processor should halt

instead of performing a possibly erroneous operation.

2. Failure status property — when a processor fails, other processors
in the system must be informed. The reason for failure must be
communicated.

3. Stable Storage Property — the storage of a processor should be par-
titioned into stable storage (which survives a processor crash,) and
volatile storage which is lost if a processor crashes.

A processor having these properties Schneider called a fail-stop pro-
cessor. The idea is that if a failure7 occurs it is pointless to continue. The
process should halt instead of continuing and possibly causing more dam-
age.
In a fault-stop processor, state is stored in either volatile or stable
memory. When the processor crashes all data in volatile memory is lost,
but all state that was stored in stable storage should be available ader the
crash.

A remarkably similar idea can be found in [38] where Gray talks about

“fail-fast” processes.

The process approach to fault isolation advocates that the pro-
cess sodware be fail-fast, it should either function correctly or
it should detect the fault, signal failure and stop operating.

7Here Schneider considers a failure as an error which cannot be corrected.

2.10. RELATED WORK

35

Processes are made fail-fast by defensive programming. They
check all their inputs, intermediate results and data structures
If any error is detected, they signal
as a matter of course.
a failure and stop.
In the terminology of [Cristian], fail-fast
sodware has small fault detection latency. — [38]

Both Schneider, and Gray, have the same essential idea; one is talking
about hardware, the other about sodware but the underlying principles are
the same.

Renzel argued that it is important that processes fail as soon as possible

ader an uncorrectable error has occurred:

A fault in a sodware system can cause one or more errors.
The latency time which is the interval between the existence
of the fault and the occurrence of the error can be very high,
which complicates the backwards analysis of an error ...
For an ecective error handling we must detect errors and fail-
ures as early as possible — [58]

Bearing in mind these arguments, and our original requirements I ad-

vocate a system with the following properties:

1. Processes are the units of error encapsulation — errors occurring in
a process will not acect other processes in the system. We call this
property strong isolation.

2. Processes do what they are supposed to do or fail as soon as possible.

3. Failure, and the reason for failure, can be detected by remote pro-

cesses.

4. Processes share no state, but communicate by message passing.

A language and system with such properties, has the necessary precon-
ditions for writing fault-tolerant sodware. Later in this thesis we will see
how these properties are satisﬁed by Erlang, and the Erlang libraries.

36

CHAPTER 2. THE ARCHITECTURAL MODEL

Many of the ideas in this thesis are not new—the fundamental principles

for making a fault-tolerant system are described in Gray’s [38] paper.

Many of the features of the Tandem computer bear a striking similar-
ity to the design principles in the OTP system, and to the fundamental
principles of Concurrency Oriented Programming which where discussed
earlier.

Here are two quotes from the paper, ﬁrstly the design principles on

page 15 of [38].

The keys to this sodware fault-tolerance are:

• Sodware modularity through processes, and messages.
• Fault containment through fail-fast sodware modules.
• Process-pairs to tolerant hardware, and transient sodware

faults.

• Transaction mechanisms to provide data, and message

integrity.

• Transaction mechanisms combined with process-pairs to
ease exception handling, and tolerate sodware faults.

Sodware modularity through processes and messages. As with
hardware, the key to sodware fault-tolerance is to hierarchi-
cally decompose large systems into modules, each module be-
ing a unit of service and a unit of failure. A failure of a module
does not propagate beyond the module.

There is considerable controversy about how to modularize
sodware. Starting with Burroughs’ Espol and continuing through
languages like Mesa and Ada, compiler writers have assumed
perfect hardware and contended that they can provide good
isolation through static compile-time type checking.
In con-
trast, operating systems designers have advocated run-time
checking combined with the process as the unit of protection
and failure.

2.10. RELATED WORK

37

Although compiler checking and exception handling provided
by programming languages are real assets, history seems to
have favored the run-time checks plus the process approach to
fault-containment. It has the virtue of simplicity—if a process or
its processor misbehaves, stop it. The process provides a clean
unit of modularity, service, fault containment and failure.
Fault containment through fail-fast sodware modules.
The process achieves fault containment by sharing no state
with other processes; its only contact with other processes is
via messages carried by a kernel message system. — [38]

If we compare this to our current Erlang system we see many striking
similarities. There are certain dicerence—in Erlang “defensive program-
ming” is not recommended since the compiler adds the necessary checks
to make this style of programming unnecessary. Gray’s “transaction mech-
anism” is provides by the mnesia data base.8 The containment and pro-
cessing of errors is managed by the “supervision tree” behaviours in the
OTP libraries.

The idea of “fail-fast” modules is mirrored in our guidelines for pro-
gramming where we say that processes should only do when they are sup-
posed to do according to the speciﬁcation, otherwise they should crash.
The supervision hierarchies in our system correspond to the hierarchies of
modules that Gray refers to. This idea can also be found in the work of
Candea and Fox [22] who talked about “crash-only sodware”—they argue
that allowing components to crash and then restart leads to a simpler fault
model and more reliable code.

More modern work with object-oriented systems has also recognised
the importance of isolating sodware components from each other. In [21]
Bryce and Razaﬁmahefa argue that is is essential to isolate programs from
one another, and from the programs which run in the host operating sys-
tem. This, they consider,
is the essential characteristic that any object
system must have. As they point out in their paper, this is a diecult
problem in an object-oriented context.

8Written in Erlang.

38

CHAPTER 2. THE ARCHITECTURAL MODEL

3 Erlang

T his chapter introduces Erlang. The treatment of the language is not

intended to be complete. For fuller treatment the reader is referred
to [5]. Developments to Erlang since [5] can be found in the OTP
documentation [34]. A more formal treatment of Erlang can be found in
the Erlang Speciﬁcation [17] and in the core Erlang speciﬁcation [23].

Erlang belongs to the class of Message-oriented languages [4] — mes-
sage oriented languages provide concurrency in the form of parallel pro-
cesses. There are no shared objects in a message-oriented language. In-
stead all interaction between processes is achieved by sending and receiv-
ing messages.

In this chapter, I present a subset of the language which provides

enough detail to understand the Erlang examples in this thesis.

3.1 Overview

The Erlang view of the world can be summarized in the following state-
ments:

• Everything is a process.

• Processes are strongly isolated.

• Process creation and destruction is a lightweight operation.

39

40

CHAPTER 3. ERLANG

• Message passing is the only way for processes to interact.

• Processes have unique names.

• If you know the name of a process you can send it a message.

• Processes share no resources.

• Error handling is non-local.

• Processes do what they are supposed to do or fail.

The use of processes as the basic unit of abstraction is motivated by the
desire to make a language which is suitable for writing large fault-tolerant
sodware systems. The fundamental problem which must be solved in
writing such sodware is that of limiting the consequences of an error—
the process abstraction provides an abstraction boundary which stops the
propagation of errors.

It is, for example, precisely this inability to limit the consequences of
errors that makes Java unsuitable for programming “safe” (sic) applications
(see page 32 for further discussion of this point).

If processes are truly isolated (which they must be to limit the conse-
quences of an error) then most of the other properties of a process, like, for
example, that the only way for processes to interact is by message passing,
etc, follow as a natural consequence of this isolation.

The statement about error handling is perhaps less obvious. When
we make a fault-tolerant system we need at least two physically separated
if it crashes, all is
computers. Using a single computer will not work,
lost. The simplest fault-tolerant system we can imagine has exactly two
computers, if one computer crashes, then the other computer should take
over what the ﬁrst computer was doing. In this simple situation even the
sodware for fault-recovery must be non-local; the error occurs on the ﬁrst
machine, but is corrected by sodware running on the second machine.

The Erlang view of the world is that “everything is a process”, when
we model our physical machines as processes we retain the idea that error
handling should be non-local. Actually, this is a modiﬁed truth, remote

3.2. EXAMPLE

41

error handling only occurs if a local attempt to ﬁx the error fails. In the
event of an exception a local process may be able to detect and correct the
fault which caused the exception, in which case as far as any other process
in the system is concerned, no error has occurred.

Viewed as a concurrent language, Erlang is very simple. Since there are
no shared data structures, no monitors or synchronised methods etc there
is very little to learn. The bulk of the language, and possible the least
interesting part of the language is the sequential subset of the language.
This sequential subset can be characterised as a dynamically typed, strict
functional programming language, which is largely free from side-ecects.
In the sequential subset there are a few operations with side-ecects, but
they are virtually never needed.

The remainder of this chapter deals ﬁrstly with the sequential subset of
the language. This is followed with sections on concurrent and distributed
programming and error handling. Finally I describe a type notation for
specifying Erlang data and function types.

To jump start the description, I start with an example of sequential

Erlang code.

3.2 Example

Figure 3.1 has a simple Erlang program. The program has the following

structure:

1. The program starts with a module deﬁnition (line 1) followed by
export and input declarations and then by a number of functions.

2. The export declaration (line 2) says that the function areas/1 is to
be exported from this module. The notation areas/1 means the
function called areas which has one argument. The only functions
which can be called from outside the module are those which are
contained in the export list.

3. The import declaration in line 3 says that the function map/2 can be

found in the module lists.

42

CHAPTER 3. ERLANG

1 -module(math).
2 -export([areas/1]).
3 -import(lists, [map/2]).

4

5 areas(L) ->

lists:sum(

map(

fun(I) -> area(I) end,
L)).

6

7

8

9

10

11 area({square, X}) ->

12

X*X;

13 area({rectangle,X,Y}) ->

14

X*Y.

Figure 3.1: An Erlang module

3.2. EXAMPLE

43

4. Lines 5 to 14 have two function deﬁnitions.

5. Line 6 is a call to the function sum in the module lists.

6. Lines 7 to 9 are a call to the function map/2 in the module lists.
Note the dicerence between this call to sum and the call to map - both
these functions are in the same module; one call uses a fully qualiﬁed
name (that is, lists:sum) whereas the other call uses an abbreviated
call sequence (that is map(...) instead of lists:map(...)). The
dicerence is accounted for by the import declaration in line 3, which
says that the function map/2 is to be found in the module lists.

7. Line 8 creates a fun which is the ﬁrst argument to map.

8. Lines 11 to 14 contain the function area/1. This function has two
clauses. The ﬁrst clause is in lines 11 to 12, the second in lines 13 to
14, the clauses are separated by a semi-colon.

9. Each clause has a head and a body. The head and body are separated

from each other by a “->” symbol.

10. A function head consists of a pattern in each argument position
and a possible guard (See Section 3.3.4). In line 13 the pattern is
{rectangle,X,Y}. In this pattern the curly bracket denote a tuple.
The ﬁrst argument of the tuple is an atom (namely “rectangle”)
and the second and third arguments are variables. Variables start
with capital letters, atoms start with small letters.

To run this program we start an Erlang shell compile the program and
enter some requests to evaluate functions, as shown in Figure 3.2. In this
ﬁgure all user input is underlined. The Erlang shell prompt is the character
“>” meaning that the system is waiting for input.

• Line 1 in ﬁgure 3.2 starts an Erlang shell.

• Line 5 compiles the module math.

44

1

2

3

4

5

6

7

8

9

10

11

12

13

CHAPTER 3. ERLANG

$ erl
Erlang (BEAM) emulator version 5.1 [source]

Eshell V5.1 (abort with ^G)
1> c(math).
ok,math
2> math : areas([{rectangle, 12, 4}, {square, 6}]).
84
3> math : area({square, 10}).
** exited: {undef,[{math,area,[{square,10}]},

{erl_eval,expr,3},
{erl_eval,exprs,4},
{shell,eval_loop,2}]} **

Figure 3.2: Compiling and running a program in the shell

• Line 7 requests a function evaluation, the shell accepts the request,

evaluates the function and prints the result in line 8.

• In line 9 we try to evaluate a function which was not exported from
the module math. An exception is generated and printed (lines 10
to 13).

3.3 Sequential Erlang

3.3.1 Data structures

Erlang has eight primitive data types:1

• Integers — integers are written as sequences of decimal digits, for
example, 12, 12375 and -23427 are integers. Integer arithmetic is

1Also called constants.

3.3. SEQUENTIAL ERLANG

45

exact and of unlimited precision.2

• Atoms — atoms are used within a program to denote distinguished
values. They are written as strings of consecutive alphanumeric char-
acters, the ﬁrst character being a small letter. Atoms can obtain any
character if they are enclosed within single quotes and an escape
convention exists which allows any character to be used within an
atom.

• Floats — ﬂoating point numbers are represented as IEEE 754 [43]
64 bit ﬂoating point numbers. Real numbers in the range ±10308
can be represented by an Erlang ﬂoat.

• References — references are globally unique symbols whose only
property is that they can be compared for equality. They are created
by evaluating the Erlang primitive make_ref().

• Binaries — a binary is a sequence of bytes. Binaries provide a
space-eecient way of storing binary data. Erlang primitives exist for
composing and decomposing binaries and for eecient input/output
of binaries. For a full treatment of binaries see [34].

• Pids — Pid is short for Process Identiﬁer—a Pid is created by the Er-
lang primitive spawn(...) Pids are references to Erlang processes.

• Ports — ports are used to communicate with the external world.
Ports are created with the BIF3 open_port. Messages can be sent to
and received from ports, but these message must obey the so-called
“port protocol.”

• Funs — Funs are function closures.4 Funs are created by expressions

of the form: fun(...) -> ... end.

And two compound data types:

2The precision of integers is only limited by available memory.
3BIF is short for Built In Function.
4Called lambda expressions in other languages.

46

CHAPTER 3. ERLANG

• Tuples — tuples are containers for a ﬁxed number of Erlang data
types. The syntax {D1,D2,...,Dn} denotes a tuple whose argu-
ments are D1, D2, ... Dn. The arguments can be primitive data types
or compound data types. The elements of a tuple can be accessed
in constant time.

• Lists — lists are containers for a variable number of Erlang data
types. The syntax [Dh|Dt] denotes a list whose ﬁrst element is
Dh, and whose remaining elements are the list Dt. The syntax []
denotes an empty list.

The syntax [D1,D2,..,Dn] is short for [D1|[D2|..|[Dn|[]]]].
The ﬁrst element of a list can be accessed in constant time. The ﬁrst
element of a list is called the head of the list. The remainder of a list
when its head has been removed is called the tail of the list.

Two forms of syntactic sugar are provided:

• Strings — strings are written as doubly quoted lists of characters,
this is syntactic sugar for a list of the integer ASCII codes for the
characters in the string, thus for example, the string "cat" is short-
hand for [97,99,116].

• Records — records provide a convenient way for associating a tag
with each of the elements in a tuple. This allows us to refer to an
element of a tuple by name and not by position. A pre-compiler
takes the record deﬁnition and replaces it with the appropriate tuple
reference.

3.3.2 Variables

Variables in Erlang are sequences of characters starting with a upper case
letter and followed by a sequence of letters or characters or the “_” char-
acter.

Variables in Erlang are either unbound, meaning they have no value, or
bound, meaning that they have a value. Once a variable has been bound

3.3. SEQUENTIAL ERLANG

47

the value can never be changed. Such variables are called single assignment
variables. Since variable values cannot ever be changed the programmer
must create a new variable every time they want to simulate the ecect of
a destructive assignment.

Thus, for example, the Erlang equivalent of the C expression:

x = 5;
x = x + 10;

is written:

X = 5;
X1 = X + 10;

Where we invent a new variable X1 since we cannot change the value

of X.

3.3.3 Terms and patterns

A Ground term is deﬁned recursively as either a primitive data type, or a
tuple of ground terms or a list of ground terms.

A Pattern is deﬁned recursively as either a primitive data type or a

variable or a tuple of patterns or a list of patterns.

A Primitive pattern is a pattern where all the variables are dicerent.
Pattern matching is the act of comparing a pattern with a ground term.
If the pattern is a primitive pattern and the ground terms are of the same
shape, and if the constants occurring in the pattern occur in the ground
term in the same places as in the pattern then the match will succeed,
otherwise it will fail. Any variables occurring in the pattern will be bound
to the corresponding data items at the same positions in the term. This
process is called uniﬁcation.

More formally if P is a primitive pattern and T is a term, then we say

that P matches T ic:

• If P is a list with head Ph and tail Pt and T is a list with Th and tail

Tt then Ph must match Th and Pt must match Tt.

48

CHAPTER 3. ERLANG

• If P is a tuple with elements {P1,P2,...,Pn} and T is a tuple with
elements {T1,T2,...,Tn} then P1 must match T1, P2 must match
T2 and so on.

• If P is a constant then T must be the same constant.

• If P is a free variable V then V is bound to T.

Here are some examples:
The pattern {P,abcd} matches the term {123,abcd} creating the

binding P 7→ 123.

The pattern [H|T] matches the term "cat" creating the bindings H 7→

99 and T 7→ [79,116].

The Pattern {abc,123} does not match the term {abc,124}.

3.3.4 Guards

Guards are expressions involving only predicates. They are written imme-
diately ader primitive patterns and introduced with the keyword when. For
example we call the program fragment:

{P,abc,123} when P == G

a guarded pattern.
Guards are written as comma-separated sequences of guard tests, where

each guard test is of the form:

T1 Binop T2

where T1 and T2 are ground terms.
The available binary operators are:

3.3. SEQUENTIAL ERLANG

49

Operator Meaning
X > Y
X < Y
X =< Y
X >= Y
X == Y
X /= Y
X =:= Y X is equal to Y
X =/= Y X is not equal to Y

X is greater than Y
X is less than Y
X is equal to or less than Y
X is greater than or equal to Y
X is equal to Y
X is not equal to Y

When a guard is used as an expression, it will always evaluate to one
of the atoms true or false. If the guard evaluates to true we say that
the evaluation succeeded otherwise it failed.

3.3.5 Extended pattern matching
In a primitive pattern all variables must be dicerent. An extended pattern
has the same syntax as a primitive pattern except that all the variables are
not required to be dicerent.

To perform pattern matching with extended patterns we ﬁrst convert
the extended pattern to a primitive pattern and guard and then match the
primitive pattern.

If the variable X is repeated in the pattern N times then rename the
second and all subsequent occurrences of X with a fresh variables.5 F1, F2
etc. For each fresh variable add a predicate Fi == X to the guard.

Using these rules

{X,a,X,[B|X]}

is transformed into

{X,a,F1,[B|F2]} when F1==X, F2==X

Finally, the pattern variable ’_’ is taken to mean the “anonymous vari-
able.” The anonymous variable matches any term and no variable binding
is created.

5A fresh variable is one that does not occur anywhere in the current lexical context.

50

CHAPTER 3. ERLANG

3.3.6 Functions

Functions obey the following rules:

1. A function is composed of one or more clauses which are separated

by semi-colons.

2. A clause has a head followed by a separator -> followed by a body.

3. A function head is composed of an atom followed by a parenthesized
set of patterns followed by an optional guard. The guard, if present,
is introduced by the when keyword.

4. A function body consists of a sequence of comma-separated expres-

sions.

Or,

FunctionName(P11,...,P1N) when G11,...,G1N ->

Body1;

FunctionName(P21,...,P2N) when G11,...,G1N ->

Body2;

...
FunctionName(PK1, PK2, ..., PKN) ->

BodyK.

Where P11, ..., PKN are the extended patterns described in the

previous section.

Here are two examples:

factorial(0) -> 1;
factorial(N) -> N * factorial(N-1).

member(H, [H|T]) -> true;
member(H, [_|T] -> member(H, T);
member(H, [])

-> false.

3.3. SEQUENTIAL ERLANG

51

Function execution is as follows:
To evaluate Fun(Arg1,Arg2,...,ArgN) we ﬁrst search for a deﬁni-
tion of the function. The corresponding deﬁnition is taken to be the ﬁrst
function whose patterns in the head of the clause match the arguments
Arg1..Argn in the function call. If the pattern match succeeds and if any
associated guard test succeeds then the body of the clause is evaluated.
All free variables occurring in the patterns in the head of the clause have
values that were obtained as a result of matching the pattern in the clause
head with the actual arguments provided in the call. As an example we
evaluate the expression member(dog,[cat,man,dog,ape]) showing all
steps taken. We assume the following deﬁnition of member:

member(H, [H1|_]) when H == H1 -> true;
member(H, [_|T] -> member(H, T);
member(H, [])

-> false.

1. Evaluate member(dog, [cat,man,dog,ape])

2. The ﬁrst clause matches with bindings {H 7→ dog, H1 7→ cat}. The

guard test then fails.

3. The second clause matches with {H 7→ dog, T 7→ [man,dog,ape]},
there is no guard test so the system evaluates member(H,T) with the
current bindings of H and T.

4. Evaluate member(dog, [man,dog,ape])

5. As before. This time the second clause matches with bindings

{H 7→ dog, T 7→ [dog,ape]}

6. Evaluate member(dog, [dog,ape])

7. The ﬁrst clause matches with bindings {H 7→ dog, H1 7→ dog}. The

guard test in the ﬁrst clause succeeds.

8. Evaluate true which is just true.

52

CHAPTER 3. ERLANG

Note that each time a function clause is entered a fresh set of variable
bindings is used, so that the values of the variable H and T in step 3 above
are distinct from those in step 5.

3.3.7 Function bodies

Function bodies are sequences of expressions. The value of a sequence of
expressions is obtained by sequentially evaluating each element in the se-
quence. The value of the body is the result of evaluating the last expression
in the sequence.

For example, suppose we deﬁne a function to manipulate a bank ac-

count:

deposit(Who, Money) ->
Old = lookup(Who),
New = Old + Money,
insert(Who, New),
New.

The body of this function consists of a sequence of four statements. If
we evaluate the expression deposit(joe, 25) then the function will be
entered with bindings {Who 7→ joe, Money 7→ 10}. Then lookup(Who)
will be called. Assume this returns W. The return value (W) is matched
against the free variable Old, the match succeeds. Ader this match we
continue with the set of bindings {Who 7→ joe, Money 7→ 10, Old 7→ W} ...

3.3.8 Tail recursion

A function call is tail-recursive if all the last calls in the function body are
calls to other functions in the system.

For example, consider the following functions:

p() ->

...
q(),

3.3. SEQUENTIAL ERLANG

53

...

q() ->

r(),
s().

At some point in the execution of p the function q is called. The ﬁnal
function call in q is a call to s. When s returns it returns a value to q, but
q does nothing with the value and just returns this value unmodiﬁed to p.
The call to s at the end of the function q is called a tail-call and on a
traditional stack machine tail-calls can be compiled by merely jumping into
the code for s. No return address has to be pushed onto the stack, since
the return address on the stack at this point in the execution is correct and
the function s will not return to q but to the correct place where q was
called from in the body of p.

A function is tail-recursive if all possible execution paths in the function

ﬁnish with tail-calls.

The important thing to note about tail-recursive functions is that they
can run in loops without consuming stack space. Such function are oden
called “iterative functions.”

Many functions can be written in either an iterative of non-iterative
(recursive) style. To illustrate this, the factorial function can be written in
these two dicerent styles. Firstly the non tail-recursive way:

factorial(0) -> 1;
factorial(N) -> N * factorial(N-1).

To write this in a tail-recursive manner requires the use of an additional

function:

factorial(N) -> factorial_1(N, 1).

factorial_1(0, X) -> X;
factorial_1(N, X) -> factorial_1(N-1, N*X).

54

CHAPTER 3. ERLANG

Many non-tail recursive functions can be made tail-recursive, by intro-

ducing an auxiliary function, with an additional argument.6

Many functions in Erlang are designed to run in inﬁnite loops—in partic-
ular the client–server model assumes that the server will run in an inﬁnite
loop. Such loops must be written in a tail-recursive manner. A typical
inﬁnite loop in a server might be written something like:

loop(Dict) ->

receive

{store, Key, Value} ->

loop(dict:store(Key, Value, Dict));

{From, {get, Key}} ->

From ! dict:fetch(Key, Dict),
loop(Dict)

end.

which is tail recursive.

3.3.9 Special forms

Two special forms are used for conditional evaluation of Erlang expression
sequences. They are the case and if statements.

3.3.10 case

case has the following syntax:

case Expression of

Pattern1 -> Expr_seq1;
Pattern2 -> Expr_seq2;
...

end

6Called an accumulator.

3.3. SEQUENTIAL ERLANG

55

case is evaluated as follows: Firstly, Expression is evaluated, assume
this evaluates to Value. Thereader Value is matched in turn against Pat-
tern1, Pattern2 ... etc. until a match is found. As soon as a match is found
to some pattern Pattern[i] then the corresponding expression sequence
Expr seq[i] is evaluated—the result of evaluating the expression sequence
Expr seq[i] becomes the value of the case statement.

3.3.11 if

A second conditional primitive if is also provided. The syntax:

if

Guard1 ->

Expr_seq1;

Guard2 ->

Expr_seq2;

...

end

is evaluated as follows: Firstly Guard1 is evaluated, if this evaluates to
true then the value of if is the value obtained by evaluating the expression
sequence Expr seq1.
is evaluated
until a match is found. At least one of the guards in the if statement must
evaluate to true otherwise an exception will be raised. Oden the ﬁnal
guard in an if statement is the atom true which guarantees that the last
form in the statement will be evaluated if all other guards have failed.

If Guard1 does not succeed Guard2...

3.3.12 Higher order functions

Higher order functions are functions which take functions as input argu-
ments or produce functions as return values. An example of the former is
the function map found in the lists module, which is deﬁned as follows:

map(Fun, [H|T]) -> [Fun(H)|map(Fun, T)];
map(Fun, [])

-> [].

56

CHAPTER 3. ERLANG

map(F, L) produces a new list by applying the function F to every

element of the list L, so for example:

> lists:map(fun(I) -> 2 *I end, [1,2,3,4]).
[2,4,6,8]

Higher order functions can be used to create control abstractions for

syntactic constructions that do not exist in the language.

For example, the programming language C provides a looping con-

struct for, which can be used as follows:

sum = 0;
for(i = 0; i < max; i++){

sum += f(i)

}

Erlang has no for loop but we can easily make one:

for(I, Max, F, Sum) when I < Max ->

for(I+1, Max, F, Sum + F(I));

for(I, Max, F, Sum) ->

Sum.

which could be used as follows:

Sum0 = 0,
Sum = for(0, Max, F, Sum0).

Functions which return new functions can also be deﬁned. The follow-

ing example in the Erlang shell illustrates this:

1> Adder = fun(X) -> fun(Y) -> X + Y end end.
#Fun<erl_eval.5.123085357>
2> Adder10 = Adder(10).
#Fun<erl_eval.5.123085357>
3> Adder(10).
15

3.3. SEQUENTIAL ERLANG

57

Here the variable Adder contains a function of X; evaluating Adder(10)

binds X to 10 and returns the function fun(Y) -> 10 + Y end.

With devilish ingenuity recursive functions can also be deﬁned, for

example, factorial:

6> Fact = fun(X) ->

G = fun(0,F) -> 1;

(N, F) -> N*F(N-1,F)

end,

G(X, G)

end.

#Fun<erl_eval.5.123085357>
7> Fact(4).
24

Functions can be referred to with the syntax fun Name/Arity. For

example, the expression:

X = fun foo/2

is shorthand for writing:

X = fun(I, J) -> foo(I, J) end

where I and J are free variables which do not occur at any other place

in the function where X is deﬁned.

3.3.13 List comprehensions

List comprehensions are expression which generate lists of values. They
are written with the following syntax:

[X || Qualifier1, Qualifier2, ...]

X is an arbitrary expression, and each qualiﬁer is either a generator or

a ﬁlter.

58

CHAPTER 3. ERLANG

• Generators are written as Pattern<-ListExpr where ListExpr

must be an expression which evaluates to a list of terms.

• Filters are either predicates or boolean expressions.

As an example, the well-known quicksort algorithm can be expressed

in terms of two list comprehensions:

qsort([]) -> [];
qsort([Pivot|T]) ->

qsort([X||X<-T,X =< Pivot]) ++
[Pivot] ++
qsort([X||X<-T,X > Pivot]).

Where ++ is the inﬁx append operator.
If you are interested in crossword puzzles and need to compute all

permutations of a string then you could use the function perms where:

perms([]) -> [[]];
perms(L) -> [[H|T] || H <- L, T <- perms(L--[H])].

Where the inﬁx operator X--Y is a copy of the list X where any element

of X which also occurs in Y have been removed.

So for example:

> perms("123").
["123","132","213","231","312","321"]

3.3.14 Binaries
Binaries are memory bucers designed for storing untyped data. Binaries
are used primarily to store large quantities of unstructured data and for
eecient I/O operations. Binaries store data in a much more space-eecient
manner than in lists or tuples. For example, a string stored as a list needs
eight bytes of storage per character, whereas a string stored in a binary
needs only one byte per character plus a small constant overhead.

3.3. SEQUENTIAL ERLANG

59

The BIF list_to_binary converts a io-list to a binary, the inverse is
computed with binary_to_list; term_to_binary converts an arbitrary
term to a binary, the inverse is binary_to_term.

Note: An io-list is a list whose elements are either small integers (a
small integer is an integer in the range 0 to 255) or binaries, or io-lists.
The BIF list-to_binary(A) ﬂattens the io-list and produces a binary
constructed from this list. binary_to_list/1 returns a ﬂat list of small
integers. binary_to_list is only a strict inverse of list_to_binary(A)
in the case where A is a ﬂat list of small integers.

Lists of binaries can be concatenated with concatenate_binaries,

and a single binary can be split into two binary with split_binary.
We can illustrate a number of operations on binaries, in the shell:

1 > B1=list_to_binary([1,2,3]).
<<1,2,3>>
2> B2=list_to_binary([4,5,[6,7],[],[8,[9]],245]).
<<4,5,6,7,8,9,245>>
3 > B3=concat_binary([B1,B2]).
<<1,2,3,4,5,6,7,8,9,245>>
4> split_binary(B3,6).
{<<1,2,3,4,5,6>>, <<7,8,9,245>>}

Expression 1 converts the list [1,2,3] to a binary B1. Here the no-
tation <<I1,I2,...>> represents the binary made from the bytes I1,I2
...

Expression 2 converts an io-list to a binary.
Expression 3 combines two binaries B1 and B2 into a single binary B3,

and expression 4 splits B4 into two binaries.

5> B = term_to_binary({hello,"joe"}).
<<131,104,2,100,0,5,104,101,108,108,111,107,

0,3,106,111,101>>
6> binary_to_term(B).
{hello,"joe"}

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CHAPTER 3. ERLANG

The BIF term_to_binary converts its argument into a binary. The
inverse function is binary_to_term which reconstructs the term from
the binary. The binary produced by term_to_binary is stored in the
so-called “external term format.” Terms which have been converted to
binaries by using term_to_binary can be stored in ﬁles, sent in messages
over a network etc and the original term from which they were made can
be reconstructed later. This is extremely useful for storing complex data
structures in ﬁles or sending complex data structures to remote machines.

3.3.15 The bit syntax

The bit syntax provides a notation for constructing binaries and for pattern
matching on the contents of binaries. To understand how binaries are
constructed I will give a number of examples in the shell:

1> X=1,Y1=1,Y2=255,Y3=256,Z=1.
1
2> <<X,Y1,Z>>.
<<1,1,1>>
3> <<X,Y2,Z>>.
<<1,255,1>>
4> <<X,Y3,Z>>.
<<1,0,1>>
5> <<X,Y3:16,Z>>.
<<1,1,0,1>>
6> <<X,Y3:32,Z>>.
<<1,0,0,1,0,1>>

In line 1 a few variables X, Y1..Y3 and Z and deﬁned. Line 2 constructs

a binary from X,Y1 and Z the result is just <<1,1,1>>.

In Line 3 Y2 is 255 and the value of Y2 is copied unchanged to the
second byte of the binary. When we try to create a binary from Y3 (which
is 256) the value is truncated, since 256 will not ﬁt into a single byte. The
quantiﬁer :16 added in line 5 corrects this problem.

If we do not quantify the size of an integer it is assumed to take up 8

bits. Line 6 shows the ecect of a 32-bit quantiﬁer.

3.3. SEQUENTIAL ERLANG

61

Not only can we specify the size of an integer, but also the byte order,

so for example:

7> <<256:32>>.
<<0,0,1,0>>
8> <<256:32/big>>.
<<0,0,1,0>>
9> <<256:32/little>>.
<<0,1,0,0>>

Line 7 says create a binary from the integer 32 and pack this integer
into 32 bits. Line 8 tells the system to create a 32-bit integer with “big
endian” byte order and line 9 says use little endian byte order.

Bit ﬁelds can also be packed:

10> <<1:1,2:7>>.
<<130>>

creates a single byte binary from a 1-bit ﬁeld followed by a 7-bit ﬁeld.
Unpacking binaries is the opposite to packing them, thus:

11> <<X:1,Y:7>> = <<130>>.
<<130>>
12> X.
1
13> Y.
2

is the inverse of line 10.
The pattern machine operation on binaries was originally designed for
processing packet data. Pattern matching is performed over a sequence of
zero or more “segments”. Each segment is written with the syntax:

Value:Size/TypeSpecifierList

Here TypeSpecifierList is a hyphen-separated list of items of the

form End-Sign-Type-Unit, where:

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CHAPTER 3. ERLANG

• End — speciﬁes the endianess of the machine and is one of big,

little or native.

• Sign — is one of signed or unsigned.

• Type — is one of integer, float, or binary.

• Unit — is of the form unit:Int where Int is a literal integer in
the range 1..256. The total size of the segment is Size x Int bits
long, this size must be a multiple of eight bits.

Any of the above items may be omitted and the items can occur in

any order.

A nice example of the use of binaries can be found in the Erlang
Open Source Distribution [34] in the section entitled “Erlang Extensions
Since 4.4.” This shows how to parse an IP version four datagram in a
single pattern-matching operation.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

-define(IP_VERSION, 4).
-define(IP_MIN_HDR_LEN, 5).

...
DgramSize = size(Dgram),
case Dgram of

<<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16,

ID:16, Flgs:3, FragOff:13,
TTL:8, Proto:8, HdrChkSum:16,
SrcIP:32,
DestIP:32, RestDgram/binary>> when HLen >= 5, 4*HLen =< DgramSize ->

OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
<<Opts:OptsLen/binary,Data/binary>> = RestDgram,
...

Lines 7–11 match the IP datagram in a single pattern-matching expres-
sion. The pattern is complex spreading over three lines and illustrating
how data which does not fall on byte boundaries can easily be extracted
(for example, the Flgs and FragOff ﬁelds which are 3 and 13 bits long
respectively).

Having pattern matched the IP datagram, the header and data part of

the datagram can be isolated (lines 12–13).

3.3. SEQUENTIAL ERLANG

63

3.3.16 Records

Records provide a method for associating a name with a particular element
in a tuple. The problem with tuples is that when the number of elements
in a tuple becomes large, it is diecult to remember which element in the
tuple means what.

In a small tuple this is rarely a problem, so we oden see programs

which manipulated tuples with a small number of elements in the tuple.

As the number of elements in the tuples becomes greater it becomes
more and more diecult to keep track of the meanings of the individual
elements in the tuple. When the number of elements in the tuple is large,
or if we wish to name the elements in the tuple for other purposes7 then
we can use records instead of tuples.

Record declarations are written with the syntax:

-record(Name, {

Key1 = Default1,
Key2 = Default2,
...

}).

Here Name is the name of the record. Key1, Key2 ... are the names
of the ﬁelds in the record. Each ﬁeld in a record can have a default value
which is used if no value for this particular ﬁeld is speciﬁed when the
record is created.

For example, we might deﬁne a person record as follows:

-record(person, {

firstName="",
lastName = "",
age}).

Once a record has been deﬁned, instance of the record can be created.

For example:

7for example, documentation.

64

CHAPTER 3. ERLANG

Person = #person{firstName="Rip",

lastname="Van Winkle",
age=793
}

creates a “Rip Van Winkel”8 person.
We can write functions which pattern match on the ﬁelds of a record
and which create new records, so when Mr. Van Winkel has his birthday
we can call:

birthday(X=#person{age=N}) ->

X#person{age=N+1}.

If the clause head matches the record then X is bound to the entire
record, and N is bound to the age ﬁeld of the record. X#person{age=K}
makes a copy of X replacing the old value of the ﬁeld named age in X with
the value K.

3.3.17 epp

Before an Erlang module is compiled it is processed by the Erlang pre-
processor epp. The Erlang pre-processor performs macro expansion and
inserts any necessary include ﬁles.

The output of the pre-processes can be saved in a ﬁle by giving the
command compile:file(M, [’P’]). This compiles any code in the ﬁle
M.erl and produces a listing in the ﬁle M.P where all macros have been
expanded and any necessary include ﬁles have been included.

3.3.18 Macros

Erlang macros are written:

-define(Constant, Replacement).
-define(Func(Var1, Var2,.., Var), Replacement).

8Even Mr. Google did not know how old Mr. Van Winkel was, so 793 is pure guesswork.

3.3. SEQUENTIAL ERLANG

65

Macros are expanded by the Erlang pre-processor epp when an ex-
pression of the form ?MacroName is encountered. Variables occurring in
macro deﬁnition match complete forms in the corresponding site of the
macro call.

-define(macro1(X, Y), {a, X, Y}).

foo(A) ->

?macro1(A+10, b)

expands into:

foo(A) ->

{a,A+10,b}.

The argument in a call to a macro and the return value from a macro
must be a complete, well-balanced expression. Thus it is not possible to
use macros like the following:

-define(start, {).
-define(stop, }).

foo(A) ->

?start,a,?stop.

In addition, there are a number of predeﬁned macros which provide

information about the current module. They are as follows:

• ?FILE expands to the current ﬁle name.

• ?MODULE expands to the current module name.

• ?LINE expands to the current line number.

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CHAPTER 3. ERLANG

3.3.19 Include ﬁles

Files can be included with the syntax:

-include(Filename).

Conventional include ﬁles have the extension .hrl. The FileName
should contain an absolute or relative path so that the preprocessor can
locate the appropriate ﬁle.

Library header ﬁles can be included with the syntax

-include_lib(Name).

For example:

-include_lib("kernel/include/file.hrl").

In which case the Erlang compiler will ﬁnd the appropriate include

ﬁles.

3.4 Concurrent programming

In Erlang, creation of a parallel process is achieved by evaluating the spawn
primitive. The expression:

Pid = spawn(F)

Where F is a fun of arity zero creates a parallel process which evaluates
F. Spawn returns a process identiﬁer (Pid) which can be used to access the
newly created process.

The syntax Pid ! Msg sends the message Msg to Pid. The message

can be received using the receive primitive, with the following syntax:

3.4. CONCURRENT PROGRAMMING

67

receive

Msg1 [when Guard1] ->

Expr_seq1;

Msg2 [when Guard2] ->

Expr_seq2;

...
MsgN [when GuardN] ->

Expr_seqN;

...
[; after TimeOutTime ->

Timeout_Expr_seq]

end

Msg1...MsgN are patterns. The patterns may be followed by optional
guards. When a message arrives at a process it is put into a mailbox
belonging to that process. The next time the process evaluates a receive
statement the system will look in the mailbox and try to match the ﬁrst item
in the mailbox with the set of patterns contained in the current receive
statement.
If no message matches then the received message is moved
to a temporary “save” queue and the process suspends and waits for the
next message. If the message matches and if any guard test following the
matching pattern also matches then the sequence of statements following
the match are evaluated. At the same time, any saved messages are put
back into the input mailbox of the process.

The receive statement can have an optional timeout. If no matching
message is received within the timeout period then the commands associ-
ated with the timeout are evaluated.

3.4.1 register

When we send a message to a process, we need to know the name of the
process. This is very secure, but somewhat inconvenient since all processes
which want to send a message to a given process have to somehow obtain
the name of that process.
The expression:

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CHAPTER 3. ERLANG

register(Name, Pid)

creates a global process, and associates the atom Name with the process
identiﬁer Pid. Thereader, messages sent by evaluating Name!Msg are sent
to the process Pid.

3.5 Error handling

Evaluating a function in Erlang will result in exactly one of two possible
outcomes: either the function will return a value, or, it will generate an
exception.

Generating exceptions is either implicit (that is, generated by the Er-
lang run-time system) or is explicitly generated by evaluating the primitive
exit(X). Implicit exceptions are described in the next section.

Here is an example of implicit exception generation. Suppose we write:

factorial(0) -> 1;
factorial(N) -> N*factorial(N-1).

Evaluating factorial(10) returns the Value 3628800, but evaluating
factorial(abc) raises the exception {’EXIT’,{badarith,...}. Ex-
ceptions cause the program to stop what it is doing and do something
else—that is why they are called exceptions. If we write:

J = factorial(I)

we expect the value of J to be the value of factorial(I) if I is an
integer. If factorial is called with a non-integer argument the statement
makes no sense. The program fragment:

I = "monday",
J = factorial(I),

is nonsense, since we cannot compute factorial("monday"). J thus

has no value and it is pointless to proceed.

Many programming languages ignore the distinction between values
and exceptions and blithely continue even though the program is non-
sense.

3.5. ERROR HANDLING

3.5.1 Exceptions

69

Exceptions are abnormal conditions which are detected by the Erlang run-
time system. Erlang programs are compiled to virtual machine instructions
which are executed by a virtual machine emulator which is part of the
Erlang run-time system.

If the emulator detects a condition where it cannot decide what to do,

it generates an exception. There are six types of exception:

1. Value errors — these are things like “divide by zero”. Here an argu-
ment to a function has the correct type, but an incorrect value.

2. Type errors — these are generated when an Erlang BIF is called
with an argument which has an incorrect type. For example, the BIF
atom_to_list(A) converts the atom A to a list of the ASCII integer
codes which represent the atom. If A is not an atom, the run-time
system generates an exception.

3. Pattern-matching errors — these are generated when an attempt is
made to match a data structure against a number of patterns and
no matching pattern is found. This can occur in function head
matching, or in matching the alternatives in a case, receive or
if statement.

4. Explicit exits — these are generated explicitly by evaluating the ex-

pression exit(Why), which generates the exception Why.

5. Error propagation — if a process receives an exit signal it may decide
to die and propagate the exit signal to all processes in its link set (see
section 3.5.6).

6. System exceptions — the run-time system might terminate a process
if it runs out of memory or if it detects an inconsistency in some in-
ternal table. Such exceptions are outside the programmer’s control.

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3.5.2 catch

CHAPTER 3. ERLANG

Exceptions can be converted to values by using the catch primitive. We
can illustrate this in the Erlang shell by trying to evaluate an illegal expres-
sion whose evaluation leads to the generation of an exception. We will try
to bind the value of 1/0 to the free variable X. This is what happens:

1> X = 1/0.

=ERROR REPORT==== 23-Apr-2003::15:20:43 ===
Error in process <0.23.0> with exit value:
{badarith,[{erl_eval,eval_op,3},{erl_eval,expr,3},
{erl_eval,exprs,4},{shell,eval_loop,2}]}
** exited: {badarith,[{erl_eval,eval_op,3},

{erl_eval,expr,3},
{erl_eval,exprs,4},
{shell,eval_loop,2}]} **

Here entering the expression X = 1/0 in the Erlang shell caused an
exception to be generated and an error message is printed to standard
output. If we try to print the value of the variable X we see the following:

2> X.
** exited: {{unbound,’X’},[{erl_eval,expr,3}]} **

X has, of course, no value, so another exception is generated and

another error message printed.

To convert the exception into a value we evaluate it within a catch

statement, as follows:

3> Y = (catch 1/0).
{’EXIT’,{badarith,[{erl_eval,eval_op,3},

{erl_eval,expr,3},
{erl_eval,exprs,4},
{shell,eval_loop,2}]}}

3.5. ERROR HANDLING

71

Now Y has a value, namely a 2-element tuple whose ﬁrst element is the
atom EXIT, and whose second value is the term {badarith,...}. Y is a
normal Erlang term, which can be freely examined and manipulated like
any other Erlang data structure. The expression:

Val = (catch Expr)

evaluates Expr in some context. If the function evaluation terminates
normally, then catch returns the value of this expression. If an exception
occurs during the evaluation then evaluation of the function stops imme-
diately and an exception is generated. The exception is an Erlang object
which describes the problem, in this case the value of catch is the value
of the exception that was generated.

If evaluating (catch Expr) returns a term of the form {’EXIT’,W}

then we say that the expression terminated with reason W.

If an exception is generated anywhere inside a catch then the value
If an exception is generated
of catch is just the value of the exception.
outside the scope of a catch then the process in which the exception was
generated will die and the exception will be broadcast to any processes
which are currently linked to the dying process. Links are created by
evaluating the BIF link(Pid).

3.5.3 exit

Explicit exceptions can be generated by calling the exit primitive. Here
is an example:

sqrt(X) when X < 0 ->

exit({sqrt,X});

sqrt(X) ->
...

which generates the exception {sqrt,X} if called with a negative ar-

gument X.

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CHAPTER 3. ERLANG

3.5.4 throw

The primitive throw is used to change the syntactic form of an exception.

• If an exception is generated by calling exit(P) within the scope of
some function F then the result of evaluating (catch F) will be a
term of the form {’EXIT’,P}.

• If an exception is generated by calling throw(Q) within the scope
of some function F then the result of evaluating (catch F) will be
the term Q.

Throw can be used for distinguishing user-generated exceptions from

exceptions produced by the run-time system.

3.5.5 Corrected and uncorrected errors

Suppose we write:

g(X) ->

case (catch h(X)) of

{’EXIT’, _} ->

10;

Val ->

Val

end.

h(cat) -> exit(dog);
h(N)

-> 10*N.

Evaluating h(cat) generates an exception, h(20) returns the value
200. Evaluating g(cat) or g(dog) returns the value 10 whereas g(10)
returns the value 100.

When we evaluate g(cat) the following sequence of events occurs:

1. h(cat) is evaluated.

3.5. ERROR HANDLING

73

2. h generates an exception.

3. The exception is caught in g.

4. g returns a value.

Evaluating g(dog) causes the following to occur:

1. h(dog) is evaluated.

2. N is bound to dog in the body of h.

3. N*10 is evaluated where N = dog.

4. An exception is generated in ’*’.

5. The exception is propagated to h.

6. The exception is caught in g.

7. g returns a value.

If we look at this carefully we observe that in evaluating d(dog) an

exception was raised but was caught and corrected in g.

Here we can say that an error did occur but that it was corrected.
If we had evaluated h(dog) directly then an error would have occurred

that was not caught and was not corrected.

3.5.6 Process links and monitors

When one process in the system dies we would like other processes in the
system to be informed; recall (page 27) that we need this in order to be
able to program fault-tolerant systems. There are two ways of doing this.
We can use either a process link or a process monitor.

Process links are used to group together sets of processes in such a way
that if an error occurs in any one of the processes then all the processes in
the group get killed.

Process monitors allow individual processes to monitor other processes

in the system.

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Process links

CHAPTER 3. ERLANG

The primitive catch is used to contain errors which occur within a process.
We now ask what happens if the top-level catch in a program does not
manage to correct an error which it has detected? The answer is that the
processes itself terminates.

The reason for failure is just the argument of the exception. When
a process fails the reason for failure is broadcast to all processes which
belong in the so-called “link set” of the failing process. A process A can
add the process B to its link set by evaluating the BIF link(B). Links are
symmetric, in the sense that if A is linked to B then B will also be linked to
A.

Links can also be created when a process is created. If the process A

creates the process B by evaluating:

B = spawn_link(fun() -> ... end),

then the process B will be linked to A. This is semantically equivalent
to evaluating spawn immediately followed by link, only the two expres-
sion are evaluated atomically, instead of sequentially. The spawn_link
primitive was introduced to correct a rare programming error which can
occur if a process dies immediately during the spawning process and does
not reach the link statement.9

If a process P dies with an uncaught exception {’EXIT’,Why} then the
exit signal {’EXIT’,P,Why} will be sent to all processes in the link set of
the process P.

So far I have not mentioned signals. Signals are things which are sent
between processes when a process terminates. The signal is a tuple of
the form {’EXIT’,P,Why} where P is the Pid of the process which has
terminated and Why is a term describing the reason for termination.

Any process which receives an exit signal will die if Why is not the atom
normal. There is one exception to this rule: if the receiving process is a
system process, then it will not die but instead the signal will be converted

9This could happen for example, if a process tries to spawn a processes which uses code in a module

which does not yet exist.

3.5. ERROR HANDLING

75

into a normal inter-process message and be added to the input mailbox of
the process. By evaluating the BIF process_flag(trap_exit,true) a
normal process can become a system process.

A typical code fragment for a system process which can handle failures

in other processes is something like this:

start() -> spawn(fun go/0).

go() ->

process_flag(trap_exit, true),
loop().

loop() ->

receive

{’EXIT’,P,Why} ->

... handle the error ...

end

One additional primitive completes the picture. exit(Pid,Why) sends
an exit signal to the process Pid with reason Why. The process evaluating
exit/2 does not terminate, so such a message can be used to “fake” the
death of a process.10

Again there is one exception to the rule that a system process will
convert all signals into messages; evaluating exit(P,kill) sends an un-
stoppable exit to the process P which will be terminated with extreme
prejudice. This use of exit/2 is needed to kill processes which refuse to
honour requests to voluntarily terminate.

Process links are useful for setting up groups of processes, which will
all die if anything goes wrong in any of the processes. Usually we just link
together all the processes which belong to an application, and let one of
the processes assume a “supervisor” role. The supervisor process is set to
trap exits. If anything goes wrong then the entire group will die except the
supervisor process which can receive messages which inform it about the
failures of the other processes in the group.

10This is a feature, not a bug!

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CHAPTER 3. ERLANG

Process Monitors

Process links are useful for entire groups of processes, but not for moni-
toring pairs of processes in an asymmetric sense. In a typical client-server
model, the relationship between the client and the servers is asymmetric as
regards error handling. Suppose that a server is involved in a number of
long-lived sessions with a number of dicerent clients; if the server crashes,
then we want to kill all the clients, but if an individual client crashes we do
not wish to kill the server.

The primitive erlang:monitor/2 can be used to set up a monitor. If

process A evaluates:

Ref = erlang:monitor(process, B)

Then if B dies with exit reason Why, A will be sent a message of the

form:

{’DOWN’, Ref, process, B, Why}

Neither A nor B have to be system processes in order to set up or

receive monitor messages.

3.6 Distributed programming

Erlang programs can be easily ported from a uni-processor to a collection
of processors. Each complete and self-contained Erlang system is called a
node. One or more Erlang nodes can run within a host operating system.
Testing a distributed application is simpliﬁed by the fact that many Erlang
nodes can be run on the same operating system. A distributed application
can be developed and tested by running all the nodes in the application on
a single processor. When the application works the dicerent nodes which
were assigned to the same processor can be moved to dicerent nodes
in a network of distributed processors. With the exception of timing all
operations in the distributed system should work in exactly the same way
as they worked in the single-node test system.

Two primitives are needed for distributed processing:

3.7. PORTS

77

• spawn(Fun,Node) — spawns a function Fun on the remote node

Node.

• monitor(Node) — is used for monitoring the behaviour of an entire

node.
monitor is analogous to link, the dicerence being that the con-
trolled object is an entire node instead of a process.

3.7 Ports

Ports provide a mechanism for Erlang programs to communicate with the
outside world. Ports are created by evaluating the BIF open_port/2. As-
sociated with each port is a so-called “controlling-process”. The controlling
processes is said to own the port. All messages from the port are sent to
the controlling process and only the controlling process is allowed to send
messages to the port. The controlling process is initially the process which
created the port, but this process can be changed.

If P is a port, and Con is the Pid of the port’s controlling process then
the port can be commanded to do something by evaluating the expression:

P ! {Con, Command}

where Command can have one of the following three possible values:

• {command,Data} — sends Data to the external object. Data must
be an io-list (see page 59 for a deﬁnition of io-lists). The io-list is
ﬂattened and the data elements in the list are sent to the external
application.

• close — closes the port. The port must respond by sending a

{P,closed} message to the controlling process.

• {connect,Pid1} — changes the controlling process to Pid. The
port must send a {Port,connected} to the original controlling
process, thereader all new messages are sent to the new controlling
process.

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CHAPTER 3. ERLANG

All data from the external application results in {Port,{data,D}}

messages being sent to the controlling process for the port.

The exact format of the messages and how the messages are framed
depends upon the details of how the port was opened. See [34] for more
details.

3.8 Dynamic code change

Erlang supports a simple mechanism for dynamic code changes. In a run-
ning Erlang node all processes share the same code. We have to consider,
therefore, what happens if we change the code in a running system.

In a sequential programming language there is only one thread of
control, so if we wish to dynamically change the code we only have to
worry about what is happening in that single thread of control. Usually in
a sequential system, if we wish to change the code, we stop the system,
change the code and re-start the program.

In a real-time control system, however, we oden do not wish to stop the
system in order to change the code. In certain real-time control systems,
we might never be able to turn oc the system in order to change the code
and so these systems have to be designed so that the code can be changed
without stopping the system. An example of such a system is the X2000
satellite control system [2] developed by NASA.

The Erlang system allows for two versions of code for every module. If
the code for a particular module has been loaded then all new processes
that call any of this code will be dynamically linked with the latest version
of the module. If the code for a particular module is subsequently changed
then processes which execute code in this module can choose either to
continue executing the old code for the module, or to use the new code.
The choice is determined by how the code is called.

If the code is called with a fully-qualiﬁed name, that is a module name
followed by a function name, then the latest version of the module will
always be called, otherwise the current version will be called. For example,
suppose we write a server loop as follows:

-module(m).

3.8. DYNAMIC CODE CHANGE

79

...
loop(Data, F) ->

receive

{From, Q} ->

{Reply, Data1} = F(Q, Data),
m:loop(data1, F)

end.

The ﬁrst time the module m is called the code for the module will be
loaded. At the end of the loop m:loop is called. Since there is only one
version of the module the code in the current module will be called.

Suppose now that we change the code for the module m and we re-
compile and re-load the code for m. If we do this, when m:loop is called
in the last clause of the receive statement, the code in the new version
of m will be called. Note that it is the programmer’s responsibility to
ensure that the new code to be called is compatible with the old code.
It is also highly advisable that all code replacement calls are tail-calls (see
section 3.3.8), since in a tail-call there is no old code to return to, and thus
ader a tail-call all old code for a module can safely be deleted.

If we want to carry on executing code in the current module and not
change to code in the new module, then we would write the loop without
the fully qualiﬁed call, thus:

-module(m).
...
loop(Data, F) ->

receive

{From, Q} ->

{Reply, Data1} = F(Q, Data),
loop(data1, F)

end.

In this case code in the new version of the module will not be called.
Judicious use of the mechanism allows processes to execute both old

and new versions of code in dicerent modules at the same time.

80

CHAPTER 3. ERLANG

Note there is a limit to two versions of the code. If a third attempt is
made to re-load the module then all processes executing code in the ﬁrst
module will be killed.

In addition to these calling conventions there are a number of BIFs

that have to do with code replacement. These are fully described in [5].

3.9 A type notation

If we build a sodware module, how can we describe how it is to be used?
The conventional answer involves the use of a so-called programming API
(Application Programming Interface). The API is usually a list of those
functions in the module which are externally callable and the types of the
inputs to the function and the types of the output.

Here is an example of how the types of a number of functions can be

speciﬁed in the Erlang type notation:

+type file:open(fileName(), read | write) ->

{ok, fileHandle()}
| {error, string()}.

+type file:read_line(fileHandle()) ->

{ok, string()}

| eof.

+type file:close(fileHandle()) ->

true.

+deftype fileName()
+deftype string()
+deftype fileHandle() = pid().

= [int()]
= [int()].

Each of the Erlang primitive data types has its own type. These primi-

tive types are:

• int() — is the integer type.

3.9. A TYPE NOTATION

81

• atom() — is the atom type.

• pid() — is the Pid type.

• ref() — is the reference type.

• float() — is the Erlang ﬂoat type.

• port() — is the port type.

• bin() — is the binary type.

List, tuple and alternation types are deﬁned recursively:

• {T1,T2,...,Tn} is the tuple type if T1 .. Tn are types. We say
that {X1,X2,...,Xn} is of type {T1,T2,...,Tn} if X1 is of type
T1 and X2 is of type T2, ... and Xn is of type Tn.

• [T] is the list type if T is a type. We say that [X1,X2,...,Xn] is of
type [T] if all the X’s are of type T. Note that the empty list [] is of
type [T] for all values of T.

• T1|T2 is the alternation type if T1 and T2 are types. We say that X

is of type T1|T2 if X is of type T1 or if X is of type T2.

New types are introduced with the notation:

+deftype name1() = name2() = ... = Type.

The names name1, name2,... follow the syntax of Erlang atoms.
Type variables are written using the syntax of Erlang variables.
so, for example, we can deﬁne:

+deftype bool()
+deftype weekday() = monday|tuesday|wednesday|

= true | false.

+deftype weekend() = saturday() | sunday().
= weekday() | weekend().
+deftype day()

thursday|friday.

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CHAPTER 3. ERLANG

Function types are written:

+type functionName(T1, T2, ..., Tn) -> T.

Where all the T’s are types. If a type variable occurs more than once
in a type deﬁnition then all the instances of the type at the position implied
by the variable must have the same type.

Here are some examples:

+deftype string() = [int()].
+deftype day()
+deftype town()

= number() = int().
= street() = string().

+type factorial(int()) -> int().
+type day2int(day())
-> int().
+type address(person()) -> {town(), street(), number()}.

Finally, function types are written:

+type fun(T1, T2, ..., Tn) -> T end

so, for example, the type of map/2 can be written:

+type map(fun(X) -> Y end, [X]) -> [Y].

The type notation here is a much simpliﬁed version of the notation

developed by Wadler & Marlow [49].

3.10 Discussion

This chapter has introduced a signiﬁcant subset of Erlang—suecient, at
least, to understand all the examples in the thesis, but I have not yet
addressed the question “Is Erlang a suitable language for programming
fault-tolerant systems in?” I believe the answer is “Yes.” I argued earlier

3.10. DISCUSSION

83

that in order to program a fault-tolerant system the programming language
used for the job had to satisfy certain properties (R1–R6 on page 27). I
now claim that Erlang does indeed satisfy these properties, for the following
reasons:

• Process are fundamental to Erlang so R1 is satisﬁed.

• R2 is satisﬁed since processes in Erlang are designed as units of error
encapsulation. If one process terminates because of a sodware error,
other process executing in the same Erlang node will not be acected
(unless, of course, they have been linked to the terminating process,
in which case the interaction is intentional).

• Processes fail immediately if functions within the processes are called
with incorrect arguments, or if the system BIFs are called with incor-
rect arguments. Immediate failure corresponds to Gray’s notion of
fail-fast processes (page 34), to Schneider’s notion of a fail-stop pro-
cessor (page 34) and to Renzel’s statement that we must detect errors
and fail as early as possible (page 35).

• When processes fail, the reason for failure is broadcast to the current

link set of the process, thus satisfying R3 and R4.

• R5 is satisﬁed by the code upgrade mechanism described in sec-

tion 3.8.

• R6 is not satisﬁed in Erlang, but it is satisﬁed in the Erlang libraries.
Stable storage makes use of either dets or mnesia. dets is a single
system disk-based storage system. If a process or node crashes then
data stored in dets should survive the crash. For stronger data pro-
tection the data should be stored on two physically separated nodes,
using the mnesia data base, which is one of the OTP applications.

I also note in passing that Schneider’s “halt on failure,” “Failure sta-
tus property” and “Stable storage property” (page 34) are satisﬁed either
directly in Erlang itself, or in the Erlang libraries.

84

CHAPTER 3. ERLANG

4 Programming

Techniques

T he previous chapter was about Erlang, but not about how to pro-

gram in Erlang. This chapter is about Erlang programming tech-
niques. The programming techniques are concerned with:

• Abstracting out concurrency — concurrent programs are, in some
sense, more diecult to write than sequential programs. Instead of
writing one module which has both concurrent and sequential code
I show how to structure the code into two modules, one of which
has all the concurrent code, the other having only purely sequential
code.

• Maintaining the Erlang view of the world — in Erlang terms every-
thing is a process. To help maintain this view I introduce the idea
of a protocol converter, which helps the programmer maintain the
illusion that everything is an Erlang process.

• The Erlang view of errors — the treatment of how errors are han-
dled in Erlang is radically dicerent from most other programming
languages, I show how error situations should be programmed in
Erlang.

• Intentional programming — this is a programming style where the
programmer can easily see from the code exactly what the pro-
grammer intended, rather than by guessing at the meaning from
a superﬁcial analysis of the code.

85

86

CHAPTER 4. PROGRAMMING TECHNIQUES

Figure 4.1: A generic component with plugins. All the concurrency and
fault-handling are handled in the generic component. The plugins are
written with purely sequential code.

4.1 Abstracting out concurrency

When we program we want to structure the code into “diecult” and “easy”
modules. The diecult modules should be few and written by expert pro-
grammers. The easy modules should be many and written by less experi-
enced programmers. Figure 4.1 shows a generic component (the diecult
part), and a number of “plugins” (the easy parts) which are used to param-
eterise the generic component.

The generic component should hide details of concurrency and mech-
anisms for fault-tolerance from the plugins. The plugins should be written
using only sequential code with well-deﬁned types.

In the following I show how to structure a client-server into a generic

component and a number of plugins.

Structuring a system into generic component and plugins is a com-
mon programming technique—what is unusual in our approach is that the
generic component can provide a rich environment in which to execute
the plugin. The plugin code itself can have errors, and the code in the
plug-in can be dynamically modiﬁed, the entire component can be moved
in a network, and all this occurs without any explicit programming in the
plugin code.

4.1. ABSTRACTING OUT CONCURRENCY

87

Abstracting out concurrency is one of the most powerful means avail-
able for structuring large sodware systems. Despite the ease with which
concurrent programs can be written in Erlang it is still desirable to restrict
code which explicitly handles concurrency to as few modules as possible.
The reason for this is that concurrent code cannot be written in a side-
ecect free manner, and as such, is more diecult to understand and analyse
than purely sequential side-ecect free code.
In a system involving large
numbers of processes, issues of message passing ordering and potential
dead- or live-lock problems can make concurrent systems very diecult to
understand and program.

The most common abstraction used by applications written in Erlang is
the client–server abstraction. In virtually all systems that are programmed
in Erlang, use of the client-server abstraction far outweighs the use of any
other abstraction. For example, on page 174 we see that 63% of all the
behaviours used in the AXD301 system were instances of the gen_server
behaviour which provides a client-server abstraction.

I will start with a simple universal client-server server1 and then show

how it can be parameterised to form a simple name service.

I will also extend the simple server in two ways. Firstly I modify
the basic server to form a fault tolerant server server2, then I extend
it to a version providing dynamic code upgrade (server3). The step-wise
reﬁnement of the server code, server1 -> server2 -> server3 leads
eventually to gen_server which is one of the standard behaviours in the
OTP libraries. The code for gen_server performs many more operations
than are performed in the simple server examples shown here. The prin-
ciples, however, of the gen_server are the same as for the simple server
examples; namely that the client-server is separated into a generic part,
which takes care of the concurrency, and a sequent plug-in modules which
merely parameterises the generic server in a particular manner to create a
speciﬁc instance of a server.

The treatment of both these extensions is deliberately simplistic. I have
ignored many implementation issues in favour of a simple explanation
which demonstrates the principles involved.

The client-server model is illustrated in ﬁgure 4.2. The client-server
model is characterized by a central server and an arbitrary number of

88

CHAPTER 4. PROGRAMMING TECHNIQUES

Figure 4.2: Client server

clients. The client server model is generally used for resource manage-
ment operations. We assume that several dicerent clients want to share
a common resource and that the server is responsible for managing the
resource.

If we ignore how the server is started and stopped and ignore all error
cases then it is possible to describe the server by means of a single function
F.

Let us suppose that the server is in a state State and that it receives a
message Query from some client. The server should respond to this query
by returning a message Reply to the client and changing state to State1.
These values are completely determined by the server function F and

are computed by the Erlang expression:

{State1, Reply} = F(Query, State)

which is evaluated within the server.
The ﬁrst universal server server1.erl is shown in ﬁgure 4.3. The
client stub routine rpc (lines 13–17) sends a messages to the server (line
14) and waits for a reply (lines 15–17). The server receives the message
sent by the client stub (line 23), computes a reply and a new state (line
24), sends the reply back to the server (line 25), and recursively calls
itself (line 26). Note that the recursive call to loop/3 (line 26) is a tail-
call (see section 3.3.8), because it is a tail-call the variable State will no

ServerClientsQueryReply4.1. ABSTRACTING OUT CONCURRENCY

89

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

-module(server1).

-export([start/3, stop/1, rpc/2]).

start(Name, F, State) ->

register(Name,

spawn(fun() ->

loop(Name, F, State)

end)).

stop(Name) -> Name ! stop.

rpc(Name, Query) ->

Name ! {self(), Query},
receive

{Name, Reply} -> Reply

end.

loop(Name, F, State) ->

receive

stop ->

void;

{Pid, Query} ->

{Reply, State1} = F(Query, State),
Pid ! {Name, Reply},
loop(Name, F, State1)

end.

Figure 4.3: A simple server.

90

CHAPTER 4. PROGRAMMING TECHNIQUES

longer be accessible by any code and thus any storage accessed by State
that cannot be reached from State1 will eventually be reclaimed by the
garbage collector. loop/3 is said therefore to run in constant space, apart,
that is, from the local storage (stored in the variable State) which is
needed to store the state of the server itself. server1.erl exports three
routines:

• start(Name, Fun, State) — starts a server with name Name.
The initial state of the server is State, Fun is a function which
completely characterises the behaviour of the server.

• stop(Name) — stops the server Name.

• rpc(Name, Q) — perform a remote procedure call on the server

called Name.

We can use this server to implement a very simple “Home Location
Register1” which we call VSHLR (Very Simple HLR). Our VSHLR has
the following interface:

• start() — starts the HLR.

• stop() — stops the HLR.

• i_am_at(Person,Loc) — tells the HLR that Person is at the loca-

tion Loc.

• find(Person) -> {ok, Loc} | error — tries to ﬁnd the position
of Person in the HLR. The HLR responds with {ok, Loc} where
Loc is the last reported location, or, error if it doesn’t know where
the person is.

vshlr1 can be implemented by parameterising server1, this is shown

in ﬁgure 4.4.

Here is a simple session using the server:

1Home location registers are widely used in the telecoms industry to keep track of the current location

of a mobile user.

4.1. ABSTRACTING OUT CONCURRENCY

91

-module(vshlr1).

-export([start/0, stop/0, handle_event/2,

i_am_at/2, find/1]).

-import(server1, [start/3, stop/1, rpc/2]).
[new/0, store/3, find/2]).
-import(dict,

start() -> start(vshlr, fun handle_event/2, new()).

stop() -> stop(vshlr).

i_am_at(Who, Where) ->

rpc(vshlr, {i_am_at, Who, Where}).

find(Who) ->

rpc(vshlr, {find, Who}).

handle_event({i_am_at, Who, Where}, Dict) ->

{ok, store(Who, Where, Dict)};
handle_event({find, Who}, Dict) ->

{find(Who, Dict), Dict}.

Figure 4.4: A Very Simple Home Location Register.

92

CHAPTER 4. PROGRAMMING TECHNIQUES

1> vshlr1:start().
true
2> vshlr1:find("joe").
error
3> vshlr1:i_am_at("joe", "sics").
ack
4> vshlr1:find("joe").
{ok,"sics"}

Even though our VSHLR program is extremely simple, it illustrates
and provides simple solutions to a number of design problems. The reader
should note the following:

• There is a total separation of functionality into two dicerent modules.
All the code that has to do with spawning processes, sending and
receiving messages etc is contained in server1.erl. All the code
that has to do with the implementation of the VSHLR is contained
in vshlr1.erl.

• The code in vshlr1.erl makes no use of any of the Erlang concur-
rency primitives. The programmer who writes this module needs to
know nothing about concurrency or fault-handling.

The second point is very important. This is an example of factoring
out concurrency—since writing concurrent programs is generally perceived
as being diecult and since most programmers are more experienced in
writing sequential code then being able to factor out the concurrency is a
distinct advantage.

Not only can we factor out concurrency but we can mask possible
errors in the code which is used to parameterise the server function. This
is shown in the next section.

4.1.1 A fault-tolerant client-server

I now extend the server by adding code for error recovery, as shown in
ﬁgure 4.5. The original server will crash if there is an error in the function

4.1. ABSTRACTING OUT CONCURRENCY

93

F/2. The term “fault-tolerance” usually applies to hardware, but here we
mean the sever will tolerate faults in the function F/2 which is used to
parameterise the server.

The function F/2 is evaluated within a catch and the client is killed if

a RPC request is made which would have killed the server.

Comparing the new server with the old we note two small changes:

the rpc code has changed to:

rpc(Name, Query) ->

Name ! {self(), Query},
receive

{Name, crash} -> exit(rpc);
{Name, ok, Reply} -> Reply

end.

and the code in the inner section of the receive statement in loop/3:

case (catch F(Query, State)) of

{’EXIT’, Why} ->

log_error(Name, Query, Why),
From ! {Name, crash},
loop(Name, F, State);

{Reply, State1} ->

From ! {Name, ok, Reply},
loop(Name, F, State1)

end

Looking at these changes in more detail we observe that if evaluat-
ing the function in the server loop raises an exception then three things
happen:

1. The exception is logged—in this case we just print out the exception
but in a more sophisticated system we would log it on non-volatile
storage.

94

CHAPTER 4. PROGRAMMING TECHNIQUES

-module(server2).
-export([start/3, stop/1, rpc/2]).

start(Name, F, State) ->

register(Name, spawn(fun() -> loop(Name,F,State) end)).

stop(Name) -> Name ! stop.

rpc(Name, Query) ->

Name ! {self(), Query},
receive

{Name, crash} -> exit(rpc);
{Name, ok, Reply} -> Reply

end.

loop(Name, F, State) ->

receive

stop -> void;
{From, Query} ->

case (catch F(Query, State)) of

{’EXIT’, Why} ->

log_error(Name, Query, Why),
From ! {Name, crash},
loop(Name, F, State);

{Reply, State1} ->

From ! {Name, ok, Reply},
loop(Name, F, State1)

end

end.

log_error(Name, Query, Why) ->

io:format("Server ~p query ~p caused exception ~p~n",

[Name, Query, Why]).

Figure 4.5: A simple server with error recovery.

4.1. ABSTRACTING OUT CONCURRENCY

95

2. A crash message is sent to the client. When the crash message is re-
ceived by the client it will raise an exception in the client code. This
is desirable since it is probably pointless for the client to continue.

3. The server continues to operate with the old value of the state vari-
able. Thus the RPC obeys “transaction semantics” that is to say, it
either succeeds in its entirety and the state of the server is updated,
or, it fails, in which case the state of the server is unchanged.

Note that the code in server2.erl is only written to protect against
an error in the characteristic function which parameterises the server. If
the server itself dies (it could, for example, be deliberately killed by some
other process in the system), then the client RPC stub will hang indeﬁnitely,
waiting for a reply message that will never be sent. If we wish to guard
against this possibility, then we could write the RPC routine as follows:

rpc(Name, Query) ->

Name ! {self(), Query},
receive

{Name, crash} -> exit(rpc);
{Name, ok, Reply} -> Reply

after 10000 ->

exit(timeout)

end.

This solution, while it solves one problem, introduces another prob-
lem: What value should we choose for the timeout?. A better solution,
which I will not elaborate here, makes use of applications and supervi-
sion trees. Server failures, should be detected not by client sodware, but
by special supervisor processes which are responsible for correcting the
errors introduced by the failure of a server.

We can now run this server parameterised with a version of VSHLR

(vshlr2) which contains a deliberate error:

A session using this modiﬁed server is shown below:

96

CHAPTER 4. PROGRAMMING TECHNIQUES

-module(vshlr2).

-export([start/0, stop/0, i_am_at/2, find/1]).

-import(server2, [start/3, stop/1, rpc/2]).
[new/0, store/3, find/2]).
-import(dict,

start() -> start(vshlr, fun handle_event/2, new()).

stop() -> stop(vshlr).

i_am_at(Who, Where) ->

rpc(vshlr, {i_am_at, Who, Where}).

find(Who) ->

rpc(vshlr, {find, Who}).

handle_event({i_am_at, Who, Where}, Dict) ->

{ok, store(Who, Where, Dict)};
handle_event({find, "robert"}, Dict) ->

1/0;

handle_event({find, Who}, Dict) ->

{find(Who, Dict), Dict}.

Figure 4.6: A home location register with a deliberate error.

4.1. ABSTRACTING OUT CONCURRENCY

97

> vshlr2:start().
true
2> vshlr2:find("joe").
error
3> vshlr2:i_am_at("joe", "sics").
ok
4> vshlr2:find("joe").
{ok,"sics"}
5> vshlr2:find("robert").
Server vshlr query {find,"robert"}
caused exception {badarith,[{vshlr2,handle_event,2}]}
** exited: rpc **
6> vshlr2:find("joe").
{ok,"sics"}

Hopefully the information in the exception is suecient to debug the

program (satisfying requirement R3 on page 27).

As a ﬁnal improvement we modify the server in ﬁgure 4.5 to allow us

to change the code in the server “on-the-ﬂy,” this is shown in ﬁgure 4.7.

I will parameterise this with vshlr3. vshlr3 is not shown here but
it is identical to vshlr2 with one exception; server2 in line three of the
module is replaced by server3.

1> vshlr3:start().
true
2> vshlr3:i_am_at("joe", "sics").
ok
3> vshlr3:i_am_at("robert", "FMV").
ok
4> vshlr3:find("robert").
Server vshlr query {find,"robert"}
caused exception {badarith,[{vshlr3,handle_event,2}]}
** exited: rpc **
5> vshlr3:find("joe").
{ok,"sics"}

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CHAPTER 4. PROGRAMMING TECHNIQUES

-module(server3).
-export([start/3, stop/1, rpc/2, swap_code/2]).

start(Name, F, State) ->

register(Name, spawn(fun() -> loop(Name,F,State) end)).

stop(Name) -> Name ! stop.

swap_code(Name, F) -> rpc(Name, {swap_code, F}).

rpc(Name, Query) ->

Name ! {self(), Query},
receive

{Name, crash} -> exit(rpc);
{Name, ok, Reply} -> Reply

end.

loop(Name, F, State) ->

receive

stop -> void;
{From, {swap_code, F1}} ->

From ! {Name, ok, ack},
loop(Name, F1, State);

{From, Query} ->

case (catch F(Query, State)) of

{’EXIT’, Why} ->

log_error(Name, Query, Why),
From ! {Name, crash},
loop(Name, F, State);

{Reply, State1} ->

From ! {Name, ok, Reply},
loop(Name, F, State1)

end

end.

log_error(Name, Query, Why) ->

io:format("Server ~p query ~p caused exception ~p~n",

[Name, Query, Why]).

Figure 4.7: A simple server with error recovery and dynamic code replace-
ment.

4.1. ABSTRACTING OUT CONCURRENCY

99

6> server3:swap_code(vshlr,

fun(I,J) -> vshlr1:handle_event(I, J) end).

ok
7> vshlr3:find("robert").
{ok,"FMV"}

The above trace illustrates how to change the code in the server “on
the ﬂy.” Lines 1-3 show the server working properly. In line 4 we trigger
the deliberate error programmed into vshlr3. Server3 handles this error
without crashing, so, for example, the response in line 5 is correct. In line
6 we send a command to change the code in the server back to the correct
version in vshlr1. Ader this command has completed, the server works
correctly as shown in line 7.

The programmer who wrote the code in vshlr3 did not need to know
anything about how server3 was implemented nor that the code in the
server could be dynamically changed without taking the server out of
service.

The ability to change sodware in a server without stopping the server
partially fulﬁls requirement 8 on page 14—namely to upgrade the sodware
in a system without stopping the system.

If we return once again to the code for server3 in ﬁgures 4.5 (the
server) and vshlr2 in ﬁgure 4.6 (the application), we observe the follow-
ing:

1. The code in the server can be re-used to build many dicerent client-

server applications.

2. The application code is much simpler than the server code.

3. To write the server code the programmer must understand all the
details of Erlang’s concurrency model. This involves name registra-
tion, process spawning, untrappable exits to a process, and sending
and receiving messages. To trap an exception the programmer must
understand the notion of an exception and be familiar with Erlang’s
exception handling mechanisms.

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CHAPTER 4. PROGRAMMING TECHNIQUES

4. To write the application, the programmer only has to understand
how to write a simple sequential program—they need to know noth-
ing about concurrency or error handling.

5. We can imagine using the same application code in a succession of
progressively more sophisticated servers. I have shown three such
servers but we can imagine adding more and more functions to the
server while keeping the server/application interface unchanged.

6. The dicerent servers (server1, server2 etc) imbue the application
with dicerent non-functional characteristics. The functional charac-
teristics for all servers are the same (that it, given correctly typed
arguments all programs will eventually produce identical results);
but the non-functional characteristics are dicerent.

7. The code which implements the non-functional parts of the system
is limited to the server (by non-function we mean things like how the
system behaves in the presence of errors, how long time function
evaluation takes, etc) and is hidden from the application program-
mer.

8. The details of how the remote procedure call is implemented are
hidden inside the server module. This means that the implementa-
tion could be changed at a later stage without changing the client
code, should this become necessary. For example, we could change
the details of how rpc/2 is implemented in ﬁgure 4.5 without hav-
ing to change any of the client sodware which calls the functions in
server2.

The division of the total server functionality into a non-functional part
and a functional part is good programming practice and gives the system
several desirable properties, some of which are:

1. Concurrent programming is oden perceived as being diecult.

In
a large programming group, where the programmers have dicerent
skill levels the expert programmers should write the generic server

4.2. MAINTAINING THE ERLANG VIEW OF THE WORLD

101

code, and the less-experienced programmers should write the appli-
cation code.

2. Formal methods could be applied to the (simpler) application code.
Work on the formal veriﬁcation of Erlang, or on type systems de-
signed to infer types from Erlang code have problems analysing con-
current programs. If the code in the generic servers is assumed to
be correct a priori then the problem of proving properties of the
system reduces to the problem of proving properties of sequential
programs.

3. In a system with a large number of client-servers all the servers can
be written with the same generic server. This makes it easier for
a programmer to understand and maintain several dicerent servers.
In section 8.3.1 we will investigate this claim when we analyse a large
system with many servers.

4. The generic servers and applications can be tested separately and
independently. If the interface remains constant over a long period
of time then both can be independently improved.

5. The application code can be “plugged into” a number of dicerent
generic servers, where these servers have dicerent non-functional
characteristics. Special servers, with the same interface, could pro-
vide an enhanced debugging environment etc. Other servers could
provide clustering, hot standby etc. This has been done in a number
of projects, for example Eddie [31] provided clustering and the Blue-
tail mail robustiﬁer [11] provided a server with hot standby facilities.

4.2 Maintaining the Erlang view of the

world

The Erlang view of the world is that everything is a process and that
processes can only interact by exchanging messages.

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CHAPTER 4. PROGRAMMING TECHNIQUES

When we interface Erlang programs to external sodware it is oden
convenient to write an interface program which maintains the illusion that
“everything is a process.”

As an example of this, we consider how to implement a web-server.
Web-servers communicate with clients using the HTTP protocol as deﬁned
in RFC2616 [36].

From the point of view of an Erlang programmer, the inner loop of
a web server would spawn a new process for each connection, accept a
request from the client and respond appropriately. The code for this would
be something like:

serve(Client) ->

receive

{Client, Request} ->

Response = generate_response(Request)
Client ! {self(), Response}

end.

Where Request and Response are Erlang terms representing HTTP

requests and HTTP responses.

The above server is very simple, it expects a single request, replies with

a single response and terminates the connection.

A more sophisticated server, would support HTTP/1.1 persistent con-

nections, the code for this is remarkably simple:

serve(Client) ->

receive

{Client, close} ->

true;

{Client, Request} ->

Response = generate_response(Request)
Client ! {self(), Response},
server(Client);

after 10000 ->

Client ! {self(), close}

end.

4.2. MAINTAINING THE ERLANG VIEW OF THE WORLD

103

Figure 4.8: A web server

This 11 line function is essentially all that is needed to make a primitive

web server with persistent connections.

The web server does not talk directly to the clients which are making
HTTP requests. To do so would signiﬁcantly clutter up the web-server
code with irrelevant detail and make the structure diecult to understand.
Instead it makes use of a “middle-man” process (see ﬁgure 4.8). The
middle-man process (an HTTP driver) converts between HTTP requests
and responses and the corresponding Erlang terms which represent these
requests and responses.

The overall structure of the HTTP driver is as follows:

relay(Socket, Server, State) ->

receive

{tcp, Socket, Bin} ->

case parse_request(State, Data) of

{completed, Request, State1} ->

Server ! {self(), {request, Req}},
relay(Socket, Server, State1);

{more, State1} ->

relay(Socket, Server, State1)

end;

{tcp_closed, Socket} ->

Server ! {self(), close};

{Server, close} ->

gen_tcp:close(Socket);

{Server, Response} ->

Data = format_response(Response),

TCPdriverDriverHTTPWebServerErlang terms TCP packets104

CHAPTER 4. PROGRAMMING TECHNIQUES

gen_tcp:send(Socket, Data),
relay(Socket, Server, State);

{’EXIT’, Server, _} ->

gen_tcp:close(Socket)

end.

If a packet comes from the client, via a TCP socket, it is parsed by
calling parse_request/2. When the response is complete an Erlang term
representing the request is sent to the server. If a response comes from the
server it is reformatted and then sent to the client. If either side terminates
the connection, or an error occurs in the server, the connection is closed
down.
If this process terminates for any reason all the connections are
automatically closed down.

The variable State is a state variable representing the state of a re-

entrant parser that is used to parse the incoming HTTP requests.

The entire code for the web-server is not shown here but can be down-

loaded from [15].

4.3 Error handling philosophy

Error handling in Erlang is radically dicerent to error handing in most
other programming languages. The Erlang philosophy for handling errors
can be expressed in a number of slogans:

• Let some other process do the error recovery.

• If you can’t do what you want to do, die.

• Let it crash.

• Do not program defensively.

4.3.1 Let some other process ﬁx the error

How can we handle errors in a distributed system? To handle hardware
errors we need replication. To guard against the failure of an entire com-
puter we need two computers.

4.3. ERROR HANDLING PHILOSOPHY

105

If computer 1 fails, computer 2 notices the failure and corrects
the error.

If the ﬁrst computer crashes the failure is noticed by the second com-
puter which will try to correct the error. We use exactly the same method
in Erlang, only instead of computers we use pairs of processes.

If process 1 fails, process 2 notices the failure and corrects the
error.

If the process Pid1 fails and if the processes Pid1 and Pid2 are linked
together and if process Pid2 is set to trap errors then a message of the
form {’EXIT’,Pid1,Why} will be delivered to Pid2 if Pid1 fails. Why
describes the reason for failure.

Note also that if the computer on which Pid1 executes dies, then an
exit message {’EXIT’,Pid1,machine_died} will be delivered to Pid2.
This message appears to have come from Pid1, but in fact comes from
the run-time system of the node where Pid2 was executing.

The reason for coercing the hardware error to make it look like a
sodware error is that we don’t want to have two dicerent methods for
dealing with errors, one for sodware errors and the other for hardware er-
rors. For reasons of conceptual integrity we want one uniform mechanism.
This, combined with the extreme case of hardware error, and the failure
of entire processors, leads to the idea of handling errors, not where they
occurred, but at some other place in the system.

Thus under all circumstances, including hardware failure it is Pid2’s
job to correct the error. This is why I say “let some other process ﬁx the
error.”

Computer 1Computer 2Pid 1Pid 2{’EXIT’,Pid,Why}106

CHAPTER 4. PROGRAMMING TECHNIQUES

This philosophy is completely dicerent to that used in a sequential
programming language where there is no alternative but to try and handle
all errors in the thread of control where the error occurs. In a sequential
language with exceptions, the programmer encloses any code that is likely
to fail within an exception handling construct and tries to contain all errors
that can occur within this construct.

Remote handling of error has several advantages:

1. The error-handling code and the code which has the error execute

within dicerent threads of control.

2. The code which solves the problem is not cluttered up with the code

which handles the exception.

3. The method works in a distributed system and so porting code from
a single-node system to a distributed system needs little change to
the error-handling code.

4. Systems can be built and tested on a single node system, but de-
ployed on a multi-node distributed system without massive changes
to the code.

4.3.2 Workers and supervisors

To make the distinction between processes which perform work, and pro-
cesses which handle errors clearer we oden talk about worker and super-
visor processes:

One process, the worker process, does the job. Another process, the
supervisor process. observes the worker. If an error occurs in the worker,
the supervisor takes actions to correct the error. The nice thing about this
approach is that:

1. There is a clean separation of issues. The processes that are sup-
posed to do things (the workers) do not have to worry about error
handling.

4.4. LET IT CRASH

107

2. We can have special processes which are only concerned with error

handling.

3. We can run the workers and supervisors on dicerent physical ma-

chines.

4. It oden turns out that the error correcting code is generic, that is,
generally applicable to many applications, whereas the worker code
is more oden application speciﬁc.

Point three is crucial—given that Erlang satisﬁes requirements R3 and
R4 (see page 27) then we can run worker and supervisor processes on
dicerent physical machines, and thus make a system which tolerates hard-
ware errors where entire processes fail.

4.4 Let it crash

How does our philosophy of handling errors ﬁt in with coding practices?
What kind of code must the programmer write when they ﬁnd an error?
The philosophy is let some other process ﬁx the error, but what does this
mean for their code? The answer is let it crash. By this I mean that in
the event of an error, then the program should just crash. But what is an
error? For programming purpose we can say that:

• exceptions occur when the run-time system does not know what to

do.

• errors occur when the programmer doesn’t know what to do.

If an exception is generated by the run-time system, but the program-
mer had foreseen this and knows what to do to correct the condition that
caused the exception, then this is not an error. For example, opening a ﬁle
which does not exist might cause an exception, but the programmer might
decide that this is not an error. They therefore write code which traps this
exception and takes the necessary corrective action.

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CHAPTER 4. PROGRAMMING TECHNIQUES

Errors occur when the programmer does not know what to do. Pro-
grammers are supposed to follow speciﬁcations, but oden the speciﬁcation
does not say what to do and therefore the programmer does not know
what to do. Here is a example:

Suppose we are writing a program to produce code for a microproces-
sor, the speciﬁcation says that a load operation is to result in opcode 1 and
a store operation should result in opcode 2. The programmer turns this
speciﬁcation into code like:

asm(load) -> 1;
asm(store) -> 2.

Now suppose that the system tries to evaluate asm(jump)—what should
happen? Suppose you are the programmer and you are used to writing
defensive code then you might write:

asm(load) -> 1;
asm(store) -> 2;
asm(X)

-> ??????

but what should the ??????’s be? What code should you write? You
are now in the situation that the run-time system was faced with when it
encountered a divide-by-zero situation and you cannot write any sensible
code here. All you can do is terminate the program. So you write:

asm(load) -> 1;
asm(store) -> 2;
asm(X)

-> exit({oops,i,did,it,again,in,asm,X})

But why bother? The Erlang compiler compiles

asm(load) -> 1;
asm(store) -> 2.

almost as if it had been written:

4.5.

INTENTIONAL PROGRAMMING

109

asm(load) -> 1;
asm(store) -> 2;
asm(X)

-> exit({bad_arg, asm, X}).

The defensive code detracts from the pure case and confuses the
reader—the diagnostic is oden no better than the diagnostic which the
compiler supplies automatically.

4.5 Intentional programming

Intentional programming is a name I give to a style of programming where
the reader of a program can easily see what the programmer intended by
their code. The intention of the code should be obvious from the names
of the functions involved and not be inferred by analysing the structure of
the code. This is best explained by an example:

In the early days of Erlang the library module dict exported a function

lookup/2 which had the following interface:

lookup(Key, Dict) -> {ok, Value} | notfound

Given this deﬁnition lookup was used in three dicerent contexts:

1. For data retrieval—the programmer would write:

{ok, Value} = lookup(Key, Dict)

here lookup is used for to extract an item with a known key from
the dictionary. Key should be in the dictionary, it is a programming
error if this is not the case, so an exception in generated if the key is
not found.

2. For searching—the code fragment:

case lookup(Key, Dict) of

{ok, Val} ->

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CHAPTER 4. PROGRAMMING TECHNIQUES

... do something with Val ...

not_found ->

... do something else ...

end.

searches the dictionary and we do not know if Key is present or
not—it is not a programming error if the key is not in the dictionary.

3. For testing the presence of a key—the code fragment:

case lookup(Key, Dict) of

{ok, _} ->

... do something ...

not_found ->

... do something else ...

end.

tests to see if a speciﬁc key Key is in the dictionary.

When reading thousands of lines of code like this we begin to worry
about intentionality—we ask ourselves the question “what did the program-
mer intend by this line of code?”—by analysing the above three usages of
the code we arrive at one of the answers data retrieval, a search or a test.
There are a number of dicerent contexts in which keys can be looked
up in a dictionary. In one situation a programmer knows that a speciﬁc
key should be present in the dictionary and it is a programming error if
the key is not in the dictionary and the program should terminate.
In
another situation the programmer does not know if the keyed item is in
the dictionary and their program must allow for the two cases where the
key is present or not.

Instead of guessing the programmer’s intentions and analyzing the

code, a better set of library routines is:

dict:fetch(Key, Dict) = Val | EXIT
dict:search(Key, Dict) = {found, Val} | not_found.
dict:is_key(Key, Dict) = Boolean

4.6. DISCUSSION

111

Which precisely expresses the intention of the programmer—here no
guesswork or program analysis is involved, we clearly read what was in-
tended.

It might be noted that fetch can be implemented in terms of search

and vice versa. If fetch is assumed primitive we can write:

search(Key, Dict) ->

case (catch fetch(Key, Dict)) of

{’EXIT’, _} ->
not_found;

Value ->

{found, Value}

end.

This is not really good code, since ﬁrst we generate an exception (which
should signal that the program is in error) and then we correct the error.

Better is:

find(Key, Dict) ->

case search(Key, Dict) of

{ok, Value} ->
Value;

not_found ->

exit({find, Key}

end.

Now precisely one exception is generated which represents an error.

4.6 Discussion

Programming is a disciplined activity. Writing clear intentional code with
apparent structure is diecult. Part of this dieculty has to do with choosing
the right abstractions. To master a complex situation we use the method
of “divide and conquer,” we split complex problems into simpler sub-
problems and then solve the sub-problems.

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CHAPTER 4. PROGRAMMING TECHNIQUES

This chapter has shown how to split a number of complex problems
into simpler sub-problems. When it comes to error-handling I have shown
how to “abstract out” the errors, and argued that the program should be
divided into “pure” code and code which “ﬁxes the errors.”

In writing a server, I have shown how to abstract out two non-functional
properties of the server. I have shown how to write a version of a server
which does not crash when there is an error in the characteristic function
which deﬁnes the behaviour of the server and I have shown how the
behaviour of the server can be changed without stopping the server.

Recovering from errors, and changing code in a running system are
typical non-functional properties that many real systems are required to
have. Oden, programming languages and systems strongly support writing
code with well-deﬁned functional behaviour, but have poor support for the
non-functional parts of the program.

In most programming languages it is easy2 to write a pure function,
whose value depends in a deterministic manner on the inputs to the func-
tion, but it is much more diecult, and sometimes impossible, to do things
like changing the code in the system, or handling errors in a generic man-
ner, or protecting one programmer’s code from failures in another part of
the system. For this reason the programmer makes use of services ocered
by the operating system—the operating system provides protection zones,
concurrency etc usually in the guise of processes.

There is a sense in which the operating system provides “what the
In a language like Erlang an
programming language designer forgot.”
operating system is hardly necessary, indeed the OS just provides Erlang
with a number of device drivers, none of the OS mechanisms for processes,
message passing, scheduling, memory management etc are needed.

The trouble with using an OS to ﬁll in for deﬁciencies in the program-
ming language is that the underlying mechanisms of the operating system
itself cannot easily be changed. The operating systems’ ideas of what a
process is or how to do interprocess scheduling cannot be changed.

By providing the programmer with lightweight processes, and primitive
mechanisms for detecting and handling errors an application programmer

2Well it should be easy.

4.6. DISCUSSION

113

can easily design and implement their own application operating system,
which is speciﬁcally designed for the characteristics of their speciﬁc prob-
lem. The OTP system, which is just an application program written in
Erlang, is an example of this.

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CHAPTER 4. PROGRAMMING TECHNIQUES

5 Programming

Fault-tolerant
Systems

Designers devote half the software in telephone switches to error detection and
correction[48]

Richard Kuhn, National Institute of Standards and Technology

W hat is a fault-tolerant system and how can we program it? This

question is central to this thesis and to our understanding of how
to build fault-tolerant systems. In this chapter we deﬁne what we
mean by “fault-tolerance” and present a speciﬁc method for programing
fault-tolerant systems. We start with a couple of quotations:

We say a system is fault-tolerant if its programs can be properly
executed despite the occurrence of logic faults. — [16]

...

To design and build a fault-tolerant system, you must under-
stand how the system should work, how it might fail, and what
kinds of errors can occur. Error detection is an essential com-
ponent of fault tolerance. That is, if you know an error has
occurred, you might be able to tolerate it by replacing the
ocending component, using an alternative means of compu-
tation, or raising an exception. However, you want to avoid
adding unnecessary complexity to enable fault tolerance be-
cause that complexity could result in a less reliable system. —
Dugan quoted in Voas [67].

115

116 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

The presentation here follows Dugan’s advice, I explain exactly what
happens when an abnormal condition is detected and how we can make a
sodware structure which detects and corrects errors.

The remainder of this chapter describes:

• A strategy for programming fault-tolerance — the strategy is to fail
immediately if you cannot correct an error and then try to do some-
thing that is simpler to achieve.

• Supervision hierarchies — these are hierarchical organisations of tasks.

• Well-behaved functions — are functions which are supposed to work
correctly. The generation of an exception in a well-behaved function
is interpreted as a failure.

5.1 Programming fault-tolerance

To make a system fault-tolerant we organise the sodware into a hierarchy
of tasks that must be performed. The highest level task is to run the ap-
plication according to some speciﬁcation. If this task cannot be performed
then the system will try to perform some simpler task. If the simpler task
cannot be performed then the system will try to perform an even simpler
task and so on. If the lowest level task in the system cannot be performed
then the system will fail.

This method is intuitively attractive. It says if we cannot do what we
want to do, then try to do something simpler. We also try to organise the
sodware so that simpler tasks are performed by simpler sodware, so that
the likelihood of success increases as the tasks become simpler.

As the tasks become simpler, the emphasis upon what operation is
performed changes—we become more interested in protecting the system
against damage than in ocering full service. At all stages our goal is
to ocer an acceptable level of service though we become less ambitious
when things start to fail.

When failures occur we become interested in protecting the system,
and logging the precise reason for failure, so that we can do something

5.1. PROGRAMMING FAULT-TOLERANCE

117

about the failure in the future. This implies that we need some kind of
stable error log which will survive a crash. In exceptional circumstances
our system might fail, but when this does happen we should never lose the
information which tells us why the system failed.

To implement our hierarchy of tasks we need some precise notion of
what is meant by the world “failure.” Evaluating a function in Erlang can
result in an exception. But exceptions are not the same as errors, and
all errors do not necessarily lead to failures. So we need to discuss the
distinction between exceptions, errors and failures.

The distinctions between exceptions, errors and failures largely has
to do with where in the system an abnormal event is detected, how it
is handled and how it is interpreted. We trace what happens when an
abnormal situation occurs in the system—this description is “bottom up” ie
it starts at the point in time where the error is detected.

• At the lowest level in the system the Erlang virtual machine detects
an internal error—it detects a divide by zero condition, or a pattern
matching error or something else. The important point about all of
these detected conditions is that it is pointless to continue evaluating
code in the processes where the error occurred. Since the virtual
machine emulator cannot continue, it does the only thing possible
and throws an exception.

• At the next level, the exception may or may not be caught. The
program fragment which traps the exception may or may not be
able to correct the error which caused the exception. If the error is
successfully corrected, then no damage is done and the process can
resume as normal. If the error is caught, but cannot be corrected,
yet another exception might be generated, which may or may not
be trapped within the process where the exception occurred.

• If there is no “catch handler” for an exception1 then the process itself
will fail. The reason for failure will be propagated to any processes
which are currently linked to the process which has failed.

1ie the current function is not evaluating within the scope of a catch statement.

118 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

• The linked processes which receive these failure signals may or may
not intercept and process these signals as if they were normal inter-
process messages.

Thus we see that what starts oc as an abnormal condition in the virtual
machine emulator propagates upwards in the system. At every point in
the upwards propagation of the error an attempt might be made to correct
the error. This attempt might succeed or fail, thus we have very ﬁne grain
control over how and where we can handle errors.

A “corrected” error, that is, a situation which has been foreseen, and
for which corrective code has successfully been executed, is not considered
a fault.

So far we have just seen how abnormal conditions, lead to exceptions,
how exceptions can be trapped, and how untrapped exceptions can lead
to process failures, and how process failures can be detected by other
processes in the system. These are the available mechanisms with which
we can implement our “hierarchy of tasks.”

5.2 Supervision hierarchies

Recall that at the beginning of this chapter we talked about the idea of a
hierarchy of tasks. The basic idea is:

1. Try to perform a task.

2. If you cannot perform the task, then try to perform a simpler task.

To each task we associate an supervisor process—the supervisor will assign
a worker to try and achieve the goals implied by the task. If the worker
process fails with a non-normal exit then the supervisor will assume that
the task has failed and will initiate some error recovery procedure. The
error recovery procedure might be to restart the worker or failing this try
to do something simpler.

Supervisors and workers are arranged into hierarchical trees, according

to the following:

5.2. SUPERVISION HIERARCHIES

119

1. Supervision trees are trees of Supervisors.

2. Supervisors monitor Workers and Supervisors.

3. Workers are instances of Behaviours.

4. Behaviours are parameterised by Well-behaved functions.

5. Well-behaved functions raise exceptions when errors occur.

Where:

• Supervision trees are hierarchical trees of supervisors. Each node in

the tree is responsible for monitoring errors in its child nodes.

• Supervisors are processes which monitor other processes in the sys-
tem. The things that are monitored are either supervisors or work-
ers. Supervisors must be able to detect exceptions generated by the
things they are monitoring, and be able to start, stop and restart the
things they are monitoring.

• Workers are processes which perform tasks.

If a worker process terminates with a non-normal exit (see page 74)
the supervisor will assume that an error has occurred and will take
action to recover from the error.

Workers in our model are not arbitrary processes, but are instances
of one of a small number of generic processes (called behaviours).

• Behaviours are generic processes whose operation is entirely char-
acterised by a small number of callback functions. These functions
must be instances of well-behaved functions.
An example of a behaviour is the the gen_server behaviour which
is used for programming a distributed fault-tolerant client-server.
This behaviour is parameterised by a number of WBFs.

All the programmer has to understand in order to program a fault-
tolerant distributed client-server is how to write a WBF. The client-
server behaviour provides a fault-tolerant framework for concurrency

120 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

Figure 5.1: Supervisor and worker symbols

and distribution. The programmer need only be concerned with
writing WBFs which parameterise this behaviour.

For convenience two particular supervision hierarchies are considered,
namely linear hierarchies and AND/OR hierarchy trees. These are shown
diagrammatically in the following sections.

5.2.1 Diagrammatic representation

Supervisors and workers can conveniently be represented using the nota-
tion in ﬁgure 5.1.

Supervisors are draw as rectangles with right-angled corners. In the top
right-hand corner there is a symbol T denoting the type of the supervisor.
The value of T will be one of O meaning “or” supervision, or A meaning
“and” supervision. These supervision types are described later.

Supervisors can supervise an arbitrary number of workers or supervi-
sors. For each entity that is supervised, the supervisor must know how to
start, stop and restart the entity that it has to supervise. This information
is kept in a SSRS which stands for “Start Stop and Restart Speciﬁcation.”
The ﬁgure on page 148 contains a simple SSRS which speciﬁes how three
dicerent behaviours are to be supervised.

Each supervisor (apart from the topmost supervisor in a hierarchy) has
exactly one supervisor directly above it in the hierarchy, we call this the
Parent of the supervisor. Conversely, processes below a given supervisor

TSupervisor nodeWorker nodeSSRSWBF5.2. SUPERVISION HIERARCHIES

121

Figure 5.2: A linear supervisor hierarchy

in the hierarchy are referred to as the children of the supervisor. Figure 5.1
shows a supervisor node with one parent and three children.

Workers are drawn as rectangles with rounded corners (see Figure 5.1).
They are parameterised by well-behaved functions (WBFs in the diagrams).

5.2.2 Linear supervision

I start with a linear hierarchy. Figure 5.2 represents a linear hierarchy of
three supervisors. Each supervisor has a SSRS for each of its children and
obeys the following rules:

• If my parent stops me then I should stop all my children.

• If any of my children dies then I must try to restart that child.

The system is started by starting the topmost supervisor. The top
level supervisor is started ﬁrst using the SSRS1 speciﬁcation. The top level
supervisor has two children, a worker and a supervisor. It starts the worker

OSSRS1OWBF2OSSRS3WBF3WBF1SSRS2122 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

Figure 5.3: An AND/OR hierarchy

(which is a behaviour) parameterising it with the well-behaved function
WBF1 and it also starts the child supervisor. The lower level supervisors
in the hierarchy are started in a similar manner until the entire system is
running.

5.2.3 And/or supervision hierarchies

We can extend our simple supervision hierarchy into a tree structure con-
taining only AND or OR nodes. Figure 5.3 shows such a tree. The de-
notation A denotes an AND supervisor, and O denotes an OR supervisor.
The rules for a supervisor in an AND/OR tree are as follows:

• If my parent stops me then I should stop all my children.

• If any child dies and I am an AND supervisor stop all my children

and restart all my children.

• If any child dies and I am an OR supervisor restart the child that

died.

AND supervision is used for dependent, or co-ordinated processes.
In the AND tree the successful operation of the system depends upon
the successful operation of all the children—thus if any child dies all the
children should be stopped and then restarted.

AWBF1WBF2OSSRS3AWBF4WBF3WBF5OSSRS5OSSRS4SSRS1SSRS25.3. WHAT IS AN ERROR?

123

OR supervision can be used to co-ordinate the activities of independent
processes. In the OR tree the activities being performed are assumed to be
independent thus the failure of one child has no consequence for the other
children—in the event of a child failure only that child process is restarted.
In concrete terms our “hierarchy of tasks” is expressed in a “supervi-

sion hierarchy”.

In our system we equate tasks with goals and goals with invariants2—
we will say that the goal has been achieved if the invariant associated
with the goal is not false. In most programs the assertion of an invariant
corresponds to the statement that evaluation of a particular designated
function did not yield an exception.

Similar work has been reported by Candea and Fox [22] who have

made a system based on “recursively-restartable Java components”

Notice that we distinguish errors into two categories, correctable and
uncorrectable errors. Correctable errors are errors in a component which
have been detected and corrected by the component. Uncorrectable errors
are errors which have been detected but for which no corrective procedure
has been speciﬁed.

The above discussion is all rather vague, since we have never said what
an error is, nor have we said how we can in practice distinguish between a
correctable and an uncorrectable error.

The situation is complicated by the fact that the vast majority of speciﬁ-
cations only specify what should happen if all the components in a system
work as planned—speciﬁcations rarely specify what should be done if a par-
ticular error occurs. Indeed if a speciﬁcation does say exactly what to do
if a particular error occurs, then it could be argued that this not an error
at all but rather a desired property of the system. This is a consequence
of the rather vague way in which the word “error” is used.

5.3 What is an error?

When we run a program the run-time system has no idea as to what
constitutes an error—it just runs code. The only indication that something

2An invariant is something that is always true.

124 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

might have gone wrong is when an exception is generated.

Exceptions are generated automatically by the run-time system when
the run-time system cannot decide what to do. For example, when per-
forming a division the run-time system might detect a “divide by zero” con-
dition in which case it generates an exception, because it does not know
what to do. A complete list of all the conditions which cause exceptions
can be found in section 3.5.1.

Exceptions do not always correspond to errors.

If, for example, a
programmer has written code which can correctly deal with a “divide by
zero” exception then the presence of such an exception may not necessarily
indicate an error.

It is the programmer who decides if an exception corresponds to an
error—in our system the programmer must explicitly say which functions
in the system are expected to never generate exceptions.

Schneider gives a number of deﬁnitions of fault-tolerance in his 1990

ACM tutorial paper. In this paper he says:

A component is considered faulty once its behaviour is no
longer consistent with its speciﬁcation — [61].

For our purposes we will deﬁne an error as a deviation between the
observed behaviour of a system and the desired behaviour of a system.
The desired behaviour is “the behaviour that the speciﬁcation says the
system should have.”

The programmer must ensure that if the system behaves in a way that
deviates from the speciﬁcation, some kind of error recovery procedure is
initiated, and that some record of this fact is recorded in a permanent error
log, so that it can be corrected later.

Building real systems is complicated by the fact that we oden don’t have
a complete speciﬁcation—in this case the programmer has some general
notion as to what constitutes a error, and what does not. In the absence of
an explicit speciﬁcation we need an implicit mechanism that corresponds
to our intuitive idea that an error is “something that causes the program to
crash.”

In the OTP system the programmer is expected to write Well-behaved
functions (WBFs)—well-behaved functions are used to parameterise the

5.3. WHAT IS AN ERROR?

125

OTP behaviours. These functions are evaluated by the code in the OTP
behaviours. If the evaluation of a parameterisation function generates an
exception, then this is deﬁned to be an error, and an error diagnostic will
be added to the error log.

At this point it is important to recall part of the inner loop of the

generic server shown on page 94, it was:

loop(Name, F, State) ->

receive
...

{From, Query} ->

case (catch F(Query, State)) of

{’EXIT’, Why} ->

log_error(Name, Query, Why),
From ! {Name, crash},
loop(Name, F, State);

{Reply, State1} ->

From ! {Name, ok, Reply},
loop(Name, F, State1)

end

end.

The callback function F is called within a catch statement.

If an
exception Why is generated, then this is assumed to be an error, and an
error message is added to the error log.

This is a very simple example, but it does illustrate the fundamental
principle of error handling used in the OTP behaviours. For example, in
the OTP gen_server behaviour, the programmer must write a callback
module M which is used to parameterise the server. The module M must,
among other things, export the callback function handle_call/2 (an ex-
ample of this is shown in line 23–29 of ﬁgure 6.1 on page 136)—this callback
function must be a well-behaved function.

126 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

5.3.1 Well-behaved functions

A well-behaved function (WBF) is a function which should not normally
generate an exception. If an exception is generated by the WBF then the
exception will be interpreted as an error.

If an exception occurs while trying to evaluate the WBF the WBF
should try to correct the condition which caused the exception. If an error
occurs within a WBF which cannot be corrected the programmer should
terminate the function with an explicit exit statement.

Well-behaved functions are written according to the following rules:

Rule: 1 — The program should be isomorphic to the speciﬁcation.

The program should faithfully follow the speciﬁcation. If the speci-
ﬁcation says something silly then the program should do something
silly. The program must faithfully reproduce any errors in the speci-
ﬁcation.

Rule: 2 — If the speciﬁcation doesn’t say what to do raise an exception.

This is very important. Speciﬁcations oden say what is to be done
if something happens, but omit to say what to do if something else
happens. The answer is “raise an exception.” Unfortunately many
programmers take this as an opportunity for creative guess-work,
and try to guess what the designer ought to have said.

If systems are written in this manner then the observation of an
exception will be synonymous with a speciﬁcation error.

Rule: 3 — If the generated exceptions do not contain enough information
to be able to isolate the error, then add additional helpful informa-
tion to the exception.

When programmers write code they should ask themselves what
information will be written into the error log if an error occurs. If the
information is insuecient for debugging purposes then they should
add enough information to the exception to allow the program to
be debugged at a later stage.

5.3. WHAT IS AN ERROR?

127

Rule: 4 — Turn non-functional requirements into assertions (invariants)
that can be checked at run-time. If the assertion is broken then raise
an exception.

An example of this might concern termination of a loop—a program-
ming error might cause a function to enter an inﬁnite loop causing
the function never to return. Such an error could be detected by
requiring that certain functions terminate within a speciﬁed time, by
checking this time and generating an exception if the function did
not terminate within the speciﬁed time.

128 CHAPTER 5. PROGRAMMING FAULT-TOLERANT SYSTEMS

6 Building an

Application

T he previous chapter introduced a general model for programming

fault-tolerant systems, and introduced the idea of a “supervision
tree” which is used to monitor the behaviour of the system. This
chapter moves from a general theory to the speciﬁc implementation of
supervisors as implemented in the OTP system.

To illustrate the supervision principles I build a simple OTP appli-
cation. This application has a single supervisor process which manages
three worker processes. The three worker processes are instances of the
gen_server, gen_event and gen_fsm behaviours.

6.1 Behaviours

Applications which use the OTP sodware are built from a number of “be-
haviours.” Behaviours are abstractions of common programming patterns,
which can be used as the building blocks for implementing systems in
Erlang. The behaviours discussed in the remainder of this chapter are as
follows:

• gen_server — this is used to build servers which are used in client-

server models.

• gen_event — this is used for building event handlers. Event han-
dlers are things like error loggers, etc. An event handler is something

129

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CHAPTER 6. BUILDING AN APPLICATION

which responds to a stream of events, it does not necessarily reply to
the processes which send events to the handler.

• gen_fsm — this is used for implementing ﬁnite state machines.

• supervisor — this is used for implementing supervision trees.

• application — this is used as a container for packaging completed

applications.

For each behaviour I will present the general principles involved, the
speciﬁc details of the programming API and give a complete example of
how to make an instance of the behaviour.

In OTP systems are built in the following hierarchical manner:

• Releases — releases are at the top of the hierarchy. A release contains
all the information necessary to build and run a system. A release
will consists of a sodware archive (packed in some form) and a set
of procedures for installing the release. The process of installing a
release can be highly complicated, since upgrade releases oden have
to be installed without stopping the target systems. An OTP release
packages all this complexity into a single abstract unit. Internally a
release consists of zero or more applications.

• Applications — applications are simpler than releases, they contain
all the code and operating procedures necessary to run a single
application, and not the entire system. When a release is composed
of multiple applications then the system should have been structured
in such a way as to ensure that the dicerent applications are largely
independent from each other, or that that the dicerent applications
have strictly hierarchical dependencies.

• Supervisors — OTP applications are commonly built from a number

of instances of supervisors.

• Workers — OTP supervisors supervise worker nodes. Worker nodes
are usually instances of the gen_server, gen_event or gen_fsm
behaviours.

6.1. BEHAVIOURS

131

Now for the application. The application is built bottom-up starting
with the worker nodes.
I will create three worker nodes (one instance
of each of the gen_server, gen_event and gen_fsm behaviours.) The
worker nodes are managed in a simple supervision tree, and the supervi-
sion tree is packaged into an application. I start with the worker nodes.

6.1.1 How behaviours are written

The OTP behaviours are written using a programming style which is simi-
lar to that used in the examples in section 4.1. There is however, one major
dicerence. Instead of parameterising the behaviour with an arbitrary func-
tion, we parameterise the behaviour with the name of a module. This
module must export a speciﬁc number of pre-deﬁned functions. Exactly
which functions must be exported depends upon the behaviour being de-
ﬁned. The complete APIs are documented in the behaviour manual pages.
As an example, assume that xyz is an instance of the gen_server

behaviour, then xyz.erl has to contain code like the following:

-module(xyz).
-behaviour(gen_server).

-export([init/1, handle_call/3, handle_cast/2,

handle_info/2, terminate/2, change_code/3]).

...

xyz.erl should export the six routines init/1 ... shown above. To

create an instance of a gen_server we call:

gen_server:start(ServerName, Mod, Args, Options)

where ServerName names the server. Mod is the atom xyz. Args is
an argument passed to xyz:init/1 and Options is an argument which
controls the behaviour of the server itself. Options is never passed as an
argument to the module xyz.

The method used for parameterising a behaviour in the examples
shown on pages 86–101 is somewhat more general than the method used

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CHAPTER 6. BUILDING AN APPLICATION

in the OTP system. The reasons for the dicerences are largely historical,
the ﬁrst behaviours being written before funs were added to Erlang.

6.2 Generic server principles

Chapter 4 introduced the idea of a generic server. The generic server
provides an “empty” server, that is, a framework from which instances of
servers can be built. The examples in Chapter 4 were deliberately short,
and illustrated the principles involved in making a generic server.

In the OTP system the Erlang module gen_server is used to make
client-server modules. gen_server can be parameterised in a number of
dicerent ways to make a range of dicerent types of servers.

6.2.1 The generic server API

To understand the gen_server API it is helpful to see the ﬂow of control
between the server and the application. I will describe the subset of the
gen_server API that I have used in the examples in this chapter.

gen_server:start(Name1,Mod,Arg,Options) -> Result where

Name1 = Name of server (see note 1).

Mod = Name of callback module (see note 3).

Arg = An argument which is passed to Mod:init/1 (see note 4).

Options = A set of options which controls the workings of the

server.

Result = The result obtained by evaluating Mod:init/1 (see note

4).

gen_server:call(Name2,Term) -> Result where

Name2 = Name of server (see note 2).

6.2. GENERIC SERVER PRINCIPLES

133

Term = Argument passed to Mod:handle_call/3 (see note 4).

Result = The result obtained by evaluating Mod:handle_call/3

(see note 4).

gen_server:cast(Name2,Term) -> ok where

Name2 = Name of server (see note2).

Term = An argument passed to Mod:handle_cast/3 (see note 4).

Notes:

1. Name1 is a term like {local,Name2} or {global,Name2}. Start-
ing a local server creates a server on a single node. Starting a global
server starts a server which can be transparently accessed from any
node in a set of distributed Erlang nodes.

2. Name2 is an atom.

3. Mod should export some or all of the following functions: init/1,
handle_call/3, handle_cast/3, terminate/2. These routines
will be called by gen_server.

4. Some of the arguments to the functions in gen_server appear un-
changed as the argument to the functions in Mod. Similarly, some of
the terms contained in the return values from the functions in Mod
reappear in the return values of the functions in gen_server.

The callback routines in Mod have the following APIs:

Mod:init(Arg) -> {ok,State}|{stop,Reason}
This routine attempts to start the server:

Arg

is the third argument supplied to gen_server:start/4.

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CHAPTER 6. BUILDING AN APPLICATION

{ok,State} means that starting the server succeeded. The inter-
nal state of the server becomes State and the original call to
gen_server:start will return {ok, Pid} where Pid identi-
ﬁes the server.

{stop,Reason}

means starting the server failed in which case the original call
to gen_server:start will return {error,Reason}.

Mod:handle_call(Term,From,State) -> {reply,R,S1}

This gets called when the user calls gen_server:call(Name,Term).

Term is any term.

From identiﬁes the client.

State is the current state of the server.

{reply,R,S1} causes the return value of gen_server:call/2 to
become R and the new state of the server to become S1.

Mod:handle_cast(Term,State) -> {noreply, S1}|{stop,R,S1}

This gets called when the user calls gen_server:cast(Name,Term).

Term is any term.

State is the current state of the server.

{noreply,S1} causes the state of the server to change to S1.

{stop,R,S1} causes the server to stop.
calling Mod:terminate(R,S1)

It will be stopped by

Mod:terminate(R, S) -> Void

This routine gets called when the server stops. The return value is
ignored.

R is the reason for termination.

S is the current state of the server.

6.2. GENERIC SERVER PRINCIPLES

135

6.2.2 Generic server example

This example makes a simple Key–Value server which is implemented
using gen_server. The Key–Value server is implemented with a call-
back module called kv1 which is shown in ﬁgure 6.1.

Line 2 of kv tells the compiler that this module is a call-back module
for the gen_server behaviour. The compiler will then issue warnings if
the module does not export the correct set of call-back routines which are
needed by gen_server.

kv.erl exports a number of client functions (line 4), and a number of
callback functions (lines 6 and 7. The client functions can be called from
anywhere within the system. The callback functions will only be called
from within the gen_server module.

kv:start() starts the server by calling gen_server:start_link/4.
The ﬁrst argument to gen_server:start_link/4 is the location of the
server. In our example the location is {local,kv} which means that the
server is a locally registered process with name kv. Several other values are
permitted for the location. These include the value {global,Name} which
would register the server with a global name (instead of a local name). Use
of a global name allows the server to be accessed in a distributed Erlang
system from any node in the system.

The remaining arguments to gen_server:start/4 are the callback
module name (kv), an initial argument (arg1) and a set of options control-
ling debugging etc ([]). Setting this to [{debug,[trace,log]}] would
turn on debugging and tracing to a log ﬁle.

When gen_server:start_link/4 is called it will initialise its internal
data structures by calling kv:init(Arg) where Arg is the third argument
that was supplied in the call to gen_server:start_link/4. For normal
operation init/1 should return a tuple of the form {ok,State}.

The client routines which are exported from kv in lines 18–21. store/2

and lookup/1 are implemented with gen_server:call/2.

Internally remote procedure calls are implemented by calling the call-
back routine handle_call/2. Lines 23–29 implement the callback rou-
tines necessary to implement the server-side of the remote procedure calls.

1Line 26 of kv.erl has a deliberate error, ignore this for now.

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CHAPTER 6. BUILDING AN APPLICATION

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-module(kv).
-behaviour(gen_server).

-export([start/0, stop/0, lookup/1, store/2]).

-export([init/1, handle_call/3, handle_cast/2,

terminate/2]).

start() ->

gen_server:start_link({local,kv},kv,arg1,[]).

stop() -> gen_server:cast(kv, stop).

init(arg1) ->

io:format("Key-Value server starting~n"),
{ok, dict:new()}.

store(Key, Val) ->

gen_server:call(kv, {store, Key, Val}).

lookup(Key) -> gen_server:call(kv, {lookup, Key}).

handle_call({store, Key, Val}, From, Dict) ->

Dict1 = dict:store(Key, Val, Dict),
{reply, ack, Dict1};

handle_call({lookup, crash}, From, Dict) ->

1/0; %% <- deliberate error :-)
handle_call({lookup, Key}, From, Dict) ->

{reply, dict:find(Key, Dict), Dict}.

handle_cast(stop, Dict) -> {stop, normal, Dict}.

terminate(Reason, Dict) ->

io:format("K-V server terminating~n").

Figure 6.1: A simple server

6.3. EVENT MANAGER PRINCIPLES

137

The ﬁrst argument to handle_call is a pattern which must match the
second argument used in the calls to gen_server:call/2. The second
In normal operation handle_call
argument is the state of the server.
should return a tuple of the form {reply,R,State1} where R is the re-
turn value of the remote procedure call (which will be returned to the
client, and become the return value of gen_server:call/2) and State1
which will become the new value of the state of the server.

gen_server:cast(kv,stop) which is called by stop/0 in line 12 is
used to stop the server. The second argument stop reappears as the ﬁrst
argument to handle_cast/2 in line 31, the second argument is the state
of the server. handle_cast returns {stop,Reason,State} which will
force the generic server to call kv:terminate(Reason,State). This is
to give the server a chance to perform any ﬁnal operations that it wishes
to perform before exiting. When terminate/2 returns, the generic server
will be stopped and all name registrations removed.

In this example we have only shown a simple example of the use of
the generic server. The manual pages for gen_server give all the options
for the values that the various arguments to the callback functions and
the control functions to gen_server can take. The generic server can be
parameterised in a large number of dicerent ways, making it easy to run
the system as a local server or as a global server in a network of distributed
Erlang nodes.

The generic server also has a number of in-built debugging aids which
are automatically made available to the programmer. In the event of an
error occurring in a server which has been built using gen_server a
complete trace dump of what went wrong will automatically be added to
the system error log. This information is usually suecient to allow post-
mortem debugging of the server.

6.3 Event manager principles

The event manager behaviour gen_event provides a general framework
for building application-speciﬁc event handling routines. Event managers
can be built for tasks like:

138

CHAPTER 6. BUILDING AN APPLICATION

• error logging.

• alarm handling.

• debugging.

• equipment management.

Event managers provide named objects to which events can be sent.
Zero or more event handlers can be installed within a given event manager.
When an event arrives at an event manager it will be processed by all
the event handlers which have been installed within the event manager.
In particular we can
Event managers can be manipulated at run-time.
install an event handler, remove an event handler or replace one event
handler with a dicerent handler.

We start with a few deﬁnitions:

• Event — something which happens.

• Event Manager — a program which coordinates processing of events
of the same category. The event manager provides a named object
to which events can be sent.

• Notiﬁcation — the act of sending an event to an event manager.

• Event Handler — a function which can process events. The event

handler must be a function of type:

State x Event -> State’

The event manager maintains a list of module × state 2-tuples of the

form {M,S}. We call such a list a module – state (MS) list.

Suppose that the internal state of the event manager is represented by

the MS list:

[{M1, S1}, {M2, S2}, ...]

when an event E is received by the event manager it replaces this list

by the list:

6.3. EVENT MANAGER PRINCIPLES

139

[{M1, S1New}, {M2, S2New}, ...].

where {ok, SiNew} = Mi:handle_event(E, Si}.
The event manager can be thought of as a generalisation of a conven-
tional ﬁnite state machine, where instead of a single state, we maintain a
“set” of states and a set of state transition functions.

As might be expected, there are also a number of interface functions in
the gen_event API to allow manipulation of the {Module,State} pairs
in the server. gen_event is more powerful than might be imagined from
this simple introduction. The full details are best appreciated by reading
the tutorial on event handling which is part of the OTP documentation.

6.3.1 The event manager API

The event manager (gen_event) exports the following routines:

gen_event:start(Name1) -> {ok, Pid}|{error,Why}

Create a new event manager.

Name1 is the name of the event manager (see note 1).

{ok,Pid} means the event manager started successfully. Pid is the

process Pid of the event manager.

{error,Why} is returned if an event manager cannot be started.

gen_event:add_handler(Name2,Mod,Args) -> ok | Error

Adds a new handler to the event manager. If the old state of the
event manager was L then if this operation succeeds the new state
of the event manager will be [{Mod,S}|L] where S is obtained by
evaluating Mod:init(Args).

Name2 is the name of the event manager (see note 1).

Mod is the name of a callback module (see note 2).

Arg = An argument which is passed to Mod:init/1.

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CHAPTER 6. BUILDING AN APPLICATION

gen_event:notify(Name2,E) -> ok

Sends an event E to the event manager.
If the state of the event
manager is a set of {Mi,Si} pairs and an event E is received then
the state of the event manager changes to a set of {Mi,SiNew} pairs
where {ok,SiNew}=Mi:handle_event(E, Si)

gen_event:call(Name2,Mod,Args) -> Reply

Perform an operation on one of the handlers in the event manager.
If the state of the event manager contains a tuple {Mod,S} then
Mod:handle_call(Args, S) is called. Reply is derived from the
return value of this call.

gen_event:stop(Name2) -> ok
Stops the event manager.

Notes:

1. Event mangers follow the same naming conventions used for generic

servers.

2. An event handler must export some of or all the following functions:

init/1, handle_event/2, handle_call/3, terminate/2.

The event handler modules have the following API:

Mod:init(Args) -> {ok,State} where

Args comes from gen_event:add_handler/3.

State is the initial value of the state associated with this event

handler.

Mod:handle_event(E,S) -> {ok,S1} where

E comes from the second argument to gen_event:notify/2.

S is the initial value of the state associated with this event handler.

6.4. FINITE STATE MACHINE PRINCIPLES

141

S1 is the new value of the state associated with this event handler.

Mod:handle_call(Args, State) -> {ok,Reply,State1} where

Args comes from the second argument to gen_event:call/2.
State is the initial value of the state associated with this event

handler.

Reply becomes the return value of gen_event:call/2
State1 is the new value of the state associated with this event

handler.

Mod:terminate(Reason, State) -> Void where

Reason tells why the event manager is being stopped.
State is the current value of the state associated with this event

handler.

6.3.2 Event manager example

Figure 6.2 shows how gen_event can be used to build a simple event
logger. The error logger keeps track of the last ﬁve error messages, and
it can display the last ﬁve error messages in response to notiﬁcation of a
report event.

Notice that the code in simple_logger.erl is purely sequential. At
this point the observant reader should also have noticed the similarity
between the forms of the arguments to gen_server and to gen_event.
In general the arguments to routines like start, stop, handle_call etc
in the dicerent behaviour modules are chosen to be as similar as possible.

6.4 Finite state machine principles

Many applications (for example protocol stacks) can be modeled as ﬁnite
state machines (FSMs). FSMs can be programmed using the ﬁnite state

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-module(simple_logger).
-behaviour(gen_event).

-export([start/0, stop/0, log/1, report/0]).

-export([init/1, terminate/2,

handle_event/2, handle_call/2]).

-define(NAME, my_simple_event_logger).

start() ->

case gen_event:start_link({local, ?NAME}) of

Ret = {ok, Pid} ->

gen_event:add_handler(?NAME,?MODULE,arg1),
Ret;
Other ->
Other

end.

stop() -> gen_event:stop(?NAME).

log(E) -> gen_event:notify(?NAME, {log, E}).

report() ->

gen_event:call(?NAME, ?MODULE, report).

init(arg1) ->

io:format("Logger starting~n"),
{ok, []}.

handle_event({log, E}, S) -> {ok, trim([E|S])}.

handle_call(report, S)

-> {ok, S, S}.

terminate(stop, _)

-> true.

trim([X1,X2,X3,X4,X5|_]) -> [X1,X2,X3,X4,X5];
trim(L)

-> L.

Figure 6.2: A simple error logger

6.4. FINITE STATE MACHINE PRINCIPLES

143

machine behaviour gen_fsm.

A FSM can be described as a set of rules of the form:

State(S) x Event(E) -> Actions (A) x State(S’)
...

Which we interpret as meaning:

If we are in the state S and the event E occurs we should
perform the actions A and make a transition to the state S’.

If we choose to program a FSM using the gen_fsm behaviour then
the state transition rules should be written as a number of Erlang functions
which follow the following convention:

StateName(Event, StateData) ->

.. code for actions here ...
{next_state, StateName’, StateData’}

6.4.1 Finite state machine API

The ﬁnite state machine behaviour (gen_fsm) exports the following rou-
tines:

gen_fsm:start(Name1,Mod,Arg,Options) -> Result

This function works exactly like gen_server:start/4 which was
discussed earlier.

gen_fsm:send_event(Name1, Event) -> ok
Send an event to FSM identiﬁed by Name1

The callback module Mod must export the following functions:

Mod:init(Arg) -> {ok,StateName,StateData}

When a FSM starts it calls init/1 this is expected to return an
initial state StateName and some data associated with the state
StateData. When gen_fsm:send_event(..., Event) is next
called the FSM will evaluate Mod:StateName(Event,StateData).

144

CHAPTER 6. BUILDING AN APPLICATION

Mod:StateName(Event,SData) -> {nextstate,SName1,SData1}

Step the FSM. StateName, Event and SData represent the current
state of the FSM. The next state of the FSM should be SName1 and
the data associated with the next state is SData1.

6.4.2 Finite state machine example

To illustrate a typical FSM application I have written a simple packet as-
sembler using gen_fsm. The packet assembler is in one of two states
waiting or collecting. When it is in the waiting state it expects to
be sent information containing a packet length, in which case it enters the
collecting state. In the collecting state it expects to be sent a number
of small data packets, which it has to assemble. When the length of all the
small data packets equals the total packet length it prints the assembled
packet and re-enters the waiting state.

Figure 6.3 is a simple packet assembler written using gen_fsm.
In
line 11 we call gen_fsm:start_link/4 to start a local instance of the
FSM behaviour—Note the similarity to gen_server:start_line/4 in
Line 10 of Figure 6.1. The third argument to gen_server:start/4 reap-
pears is passed as an argument to init/1 in line 17.

The waiting state is modeled by the function waiting/2 (line 21)
and the collecting state by collecting/2 (lines 24–34). The data as-
sociated with the state is stored in the second argument to these functions.
The ﬁrst argument of both these functions is provided by the second ar-
gument in the calls made to gen_fsm:send_event/2. So, for example,
send_data/1 calls gen_fsm:send_event/2 with a second argument Len.
This argument reappears as the ﬁrst argument to waiting/2 in line 21.

The state of the FSM is represented by a 3-tuple {Need,Len,Buff}.
When collecting data, Need is the total number of bytes that should be
collected, Len is the number which have actually been collected, and
Buff is a bucer containing the bytes which have actually been collected.
This 3-tuple appears as the second argument to collect/2 in line 24.

We can illustrate the use of the packet assembler in a session with the

Erlang shell:

6.4. FINITE STATE MACHINE PRINCIPLES

145

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25

26

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28

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32

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34

35

36

37

38

39

-module(packet_assembler).
-behaviour(gen_fsm).

-export([start/0, send_header/1, send_data/1]).

-export([init/1,terminate/3,waiting/2,collecting/2]).

-define(NAME, my_simple_packet_assembler).

start() ->

gen_fsm:start_link({local, ?NAME},?MODULE,arg1,[]).

send_header(Len) -> gen_fsm:send_event(?NAME, Len).

send_data(Str)

-> gen_fsm:send_event(?NAME, Str).

init(arg1) ->

io:format("Packet assembler starting~n"),
{ok, waiting, nil}.

waiting(N, nil) ->

{next_state, collecting, {N,0,[]}}.

collecting(Buff0, {Need, Len, Buff1}) ->

L = length(Buff0),
if

L + Len < Need ->

{next_state, collecting,

{Need, Len+L, Buff1++Buff0}};

L + Len == Need ->

Buff = Buff1 ++ Buff0,
io:format("Got data:~s~n", [Buff]),
{next_state, waiting, nil}

end.

terminate(Reason, State, Data) ->

io:format("packet assembler terminated:"
"~p ~n", [Reason]),

true.

Figure 6.3: A simple packet assembler

146

CHAPTER 6. BUILDING AN APPLICATION

> packet_assembler:start().
{ok,<0.44.0>}
> packet_assembler:send_header(9).
ok
> packet_assembler:send_data("Hello").
ok
> packet_assembler:send_data(" ").
ok
> packet_assembler:send_data("Joe").
Got data:Hello Joe
ok

Again gen_fsm has more functionality than can be described here.

6.5 Supervisor principles

Up to now we have concentrated on primitive behaviours which solve typ-
ical application problems, and a large part of an application can be written
using the primitive client-server, event handling and FSM behaviours. The
gen_sup behaviour is the ﬁrst meta-behaviour which is used to glue to-
gether primitive behaviours into supervision hierarchies.

6.5.1 Supervisor API

The supervisor API is extremely simple:

supervisor:start_link(Name1,Mod,Arg) -> Result
starts a supervisor but calling Mod:init(Arg)

Mod must export init/1, where

Mod:init(Arg) -> SupStrategy SupStrategy is a term describing

the supervision tree.

6.5. SUPERVISOR PRINCIPLES

147

SupStrategy is a term describing how workers in a supervision tree
should be started, stopped or restarted.
I will not describe this in any
detail here. The example which follows contains a simple supervision tree
which is explained here. Complete detail can be found in the supervisor
manual pages.

6.5.2 Supervisor example

The example in ﬁgure 6.4 monitors the three worker behaviours which
were introduced earlier in this chapter. Recall that kv.erl included a
deliberate error (Line 26 of ﬁgure 6.1) and that simple_logger.erl also
had an error2 (which I didn’t mention). We will see what happens when
these errors occur at run-time.

The module simple_sup.erl (Figure 6.4) deﬁnes the behaviour of
the supervisor. It starts in line 7 by calling supervisor:start_link/3
— the calling conventions harmonise with the start and start_link
functions exported by the other behaviours in the system. ?MODULE is a
macro which expands to the current module name simple_sup. The ﬁnal
argument is set to nil. The supervisor method starts by calling init/1 in
the speciﬁed callback module, with argument nil (the third argument to
start_link/3).

init/1 returns a data structure deﬁning the shape of the supervision
tree and the strategy to be used. The term {one_for_one,5,1000} (Line
11) tell the supervisor to construct an “or” supervision tree (see page 122)—
this is because the three activities being supervised are considered unre-
lated. The numbers 5 and 1000 specify a restart frequency—if the super-
visor has to restart the processes which it is monitoring more than 5 times
in 1000 seconds then it itself will fail.

There are three objects in our supervision tree, but I will only describe
how the packet assembler is added to the supervision tree. The other two
behaviours follow the same principles as for the packet assembler.
Lines 13–15 specify the behaviour of the packet assembler.
The ﬁrst item in the tuple describes how the packet assembler should

2You did notice the error, I hope.

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CHAPTER 6. BUILDING AN APPLICATION

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21

-module(simple_sup).
-behaviour(supervisor).

-export([start/0, init/1]).

start() ->

supervisor:start_link({local, simple_supervisor},

?MODULE, nil).

init(_) ->

{ok,

{{one_for_one, 5, 1000},

[

{packet,

{packet_assembler, start, []},
permanent, 500, worker, [packet_assembler]},

{server,

{kv, start, []},
permanent, 500, worker, [kv]},

{logger,

{simple_logger, start, []},
permanent, 500, worker, [simple_logger]}]}}.

Figure 6.4: A simple supervisor

6.5. SUPERVISOR PRINCIPLES

149

be supervised. This starts on line 13. The atom packet is an arbitrary
name (which must be unique for this instance of a supervisor) which can
be used to refer to the node in the supervision tree.

Because the objects which are being supervised are themselves in-
stances of OTP behaviours then plugging them into the supervision tree
is simple. The next argument (line 14) is a 3-tuple {M,F,A} which will be
called by the supervisor to start the speciﬁed processes. The supervisor
calls apply(M,F,A) when it wants to start a supervised process.

The ﬁrst argument permanent on line 15 says that the monitored pro-
cess is to be a so-called “permanent” process. A permanent process will
be automatically restarted by the supervisor if it fails.

In addition to specifying how to start a supervised process, the super-
vised process itself must be written in a particular manner.
It must for
example, be possible for the supervisor to request the supervised process
to terminate in an orderly manner. To do this the supervised processes
must obey the so-called “Shutdown protocol.”

To terminate a worker process the supervisor calls shutdown(P,How)
where P is the Pid of the worker and How determines how the worker is to
be stopped. shutdown is deﬁned as follows:

shutdown(Pid, brutal_kill) ->

exit(Pid, kill);

shutdown(Pid, infinity) ->

exit(Pid, shutdown),
receive

{’EXIT’, Pid, shutdown} -> true

end;

shutdown(Pid, Time) ->

exit(Pid, shutdown),
receive

{’EXIT’, Pid, shutdown} ->

true

after Time ->

exit(Pid, kill)

end.

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CHAPTER 6. BUILDING AN APPLICATION

If How is brutal_kill then the worker process is killed (see page 75).
If How is infinity then a shutdown signal is sent to the worker process

which should respond with an {’EXIT’,Pid,shutdown} message.

If How is an integer T when the process is given T milliseconds to termi-
nate. If a {’EXIT’,Pid,shutdown} is not received within T milliseconds
then the process is unconditionally killed.

In line 15 of ﬁgure 6.4 the integer 500 is a “shutdown time” which
is needed in the shutdown protocol. It says that if the supervisor wishes
to stop a supervised processe then it is to allow the process up to 500
milliseconds to stop what it is doing.

The worker argument means that the supervised process is a worker
process (recall from page 119 that a supervised process can be either a
worker or a supervisor process) and [packet_assembler] is a list of all
modules used by this behaviour (this argument is needed for synchronising
code change operations).

Once everything has been deﬁned, we can compile and run the super-
visor. In the following transcript, I start a supervisor and trigger a number
of errors in the supervised processes. The supervised processes die and
are automatically restarted by the supervisor.

The ﬁrst example shows what happens when an error occurs in the
packet assembler. We start the supervisor, and check the Pid of the packet
assembler.

1> simple_sup:start().
Packet assembler starting
Key-Value server starting
Logger starting
{ok,<0.30.0>}
2> whereis(my_simple_packet_assembler).
<0.31.0>

The printout shows that the servers have started.
Now we send a 3-byte length count, followed by 4 bytes of data:3

3This was the second deliberate error and I’m sure you have already noticed it!

6.5. SUPERVISOR PRINCIPLES

151

3> packet_assembler:send_header(3).
ok
4> packet_assembler:send_data("oops").
packet assembler terminated:

{if_clause,

[{packet_assembler,collecting,2},

{gen_fsm,handle_msg,7},
{proc_lib,init_p,5}]}

ok
Packet assembler starting
=ERROR REPORT==== 3-Jun-2003::12:38:07 ===
** State machine my_simple_packet_assembler terminating
** Last event in was "oops"
** When State == collecting
**
** Reason for termination =
** {if_clause,[{packet_assembler,collecting,2},

Data == {3,0,[]}

{gen_fsm,handle_msg,7},
{proc_lib,init_p,5}]}

This results in quite a lot of output. Firstly the packet assembler crashes,
which accounts for the ﬁrst error printout. Secondly, the supervisor detects
that the packet assembler has crashed and so it restarts it—the message
“Packet assembler starting” is printed when the process restarts. Fi-
nally there is a long and hopefully informative error message.

The error message contains information about the state of the FSM
It tells us that the state of the FSM was
at the point when it crashed.
collecting and that the data associated with this state was the 3-tuple
{3,0,[]} and that the event which caused the FSM to crash was "oops".
This information should be suecient to debug the FSM machine.

In this case the output from the error logger is directed to standard
output. In a production system the error logger would be conﬁgured to
direct its output to stable storage. Analysis of the error log should be
suecient for post-mortem debugging of the system.

We can conﬁrm that the supervisor has correctly restarted the packet

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CHAPTER 6. BUILDING AN APPLICATION

assembler; evaluating whereis(my_simple_packet_assembler) returns
the Pid of the newly started packet assembler.

6> whereis(my_simple_packet_assembler).
<0.40.0>
7> packet_assembler:send_header(6).
ok
8> packet_assembler:send_header("Ok now").
Got data:Ok now
ok

In a similar manner we can evoke the deliberate error in the Key–Value

server:

12> kv:store(a,1).
ack
13> kv:lookup(a).
{ok,1}
14> spawn(fun() -> kv:lookup(crash) end).
<0.49.0>
K-V server terminating
Key-Value server starting
15>
=ERROR REPORT==== 3-Jun-2003::12:54:10 ===
** Generic server kv terminating
** Last message in was {lookup,crash}
** When Server state == {dict,1,

... many lines removed ...

16,
16,

** Reason for termination ==
** {badarith,[{kv,handle_call,3},{proc_lib,init_p,5}]}

15> kv:lookup(a).
error

6.6. APPLICATION PRINCIPLES

153

Note the kv:lookup(crash) must be evaluated in a temporary pro-
cesses which is not linked to the query shell. This is because the supervisor
was started with the call supervisor:start_link/4 and is thus linked
to the query shell. Evaluating kv:lookup(crash) directly in the shell will
crash the supervisor, which is probably not what was intended.4

Note how the supervisor and pre-deﬁned behaviours work together.
The supervisor and the primitive behaviours were not designed in isola-
tion, but were designed to complement each other.

The default behaviour is to provide as much helpful information as

possible in the error log and to put the system into a safe state.

6.6 Application principles

We have now constructed three primitive servers and built them into a
supervision tree; what remains is to build everything into an application.
An application is a container for everything that is needed to deliver a
particular application.

The way in which applications are programmed dicers from the earlier
behaviours. The earlier behaviours use callback modules, which must
export a number of pre-deﬁned functions.

Applications do not use callback functions but instead assume a spe-
ciﬁc organisation of ﬁles, directories and sub-directories in the ﬁle system.
The most important part of the application is contained in the application
descriptor ﬁle (a ﬁle with extension .app) which describes all the resources
needed by the application.

6.6.1 Applications API

Applications are described using an application descriptor ﬁle. Application
descriptor ﬁles have the extension .app. The MAN (4) manual page for
an application deﬁnes the structure of a .app ﬁle to have the following
structure:

4My original attempt at this failed, but Chandrashekhar Mullaparthi on the Erlang mailing list was kind

enough to point out why my behaviours were behaving badly.

154

CHAPTER 6. BUILDING AN APPLICATION

{application, Application,

[{description, Description},

Vsn},
{vsn,
Id},
{id,
[Module1, .., ModuleN]},
{modules,
MaxT},
{maxT,
{registered,
[Name1, .., NameN]},
{applications, [Appl1, .., ApplN]},
{included_applications, [Appl1, .., ApplN]},
{env,
{mod,
{start_phases,

[{Par1, Val1}, .., {ParN, ValN}]},
{Module, StartArgs}},

[{Phase1, PhaseArgs1}, ..,
{PhaseN, PhaseArgsN}]}]}.

All keys in the application association list are optional.

If omitted,

reasonable default values are used.

6.6.2 Application example

To package our application which consists of three primitive behaviours
and one supervisor we use the application ﬁle simple.app shown in ﬁg-
ure 6.5.

In our example the structure of the .app ﬁle is straight-forward.
The main purpose of the application ﬁles is to name and describe the
application and to list all the modules and registered process names that
the application uses.

In addition to the simple.app we need a main program which we use
to “launch” the application; we can use simple.erl which in ﬁgure 6.6.
simple.erl has a couple of calls to start and stop the application.

Now we are ready to run the application. Assuming that all the Erlang
ﬁles are compiled and in the same directory as the .app ﬁle then we can
start the application, and test one of the servers as follows:

1> application:start(simple, temporary).
Packet assembler starting

6.6. APPLICATION PRINCIPLES

155

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7

{application, ’simple’,

[{description, "A simple application"},

{vsn,
{modules,

{maxT,
{registered,

"1.0"},

[simple,kv,packet_assembler,
simple_sup,simple_logger]},

infinity},
[kv, my_simple_event_logger,

my_simple_packet_assembler]},

{applications, []},
{included_applications, []},
{env,
{mod,

[]},
{simple, go}}]}.

Figure 6.5: simple.app - a simple application

-module(simple).
-behaviour(application).

-export([start/2).

start(_, _) -> simple_sup:start().

Figure 6.6: simple.erl - a simple application

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CHAPTER 6. BUILDING AN APPLICATION

Key-Value server starting
Logger starting
ok
2> packet_assembler:send_header(2).
ok
3> packet_assembler:send_data("hi").
ok
Got data:hi

Now we can stop the application:

4> application:stop(simple).

=INFO REPORT==== 3-Jun-2003::14:33:26 ===

application: simple
exited: stopped
type: temporary

ok
Ader stopping the application all processes running within the applica-

tion will be closed down in an orderly manner.

6.7 Systems and releases

The development of this chapter has been “bottom-up.” I start with sim-
ple things, combining them into larger and more complicated units.
I
started with a number of primitive servers, gen_server, gen_event and
gen_fsm. I organised these primitive behaviours into a supervision hierar-
chy, and then built the supervision hierarchy into an application.

The ﬁnal stage, which is not shown here, is to build the application
into a release. A release packages a number of dicerent applications into
a single conceptual unit. The result is small number of ﬁles which can be
moved to a target environment.

Building a complete release is a complex procedure—not only must a
release describe the current state of the system it must also know about
previous versions of the system.

6.8. DISCUSSION

157

Releases contain not only information about the current version of the
sodware but also information about previous releases of the sodware. In
particular they contain procedures for upgrading the system from an earlier
version of the sodware to the current version of the sodware. This upgrade
must oden be performed without stopping the system. A release must also
handle the situation where the installation of the new sodware fails for
some reason. If a new release fails, then the system should revert back to
a previous stable state. All this is handled by the release management part
of the OTP system.

When we look at the AXD301 project in chapter 8, we will see that
there were 122 instances of gen_server, 36 instances of gen_event and
10 instances of gen_fsm. There were 20 supervisors and 6 applications.
All this is packaged into one release.

In my opinion the simplest of these behaviours is gen_server which
also happens to be the single most used design pattern. Errors in a
gen_server callback module should result in informative error messages
of suecient quality to enable post-hoc debugging of the system.

Using the above behaviours is a compromise between design and pro-
gramming eeciency. It is much quicker to design and program a system
using design patterns, but the resultant code is marginally less eecient than
hand-written code which solves exactly the same problem.

6.8 Discussion

• In the OTP system the generic modules which implemented the
behaviours themselves were written by expert Erlang programmers.
These modules are based on several years of experience and repre-
sent “best practice” in writing code for the particular problem.

• Systems built using the OTP behaviours have a very regular struc-
ture. For example, all client-servers and supervision trees have an
identical structure. The use of the behaviour forces a common struc-
ture in the solution of the problem. The applications programmer
has to provide that part of the code which deﬁnes the semantics of

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CHAPTER 6. BUILDING AN APPLICATION

their particular problem. All the infrastructure is provided automat-
ically by the behaviour.

• It is relatively easy for a new programmer joining an existing team
to understand behaviour-based solutions to problems. Once they
have gained familiarity with the behaviours they can easily recognize
situations where a particular behaviour should be used.

• Most of the “tricky” systems programming can be hidden within the
implementation of the behaviours (which are actually much more
complicated than described here).
It you look back to the client-
server and event handler behaviours you will see that all the code to
do with concurrency, message passing etc is isolated in the “generic”
part of the behaviour. The “problem speciﬁc” code only has pure
sequential functions with well-deﬁned types.
This is a highly desirable state of acairs—concurrent programs which
are “diecult” are isolated to small well-deﬁned parts of the system.
The vast majority of the code in the system can be written using
sequential programs having well-deﬁned types.

In our system the behaviours solve orthogonal problems—for example,
client–server has nothing to do with worker-supervisor. In building a real
system we pick and mix between behaviours and combine them in many
dicerent ways to solve problems.

Ocering a small and ﬁxed set of behaviours to a sodware designer has

several beneﬁts:

• It focuses attention on a small set of well-proven techniques. We
know in advance that the individual techniques work well in prac-
tice. Given totally unconstrained choice and complete freedom of
action in a design, the designer may be tempted to produce some-
thing which is unnecessarily complex or something which cannot be
implemented.

• It allows the designer to structure and talk about the design in a

precise manner. It provides a vocabulary for discourse.

6.8. DISCUSSION

159

• It completes the feedback cycle between design and implementation.
All the behaviours presented here work in practice. They are all
used, for example, in the Ericsson AXD301 product.

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CHAPTER 6. BUILDING AN APPLICATION

7 OTP

T he Open Telecom Platform (OTP) is a development system de-

signed for building and running telecommunications systems. A
block diagram of the system is shown in ﬁgure 7.1, which is taken
from the article in [66]. As can be seen from ﬁgure 7.1 the OTP sys-
tem is a so-called “middleware platform” designed to be run on top of a
conventional operating system.

The OTP system was developed internally at Ericsson. Most of the
sodware was released into the public domain subject to the Erlang public
license.1

Included in the OTP release are the following components:

1. Compilers and development tools for Erlang.

2. Erlang run-time systems for a number of dicerent target environ-

ments.

3. Libraries for a wide range of common applications.

4. A set of design patterns for implementing common behavioural pat-

terns.

5. Educational material for learning how to use the system.

6. Extensive documentation.

1Very similar to an open source license.

161

162

CHAPTER 7. OTP

Figure 7.1: The OTP system architecture

OTP has been ported to a number of dicerent operating systems, these
include all the major Unix-like systems (Linux, FreeBSD, Solaris, OS-X ..),
most of the Windows operating systems (Windows 95, 98, NT, ...) and a
number of embedded operating systems like VxWorks.

The Erlang run-time system is a virtual machine designed to run the in-
termediate code produced by the Erlang BEAM compiler. It also provides
run-time support services for a native code Erlang compiler.

The Erlang BEAM compiler replaced the original JAM compiler in
1998. The BEAM compiler [41, 42] compiles Erlang code into sequences
of instructions for a 32-bit word threaded interpreter. The original JAM
machine was a non-threaded byte code interpreter.

For additional eeciency Erlang programs can be compiled to native
code using the HIPE compiler [47] developed at the University of Uppsala.
Both interpreted BEAM and compiled code can be freely intermixed at
a module level, ie, entire modules can be compiled to either BEAM or
HIPE code, but code within an individual module cannot be intermixed.
Both the beam and HIPE machines use common code in the Erlang
run-time system for memory management, input/output, process manage-
ment, and garbage collection etc.

Applications written in Applicationswritten in C or otherlanguagesErlangMnesiaSNMPAgentWeb serverErlang Run−time systemCommercial Operating System and computer hardware7.1. LIBRARIES

163

The Erlang run-time system ocers many of the services which are tra-
ditionally ocered by an operating system, so by comparison with the run-
time support needed for a purely sequential language the run-time system
is fairly complex. All Erlang processes are managed by the Erlang run-time
system—even when there are several tens of thousands of Erlang processes
running under control of the Erlang run-time system the host operating
system will only think that there is one process running, that being the
Erlang run-time system itself.

The Erlang compiler, on the other hand is rather simple, compared
to most other languages. Compilation is oden a simple translation of Er-
lang code into an appropriate primitive in the virtual machine. Thus,
for example, the spawn primitive in Erlang compiles to a single opcode
in the virtual machine (the spawn primitive) and great care was taken to
implement this as eeciently as possible.

7.1 Libraries

The OTP release contains a large set of libraries, all of which for release
purposes are considered instances of OTP applications. Release R9B con-
tains the following applications:

• appmon — a graphical utility to observe and manipulate supervision

trees.

• asn1 — a compiler and run-time support for decoding and encoding

packets deﬁned in ASN.1.

• compiler — the Erlang compiler.

• crypto — a set of functions for encrypting and decrypting data and

for computing message digests.

• debugger — an Erlang source code debugger.

• erl_interface — a set of libraries for communicating with dis-

tributed Erlang nodes.

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CHAPTER 7. OTP

• erts — the Erlang run-time system.

• et — the event tracer and tools to record and give a graphical pre-

sentation of event data.

• eva — the “event and alarm” handling application.

• gs — a graphics system. A set of graphics routines for building GUIs.

• ic — Erlang IDL compiler.

• inets — an HTTP server and an FTP client.

• jinterface — a tool to create Java to Erlang interfaces.

• kernel — one of the two basic libraries needed to run the system
(the other is stdlib). kernel contains code for ﬁle servers, code
servers etc.

• megaco — libraries for the Megaco2/H248 protocols.

• mnemosyne — a database query language for mnesia.

• mnesia — a distributed DBMS with sod real-time properties for Er-

lang.

• observer — tools for tracing and observing the behaviour of a dis-

tributed system.

• odbc — an Erlang interface ODBC interface to SQL databases.

• orber — an Erlang implementation of a CORBA object request
broker. Note: there are also separate applications to provide access
to various CORBA services, such as the events, notiﬁcations, ﬁle
transfers etc.

• os_mon — a tool to monitor resource usage in the external operating

system.

2Media Gateway Control.

7.1. LIBRARIES

165

• parsetools — tools for parsing Erlang. Includes yecc an LALR(1)

parser generator.

• pman — a graphic tool to inspect the state of the system. Pman can

be used to observe local or remote Erlang nodes.

• runtime_tools — miscellaneous small routines needed in the run-

time system.

• sasl — short for “System Architecture Support Libraries.” This
application contains support for alarm handling, managing releases
etc.

• snmp — an Erlang implementation of the Simple Network Manage-
ment Protocol [24]. This application includes a MIB compilers and
tools for building MIBs etc.

• ssl — an Erlang interface to the secure sockets layer.

• stdlib — the “pure” Erlang libraries needed to run the system. The

other obligatory application is kernel.

• toolbar — a graphical

toolbar from which applications can be

started.

• tools — a package of stand-alone applications for analysing and
monitoring Erlang programs. This includes tools for proﬁling, cov-
erage analysis, cross reference analysis etc.

• tv — a “table viewer.” The table viewer is a graphic application to

allow graphic browsing of tables in the mnesia database.

• webtool — a system for managing web-based tools (such as inets).

The OTP libraries provide a highly sophisticated tool set and are a
good starting point for any commercial product, they are however, fairly
complex.

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CHAPTER 7. OTP

Recall that all of chapter 6 was devoted to a simpliﬁed explanation
of ﬁve behaviours (gen_server, gen_event, gen_fsm, supervisor and
application) and a complete explanation of any one of these behaviours
is outside the scope of this thesis. The principles behind one of these
behaviours (gen_server) was the subject of pages 86–101.

stdlib in release R9B has some 71 modules—four of them have been

described here.

8 Case Studies

T his section of the thesis presents studies of some systems which

were written using the Erlang/OTP system. The ﬁrst is the Erics-
son AXD301 system—the AXD301 is a high-capacity ATM1 switch.
The version of the AXD301 system studied here has over 1.1 million lines
of Erlang, which makes it one of the largest programs ever to be written in
a functional style of programming. The AXD makes extensive use of the
OTP libraries, so it provides a good test of the functionality of the libraries.
Following this I study a number of small products made by Bluetail
AB or Alteon Web Systems/Nortel Networks. To avoid confusion, I might
add that Bluetail AB was founded by most of the “Erlang people” who led
the Ericsson CSLab (myself included) and that Bluetail was subsequently
acquired by Alteon Web Systems, and that Nortel Networks then acquired
Alteon. The products, however, were all developed by the same core
group of people.

These products include the Bluetail Mail Robustiﬁer (BMR) and the
“SSL2 accelerator,” made by Alteon Web Systems and sold by Nortel
Networks. The SSL accelerator was developed in a remarkably short time
(9 months) and rapidly became the “market leader” in the niche market for
“embedded secure socket layer devices.” The SSL accelerator also makes
extensive use of the Erlang/OTP system and libraries.

These projects represent two extremes. The AXD301 was done with

1Asynchronous Transfer Mode.
2Secure Socket Layer.

167

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CHAPTER 8. CASE STUDIES

a large group of programmers; over 40 programmers have been involved
with the code over a 4-year period. Large sodware projects are notoriously
diecult to manage, and the resulting code is oden diecult to understand;
so one of the matters that I am concerned with is how well (or poorly) the
OTP design methodology supports the construction of large systems.

The second set of projects was programmed by a much smaller group
of programmers (5-10 depending upon the product) and was completed
within a much shorter time frame (six months). The developers were all
highly experienced Erlang programmers. Two of the developers, Magnus
Fr¨oberg and Martin Bj¨orklund, designed and programmed the original
OTP behaviours. One of the developers, Claes Wikstr¨om was a co-author
of the second edition of the Erlang book. Wikstr¨om was also the main
implementor of distributed Erlang, and of the mnesia data base.

8.1 Methodology

In the case studies, I am interested in the following:

• Problem domain — what was the problem domain. Is this the kind

of problem that Erlang/OTP was designed to solve?

• Quantitative properties of the code — how many lines of code were
written? How many modules? How was the code organised? Did
the programmers follow the design rules? Were the design rules
helpful? What was good? What was bad?

• Evidence for fault-tolerance — Is the system fault-tolerant? The rai-
son d’ˆetre for Erlang was to build fault-tolerant systems.
Is there
evidence that errors have occurred at run-time and been success-
fully corrected? Is the information produced when a programming
error occurred good enough to subsequently correct the program?

• Understanding the system — Is the system understandable? Can it

be maintained?

8.1. METHODOLOGY

169

Rather than ask general questions about the properties of the system
I am looking for speciﬁc evidence that the system behaves in a desirable
manner. In particular:

1. Is there evidence to show that the system has crashed due to a
programming error and that the error was corrected and that the
system subsequently recovered from the error and behaved in a
satisfactory manner ader error correction had occurred?

2. Is there evidence to show that the system has run for an extended
period of time and that sodware errors have occurred but that the
system is still stable?

3. Is there evidence that code in the system has been updated “on the

ﬂy?”

4. Is there evidence that garbage collection etc works (ie that we have
run a garbage-collected system for a long time without getting garbage
collection errors?)

5. Is there evidence that the information in the error logs is suecient

for post-crash localization of the error?

6. Is there evidence that the code in the system can be structured in
such a way that the majority of programmers do not need to be
concerned with the details of the concurrency patterns used in the
system?

7. Is there evidence that the supervision trees work as expected?

8. Is the code structured in a clean/dirty style?

Items 1, 2 and 5 above are included because we wish to test that our

ideas about programming fault-tolerant systems work in practice.

Item 4 tests the hypothesis that long-term garbage collection can be
used for real-time systems which have to operate over extended periods of
time.

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CHAPTER 8. CASE STUDIES

Item 6 is a measure of the abstraction power of the OTP behaviours.
It is desirable for a number of reasons to “abstract out” details of concur-
rency for commonly recurring situations. The set of OTP behaviours is an
attempt to do this. The extent to which programmers can to a ﬁrst approx-
imation ignore concurrency is an important measure of how suitable the
OTP behaviours are for making system sodware. We can assess the extent
to which concurrency can be ignored by observing how oden program-
mers are forced to use explicit message passing and process manipulation
primitives in their code.

Item 7 tests if the supervisor strategies work as expected.
Item 8 tests if it is possible to program according to the rules we gave
in Appendix B. In particular the guidelines stress the importance of struc-
turing the system in a clean/dirty manner. “Clean” code for our purposes
is assumed to be side-ecect free code, such code is much easier to under-
stand than “dirty” code, ie code having side-ecects.

Our entire system is concerned with the manipulation of hardware.
This manipulation of hardware involves side-ecects. Our concern there-
fore, is not whether side-ecects can be avoided, but rather to what extent
code with side-ecects can be restricted to a small number of modules.
Rather than having code with side-ecects scattered in a uniform manner
all over the system, it is desirable to have a small number of “dirty” mod-
ules with a large number of side-ecects, combined with a much larger
number of modules which are written in a side-ecect free manner. An
analysis of the code will reveal if such a structuring was possible.

Also of interest is “counter evidence.” We would like to know about
any cases where our paradigm breaks down, and if this breakdown was a
major problem.

8.2 AXD301

The AXD301 [18] is a high-performance Asynchronous Transfer Mode
(ATM) switch produced by Ericsson. The system is built from a number
of scalable modules—each module provides 10 GBit/s of switching capacity
and up to 16 modules can be connected together to form a 160 GBit/s

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

171

switch.

The AXD301 is designed for “carrier-class” non-stop operation [70].
The system has duplicated hardware which provides hardware redundancy
and hardware can be added or removed from the system without inter-
rupting services. The sodware has to be able to cope with hardware and
sodware failures. Since the system is designed for non-stop operation it
must be possible to change the sodware without disturbing traec in the
system.

8.3 Quantitative properties of the sod-

ware

Here I report the result of an analysis of a snapshot of the AXD301 sod-
ware. The snapshot represents the state of the system as it was on 5
December 2001.

The analysis is only concerned with the quantitative properties of the
Erlang code in the system. The gross properties of the system are as
follows:

Number of Erlang modules
Clean modules
Dirty modules
Number of lines of code
Total number of Erlang functions
Total dumber of clean functions
Total number of dirty functions
Percentage of dirty functions/code lines

2248
1472
776
1136150
57412
53322
4090
0.359%

In the above table the code has been subject to a simple analysis which
superﬁcially classiﬁes each module or function as “clean” or “dirty.” A
module is considered dirty if any function in the module is dirty, otherwise
it is clean. To simplify matters I say that a function is dirty if it sends or
receives a message or if it calls one of the following Erlang BIFs:

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CHAPTER 8. CASE STUDIES

apply, cancel_timer, check_process_code,
delete_module, demonitor, disconnect_node, erase,
group_leader, halt, link, load_module, monitor_node,
open_port, port_close, port_command, port_control,
process_flag, processes, purge_module, put, register,
registered, resume_process, send_nosuspend, spawn,
spawn_link, spawn_opt, suspend_process, system_flag,
trace, trace_info, trace_pattern, unlink, unregister,
yield.

The reason for this classiﬁcation is that any code fragment which calls

one of these BIFs is potentially dangerous.

Notice that I have chosen a particularly simple deﬁnition of “dirty.” At
ﬁrst sight it might appear that it would be better to recursively deﬁne a
module as being dirty if any function in the module calls a “dangerous”
BIF or a dirty function in another module. Unfortunately with such a
deﬁnition virtually every module in the system would be classiﬁed as dirty.
The reason for this is that if you compute the transitive closure of all
functions calls exported from a particular module, the transitive closure
will include virtually every module in the system. The reason why the
transitive closure is so large is due to “leakage” which occurs from many
of the modules in the Erlang libraries.

We take the simplifying view that all modules are well-written and
tested, and that if they do contain side-ecects, that the module has been
written in such a way so that the side ecects do not leak out from the
module to adversely acect code which calls the module.

With this deﬁnition 65% of all modules are clean. Since any module is
considered dirty if it contains a single dirty function, it is more interesting
to look at the ratio of clean/dirty functions. A function will be considered
dirty if a single call is made to unclean BIF. Viewed at a function level 92%
of all functions appear to be written in a side-ecect free manner.

Note that there were a total of 4090 dirty functions contained in 1.13
million lines of code, this is less than four dirty functions per thousand
lines of code.

The distribution of dirty functions is shown in Figure 8.1. The distri-

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

173

200

180

160

140

120

# Mods

100

80

60

40

20

0

0

10

20

30

40

50

60

70

# dirty functions

Figure 8.1: Distribution of dirty functions

bution of dirty functions is both encouraging and discouraging. The good
news is that 95% of all dirty functions are found in slightly over 1% of the
modules. The bad news is that there are a large number of modules with
a very small number of impure functions. For example, there are 200
modules with 1 dirty function, 156 with 2 dirty functions etc.

The interesting thing about this data is that there has been no system-
atic ecort to make the code “pure.” The “emergent style” of programming
seems therefore, to favour a style of programming where a small num-
ber of modules have a large number of side-ecects, together with a larger
number of modules having very few side-ecects.

The Erlang programming rules favour this style of programming, the
intention is to get the more experienced programmers to write and test
the code that has lots of side-ecects. Based on the AXD301 code it might
be a good idea to explicitly deﬁne which modules are allowed to contain
side-ecects and enforce this with some kind of quality control.

If we look in detail at which primitives were called that could introduce

side-ecects, we get the following ordering:

put (1904), apply (1638), send (1496), receive (760), erase (336),

process_flag (292), spawn (258), unlink (200), register (154),

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CHAPTER 8. CASE STUDIES

spawn_link (126), link (106), unregister (38), open_port (20),
demonitor (14), processes (14), yield (12), halt (10),
registered (6), spawn_opt (4), port_command (4), trace (4),
cancel_timer (2), monitor_node (2).

The most common primitive was put which was used 1904 times etc.
From this we can see that some of the Erlang primitives in our “black
list” have never been used at all. The most common primitive which in-
troduces a side-ecect is put—depending upon how put is used this may or
may not be dangerous. One common use of put is to assert a global prop-
erty of a process which is used for debugging purposes. This is probably
safe, though no automatic analysis program could infer this fact.

The dangerous side-ecects are those which change the concurrency
structure of the applications, thus modules which call the link, unlink,
spawn or spawn_link, primitives must be carefully checked.

Even more dangerous is code that might evaluate halt or processes—

I would assume that such code is very carefully checked.

8.3.1 System Structure

The AXD301 code is structured using the OTP supervision trees and the
overall structure of the AXD301 code can be inferred primarily from the
shape of these trees. Interior nodes in the supervision tree are themselves
supervisor nodes, terminal nodes in the tree are OTP behaviours or more
specialised application speciﬁc processes.

The AXD system supervision tree had 141 nodes and used 191 in-
stances of the OTP behaviours. The number of instances of the behaviours
were:

gen_server (122), gen_event (36), supervisor (20), gen_fsm (10),

application (6).

gen_server and to a lesser extent gen_event dominate—there being
122 instances of the generic server. One interesting point to note is how
few behaviours are required. The client-server abstraction (gen_server)
is so useful that 63% of all generic objects in the system are instances of
client-servers.

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

175

In the OTP libraries a supervisor starts an application by calling a
function in the so-called child_spec of the processes which the super-
visor controls. Among other things the “child speciﬁcation” contains a
{Mod,Func,Args} tuple which is used to specify how to start the super-
vised process.

This method of starting a supervised process is completely general,
In the
since any arbitrary function can be started by the supervisor.
AXD301 case, the full generality of this method was not used,
instead
one of three dicerent startup methods was used for all supervision hierar-
chies. Of these three methods, one method dominated, and was used for
all except three of the supervision trees.

The AXD301 architects deﬁned one master supervisor, which could be
parameterised in a number of standardised ways. The AXD301 supervi-
sors were packaged as conventional OTP applications, whose behaviour
was described in so-called .app ﬁles [34]. Analysing all the AXD301 apps
gives us a good overall idea of the static structure of the AXD sodware.

There were a total of 141 .app ﬁles the AXD sodware. These 141 ﬁles
represent 11 disjoint supervision trees. Most of the supervision trees are
very ﬂat and do not have a complex structure.

A simple way to show this structure is to plot the trees in a simple
ASCII display. Here, for example, is one of the eleven top-level trees,
responsible for “Standby” services

|-- chStandby
|
|
|
|
|
|
|
|
|
|
|
|
|
|

|-- stbAts
|
|
|
|
|
|
|
|
|
|
|
|-- stbSws
|

|-- aini_sb
|-- cc_sb
|-- iisp_sb
|-- mdisp_sb
|-- pch_sb
|-- pnni_sb
|-- reh_sb
|-- saal_sb
|-- sbm_sb
|-- spvc_sb
|-- uni_sb

|-- cecpSb

Standby top application
Standby parts of ATS Subsystem
Protocol termination of AINI, ...
Call Control.Standby role
Protocol termination of IISP ...
Message Dispatcher, standby role
Permanent Connection Handling ...
Protocol termination of PNNI ...
Standby application for REH
SAAL, standby application.
Standby Manager, start standby role
Soft Permanent Connection ...
Protocol termination of UNI, ...
Standby applications - SWS
Circuit Emulation for AXD301, ...

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CHAPTER 8. CASE STUDIES

|
|

|
|

|-- cnh_sb
|-- tecSb

Connection Handling on standby node
Tone & Echo for AXD301, SWS, ...

As can be seen the tree has a very simple structure, being ﬂat rather
than deep. There are two main sub-nodes in the tree, and the supervisor
structure underneath the sub-nodes is ﬂat.

Note that displaying the data in this manner only shows the organ-
isation of the supervisor nodes themselves. The actual processes which
were sub-nodes to the leafs in the tree are not shown, nor is the type of
supervision (ie and or or supervision).

The reason why the trees are ﬂat rather than deep reﬂects experience
gained with the AXD sodware. Put simply “Cascading restarts do not
work.”

Wiger [69] who was the chief sodware architect of the AXD301 found
that restarting a failed process with the same arguments oden worked,
but that if the simple restart procedure failed, then cascading restarts (ie
restarting the level above) generally did not help.

Interestingly, in 1985, Gray had observed that most hardware faults
were transient and could be corrected by reinitialising the state of the
hardware and retrying the operation. He conjectured that this would also
be true for sodware:

I conjecture that there is a similar phenomenon in sodware —
most production faults are sod. If the program state is reinitial-
ized and the failed operation retried, the operation will usually
not fail the second time. — [38]

The full generality of the model presented in this thesis, namely that of
trying a simpler strategy in the event of failure was only partially exploited.
This particular exploitation was accidental rather than by design—the OTP
libraries themselves which provide interfaces to the ﬁle system and to
system level services like sockets etc are written in such a manner as to
protect the integrity of the system in the event of a failure. So, for example,
ﬁles or sockets are automatically closed if the controlling processes for the
ﬁle or socket terminates for any reason.

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

177

The level of protection provided by the OTP library services automati-
cally provides the “simpler level of service” which is implied by our model
for fault-tolerant computing.

8.3.2 Evidence for fault recovery

In the following sections I present evidence that the error-recovery mech-
anism have worked as planned. This evidence is based on an analysis of
entries contained in the Ericsson trouble report database. When I have
quoted from the data in the trouble report database I have not added
anything to the entries, but I have removed irrelevant detail.

8.3.3 Trouble report HD90439

Trouble report HD90439 from 14 May 2003 has the following information:

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

1. Description

Heading: CRASH REPORT - Performance measurements on ET2 issue
Priority: C:
Status: FI:
Hot TR: NO

3 M, Minor fault or opinion: no traffic disturbance

Finish

...

2. Observation

Top of page

EFFECT:
CRASH REPORT - Performance measurements on ET2 issue

DESCRIPTION:
Node: AXD305 R7D PP6
Customer: ***************

=CRASH REPORT==== 14-May-2003::14:05:00 ===

crasher:

pid: <0.5605.0>
registered_name: []
error_info: {function_clause,[{etcpPrm,get_hwm_base,[ne_cv_l_ga,et2]},

{prmMibExt,create_mep,3},
{prmMibExt,get_counter_values,4},
{perfCollect,collect_group,2},
{proc_lib,init_p,5}]}

initial_call: {perfCollect,collect_generic,

[{observed_object_group,

{groupCb,prmMibExt},
1504,
[127],
undefined},

63220104300]}

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CHAPTER 8. CASE STUDIES

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

ancestors: [perfServer,perfSuper,omsSuper,omAxd301Super,<0.4396.0>]
messages: []
links: [<0.4523.0>]
dictionary: [{{eqm_mi_apply,{em,70},if_type_to_sublayer,1},

{intfIfDbase,if_type_to_sublayer_int}}]

trap_exit: false
status: running
heap_size: 121393
stack_size: 23
reductions: 1332403

neighbours:

MEASURES:
Performance measurements were turned off and the crash report stopped
occurring

...

4. Answer

...
P R O B L E M & C O N S E Q U E N C E
=========================================================

A combination of the wildcard implementation together with a DS1
measurement can cause the described crash. The effect is that the
measurement fails.

S O L U T I O N
=========================================================

The fault has been detected, but it will not be released in R7D unless
it is important for a customer.

The wildcard implementation is greatly improved in R8B where this problem
does not exist. In R7D, we recommend to specify the PDH interfaces to be
included
in the DS1 measurement.

Crash number 90439 is fairly typical, it illustrates the situation where a
hardware errors occurred, is corrected and the system reverted to normal
use. The crash report is in lines 23–27 of the error log and it contains the
following information:

{function_clause,

[{etcpPrm,get_hwm_base,[ne_cv_l_ga,et2]},

{prmMibExt,create_mep,3},
{prmMibExt,get_counter_values,4},
{perfCollect,collect_group,2},
{proc_lib,init_p,5}]}

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

179

When etcpPrm:get_hw_base(ne_cv_l_ga,et2) is called the func-
tion call fails with a pattern matching error.
Interestingly, this error oc-
curred on 14 April 2003, the snapshot of the system that I had access to
is from 5 December 2001, and therefore I reasoned that the error might
have been present in my snapshot code. To satisfy my curiosity I checked
the code in etcpPrm. The code looked like this:

get_hwm_base(locd_cnt, et155) ->

?et155dpPmFm_MEPID_Frh_LOCDEvt;

... 386 lines omitted ...

get_hwm_base(fe_pdh2_uat_tr, et2x155ce) ->

?et2x155cedpPmFm_MEPID_Frh_FE_E1_UAT_Tr.

Indeed there was no pattern which would match the calling arguments,
so clearly this error would occur if called with the arguments shown in the
error log. I was able to clearly locate the error and understand why the
program has failed even though I didn’t have the faintest idea what the
error means.

It is also encouraging to note that the programmer who wrote this code
had not programmed in a defensive manner, ie they had not written code
like this:

get_hwm_base(locd_cnt, et155) ->

?et155dpPmFm_MEPID_Frh_LOCDEvt;

... 386 lines omitted ...
get_hwm_base(fe_pdh2_uat_tr, et2x155ce) ->

?et2x155cedpPmFm_MEPID_Frh_FE_E1_UAT_Tr;

get_hw_base(X, Y) ->
exit(....).

Instead they had written their code exactly in the style which I recom-
mended on page 108. Recall that the motivation for writing code in this
manner is that a defensive style of programming is unnecessary since the
Erlang compiler automatically adds additional information that is suitable
for debugging the program.

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CHAPTER 8. CASE STUDIES

8.3.4 Trouble report HD29758

Crash number HD29758 is more interesting. Reading the log and the
subsequent comments of the engineers who studied the problem we can
see that an error occurred at run-time which must have been corrected.
Even though an error occurs it does not acect the traec, and it is deemed
not worthy of correction.

Here is a section extracted from the trouble report:

2

T R O U B L E D E S C R I P T I O N

R7D NIT test case 3.4.1.2, takeover of OM process by CH CP.
Traffic continues to run successfully,
but we get several ERROR REPORTS in the
CP being blocked.

Here’s an example, see enclosure for erlang log:

=ERROR REPORT==== 5-Apr-2002::09:03:55 ===

error_info: {{case_clause,[]},

[{mdispGenServ,from_plc,4},

{mdispGenServ,handle_info,2},
{gen_server,handle_msg,6},
{proc_lib,init_p,5}]}

msg_info_Tag: from_plc
msg_info_MFA: {mdispGenServ,from_plc,

[old_hc,
{hcid,

{ncs,mlgCmHcc,{97575,0}},
msgQueue,
undefined}]}

msg_info_PlcResult: true
node: ’axd301@cp1-1’
proc_info: [{pid,{<0.20804.4>,"MDISP Server"}},
{message_queue_len,0},
{dictionary,[{mdispPerfPid,<0.20805.4>},
{’$ancestors’,

... many lines omitted ...

4.2 Answer Text

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31

32

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

181

33

34

35

36

37

38

39

40

The fault has been solved in version R8B of block
MDISP. No solution is planned in earlier versions
for mainly two reasons:

1) The fault has not yet given any obvious negative
effects on traffic handling.

...

Lines 10–28 show that an error has occurred. Line 4 and lines 37–
38 show that despite the error the system functions without any “obvious
negative ecects on traec handling.”

This error is rather nice, it shows that for a certain execution path an
error is encountered. The system detects and recovers from the error, so
for this particular error the system behaved in a reasonable manner in the
presence of a sodware error.

8.3.5 Deﬁciencies in OTP structure

One interesting area in which the Erlang model of programming did not
work was in the handling of call setup and call termination used in the
AXD301 sodware.

In the section on the philosophy of COP, I argued for a 1:1 mapping
of the problem structure onto the sodware architecture. In one important
part of the AXD301 sodware this mapping was not possible. This part of
the sodware had to do with the setup and termination of calls.

In order to understand this point I must go into a little detail about
one of the most important services ocered by the AXD301 switch. The
AXD301 is a switching system, as such it is responsible for the maintenance
of a large number of connections.

At any one time, one module in the system will be handling a large
number of virtual channels. Each channel represents a single connection.
Typically, a single node might handle up to 50,000 connections. Connec-
tions are in one of three possible states:

1. Setup — In this state a new connection being established. There is

intensive signalling.

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CHAPTER 8. CASE STUDIES

2. Connected — In this state the connection is established. There is
very little signalling, only monitoring of the on-going connection.

3. Terminating — In this state the connection is terminating. There is

some signalling between the end-points of the connection.

The setup and termination phases of a connection are very quick, and
typically take a few milliseconds. Termination is somewhat simpler than
setup. In the connection phase only monitoring is involved, the connection
phase is typically many orders of magnitude longer than the setup and
termination phases. Connections could range from seconds to hours, or
even years.

At any time there might be up to 50,000 connections per node, the
vast majority of which will be in the connected state. The system is di-
mensioned for a maximum of 120 calls/second—this number refers to the
number of simultaneous calls which can be in the setup or termination
phase, not to the number of calls which are in the connected phase.

Modelling the natural concurrency of the problem needs about six
concurrent processes per call setup/termination. Having six processes per
connection during setup and (say) a couple of processes per connected call
requires a few hundred thousand Erlang processes. Most of these processes
do nothing and are suspended for a very long time (these are the processes
which monitor the connected calls)—this could not be implemented for a
number of reasons.

Firstly, processes take space, even if they are not doing anything. Sec-
ondly, any state involved in the connected phase must be replicated across
a physical hardware boundary. This is in order to provide for continuous
service in the presence of a hardware failure.

The AXD301 tries to minimise the number of active processes used for

connection handling and uses the following strategy.

1. During call setup six processes are created to handle each call.

2. If the call setup succeeds, then all relevant information about the
call is reduced to a “call record” which describes the call. The six
processes used in call setup are destroyed and the call record which

8.3. QUANTITATIVE PROPERTIES OF THE SOFTWARE

183

describes the call is stored locally. Finally, an asynchronous message
containing a copy of the call record is sent to the backup node for
the current node.

3. At call termination, the process structure required to terminate the
call is created and initialized from the data in the call record. Call
termination processing occurs and all the processes involved are de-
stroyed.

Because the call setup process is well understood, the memory require-
ments for a process in the setup phase is well understood. The maximum
stack and heap size needed by the setup processes is well known and has
been established by a number of measurements. Using this data it is pos-
sible to initialise the six processes needed for call setup with sueciently
large initial stacks and heaps so that no garbage collection occurs during
call setup.

During setup no attempt is made to replicate the state of the calls across
a physical boundary, so if the system crashes, the call will be lost. In this
event the client will detect the failure and merely try again. In the event of
hardware failure the retry will be directed to a new hardware module and
should succeed. Since call setup is so quick, this is hardly a problem.

When the setup phase is complete, all information is reduced to a “call
record” (this is about 1 KB/call) and the six processes used in call setup
are destroyed. An asynchronous message containing the call record is sent
to the backup processor. Hardware units are always conﬁgured in pairs.
For the pair (A, B), the machine A is considered a backup machine for
machine B and machine B is a backup for machine A.

Signals from a particular client are arbitrated by a hardware dispatcher
unit, which sends all signals for a particular call to the “master” unit for
the call. If the master unit fails and the call is in the connected state then
the arbitrator will direct signals to the backup unit.

Call termination, or modiﬁcation to the call is the inverse of call setup.
When the system detects that some operation is to be performed on the
call, the call record is retrieved and the process structure necessary to
manipulate the call is created and initialized with data from the call record.
Thereader, call processing proceeds as for call setup.

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CHAPTER 8. CASE STUDIES

This model minimizes the number of processes to the active set of
processes needed to perform a particular operation. When the application
reaches a point in time where there is little active processing of the data,
the processes are destroyed and the relevant data needed to re-create the
process structure is stored in a database and is asynchronously replicated
on a backup machine.

This approach has the advantages of having a ﬂexible process structure
which is needed in the complex phases of call setup or termination—but
when the processes are inactive, storage is reclaimed by deleting the pro-
cesses and storing their state data in stable storage.

The fact that the state information is asynchronously replicated in-
creases throughput in the system without compromising the integrity of
the system.

This solution ﬁts nicely with the Erlang processes model. Since pro-
cesses are light-weight, we can create and destroy them “on-demand.” Sys-
tems with heavy-weight processes could use a similar solution, where in-
stead of destroying a process when it has ﬁnished, the process is recycled,
and a pool of processes is maintained by the system.

At the time when the AXD301 sodware was developed there was an
upper limit on the total number of Erlang processes which could run at the
same time in the system. This number, while large, was not large enough
to allow hundreds of thousands of processes in the system. Later versions
of the system allow hundreds of thousands of Erlang processes, though not
millions of processes.

Even though the designer of a system should not, at a ﬁrst approxima-
tion have to worry about the total number of processes in the system, they
should be able to crudely dimension the system and have a rough idea
as to how many processes will be needed throughout the operation of an
application.

For very long-lived processes, storing the process state in a data base
when the process is not active, is an attractive alternative. This method
has the added advantage that by replicating the state on stable storage the
same sodware can be made fault-tolerant.

8.4. SMALLER PRODUCTS

185

8.4 Smaller products

The second case study concerns two products developed by Bluetail AB.
The ﬁrst of these products was the “Bluetail Mail Robustiﬁer” (BMR) which
is described in [11].

8.4.1 Bluetail Mail Robustiﬁer

The Bluetail Mail Robustiﬁer was a product designed to increase the
reliability of existing e-mail services. The BMR was designed as a proxy
which was placed between the clients which wished to make use of some
e-mail services, and a number of e-mail servers.

As far as the clients were concerned all the servers had the same IP ad-
dress (the address of the proxy)—internally the proxy had at least two physi-
cal machines (for fault-tolerance) which could intercept and relay messages
to the back-end servers. The back-end servers were themselves machines
running standardised mail servers. The BMR concentrated on three mail
protocols SMTP [57], POP3 [52] and IMAP4 [27].

The BMR was shipped to its customers as a “turn-key” system. The

requirements [11] for the BMR were given as:

1. Down times should be a few minutes per decade.

2. If a mail server fails some other server should take over
with a minimum of delay, clients should not notice that
the server has failed.

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CHAPTER 8. CASE STUDIES

3. It should be possible to remotely manage the system. We
would like to add or remove mail servers or take them
out of service without interrupting the quality of service.
4. It should be possible to upgrade the BMR sodware itself

without stopping the system.

5. In the event of an error in the BMR sodware it should
be possible to revert to a previous version of the sodware
without stopping the system.

6. The system must work together with and improve the

performance of existing mail systems.

7. The system should work on existing hardware and run

on existing operating systems.

8. The system should be at least as eecient as a conven-

tional imperative language implementation.

9. The system should have a conventional Unix command
line interface, and conventional Unix manual pages.

10. The system should have a GUI interface.

Points 1, 3 and 4 are typical requirements for a sod real-time system,
in particular point 4 is a typical requirement for this kind of system but
is rarely found in the requirements for a system that is only to be used
for a short period of time. Point 5 is related to point 4—as far as possible
we want the operation of the sodware to be entirely autonomous and to
require minimal operator intervention. The fact that the BMR could be
remotely managed and that upgrades and downgrades to the system could
be handled automatically and remotely was an important factor in selling
the product. Indeed, one of the main reasons for buying the BMR was
to relieve the burden of manually monitoring and maintaining an e-mail
system.

The BMR was written using 108 Erlang modules and had 36,285 lines
of Erlang code.
It was written from scratch and delivered to the ﬁrst
customer within six months of the project start. The BMR has been in live

8.4. SMALLER PRODUCTS

187

operation with the Swedish Telenordia ISP since 1999 and handles millions
of emails per year.

BMR implemented its own release management system, as an exten-
sion to the release behaviour in the OTP system. The BMR system is an
intrinsically distributed system. It is desirable that sodware upgrade in a
distributed system has “transaction” semantics, that is, either the sodware
upgrade works in its entirety on all nodes of the system, or, that it fails and
no sodware is changed on any node.

In BMR two versions of the entire system could co-exist, a old version
and a new version. Adding a new version of the sodware, makes the
current version of the sodware the old version, and the added version
becomes the new version.

To achieve this, all BMR sodware upgrade packages were written in a
reversible manner, ie it should be possible to not only perform a dynamic
upgrade from an old version of the sodware to a new version, but also to
convert from a new version back to the old version.

Upgrading the sodware on all nodes was done in four phases.

1. In phase one the new sodware release is distributed to all nodes—this

always succeeds.

2. In phase two the sodware on all nodes is changed from the old
version to the new version. If the conversion on any node fails, then
all nodes running new versions of the sodware are backed to run the
old version of the sodware.

3. In phase three all nodes in the system run the new sodware, but
should any error occur, then all nodes are backed to run the old
sodware. The system has not yet committed to run the new sodware.

4. Ader the new system has run successfully for a suecient time period
the operator can “commit” the sodware change. Committing the
system (phase four) changes the behaviour of the system. If an error
occurs ader a commit then the system will restart using the new
sodware and not revert to the old sodware.

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CHAPTER 8. CASE STUDIES

Interestingly, almost exactly the same mechanism is used in the X2000
system [63] developed by NASA, for their deep-space applications, where
sodware applications also have to be upgraded without stopping the sys-
tem.

In addition the BMR upgrade system had to allow for the possibility
that a node in the distributed system was “out-of-service” at the time when
the sodware upgrade was being performed. In this case, when the node
was re-introduced to the system it would learn about any changes that
had been made to the system during the time it was unavailable, and any
necessary sodware upgrades would be performed.

8.4.2 Alteon SSL accelerator

The Alteon SSL3 Accelerator was the ﬁrst product to be produced ader
Bluetail AB was acquired by Alteon Web Systems. An SSL accelerator is a
hardware appliance containing special purpose hardware for speeding up
cryptographic computations. The SSL accelerator is marketed and sold by
Nortel Networks. The control system for the SSL accelerator is written in
Erlang.

According to Infonetics Research, the Alteon SSL Accelera-
tor is the leading SSL Accelerator appliance in the market.
With more SSL Accelerators deployed than any other vendor,
Nortel Networks leads the market with innovative new appli-
cations and features such as back-end encryption, integrated
load balancing, session persistence, application address trans-
lation, Layer 7 ﬁltering, and secure global server load balanc-
ing (GSLB). Ader winning all evaluation categories, Network
Computing named Nortel Networks Alteon SSL Accelerator
“King of the Hill” in their latest SSL Accelerator bake-oc, cit-
ing industry-leading performance, features, and manageability
as distinguishing attributes. — [53]

3Secure Socket Layer.

8.4. SMALLER PRODUCTS

189

The SSL accelerator is produced as a hardware appliance. Interestingly
it was produced in a very short time (less than one year) and rapidly
became market leader having won the “best in test” awards in all categories
awarded by “Network Computing.”

The sodware architecture for the SSL accelerator was derived from the

generic architecture used in the Bluetail Mail Robustiﬁer.

8.4.3 Quantitative properties of the code

Unfortunately, Nortel Networks would not let me analyse their source code
in any detail, so I can only report on the grossed up properties of their
code. This data is derived for all Bluetail/Alteon Erlang products. It does
not distinguish the individual products:

Number of Erlang modules
Clean modules
Dirty modules
Number of lines of code
Total number of Erlang functions
Total number of clean functions
Total number of dirty functions
Percentage of dirty functions/code lines

253
122
131
74440
6876
6266
610
0.82%

The products made extensive use of the OTP behaviours using 103
behaviours in all. Again gen_server dominated. The number of times
the dicerent behaviours were used were:

gen_server (56), supervisor (19), application (15), gen_event (9),

rpc_server (2), gen_fsm (1), supervisor_bridge (1).

There is not so much to say about these ﬁgures, apart from the fact that
at ﬁrst sight the AXD project used relatively longer functions and relatively
fewer behaviours than the Bluetail/Alteon projects.

I interviewed the people who had programmed the SSL accelerator.
They told me that in practice the architecture worked well and that failures
due to sodware unrecoverable sodware errors had not occurred, but that
they kept no records which could conﬁrm this observation.

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CHAPTER 8. CASE STUDIES

They also said that the product upgrade mechanism had evolved be-
yond that developed in the OTP system. Any product release increment
had to be designed in such a way that it was totally reversible. That is,
when planning to go from version N of the system to version N+1 they
would not only have to write code to go from version N to version N+1 but
also they would have to write code to go from version N+1 to version N.

This discipline was strictly followed, it was therefore possible to “roll-
back” the system from its current incarnation to the very ﬁrst release. For
commercial reasons, Nortel did not wish to release any detailed informa-
tion about the details of this process.

8.5 Discussion

Before I started these case studies I had a fairly clear idea as to what pa-
rameters it would be possible to measure. The ultimate test of a technology
is ask the users of the technology the following question:

“Does it work?”
If you ask this question to the people at Ericsson or Nortel they shake

their heads in amazement and say:

“Of course it works!”
I had hoped that they would be able to provide me with clearly doc-
umented evidence that the system does indeed work in the manner that I
have described in this thesis. I would liked to have found evidence in the
log ﬁles that the code upgrades had worked, that the system had crashed
and recovered, that the system had run for thousands of hours without
interruption etc.

The people who I interviewed were happy to tell me that this was the
case, that code-upgrades etc did work as expected. But they could not
produce any collaborative evidence that this was the case (ie there were
no records in the log ﬁles that code upgrades etc had occurred).

The reason that there was no documentary evidence was that the sys-
tem had not been instrumented in such a way as to report to the log ﬁles
that things like code upgrade had occurred, and thus there was no lasting
evidence. Interestingly there was no counter-evidence to suggest that the

8.5. DISCUSSION

191

procedures had failed.
I assume that if the procedures had failed then
there would have been a lot of trouble reports etc which documented this
fact.

Evidence for the long-term operational stability of the system had also
not been collected in any systematic way. For the Ericsson AXD301 the
only information on the long-term stability of the system came from a
power-point presentation showing some ﬁgures claiming that a major cus-
tomer had run an 11 node system with a 99.9999999% reliability, though
how these ﬁgure had been obtained was not documented.

In the case of the BMR and the SSL accelerator there was no hard
evidence that the system had run reliably for extended periods of time.
The reason for this is that such information is not recorded in the log ﬁles.
The BMR is, of course, a fault-tolerant distributed system, having sev-
eral interconnected nodes. In such a system the failure of an individual
node is not a “noteworthy event,” since the system is designed from the
beginning to survive the crash of a single node.

If single nodes do crash (which they obviously do, since there is anec-
dotal evidence that this is the case) then the performance of the system
as a whole is slightly degraded, but not to the point where it becomes an
operational problem.

I am unaware of any entire system failures (where all nodes fail)—were
this to be the case I would suspect that this is due to a massive hardware
failure acecting all machines in the system. Such failures appear to be
extremely rare, and even if they did occur the cause of failure has nothing
to do with the sodware, but is an entirely dicerent hardware issue.

In a distributed system the perception of failure, and even our language
for talking about failure need modiﬁcation. It hardly makes sense to talk
about total system failure (since this is such an uncommon event)—instead
we need to talk about some measure of degradation of service quality.

The sodware systems in the case study are so reliable that the people
operating these system are inclined to think that they are error-free. This is
not the case, indeed sodware errors did occur at run-time but these errors
were quickly corrected so that nobody ever noticed that the errors had
occurred. To get accurate statistics on the long-term stability one would
have to record all start and stopping times of the system and measure a

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CHAPTER 8. CASE STUDIES

number of parameters that indicated the “health” of the system. Since the
system appears to be entirely “healthy” such statistics never seem to have
been collected in any systematic way.

As regards an analysis of the AXD301 code base I had hoped to ﬁnd
evidence that the programming rules were being used correctly—I would
liked, for example, to have seen a clear separation into “clean” and “dirty”
code.
I accept that certain parts of the system will always be written
in a suspect manner (for reasons of eeciency, or for some other reason)
but I would have liked this code to be separated in a clear manner from
the main body of the code, and for more stringent test procedures to be
applied to this code etc.

This was not the case. A majority of the code was clean, but the
distribution of dirty code was not a pure “step” function (ie there was no
clear division where I could say, “this code is bad, stare at it carefully” and
“this code is good”) but rather a spread out distribution where there were
a small number of modules which had a lot of side-ecects (this doesn’t
worry me), but more troublesome a largish number of modules with only
one or two calls to dirty primitives.

Without a deeper understanding of the code than is possible here it is
impossible to say if this is in the nature of the problem, and that these calls
with potential side-ecects introduce problems into the system, or if they
are harmless.

In any case, the message is clear. Programming rules alone are insue-
cient to cause the system to be written in a particular way. If one wished
to enforce the partitioning of code into clean and dirty code then tool sup-
port would have to be added and the policies would have to be enforced
in some manner. Whether or not this is desirable is debatable—probably
the best approach is to allow the rules to be broken and hope that the
programmers know what they are doing when they break a programming
rule.

The ultimate test of a technology is, of course, the “did-it-work test.” In
the case of the AXD301 and the Nortel SSL accelerator the answer was a
resounding “yes.” Both these products are small niche products, but both
of them are market leaders in their niches.

9 APIs and

Protocols

W hen we have built a sodware module we need to describe how

it is to be used. One such method is to deﬁne a programming
language API for all the exported functions that are in the module.

To do this in Erlang we could use the type system outlined on page 80.

This method of deﬁning API’s is widespread. The details of the type
notation vary from language to language. The degree to which the type
system is enforced by the underlying language implementation varies from
system to system. If the type system is strictly enforced then the language
is called “strongly typed,” otherwise it is called “untyped”—this point oden
causes some confusion, since many languages which need type declarations
have type systems which can easily be broken. Languages like Erlang,
need no type declarations, but are “type safe,” meaning that the underlying
system cannot be broken in such a way as to damage the system.

Even if our language is not strongly typed, the existence of type dec-
larations provides valuable documentation, and can be used as input to a
dynamic type checker which can be used for run-time type checking.

Unfortunately, API’s written in the conventional manner are not sue-
cient to understand the operation of a program. For example, consider
the following code fragment:

silly() ->

{ok, H} = file:open("foo.dat", read),
file:close(H),
file:read_line(H).

193

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CHAPTER 9. APIS AND PROTOCOLS

According to the type system and the API given in the example on
page 80 this is a perfectly legal program and yet it is obviously total non-
sense, since we cannot expect to read from a ﬁle once it has been closed.
To remedy the above problem we could add an additional state pa-
rameter. In a fairly obvious notation, the API could be written something
like this:

+type start x file:open(fileName(), read | write) ->

{ok, fileHandle()} x ready
| {error, string()} x stop.

+type ready x file:read_line(fileHandle()) ->
{ok, string()} x ready

| eof x atEof.

+type atEof | ready x file:close(fileHandle()) ->

true x stop.

+type atEof | ready x file:rewind(fileHandle()) ->

true x ready

This model of the API uses four state variables start, ready, atEof
and stop. The state start means the ﬁle has not been opened. The
state ready means the ﬁle is ready to be read, atEof means that the ﬁle
is positioned at end-of-ﬁle. The server always starts in the state start and
stops in the state stop.

The API now says that, for example, when we are in a state ready, that
a file:read_line function call is legal. This will either return a string,
in which case we stay in the state ready or it will return eof and we will
be in the state atEof.

In state atEof we can close or rewind the ﬁle, all other operations are
illegal. If we choose to rewind the ﬁle, the state will change back to ready
in which case a read_line operation becomes legal again.

Augmenting the API with state information provides us with a method
for determining if the sequence of allowed operation is in accordance with
the design of a module.

9.1. PROTOCOLS

9.1 Protocols

195

Having seen how we can specify sequencing in an API, there is an equiva-
lent idea that is applicable to protocols.

Consider two components which communicate by pure message pass-
ing, at some level of abstraction we need to be able to specify the protocol
of the messages which can ﬂow between the components.

The protocol P between two components A and B can be described in

terms of a non-deterministic ﬁnite state machine.

Assume that process B is a ﬁle server, and that A is a client program
which makes use of the ﬁle server, assume further that sessions are con-
nection oriented. The protocol that the ﬁle server obeys can be speciﬁed
as follows:

+state start x {open, fileName(), read | write} ->
{ok, fileHandle()} x ready
| {error, string()} x stop.

+state ready x {read_line, fileHandle()} ->

{ok, string()} x ready

| eof x atEof.

+state ready | atEof x {close, fileHandle()}->

true x stop.

+state ready | atEof x {rewind, fileHandle()) ->

true x ready

This speciﬁcation says that if the ﬁle server is in the state start then
it can receive a message of type {open, fileName(), read|write}, it

PAB196

CHAPTER 9. APIS AND PROTOCOLS

Figure 9.1: Two processes and a protocol checker

will then respond by sending out a message of type {ok, fileHandle()}
and move to the state ready or it will respond by replying with the mes-
sage {error, string()} and move into the state stop.

Given a protocol which is speciﬁed in a manner similar to the above
it is possible to write a simple “protocol checking” program which can be
placed between any pair of processes. Figure 9.1 shows a protocol checker
C placed between two processes X and Y.

When X sends a message Q (a query) to Y, Y responds with a response
R and with a new state S. The pair {R,S} can be type-checked against the
rules in the protocol speciﬁcation. The protocol checker C sits between X
and Y and checks that all the messages between X and Y are according to
the protocol speciﬁcation.

In order to check the protocol rules the checker needs to have access
to the state of the server, this is because the protocol speciﬁcation might
contain productions like:

+state Sn x T1 -> T2 x S2 | T2 x S3

In which case the observation of a reply message of type T2 is not

suecient to distinguish between the output states S2 and S3.

If we recall the simple generic server, shown on page 89, the main loop

of the program was:

loop(State, Fun) ->

receive

CXY{R,S}{R,S}PQQ9.2. APIS OR PROTOCOLS?

197

{ReplyTo, ReplyAs, Q} ->

{Reply, State1} = F(State, Q),
Reply ! {ReplyAs, Reply},
loop(State1, Fun)

end.

Which can easily be changed to:

loop(State, S, Fun) ->

receive

{ReplyTo, ReplyAs, Q} ->

{Reply, State1, S1} = F(State, S, Q),
Reply ! {ReplyAs, S1, Reply},
loop(State1, S1, Fun)

end.

Where S and S1 represent the state variable which was speciﬁed in the
protocol speciﬁcation. Note that the state of the interface (ie the value of
the state variable used in the protocol speciﬁcation) is not the same as the
state of the server State.

Given such a change, the generic server becomes re-cast in a form
which allows a dynamic protocol checker to be inserted between the client
and the server.

9.2 APIs or protocols?

Up to now we have shown what are essentially two equivalent ways of
doing the same thing. We can impose a type system on our programming
language, or we can impose a contract checking mechanism between any
two components in a message passing system. Of these two methods I
prefer the use of a contract checker.

The ﬁrst reason for this has to do with how we structure systems. In our
model of programming we assume isolated components and pure message
passing. The components themselves are considered “black boxes.” From
outside the black box how a computation is performed inside the black

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CHAPTER 9. APIS AND PROTOCOLS

box is totally irrelevant. The only thing which is important is whether or
not the black box behaves according to its protocol speciﬁcation.

Inside the black box, it may be desirable, for reasons of eeciency or
otherwise to program using obscure programming methods and to break
all rules of common sense and good programming practice. This does not
matter in the slightest, provided the external behaviour of the system is
consistent with the protocol speciﬁcation.

By simple extension the protocol speciﬁcation can be extended to spec-
ify the non-functional properties of a system. For example, we might add
to our protocol speciﬁcation language a notion of time, then we could say
things like:

+type Si x {operation1, Arg1} ->

value1() x Sj within T1

| value2() x Sk after T2

Meaning that operation1 should return a value1() type data struc-
ture within time T1 or return something of type value2() ader time T2
etc.

The second reason has to do with where we do things in the system.
Placing the contract checker outside a component in no way interferes with
the construction of the component itself, moreover it allows a ﬂexible way
of adding or removing introspective testing powers to the system, which
can be checked at run-time and which can be conﬁgured in a number of
dicerent ways.

9.3 Communicating components

How should Erlang talk to the outside world? — this question becomes
interesting if we want to build distributed applications where Erlang is one
of a number of communicating components. In [35] we can read that:

The two fundamental building blocks underlying any PLITS
system are modules and messages. A module is a self-contained

9.4. DISCUSSION

199

entity, something like a Simula or Smalltalk class, a SAIL pro-
cess or a CLU module.
It is not important for the moment
which programming language is used to encode the body of
the module; we wish to explicitly account for the case in which
the various modules are coded in dicerent languages on a va-
riety of machines — [35]

In order to make a system of communication components we have to

agree on a number of dicerent things. We need:

• A transport format and a way of mapping language entities into

entities in the transport format.

• A system of types, built on top of the entities in the transport format.

• A protocol description language in terms of the system of types.

A method for doing this involving a transport format called UBF (short
for Universal Binary Format), which was designed for rapid parsing was
presented in [13]. A slightly revised version of the paper can be found in
appendix C.

9.4 Discussion

I want to return to the central question of the thesis—How can we make a
reliable system in the presence of errors? Stepping outside the system and
viewing the system as a set of communicating black boxes is very helpful
here.

If we formally describe the protocol to be obeyed on a communication
channel between two black boxes then we can use this as a means for de-
tecting and identifying errors, we can also say precisely which component
has failed.

Such an architecture satisﬁes requirements R1–R4 on page 27—and
therefore, by my reasoning can be used for programming error-resilient
systems.

200

CHAPTER 9. APIS AND PROTOCOLS

The “try something simpler” idiom (page 116) also applies. If a black-
box implementation of a function fails to work, then we could revert to a
simpler implementation, also implemented as a black box. The protocol
checking mechanism could be used for making meta-level decisions as to
which implementation should be used, choosing a simpler implementation
if errors are observed. When the components reside on physically sep-
arated machines, the property of strong isolation is almost automatically
enforced.

10 Conclusions

T heses are never completed—at some point, however, you just give

up... In this thesis I have presented a number of things—a new pro-
gramming language and a method for programming fault-tolerant
systems. Erlang appears to be a useful tool for programming reliable
systems—amazingly it is being used for a large number of applications
which are far removed from the the problem domain that Erlang was de-
signed to be used for.

10.1 What has been achieved so far?

The work described in this thesis, and related work performed elsewhere
has demonstrated a number of dicerent things, namely:

• That Erlang and the associated technology works. This is, in itself,
an interesting result. Many people have argued that languages like
Erlang cannot be used for full-scale industrial sodware development.
Work on the AXD301 and on the Nortel range of products shows
that Erlang is an appropriate language for these types of product
development. The fact that not only are these products successful,
but also that they are market leaders in their respective niches is also
signiﬁcant.

• That programming with light-weight processes and no shared mem-
ory works in practice and can be used to produce complex large-
scale industrial sodware.

201

202

CHAPTER 10. CONCLUSIONS

• That it is possible to construct systems that behave in a reasonable

manner in the presence of sodware errors.

10.2 Ideas for future work

It would be a shame if the Erlang story had no future chapters, and there
are a number of directions in which I would like to see the language
develop.

• Conceptual integrity — can we develop Erlang so as to reinforce the
everything is a process view of the world? Can we make the system,
and Erlang code more regular and easier to understand?

• Kernel improvements — the statement on page 83 is a modiﬁed
truth. The property of strong isolation is not strictly adhered to in
any known implementation of Erlang. One process could acect an-
other process in the system by allocating vast amounts of memory
or by sending large numbers of messages to another process. A
malicious process could destroy the entire system by creating large
numbers of atoms and overﬂow the atom table etc. Current imple-
mentations of Erlang are not designed to protect the system from
malevolent attacks. Kernel improvements are possible which would
guard against many such attacks.

• How can we program components — the idea of a set of communicat-
ing processes leads naturally to the idea of writing the components
in dicerent languages. How can we do this?

10.2.1 Conceptual integrity

How can we make the system easier to understand? The Erlang program-
ming model is “Everything is a process.” We can emphasise this view by
making a few minor changes to the languages. With these changes, most
(if not all) BIFs become unnecessary. Let me assume the following:

• Everything is a process.

10.2.

IDEAS FOR FUTURE WORK

203

• All processes have names.

• Message passing is explicit.

I also introduce a new inﬁx remote procedure call operator, called

“bang bang” and written:

A !! B

If A is a Pid then this is short for:

A ! {self(), B},
receive

{A, Reply} ->

Reply

end

In fact A can be a Pid, an atom, a string or a list of Pid’s atoms or

strings.

[A1,A2,A3,..]!!X returns [V1,V2,V3,...] where V1 = A1!!X and

V2 = A2!!X, etc. All the RPCs are performed in parallel.

10.2.2 Files and bang bang

Recall that I said earlier that everything is a process, and that all processes
have names, thus ﬁles are processes.

If F is a ﬁle, then F !! read reads the ﬁle, and F !! {write, Bin}

writes the ﬁle.

The following code fragment reads a ﬁle and copies it to a new location:

{ok, Bin} = "/home/joe/abc" !! read,
"/home/joe/def" !! {write, Bin}

This example reads three ﬁles in parallel:

204

CHAPTER 10. CONCLUSIONS

[A, B, C] =

["/home/joe/foo",
"http://www.sics.se/~joe/another_file.html",
"ftp://www.some.where/pub/joe/a_file"] !! read

With some fairly obvious syntactic sugar.
Now suppose I am working at home, and keep a copy of my work
on a remote host. The following code compares my local copy of the ﬁle
with the copy on a backup machine, and if they are dicerent updates the
backup copy:

L = ["/home/joe/foo",

Remote= "ftp://www.sics.se/~joe/backup/foo"],

case L !! read of

-> true;

{X, X}
{{ok,Bin},_} -> Remote !! {write, Bin};
_ -> error

end

10.2.3 Distribution and bang bang

If a process name is a string of the form "erl://Node/Name" then the
operation will be performed on a remote node instead of on the local
node, that is:

"erl://Node/Name" !! X

means evaluate Name !! X on the node Node and return the result to

the user. Thus:

{ok,Bin} =

"erl://joe@enfield.sics.se/home/joe/abc" !! read,

"/home/joe/def" !! {write, Bin}

reads a remote ﬁle and stores it on the local computer.

10.2.

IDEAS FOR FUTURE WORK

205

10.2.4 Spawning and bang bang

In the beginning of the universe several “magic” processes are assumed to
exist. These have names like "/dev/..."—these processes are pre-deﬁned
and do magic things.

In particular, "/dev/spawn" is a process spawning device. If you send

"/dev/spawn" an Erlang fun it will send you back an Erlang Pid, so:

Pid1 = "/dev/spawn" !! fun() -> looper() end),
Pid2 = "erl://joe@bingbang.here.org/dev/spawn"

!! fun() -> ... end

Creates two Erlang processes, one on bingbang.here.org. and the

other on the local node.

Now we can throw away the Erlang primitive spawn. It is replaced by

the spawn device.

10.2.5 Naming of processes

I talked about "/dev/spawn", here are a few more devices in the same
ﬂavour:

process spawner
stdout

/dev/spawn
/dev/stdout
/dev/log/spool/errors error logger
code server
/dev/code
processes
/proc/Name
ﬁles
/Path/To/File
dets tables
/dev/dets
ets tables
/dev/ets
mnesia tables
/dev/mnesia
read only ﬁles
http://Host/File
remote processes
erl://Node@Hist/..

We also need to extend the process registration primitive, to allow us

to register a string as a name for a process.

206

CHAPTER 10. CONCLUSIONS

-module(vshlr4).

-export([start/0, stop/0, handle_rpc/2, handle_cast/2]).
-import(server1, [start/3, stop/1, rpc/2]).
[new/0, store/3, find/2]).
-import(dict,

start() -> start(vshlr4, fun handle_event/2, new()).

stop() -> stop(vshlr4).

handle_cast({i_am_at, Who, Where}, Dict) ->

store(Who, Where, Dict).

handle_rpc({find, Who}, Dict) ->
{find(Who, Dict), Dict}.

Figure 10.1: A Very Simple Home Location Register (revisited)

10.2.6 Programming with bang bang

I start with a simple example. Figure 10.1 is my old friend the home
location register, revisited yet again. This time vshlr4 is written as a pure
Erlang function, and no attempt is made to hide the access functions inside
stub function. Instead we expose the interface.

With the bang bang notation, the shell dialog shown on page 90 be-

comes:

1> vshlr4:start().
true
2> vshlr4 !! {find, "joe"}.
error
3> vshlr4 ! {i_am_at, "joe", "sics"}.

10.3. EXPOSING THE INTERFACE - DISCUSSION

207

ack
4> vshlr4 !! {find, "joe"}.
{ok,"sics"}

We achieve the same results as before, but this time we have not en-
capsulated the remote procedure call in a stub function. Instead we have
exposed it to the programmer.

10.3 Exposing the interface - discussion

Should we expose the interfaces or not? Let’s imagine two ways of do-
ing things. Suppose we have a remote ﬁle server, accessible through a
variable F1, suppose also that the protocol between the ﬁle server and
the client code is hidden in some module file_server.erl.
Inside
file_server.erl we ﬁnd code like this:

-module(file_server).
-export([get_file/2]).
...
get_file(Fs, FileName) ->

Fs ! {self(), {get_file, Name}},
receive

{Fs, Reply} ->
Reply

end

The access function get_file/2 is used by client code to fetch a ﬁle

from the ﬁle server. The statement:

File = file_server:get_file(F1, "/home/joe/foo")

is used to fetch a ﬁle. The code which does this is nicely hidden
inside file_server and the user has no idea or need to know about the
underlying protocol.
In Concurrent Programming in Erlang we argued
that hiding the details of an interface was good programming practice:

208

CHAPTER 10. CONCLUSIONS

The purpose of interface functions is to create abstractions
which hide speciﬁc details of the protocol used between the
clients and the server. A user of a service does not need to
know the details of the protocols used to implement the ser-
vice, or the internal data structures and algorithms used in the
server. An implementor of the service is then free to change
any of these internal details at any time while maintaining the
same user interface.—[5] (pages 81–82)

An unfortunate consequence of this encapsulation, is that we cannot
easily parallelise simultaneous calls to dicerent ﬁle servers, nor can we
stream requests to a single ﬁle server.

If we wish to perform several RPCs in parallel it might be better to
dispatch all the requests, and then gather all the responses, but this cannot
be done in all situations unless the interface is exposed.

10.4 Programming communicating com-

ponents

Interestingly the dominant way of building Internet applications involves
the use of:

• Isolated components.

• Pure message passing.

• Informal protocols.

In the Internet case the components really are physically isolated, for
example, a client in Sweden might make use of the services of a server in
Australia. Pure message passing with no shared data is the norm. This is
possibly because just about the only way that the system builder can under-
stand and build distributed applications involves the use of asynchronous

10.4. PROGRAMMING COMMUNICATING COMPONENTS

209

protocols described informally by the use of RFCs.1 The problem with
RFCs is that virtually every RFC deﬁnes its own ad hoc syntax for data
transit, and that the allowed sequences of messages are oden never de-
scribed formally, but must be guessed by the implementor.

The use of a standard syntax for describing RFCs (for example, XML,
lisp S-expressions or UBF terms) would be a great step forward, enabling a
single parser to be used for all applications. The use of a contract checker
as proposed in appendix C would also make programming Internet ap-
plications signiﬁcantly easier, and hopefully would make the applications
much more reliable.

Interestingly, the Internet model works well even without notiﬁcation
for the reasons for failure. If we have a pathological distrust of the com-
ponent we are talking to,
then this might be sensible, but for normal
applications notiﬁcation of failure reason would make implementing and
debugging applications a lot easier.

The use of the contract checker also accurately pinpoints exactly where
errors have occurred, this becomes increasingly important as more com-
municating components are added to the system.

While the model of communicating components is widely used for
distributed applications, it is infrequently used for structuring single-node
applications. In the interest of eeciency, designers reject the idea of us-
ing processes as protection domains for their sodware, and favour shared
object models.

I believe that the model for programming single-node applications
should be exactly the same as that used for programming distributed ap-
plications. Dicerent components should be protected from each other by
It should be impossible for the components to damage each
processes.
other and the components should communicate using deﬁned protocols
which are speciﬁed by a contract and enforced by a contract checker.

This way we can build reliable applications.

1Requests For Comments—a set of memos describing various aspects of the Internet, including all the

major application protocols.

210

CHAPTER 10. CONCLUSIONS

A Acknowledgments

The Erlang system, the OTP libraries, and all the applications that have
been built in Erlang are the result of a massive amount of work done by a
large number of people.

It is always diecult to write the acknowledgments section, since I don’t
want to miss out anybody who has contributed to the system, and I don’t
want to make any errors—I shall try to be as accurate as possible, for any
omissions or inaccuracies I apologise in advance.

I would like to start by thanking all the “management types” who made
this possible in the ﬁrst place. Bjarne D¨acker who was my boss at Ericsson
has alway been enthusiastic about our work, and fought many battles on
our behalf—thank you Bjarne. Torbj¨orn Jonsson (who was Bjarne’s boss)
fought even more battles on Bjarne’s behalf—thanks Torbj¨orn I learnt a
lot from you. Jane Walerud—(Jane was the woman behind Open Source
Erlang), she was the managing director of Bluetail, and taught me all I
know about running a small business.

Now we get to the developers of Erlang which is a much longer list.
The original Erlang team was myself, Robert Virding, and Mike Williams.
I originally wrote the Erlang compiler, Robert wrote the libraries, and
Mike wrote the JAM emulator. Robert and I wrote several dicerent Erlang
emulators, mostly in Prolog. Mike re-wrote the emulator in C. Ader a cou-
ple of years Claes Wikstr¨om (Klacke) came along and added distribution
to Erlang and Bogumil Hausman invented an improved Erlang machine,
the BEAM.1

1Bogdans Erlang Abstract machine.

211

212

APPENDIX A. ACKNOWLEDGMENTS

Many members of the CSLab “dropped into” the project, wrote a few
programs in Erlang, then went on do other things. Carl Wilhelm Wellin
wrote yecc. Klacke and Hans Nilsson wrote mnesia, and mnemosyne, and
Tony Rogvall and Klacke added binaries to the language, and generally
did amazing things with networks. Per Hedeland with amazing patience
answered all my stupid question about Unix, and made sure our systems
always worked beautifully. He also re-wrote the tricky bits in the Erlang
emulator when nobody was looking. Magnus Fr¨oberg wrote the debugger,
and Torbj¨orn T¨ornkvist wrote the interface generator, so that you could
interface Erlang with C.

When Erlang moved out of the lab and OTP was born, the group ex-
tended and reshaped. Magnus Fr¨oberg, Martin Bj¨orklund and I designed
the OTP library structure and structure of the behaviours. The OTP be-
haviours were based on a number of ideas that had been ﬂoating around
the lab. Klacke had written a “generic server” similar to gen_server,
and Peter H¨ogfelt had written a generic server, and a early version of the
supervision tree. Many of the ideas about process supervision came from
Peter’s work in the mobility server project.

Ader I led Ericsson, the day-to-day maintenance, and development of
the system moved to a new generation of programmers. Bj¨orn Gustavsson
maintains the Emulator, and the OTP libraries are maintained by Lars
Thors´en, Kenneth Lundin, Kent Boortz, Raimo Niskanen, Patrik Nyblom,
Hans Bolinder, Richard Green, H˚akan Mattsson, and Dan Gudmundsson.
Now to our users—the Erlang/OTP system has been signiﬁcantly im-

proved by interaction with our faithful band of users.

The ﬁrst set of users, who built the ﬁrst major product in Erlang were,
Mats Persson, Kerstin ¨Odling, Peter H¨ogfeld, ˚Ake Rosberg, H˚akan Karls-
son, and H˚akan, Larsson.

In the AXD301 Ulf Wiger, Stacan Blau, did magniﬁcent work pioneer-

ing the use of Erlang for carrier-class applications.

Both inside Ericsson, and outside Ericsson, our users did amazing
things. Sean Hinde in the UK became a one-man Erlang factory inside
“one-2-one” (now T-mobile).

Almost ﬁnally the Erlang mailing list has been a source of inspiration
and encouragement. Today if anybody wants to know anything about Er-

213

lang they just “ask the Erlang list,” and usually get a accurate and informed
reply within a hour or so. Thanks to all the people on the list and espe-
cially to those who I have never met, but with whom I have exchanged
many many long and interesting e-mails.

Finally thanks to my friends and colleagues at SICS—to Prof. Seif
Haridi for supervising this thesis. To Per Brand for encouraging me to
write the thesis, and for all the other members of the Distributed Systems
Laboratory with whom I have had many stimulating discussions.

Thanks everybody.

214

APPENDIX A. ACKNOWLEDGMENTS

Programming
Rules and
Conventions

B

Program Development Using Erlang
Programming Rules and Conventions.1

K Eriksson, M Williams, J Armstrong
13 March 1996

Abstract

This is a description of programming rules and advice for how to write
systems using Erlang.

Note

This document is a preliminary document and is not complete.

The requirements for the use of EBC’s “Base System” are not docu-
mented here, but must be followed at a very early design phase if the
“Base System” is to be used. These requirements are documented in
1/10268-AND 10406 Uen “MAP - Start and Error Recovery.”

1This is a reformatted version of the Ericsson internal Document: EPK/NP 95:035—The document was

released into the public domain as part of the Open Source Erlang distribution

215

216 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

Contents

1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

2. Structure and Erlang Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 216

3. SW Engineering Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

4. Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

5. Processes, Servers and Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

6. Various Erlang Speciﬁc Conventions . . . . . . . . . . . . . . . . . . . . . . . . 233

7. Speciﬁc Lexical and Stylistic Conventions . . . . . . . . . . . . . . . . . . . 237

8. Documenting Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

9. The Most Common Mistakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

10. Required Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

1 Purpose

This paper lists some aspects which should be taken into consideration
when specifying and programming sodware systems using Erlang. It does
not attempt to give a complete description of general speciﬁcation and
design activities which are independent of the use of Erlang.

2 Structure and Erlang Terminology

Erlang systems are divided into modules. Modules are composed of
functions and attributes. Functions are either only visible inside
they can also be called by other
a module or they are exported i.e.
functions in other modules. Attributes begin with “-” and are placed in
the beginning of a module.

217

The work in a system designed using Erlang is done by processes. A
process is a job which can use functions in many modules. Processes com-
municate with each other by sending messages. Processes receive
messages which are sent to them, a process can decide which messages
it is prepared to receive. Other messages are queued until the receiving
process is prepared to receive them.

A process can supervise the existence of another process by setting
up a link to it. When a process terminates, it automatically sends exit
signals to the process to which it is linked. The default behaviour of a
process receiving an exit signal is to terminate and to propagate the signal
to its linked processes. A process can change this default behaviour by
trapping exits, this causes all exit signals sent to a process to be turned
into messages.

A pure function is a function that returns the same value given the
same arguments regardless of the context of the call of the function. This
is what we normally expect from a mathematical function. A function that
is not pure is said to have side ecects.

Side ecects typically occur if a function a) sends a message b) receives
a message c) calls exit d) calls any BIF which changes a process’s en-
vironment or mode of operation (e.g. get1, put2, erase1, process ﬂag2
etc).

Warning: This document contains examples of bad code.

3 SW Engineering Principles

3.1 Export as few functions as possible from a module

Modules are the basic code structuring entity in Erlang. A module can
contain a large number of functions but only functions which are included
in the export list of the module can be called from outside the module.

Seen from the outside, the complexity of a module depends upon the
number of functions which are exported from the module. A module
which exports one or two functions is usually easier to understand than a
module which exports dozens of functions.

Modules where the ratio of exported/non-exported functions is low

218 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

are desirable in that a user of the module only needs to understand the
functionality of the functions which are exported from the module.

In addition, the writer or maintainer of the code in the module can
change the internal structure of the module in any appropriate manner
provided the external interface remains unchanged.

3.2 Try to reduce intermodule dependencies
A module which calls functions in many dicerent modules will be more
diecult to maintain than a module which only calls functions in a few
dicerent modules.

This is because each time we make a change to a module interface,
we have to check all places in the code where this module is called. Re-
ducing the interdependencies between modules simpliﬁes the problem of
maintaining these modules.

We can simplify the system structure by reducing the number of dicer-

ent modules which are called from a given module.

Note also that it is desirable that the inter-module calling dependencies

form a tree and not a cyclic graph. Example:

But not

Function AModule AFunction BModule BFunction CModule CFunction AModule AFunction BModule BFunction CModule C3.3 Put commonly used code into libraries

219

Commonly used code should be placed into libraries. The libraries should
be collections of related functions. Great ecort should be made in ensur-
ing that libraries contain functions of the same type. Thus a library such
as lists containing only functions for manipulating lists is a good choice,
whereas a library, lists_and_maths containing a combination of func-
tions for manipulating lists and for mathematics is a very bad choice.

The best library functions have no side ecects. Libraries with functions

with side ecects limit the re-usability.

3.4 Isolate “tricky” or “dirty” code into separate modules
Oden a problem can be solved by using a mixture of clean and dirty code.
Separate the clean and dirty code into separate modules.
Dirty code is code that does dirty things. Example:

• Uses the process dictionary.

• Uses erlang:process_info/1 for strange purposes.

• Does anything that you are not supposed to do (but have to do).

Concentrate on trying to maximize the amount of clean code and
Isolate the dirty code and clearly
minimize the amount of dirty code.
comment or otherwise document all side ecects and problems associated
with this part of the code.

3.5 Don’t make assumptions about what the caller will do
with the results of a function

Don’t make assumptions about why a function has been called or about
what the caller of a function wishes to do with the results.

For example, suppose we call a routine with certain arguments which
may be invalid. The implementer of the routine should not make any
assumptions about what the caller of the function wishes to happen when
the arguments are invalid.

220 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

Thus we should not write code like:

do_something(Args) ->

case check_args(Args) of

ok ->

{ok, do_it(Args)};

{error, What} ->

String = format_the_error(What),
%% Don’t do this
io:format("* error:~s\n", [String]),
error

end.

Instead write something like:

do_something(Args) ->

case check_args(Args) of

ok ->

{ok, do_it(Args)};

{error, What} ->
{error, What}

end.

error_report({error, What}) ->

format_the_error(What).

In the former case the error string is always printed on standard output,
in the latter case an error descriptor is returned to the application. The
application can now decide what to do with this error descriptor.

By calling error_report/1 the application can convert the error de-
scriptor to a printable string and print it if so required. But this may not
be the desired behaviour - in any case the decision as to what to do with
the result is led to the caller.

221

3.6 Abstract out common patterns of code or behaviour

Whenever you have the same pattern of code in two or more places in the
code try to isolate this in a common function and call this function instead
of having the code in two dicerent places. Copied code requires much
ecort to maintain.

If you see similar patterns of code (i.e. almost identical) in two or more
places in the code it is worth taking some time to see if one cannot change
the problem slightly to make the dicerent cases the same and then write
a small amount of additional code to describe the dicerences between the
two.

Avoid “copy” and “paste” programming, use functions!

3.7 Top-down

Write your program using the top-down fashion, not bottom-up (starting
with details). Top-down is a nice way of successively approaching details
of the implementation, ending up with deﬁning primitive functions. The
code will be independent of representation since the representation is not
known when the higher levels of code are designed.

3.8 Don’t optimize code

Don’t optimize your code at the ﬁrst stage. First make it right, then (if
necessary) make it fast (while keeping it right).

3.9 Use the principle of “least astonishment”

The system should always respond in a manner which causes the “least
astonishment” to the user - i.e. a user should be able to predict what will
happen when they do something and not be astonished by the result.

This has to do with consistency, a consistent system where dicerent
modules do things in a similar manner will be much easier to understand
than a system where each module does things in a dicerent manner.

If you get astonished by what a function does, either your function

solves the wrong problem or it has a wrong name.

222 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

3.10 Try to eliminate side ecects
Erlang has several primitives which have side ecects. Functions which use
these cannot be easily re-used since they cause permanent changes to their
environment and you have to know the exact state of the process before
calling such routines.

Write as much as possible of the code with side-ecect free code.
Maximize the number of pure functions.
Collect together the functions which have side ecects and clearly doc-

ument all the side ecects.

With a little care most code can be written in a side-ecect free manner

- this will make the system a lot easier to maintain, test and understand.

3.11 Don’t allow private data structure to “leak” out of a
module

This is best illustrated by a simple example. We deﬁne a simple module
called queue - to implement queues:

-module(queue).
-export([add/2, fetch/1]).

add(Item, Q) ->

lists:append(Q, [Item]).

fetch([H|T]) ->
{ok, H, T};

fetch([]) ->

empty.

This implements a queue as a list, but unfortunately to use this the user
must know that the queue is represented as a list. A typical program to
use this might contain the following code fragment:

NewQ = [], % Don’t do this
Queue1 = queue:add(joe, NewQ),
Queue2 = queue:add(mike, Queue1), ....

223

This is bad - since the user a) needs to know that the queue is repre-
sented as a list and b) the implementer cannot change the internal repre-
sentation of the queue (they might want to do this later to provide a better
version of the module).

Better is:

-module(queue).
-export([new/0, add/2, fetch/1]).

new() ->
[].

add(Item, Q) ->

lists:append(Q, [Item]).

fetch([H|T]) ->
{ok, H, T};

fetch([]) ->

empty.

Now we can write:

NewQ = queue:new(),
Queue1 = queue:add(joe, NewQ),
Queue2 = queue:add(mike, Queue1), ...

Which is much better and corrects this problem. Now suppose the user

needs to know the length of the queue, they might be tempted to write:

Len = length(Queue) % Don’t do this

since they know that the queue is represented as a list. This is bad
programming practice which leads to code which is very diecult to main-
tain and understand. If they need to know the length of the queue then a
length function must be added to the module, thus:

224 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

-module(queue).
-export([new/0, add/2, fetch/1, len/1]).

new() -> [].

add(Item, Q) ->

lists:append(Q, [Item]).

fetch([H|T]) ->
{ok, H, T};

fetch([]) ->

empty.

len(Q) ->

length(Q).

Now the user can call queue:len(Queue) instead.
Here we say that we have “abstracted out” all the details of the queue

(the queue is in fact what is called an “abstract data type”).

Why do we go to all this trouble? The practice of abstracting out inter-
nal details of the implementation allows us to change the implementation
without changing the code of the modules which call the functions in the
module we have changed. So, for example, a better implementation of the
queue is as follows:

-module(queue).
-export([new/0, add/2, fetch/1, len/1]).

new() ->

{[],[]}.

add(Item, {X,Y}) -> % Faster addition of elements

{[Item|X], Y}.

225

fetch({X, [H|T]}) ->
{ok, H, {X,T}};

fetch({[], []) ->

empty;

fetch({X, []) ->

% Perform this heavy computation only sometimes.
fetch({[],lists:reverse(X)}).

len({X,Y}) ->

length(X) + length(Y).

3.12 Make code as deterministic as possible

A deterministic program is one which will always run in the same man-
ner no matter how many times the program is run. A non-deterministic
program may deliver dicerent results each time it is run. For debugging
purposes it is a good idea to make things as deterministic as possible. This
helps make errors reproducible.

For example, suppose one process has to start ﬁve parallel processes
and then check that they have started correctly, suppose further that the
order in which these ﬁve are started does not matter.

We could then choose to either start all ﬁve in parallel and then check
that they have all started correctly but it would be better to start them one
at a time and check that each one has started correctly before starting the
next one.

3.13 Do not program “defensively”

A defensive program is one where the programmer does not “trust” the
input data to the part of the system they are programming. In general one
should not test input data to functions for correctness. Most of the code
in the system should be written with the assumption that the input data to
the function in question is correct. Only a small part of the code should

226 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

actually perform any checking of the data. This is usually done when data
“enters” the system for the ﬁrst time, so once data has been checked as it
enters the system it should thereader be assumed correct.

Example:

%% Args: Option is all | normal
get_server_usage_info(Option, AsciiPid) ->

Pid = list_to_pid(AsciiPid),
case Option of

all -> get_all_info(Pid);
normal -> get_normal_info(Pid)

end.

The function will crash if Option neither normal nor all, and it

should do that. The caller is responsible for supplying correct input.

3.14 Isolate hardware interfaces with a device driver

Hardware should be isolated from the system through the use of device
drivers. The device drivers should implement hardware interfaces which
make the hardware appear as if they were Erlang processes. Hardware
should be made to look and behave like normal Erlang processes. Hard-
ware should appear to receive and send normal Erlang messages and
should respond in the conventional manner when errors occur.

3.15 Do and undo things in the same function

Suppose we have a program which opens a ﬁle, does something with it
and closes it later. This should be coded as:

do_something_with(File) ->

case file:open(File, read) of,

{ok, Stream} ->
doit(Stream),
file:close(Stream) % The correct solution

Error -> Error

end.

Note how we open the ﬁle (file:open)and close it (file:close) in
the same routine. The solution below is much harder to follow and it is
not obvious which ﬁle is closed. Don’t program it like this:

227

do_something_with(File) ->

case file:open(File, read) of,

{ok, Stream} ->
doit(Stream)
Error -> Error

end.

doit(Stream) ->

....,
func234(...,Stream,...).
...

func234(..., Stream, ...) ->

...,
file:close(Stream) %% Don’t do this

4 Error Handling

4.1 Separate error handling and normal case code

Don’t clutter code for the “normal case” with code designed to handle
exceptions. As far as possible you should only program the normal case.
If the code for the normal case fails, your process should report the error
and crash as soon as possible. Don’t try to ﬁx up the error and continue.
The error should be handled in a dicerent process.
(See “Each process
should only have one role” on page 229).

Clean separation of error recovery code and normal case code should

greatly simplify the overall system design.

The error logs which are generated when a sodware or hardware error
is detected will be used at a later stage to diagnose and correct the error.
A permanent record should be kept of any information that will be helpful
in this process.

228 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

4.2 Identify the error kernel

One of the basic elements of system design is identifying which part of the
system has to be correct and which part of the system does not have to be
correct.

In conventional operating system design the kernel of the system is
assumed to be, and must be, correct, whereas all user application programs
do not necessarily have to be correct. If a user application program fails
this will only concern the application where the failure occurred but should
not acect the integrity of the system as a whole.

The ﬁrst part of the system design must be to identify that part of the
system which must be correct; we call this the error kernel. Oden the
error kernel has some kind of real-time memory resident data base which
stores the state of the hardware.

5 Processes, Servers and Messages

5.1 Implement a process in one module

Code for implementing a single process should be contained in one mod-
ule. A process can call functions in any library routines but the code for
the “top loop” of the process should be contained in a single module. The
code for the top loop of a process should not be split into several modules -
this would make the ﬂow of control extremely diecult to understand. This
does not mean that one should not make use of generic server libraries,
these are for helping structuring the control ﬂow.

Conversely, code for no more than one kind of process should be
implemented in a single module. Modules containing code for several
dicerent processes can be extremely diecult to understand. The code for
each individual process should be broken out into a separate module.

5.2 Use processes for structuring the system

Processes are the basic system structuring elements. But don’t use pro-
cesses and message passing when a function call can be used instead.

229

5.3 Registered processes

Registered processes should be registered with the same name as the mod-
ule. This makes it easy to ﬁnd the code for a process.

Only register processes that should live a long time.

5.4 Assign exactly one parallel process to each true concur-
rent activity in the system

When deciding whether to implement things using sequential or parallel
processes then the structure implied by the intrinsic structure of the prob-
lem should be used. The main rule is:

“Use one parallel process to model each truly concurrent activity in the

real world.”

If there is a one-to-one mapping between the number of parallel pro-
cesses and the number of truly parallel activities in the real world, the
program will be easy to understand.

5.5 Each process should only have one “role”
Processes can have dicerent roles in the system, for example in the client-
server model.

As far as possible a process should only have one role, i.e. it can be a

client or a server but should not combine these roles.
Other roles which processes might have are:

Supervisor watches other processes and restarts them if they fail.

Worker a normal work process (can have errors).

Trusted Worker not allowed to have errors.

5.6 Use generic functions for servers and protocol handlers
wherever possible

In many circumstances it is a good idea to use generic server programs
such as the generic server implemented in the standard libraries. Con-

230 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

sistent use of a small set of generic servers will greatly simplify the total
system structure.

The same is possible for most of the protocol handling sodware in the

system.

5.7 Tag messages

All messages should be tagged. This makes the order in the receive state-
ment less important and the implementation of new messages easier.

Don’t program like this:

loop(State) ->

receive
...
{Mod, Funcs, Args} -> % Don’t do this

apply(Mod, Funcs, Args},
loop(State);

...

end.

The new message {get_status_info, From, Option} will intro-

duce a conﬂict if it is placed below the {Mod, Func, Args} message.

If messages are synchronous, the return message should be tagged
with a new atom, describing the returned message. Example: if the incom-
ing message is tagged get_status_info, the returned message could be
tagged status_info. One reason for choosing dicerent tags is to make
debugging easier.

This is a good solution:

loop(State) ->

receive
...
% Use a tagged message.
{execute, Mod, Funcs, Args} ->

apply(Mod, Funcs, Args},

231

loop(State);

{get_status_info, From, Option} ->

From ! {status_info,

get_status_info(Option, State)},

loop(State);

...

end.

5.8 Flush unknown messages

Every server should have an Other alternative in at least one receive
statement. This is to avoid ﬁlling up message queues. Example:

main_loop() ->

receive

{msg1, Msg1} ->

...,
main_loop();
{msg2, Msg2} ->

...,
main_loop();

Other -> % Flushes the message queue.

error_logger:error_msg(

"Error: Process ~w got unknown msg ~w~n.",
[self(), Other]),

main_loop()

end.

5.9 Write tail-recursive servers

All servers must be tail-recursive, otherwise the server will consume mem-
ory until the system runs out of it.

Don’t program like this:

loop() ->
receive

232 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

{msg1, Msg1} ->

...,
loop();

stop ->
true;
Other ->

error_logger:log({error, {process_got_other,

self(), Other}}),

loop()

end,
% Don’t do this!
% This is NOT tail-recursive
io:format("Server going down").

This is a correct solution:

loop() ->
receive

{msg1, Msg1} ->

...,
loop();

stop ->

io:format("Server going down");

Other ->

error_logger:log({error, {process_got_other,

self(), Other}}),

loop()

end. % This is tail-recursive

If you use some kind of server library, for example generic, you

automatically avoid doing this mistake.

5.10 Interface functions

Use functions for interfaces whenever possible, avoid sending messages
directly. Encapsulate message passing into interface functions. There are

cases where you can’t do this.

The message protocol is internal information and should be hidden to

other modules.

Example of interface function:

233

-module(fileserver).
-export([start/0, stop/0, open_file/1, ...]).

open_file(FileName) ->

fileserver ! {open_file_request, FileName},
receive

{open_file_response, Result} -> Result

end.

...<code>...

5.11 Time-outs

Be careful when using after in receive statements. Make sure that
you handle the case when the message arrives later (See “Flush unknown
messages” on page 231).

5.12 Trapping exits

As few processes as possible should trap exit signals. Processes should
either trap exits or they should not. It is usually very bad practice for a
process to “toggle” trapping exits.

6 Various Erlang Speciﬁc Conventions

6.1 Use records as the principle data structure

Use records as the principle data structure. A record is a tagged tuple and
was introduced in Erlang version 4.3 and thereader (see EPK/NP 95:034).
It is similar to struct in C or record in Pascal.

234 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

If the record is to be used in several modules, its deﬁnition should be
placed in a header ﬁle (with suex .hrl) that is included from the modules.
If the record is only used from within one module, the deﬁnition of the
record should be in the beginning of the ﬁle where the module is deﬁned.
The record features of Erlang can be used to ensure cross module
consistency of data structures and should therefore be used by interface
functions when passing data structures between modules.

6.2 Use selectors and constructors

Use selectors and constructors provided by the record feature for managing
instances of records. Don’t use matching that explicitly assumes that the
record is a tuple. Example:

demo() ->

P = #person{name = "Joe", age = 29},
#person{name = Name1} = P,% Use matching, or...
Name2 = P#person.name.

% like this.

Don’t program like this:

demo() ->

P = #person{name = "Joe", age = 29},
% Don’t do this
{person, Name, _Age, _Phone, _Misc} = P.

6.3 Use tagged return values

Use tagged return values.

Don’t program like this:

keysearch(Key, [{Key, Value}|_Tail]) ->

Value; %% Don’t return untagged values!

keysearch(Key, [{_WrongKey,_WrongValue}|Tail]) ->

keysearch(Key, Tail);

keysearch(Key, []) ->

false.

235

Then the Key, Value cannot contain the false value. This is the correct
solution:

keysearch(Key, [{Key, Value}|_Tail]) ->

{value, Value}; %% Correct. Returns tagged value.

keysearch(Key, [{_WrongKey, _WrongValue}|Tail]) ->

keysearch(Key, Tail);

keysearch(Key, []) ->

false.

6.4 Use catch and throw with extreme care

Do not use catch and throw unless you know exactly what you are doing!
Use catch and throw as little as possible.

Catch and throw can be useful when the program handles compli-
cated and unreliable input (from the outside world, not from your own
reliable program) that may cause errors in many places deeply within the
code. One example is a compiler.

6.5 Use the process dictionary with extreme care

Do not use get and put etc. unless you know exactly what you are doing!
Use get and put etc. as little as possible.

A function that uses the process dictionary can be rewritten by intro-

ducing a new argument.

Example:
Don’t program like this:

tokenize([H|T]) ->

...;

tokenize([]) ->

% Don’t use get/1 (like this)
case get_characters_from_device(get(device)) of

eof -> [];
{value, Chars} ->

236 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

tokenize(Chars)

end.

The correct solution:

tokenize(_Device, [H|T]) ->

...;

tokenize(Device, []) ->

% This is better
case get_characters_from_device(Device) of

eof -> [];
{value, Chars} ->

tokenize(Device, Chars)

end.

The use of get and put might cause a function to behave dicerently
when called with the same input arguments. This makes the code hard
to read since it is non-deterministic. Debugging will be more complicated
since a function using get and put is a function of not only of its input
arguments, but also of the process dictionary. Many of the run time errors
in Erlang (for example bad_match) include the arguments to a function,
but never the process dictionary.

6.6 Don’t use import

Don’t use -import, using it makes the code harder to read since you
cannot directly see in what module a function is deﬁned. Use exref
(Cross Reference Tool) to ﬁnd module dependencies.

6.7 Exporting functions

Make a distinction of why a function is exported. A function can be
exported for the following reasons (for example):

• It is a user interface to the module.

237

• It is an interface function for other modules.

• It is called from apply, spawn etc. but only from within its module.

Use dicerent -export groupings and comment them accordingly. Ex-

ample:

%% user interface
-export([help/0, start/0, stop/0, info/1]).

%% intermodule exports
-export([make_pid/1, make_pid/3]).
-export([process_abbrevs/0, print_info/5]).

%% exports for use within module only
-export([init/1, info_log_impl/1]).

7 Speciﬁc Lexical and Stylistic Conventions

7.1 Don’t write deeply nested code

Nested code is code containing case/if/receive statements within other
case/if/receive statements. It is bad programming style to write deeply
nested code - the code has a tendency to drid across the page to the
right and soon becomes unreadable. Try to limit most of your code to a
maximum of two levels of indentation. This can be achieved by dividing
the code into shorter functions.

7.2 Don’t write very large modules

A module should not contain more than 400 lines of source code. It is
better to have several small modules than one large one.

238 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

7.3 Don’t write very long functions

Don’t write functions with more than 15 to 20 lines of code. Split large
functions into several smaller ones. Don’t solve the problem by writing
long lines.

7.4 Don’t write very long lines

Don’t write very long lines. A line should not have more than 80 charac-
ters. (It will for example ﬁt into an A4 page.)

In Erlang 4.3 and thereader string constants will be automatically con-

catenated. Example:

io:format("Name: ~s, Age: ~w, Phone: ~w ~n"

"Dictionary: ~w.~n", [Name, Age, Phone, Dict])

7.5 Variable names

Choose meaningful variable names - this is very diecult.

If a variable name consists of several words, use “ ” or a capitalized

letter to separate them. Example: My_variable or MyVariable.

Avoid using “ ” as don’t care variable, use variables beginning with
“ ” instead. Example: _Name.
If at a later stage you need the value of
the variable, you just remove the leading underscore. You will have no
problem ﬁnding what underscore to replace and the code will be easier to
read.

7.6 Function names

It
The function name must agree exactly with what the function does.
should return the kind of arguments implied by the function name.
It
should not surprise the reader. Use conventional names for conventional
functions ( start, stop, init, main_loop).

Functions in dicerent modules that solve the same problem should

have the same name. Example: Module:module_info().

239

Bad function names are one of the most common programming errors

- good choice of names is very diecult!

Some kind of naming convention is very useful when writing lots of
dicerent functions. For example, the name preﬁx “is_” could be used to
signify that the function in question returns the atom true or false.

is_...() -> true | false
check_...() -> {ok, ...} | {error, ...}

7.7 Module names

Erlang has a ﬂat module structure (i.e. there are no modules within mod-
ules). Oden, however, we might like to simulate the ecect of a hierarchical
module structure. This can be done with sets of related modules having
the same module preﬁx.

If, for example, an ISDN handler is implemented using ﬁve dicerent

and related modules. These module should be given names such as:

isdn_init
isdn_partb
isdn_...

7.8 Format programs in a consistent manner

A consistent programming style will help you, and other people, to under-
stand your code. Dicerent people have dicerent styles concerning inden-
tation, usage of spaces etc.

For example you might like to write tuples with a single comma be-

tween the elements:

{12,23,45}

Other people might use a comma followed by a blank:

{12, 23, 45}

Once you have adopted style - stick to it.
Within a larger project, the same style should be used in all parts.

240 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

8 Documenting Code

8.1 Attribute code

You must always correctly attribute all code in the module header. Say
where all ideas contributing to the module came from - if your code was
derived from some other code say where you got this code from and who
wrote it.

Never steal code - stealing code is taking code from some other module

editing it and forgetting to say who wrote the original.

Examples of useful attributes are:

-revision(’Revision: 1.14 ’).
-created(’Date: 1995/01/01 11:21:11 ’).
-created_by(’eklas@erlang’).
-modified(’Date: 1995/01/05 13:04:07 ’).
-modified_by(’mbj@erlang’).

8.2 Provide references in the code to the speciﬁcations

Provide cross references in the code to any documents relevant to the
understanding of the code. For example, if the code implements some
communication protocol or hardware interface give an exact reference
with document and page number to the documents that were used to
write the code.

8.3 Document all the errors

All errors should be listed together with an English description of what
they mean in a separate document (See “Error Messages” on page 246.)
By errors we mean errors which have been detected by the system.
At a point in your program where you detect a logical error call the

error logger thus:

error_logger:error_msg(Format,

{Descriptor, Arg1, ....})

241

And make sure that the line {Descriptor, Arg1,...} is added to

the error message documents.

8.4 Document all the principle data structures in messages

Use tagged tuples as the principle data structure when sending messages
between dicerent parts of the system.

The record features of Erlang (introduced in Erlang versions 4.3 and
thereader) can be used to ensure cross module consistency of data struc-
tures.

An English description of all these data structure should be docu-

mented (See “Message Descriptions” on page 246.)

8.5 Comments

Comments should be clear and concise and avoid unnecessary wordiness.
Make sure that comments are kept up to date with the code. Check that
comments add to the understanding of the code. Comments should be
written in English.

Comments about the module should not be indented and should start
with three percent characters (%%%), (See “File Header, description” on
page 243).

Comments about a function should not be indented and start with two

percent characters (%%), (See “Comment each function” on page 242).

Comments within Erlang code should start with one percent character
(%). If a line only contains a comment, it should be indented as Erlang
code. This kind of comment should be placed above the statement it
refers to. If the comment can be placed on the same line as the statement,
this is preferred.

%% Comment about function
some_useful_functions(UsefulArgugument) ->

another_functions(UsefulArgugument),
% Comment about complicated_stmnt at the same level of indentation
complicated_stmnt,

% Comment at end of line

......

242 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

8.6 Comment each function

The important things to document are:

• The purpose of the function.

• The domain of valid inputs to the function. That is, data structures

of the arguments to the functions together with their meaning.

• The domain of the output of the function. That is, all possible data

structures of the return value together with their meaning.

• If the function implements a complicated algorithm, describe it.

• The possible causes of failure and exit signals which may be gener-
ated by exit/1, throw/1 or any non-obvious run time errors. Note
the dicerence between failure and returning an error.

• Any side ecect of the function.

Example:

%%----------------------------------------------------------------------
%% Function: get_server_statistics/2
%% Purpose: Get various information from a process.
%% Args:
%% Returns: A list of {Key, Value}
%%
%%----------------------------------------------------------------------
get_server_statistics(Option, Pid) when pid(Pid) ->

or {error, Reason} (if the process is dead)

Option is normal|all.

......

8.7 Data structures

The record should be deﬁned together with a plain text description. Ex-
ample:

%% File: my_data_structures.h

%%---------------------------------------------------------------------
%% Data Type: person
%% where:
%%
%%
%%
%%
%%
%%----------------------------------------------------------------------
-record(person, {name, age, phone = [], dict = []}).

name: A string (default is undefined).
age: An integer (default is undefined).
phone: A list of integers (default is []).
dict:

A {Key, Value} list (default is the empty list).

A dictionary containing various information about the person.

8.8 File headers, copyright

Each ﬁle of source code must start with copyright information, for example:

243

%%%---------------------------------------------------------------------
%%% Copyright Ericsson Telecom AB 1996
%%%
%%% All rights reserved. No part of this computer programs(s) may be
%%% used, reproduced,stored in any retrieval system, or transmitted,
%%% in any form or by any means, electronic, mechanical, photocopying,
%%% recording, or otherwise without prior written permission of
%%% Ericsson Telecom AB.
%%%---------------------------------------------------------------------

8.9 File headers, revision history

Each ﬁle of source code must be documented with its revision history
which shows who has been working with the ﬁles and what they have
done to it.

%%%---------------------------------------------------------------------
%%% Revision History
%%%---------------------------------------------------------------------
%%% Rev PA1 Date 960230 Author Fred Bloggs (ETXXXXX)
%%% Initial pre release. Functions for adding and deleting foobars
%%% are incomplete
%%%---------------------------------------------------------------------
%%% Rev A Date 960230 Author Johanna Johansson (ETXYYY)
%%% Added functions for adding and deleting foobars and changed
%%% data structures of foobars to allow for the needs of the Baz
%%% signalling system
%%%---------------------------------------------------------------------

8.10 File Header, description

Each ﬁle must start with a short description of the module contained in
the ﬁle and a brief description of all exported functions.

%%%---------------------------------------------------------------------
%%% Description module foobar_data_manipulation
%%%---------------------------------------------------------------------
%%% Foobars are the basic elements in the Baz signalling system. The
%%% functions below are for manipulating that data of foobars and for
%%% etc etc etc
%%%---------------------------------------------------------------------
%%% Exports
%%%---------------------------------------------------------------------
%%% create_foobar(Parent, Type)
%%%
%%%
%%%---------------------------------------------------------------------

returns a new foobar object
etc etc etc

244 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

If you know of any weakness, bugs, badly tested fea-
tures, make a note of them in a special comment, don’t
try to hide them. If any part of the module is incomplete,
add a special comment. Add comments about anything which will
be of help to future maintainers of the module. If the product of which the
module you are writing is a success, it may still be changed and improved
in ten years time by someone you may never meet.

8.11 Do not comment out old code - remove it

Add a comment in the revision history to that ecect. Remember the source
code control system will help you!

8.12 Use a source code control system

All non trivial projects must use a source code control system such as RCS,
CVS or Clearcase to keep track of all modules.

9 The Most Common Mistakes

• Writing functions which span many pages (See “Don’t write very

long functions” on page 238).

• Writing functions with deeply nested ifs receives, cases etc (See

“Don’t write deeply nested code” on page 237).

• Writing badly typed functions (See “Use tagged return values” on

page 234).

• Function names which do not reﬂect what the functions do (See

“Function names” on page 238).

• Variable names which are meaningless (See “Variable names” on

page 238).

245

• Using processes when they are not needed (See “Assign exactly one
parallel process to each true concurrent activity in the system” on
page 229).

• Badly chosen data structures (Bad representations).

• Bad comments or no comments at all (always document arguments

and return value).

• Unindented code.

• Using put/get (See “Use the process dictionary with extreme care”

on page 235).

• No control of the message queues (See “Flush unknown messages”

on page 231 and “Time-outs” on page 233).

10 Required Documents

This section describes some of the system level documents which are nec-
essary for designing and maintaining system programmed using Erlang.

10.1 Module Descriptions

One chapter per module. Contains description of each module, and all
exported functions as follows:

• the meaning and data structures of the arguments to the functions

• the meaning and data structure of the return value

• the purpose of the function

• the possible causes of failure and exit signals which may be gener-

ated by explicit calls to exit/1.

Format of document to be deﬁned later:

246 APPENDIX B. PROGRAMMING RULES AND CONVENTIONS

10.2 Message Descriptions

The format of all inter-process messages except those deﬁned inside one
module.

Format of document to be deﬁned later:

10.3 Process

Description of all registered servers in the system and their interface and
purpose.

Description of the dynamic processes and their interfaces.
Format of document to be deﬁned later:

10.4 Error Messages

Description of error messages

Format of document to be deﬁned later:

C UBF

Getting Erlang to talk to the outside
world.1

Joe Armstrong
7 May 2002

Abstract

How should Erlang talk to the outside world? — This question becomes
interesting if we want to build distributed applications where Erlang is one
of a number of communicating components.

We assume these components interact by exchanging messages - at this
level of abstraction, details of programming language, operating system
and host architecture are irrelevant. What is important is the ease with
which we can construct such systems, and the precision with which we
can isolate faulty components in such a system. Also of importance is the
eeciency (both in terms of CPU and bandwidth requirements) with which
we can send and receive messages in the system.

One widely adopted solution to this problem involves the XML family
of standards (XML, XML-schemas, SOAP and WSDL) - we argue that

1This is a reformatted and slightly revised version of the the paper presented at the ACM SIGPLAN

Erlang Workshop—2002 Pittsburg, PA USA[13].

247

248

APPENDIX C. UBF

Figure C.1: Black boxes and Contract Checkers

this is ineecient and overly complex and propose basing our system on a
simpler binary scheme called UBF (Universal Binary Format). The UBF
scheme has the expressive power of the XML set of standards - but is
considerably simpler.

UBF has been prototyped in Erlang - the entire scheme (equivalent in
semantic power to the XML series of standards) was implemented in a
mere 1100 lines of Erlang. UBF encoding of terms is also shown to be
more space eecient than the existing “Erlang term format”. For example,
UBF encoded parse trees of Erlang programs are on average about 60% of
the size of the equivalent ETS format encoding which is used in the open
source Erlang distribution.

Categories and Subject Descriptors

C.2.2 [Computer Communications]: Network Protocols; D.1 [Sod-
ware]: Programming Techniques; D.1 [Sodware]: Programming Lan-
guages

1 Introduction

We are interested in building reliable distributed systems out of asyn-
chronously communicating components. We assume that the components
are written in dicerent programming languages, run on dicerent operating
systems and operate anywhere in the network. For example, some compo-
nents may be written in Erlang, others in Java, others in C; the components
might run on Unix, or Windows or Solaris.

BBBBBCCCC249

We ask the questions “How should such systems interact?” and “Can
we create a convenient language-neutral transport layer to allow such ap-
plications to be easily constructed?”

Suppose further that we have several dicerent components and that
they collaborate to solve some problem - each individual component has
been tested and is assumed to work, and yet the system as a whole does
not work. Which component is in error?

There are a number of conventional methods for solving parts of this
problem, for example, we could use an interface description language (like
Sun XDR [62] or ASN.1 [44]) or we could use a more complex framework
like Corba [56]. All these methods have associated problems - many of
these methods are supposedly language neutral but in practice are heavily
biased to languages like C or C++ and to 32-bit word length processor
architectures. The more complex frameworks (like Corba) are diecult
to implement and are inappropriate for simple applications. Proprietary
solutions for component interaction (like Microsod’s COM and DCOM)
are not considered, since they are both complex and, more damagingly,
not available on all platforms.

Out of this mess a single universal panacea has emerged - XML. The
XML series of standards, notably XML[20], XML-schemas[32], [65] with
SOAP[54], [39], [40] and WSDL[25] has emerged as the universal solution
to this problem.

The XML solution involves three layers:

• A transport layer - XML provides a simple transport layer. XML
encoded terms can be used to encode complex structured objects.

• A type system - XML schemas provides a type schema for describing

the type of the content of a particular XML tag.

• A protocol description language - SOAP deﬁnes how simple remote
procedure calls can be encoded as XML terms. More complex
interactions between components can be described using the Web
Service Description Language (WSDL).

The above architectural layering is desirable, in so much as it sepa-
rates transport of data (XML), the types of the data (XML-schemas) and

250

APPENDIX C. UBF

the semantics of interactions between dicerent components in the network
(SOAP and WDSL).

Unfortunately, while the architecture is essentially correct, the details
leave much to be desired. The individual components sucer from a num-
ber of signiﬁcant problems.

We argue in the next section of the paper that XML is overly complex
and overly verbose. Following this section we propose a simpler and more
eecient but equally expressive binary format, which could be used as a
complement to XML.

Our proposed schema has been implemented fully in Erlang and par-
tially in Java and C - we present some preliminary results in the ﬁnal section
of the paper.

Our type system has an expressive power similar to that of the expres-
sive power of XML-schemas, though we believe our scheme to be much
simpler. Our contract language has many similarities to WSDL but again
we believe it to be simpler and more expressive.

Our architecture also has many similarities to the .NET architecture,

though we believe our architecture to be simpler and more powerful.

The remainder of the paper describes the system in detail, gives some
performance ﬁgures and describes our initial experience with the system.

2 Problems with XML

2.1 Complexity

XML, XML-schemas, SOAP and WDSL are a complex set of inter-related
standards. A full implementation of the above standards requires many
tens of thousands of lines of code and the implementation of a number of
minor standards (like XML-name-spaces and XML-path) etc.

The XML standard itself has a grammar of 89 productions and requires
many pages of explanatory text - entire text books have been written just
to explain the (simple) standard. Having implemented three XML parsers
in Erlang I am in the position to say that XML is decidedly not simple to
implement - amazingly, most of the complexity occurs in the implementa-
tion of a number of features which the vast majority of programmers will

251

never use (these are antediluvian hang-backs to SGML).

The original design of XML had a notion of structure (described by
a regular grammar) but no notion of type. Structure was described using
DTDs (Data Type Descriptions) - but the DTD’s did not themselves have
an XML syntax. This was viewed by some as a disadvantage - XML-
schemas came to the rescue - using XML-schemas XML structures could
be described in XML itself, and a rich set of types was added.

What was been described by the XML DTD[26]

<!ELEMENT ROOT (A?,B+,C*)>

became in XML-schemas:

<element name="ROOT">

<complexType content="elementOnly">

<choice>

<element ref="t:A">
<sequence>

<element ref="t:B">
<element ref="t:C">

</sequence>

</choice>

</complexType>

</element>

The notation for saying that the content of a tag should be of a partic-

ular type is equally verbose.

XML-schemas has 19 built-in (or primitive) types and three type con-
structors The net result of this is that, if you want to express types you
have to use XML-schemas. Unfortunately, the verbosity of the speciﬁca-
tion makes the schemas diecult to read.

In retrospect, a much simpler alternative would have been to extend

XML with a small number of primitive data types.

For example, XML has a construct like:

<!ELEMENT xxx (#PCDATA)>

252

APPENDIX C. UBF

it would have been easy to extend this with expressions like:

<!ELEMENT xxx (#INTEGER32)>

Meaning that xxx is a 32 bit integer.
Such an extension would have provided a succinct and readable alter-

native to XML schemas.

2.2 Verbosity

XML encodings are incredibly verbose. The designers of XML excuse
themselves with the words: “Terseness in XML markup is of minimal
importance.”[20] Unfortunately, the very verbosity of XML makes eecient
parsing impossible, since at the very least the parser must examine every
single input character. This property limits the usefulness of XML as a
transport format for mobile devices with limited bandwidth.

Interestingly, one of the most common XML applications designed for
such devices, namely WAP, uses an ad hoc method [50] to compress XML
WAP programs, providing striking evidence that raw XML is inappropriate
as a universal format for low-bandwidth devices.

Another strange property of XML is that binary data must be encoded
prior to transmission. For example, a JPEG image must ﬁrst be base64
encoded. Base64 encoding processes data in 24 bit groups, replacing each
lines are
3 byte sequence on input with a 4 byte sequence on output,
limited to 76 characters and only printable characters are transmitted.

This is all very strange and highly ineecient (especially considering
that SOAP uses TCP/IP for data transport and TCP/IP itself is designed
for eecient transport of binary data) - the bit about 76 characters probably
has something to do with punched cards, and the restriction to printable
characters has something to do with transmission systems that may only
pass seven bits of a byte in a transparent manner.

Unfortunately the weird quoting rules of SGML apply to XML - you
might naively think that binary data could be transmitted “as is” - un-
fortunately you can’t just quote binary data in XML - if the binary data
just happened to contain a valid XML end tag then chaos would ensue.

253

Figure C.2: Client/server with contract checker

Most programming languages have quoting conventions which allow an
arbitrary sequence of characters to be quoted, XML does not; thus, for
example, any data can be placed within a CDATA block except data con-
taining the string ]]> - this fact severely limits the usefulness of CDATA
section, making it impossible to (say) quote an arbitrary XML program -
since it itself might contain a CDATA section. One wonders why such a
convention was adopted.

3. Our architecture

Our architecture is shown in Figures D.1 and D.2. Figure D.1 shows
a number of communicating components. The components are assumed
to be black boxes, at any particular time a component might behave as a
client or as a server. Between the components we place an entity which
we call a contract checker (shown in the diagram as a black blob), the
contact checker checks the legality of the ﬂow of messages between the
components.

We assume the contract checker starts in state S (see Figure D.2); the
client sends a message X intended for the server, the contract checker
checks that X is of the correct type and that it is expected in state S, if so
it is sent to the server. The server responds with a Message × State tuple
{M, S1} the contract checker checks that this message is an expected
response in the current state, if so {M, S1} is sent to the client and the

Client(M,S1)XXServer(M,S1)254

APPENDIX C. UBF

state of the contract checker updated to S1.2

The contract checker is parameterised with a contract that speciﬁes the
ordering and types of the allowed message sequences between the client
and the server.

The contract is written using a simple non-deterministic ﬁnite state

machine and a simple type language.

The contract is modeled as a set of four tuples of the form:

{Sin, Tin, Tout, Sout}

This means that if the server is in state Sin and it receives a message
of type Tin then it may possibly respond with a message of type Tout and
change its state to Sout.

The contract checker assumes that the start state of the server is start

which is assigned to the state variable S.

If the client sends the server a message X the contract checker checks
that there are some rules in the contract where S = Sin and typeof(X) =
Tin - if there are any such rules then the client is said to follow the contract
and the message X can be safely sent to the server. If no such rules match,
then the client is said to have broken the contract and both client and
server are informed about this.

If the client has sent a valid message then the set of expected output

responses of the server is pruned to a set of two tuples

{Tout, Sout}

being the allowed set of Type × State tuples that the server can respond

with.

The server must respond with a {Msg, State} tuple - the contract
manager checks if there is a tuple in the response set where State = Sout
and typeof(Msg) = Tout.

If there is such a tuple then the response is accepted and Msg is sent
back to the client and the global value of the state S is updated to State.

2Note that this is unlike the convention RPC mechanism, where a server responds with a message in

response to a particular query, and the next state of the server (if it is statefull) is implied by the protocol.

255

Note that the contract checker operates transparently in normal opera-
tion. In the case where both the client and server follow the contract no
changes are made to the messages passed between the client and server -
the only possible dicerence between client/server interaction using a con-
tract checker and not using a contract checker is a slight timing dicerence.

4. UBF - a universal binary format

Contracts are written in a language we call UBF which has two compo-
nents:

• UBF(A) is a data transport format, roughly equivalent to well-

formed XML.

• UBF(B) is a programming language for describing types in UBF(A)
and protocols between clients and servers. UBF(B) is roughly equiv-
alent to veriﬁed XML, XML-schemas, SOAP and WDSL.

While the XML series of languages had the goal of having a human
readable format the UBF languages take the opposite view and provide a
“machine friendly” format.

UBF is designed to be easy to implement. As a proof of concept —
UBF drivers for Erlang, Oz, Java and TCL can be found at the author’s
web site [6]. Implementors are welcome to add new languages.

UBF is designed to be “language neutral” — UBF(A) deﬁnes a language
neutral binary format for transporting data across a network. UBF(B) is a
type system for describing client/server interactions which use UBF(A).

5. UBF(A) - a binary transport format

UBF(A) is a transport format, it is designed to be easy to parse and to be
easy to manipulate with a text editor. UBF(A) is based on a byte encoded
virtual machine, 26 byte codes are reserved. Instead of allocating the byte
codes from 0 we use the printable character codes to make the format easy
to read and edit.

256

APPENDIX C. UBF

Simplicity is the goal, so we deﬁne a minimal set of primitive types
(four, compared with XML-schemas which have 19) and two types of
“glue” for building complex types from more simple types.

5.1 Primitive types

UBF(A) has four primitive types. When a primitive tag is recognized it is
pushed onto the “recognition stack” in our decoder. The primitive types
are:

Integers - integers are written as sequences of bytes described by the
regular expression [-][0-9]+. That is, an optional minus (to denote
a negative integer) followed by a sequence of one or more digits. No
restrictions are made as to the precision of the integer, precision
issues are be dealt with in UBF(B).

Strings — strings are written enclosed in double quotes, thus:

"...."

Within a string two quoting conventions are observed, " must be
written \" and \ must be written \\ - no other quotings are allowed
(this is so we can write a double quote within a string).

Binary Data — binary data is encoded, thus:

Int ~....~

First an integer, representing the length of the binary data is en-
coded, followed by a tilde, the data itself, which must be exactly the
length given in the integer, and than a closing tilde. The closing tilde
has no signiﬁcance and is retained for readability. White space can
be added between the integer length and the data for readability.

Constants — constants are encoded as strings, only using a single quote

instead of a double quote.

257

Constants are commonly found in symbolic languages like Lisp, Pro-
log or Erlang. In C they would be represented by hashed strings.
The essential property of a constant is that two constants can be
compared for equality in constant time.

In addition any item can be followed by a semantic tag written ‘...‘.
This tag has no meaning in UBF(A) but might have a meaning in UBF(B).
For example:

12456 ~....~ ‘jpg‘

Represents 12456 bytes of raw data with the semantic tag “jpg.” UBF(A)
does not know what “jpg” means - this is passed on to UBF(B) which might
know what it means - ﬁnally the end application is expected to know what
to do with an object of type “jpg,” it might for example know that this
represents an image. UBF(A) will just encode the tag, UBF(B) will type
check the tag, and the application should be able to understand the tag.

5.2 Compound types

Having deﬁned our four simple types we deﬁne two type of “glue” for
making compound objects.

Structs — structures are written:

{ Obj1 Obj2 ... Objn }

Where Obj1..Objn are arbitrary UBF(A) objects and the byte codes
for { and } are used to delimit a structure. The decoder and encoder
must map UBF(A) objects onto an appropriate representation in the
application programming language (for example structs in C, arrays
in Java, tuples in Erlang etc.).

Structs are used to represent Fixed numbers of objects

Lists —lists are used to represent variable numbers of objects. They are

written with the syntax:

258

APPENDIX C. UBF

# ObjN & ObjN-1 & ... & Obj2 & Obj1

This represents a list of objects - the ﬁrst object in the list is Obj1
the second Obj2 etc.- Note that the objects are presented in reverse
order. Lisp programmers will recognize # as an operator that pushes
NIL (or end of list) onto the recognition stack and & as an operator
that takes the top two items on the recognition stack and replaces
them by a list cell.

Finally we need to know when an object has ﬁnished. The operator
$ signiﬁes end of object. When $ is encountered there should be only one
item on the recognition stack.

5.3 White space

For convenience, blank, carriage return, line feed, tab and comma are
treated as white space. Comments can be included in UBF(A) with the
syntax %...% the usual quoting convention applies.

5.4 Caching optimizations

So far we have used exactly 26 control characters, namely:

%"~’‘{}#&\s\n\t\r,-01234567890

This leaves us with 230 unallocated byte codes. These are used as

follows: The byte code sequence

>C

Where C is not one of the reserved byte codes or > means store the
top of the recognition stack in the register reg[C] and pop the recognition
stack.

Subsequent reuse of the single character C means “push reg[C] onto

the recognition stack.”

259

Figure C.3: Client and Server in dicerent
checker.

languages with a contract

6. Programming by Contract

Central to UBF is the idea of a contract. The contract regulates the set

of legal conversations that can take place between a client and a server.

A sodware component (the contract checker) is placed between a client
and server and it checks that all interactions between the client and server
are legal.

The contract is written using types - the contract says (in a formal

language) something like:

“If I am in state S and you send me a message of type T1 then
I will reply with a message type T2 and move to state S1, or,
I will reply with a message of type T3 and move to state S2 ...
etc.”

The contract checker dynamically checks that both sides involved in a
transaction obey the contract. Our contracts are expressing in a language
we call UBF(B).
UBF(B) has:

A type system - for describing the types of UBF(A) objects.

A protocol description language - for describing client-server in-
teraction in terms of a non-deterministic ﬁnite state machine.

An LALR(1) grammar for UBF can be found in appendix A.

ClientContractUBF driverServercheckerUBF DriverUBF(A)objectsUBF(A)objectsJavaobjectsContractUBF(B)JavaErlang termsErlang260

APPENDIX C. UBF

6.1 The type system

The type system used here to describe the type of UFB(A) encoded objects
is a simpliﬁed version of the type system used to describe Erlang terms[8].
The notation:

• int() Means a UBF(A) integer.

• string() Means a UBF(A) string.

• constant() Means a UBF(A) constant.

• bin() Means a UBF(A) binary data item.

• X() Means an Object of type X

UBF(A) literals are written as follows:

• "..." - denotes a UBF(A) string.

• [a-z][a-zA-Z0-9_]* - denotes a UBF(A) constant.

• [-][0-9]+ - denotes a UBF(A) integer.

Complex types are deﬁned recursively:

{T1, T2, ..., Tn} Is the tuple type if T1 .. Tn are types. We say that

{X1, X2, ..., Xn} is of type
{T1, T2, ..., Tn} if X1 is of type T1, X2 is of type T2, ... and Xn
is of type Tn.

[T ]

Is the list type if T is a type. We say that
# Xn & Xn-1 & ... X2 & X1 is of type [T] if all Xi are of type T.

T 1|T 2 Is the alternation type if T1 and T2 are types. We say that X is of

type T1 | T2 if X is of type T1 or if X is of type T2.

261

6.2 New types

New types are introduced in UBF(B) with the notation:

+TYPES X() = Type1; Type2; ...

Where Type1, Type2, ... are simple types, literal types or complex

types.

Examples of types are:

+TYPES

person()

= {person,

firstname(),
lastname(),
sex(),
age()};
firstname() = string();
lastname() = string();
age()
sex()
people()

= int();
= male | female;
= [person()].

This type schema deﬁnes a number of dicerent types. For example, it

is easily seen that:

’person’ >p
# {p "jim" "smith" ’male’ 10} &

{p "susan" "jones" ’female’ 14} & $

Is of type people().
Note that unlike XML UBF(A) encoded terms do not contain any tag
information. To make this clearer, suppose we had made an XML data
structure to represent the same information, this might be something like:

<people>

<person>

262

APPENDIX C. UBF

<firstname>jim</firstname>
<lastname>smith</lastname>
<sex>male</sex>
<age>10</age>

</person>
<person>

<firstname>susan</firstname>
<lastname>jones</lastname>
<sex>female</sex>
<age>14</age>

</person>

</people>

The XML data structure contains a large number of redundant tags -
whereas our representation omits all the tags The sizes of the ﬁrst repre-
sentation is 65 bytes and the second 215 (ignoring white space which is
redundant) - we might thus expect that parsing the UBF expression would
be at least three times as fast as parsing the XML expression.

Note that UBF(B) type is a language independent type schema. It deﬁnes
the types of messages ader encoding, and is thus universally applicable to
any programming language which produces UBF encoded data.

Language independent type schemas are the basis of Contracts between

clients are servers.

6.3 The Contract Language

We start with a simple example:

+NAME("file_server").
+VSN("ubf1.0").

+TYPES
info()
description() = description;
services()

= services;

= info;

263

contract()

= contract;

file()
ls()
files()
getFile()
noSuchFile()

+STATE start

= string();
= ls;
= {files, [file()]};
= {get, file()};
= noSuchFile.

=> files()

ls()
getFile() => binary()

& start;
& start
| noSuchFile() & stop.

+ANYSTATE

=> string();
info()
description() => string();
contract()

=> term().

The program starts with a sequence of type deﬁnitions (they follow the
TYPES keyword) - these deﬁne the types of the message that are visible
across the interface to the component.

Here, for example we see type getFile() is deﬁned as {get,file()}

where file() is of type string().

Given this deﬁnition it can easily be seen that the UBF(A) sequence of

characters {’get’ "image.jpg"}$ belongs to this type.

Reading further (in the STATE part of the program) we see the rule:

+STATE start

ls()
getFile() => bin()

=> files()

& start;
& start
| noSuchFile() & stop.

In English, this rule means:

If the system is in the state start and if it receives a message
of type ls() then respond with a message of type files()

264

APPENDIX C. UBF

and move into the start state, otherwise, if a message of type
getFile() is received then either respond with a message of
type bin() and move to the state start, or respond with a
message of type noSuchFile() and move to the state stop.

To continue with our example, we requested a ﬁle named image.jpg

the valid responses are of type bin() or
noSuchFile() which corresponds to UBF(A) encoded sequences like
NNN~ ... ~$ or ’noSuchFile’$.

Note that it might not always be possible for a component to distinguish
between two dicerent state transitions on the basis of the response alone.
Consider the following fragment of a contract:

+TYPES running() = string();
= string().

error()

+STATE running

request() => ok() & running;

| error() & stopping.

If we knew a component was in the state running and we sent it
a message of type request() then we would expect it to respond with
one of the types ok() or error() - unfortunately these types are indistin-
guishable, since both are represented as strings in UBF(A). For this reason
we require that the server responds with a State X Message pair, not
just a message. The server explicitly reveals its next state to the contract
checker.

7. Implementation details

The entire UBF system has been prototyped in Erlang. The entire system
is about 1100 lines of commented Erlang code.

• UBF encoding/decoding 391 lines.

• Contract parsing 270 lines.

265

• Contract checker and type checker - 301 lines.

• Run-time infrastructure and support libraries - 130 lines.

This compares favourably with the complexity of an XML implementa-
tion. As an example an incomplete implementation of XML which I wrote
two years ago has 2765 lines of Erlang code. This should be compared
with the 391 lines of code in the UBF implementation.

8. Performance

So far, the system has been implemented entirely in Erlang and no thought
given to embedding the UBF encoding/decoding sodware and the type
checking sodware into the Erlang run-time system.

The only measure of performance we give here concerns the packing

density of UBF encoded Erlang terms.

As a simple check we compared the size of the encoding of the parse
tree of a number of Erlang modules, with the size of the a binary produced
by evaluating the expression:

size(term_to_binary(epp:parse_file(F, [], [])))

The algorithm used to serialize the term representing the parse tree
was a simple two-pass optimizer which cached the most frequently used
constants which occurred in the program.

Based on a sample of 24 ﬁles we observed that on average the UBF(A)
encoded data was 59% of the size of the corresponding data when encoded
in the Erlang external term format. In applications where bandwidth is ex-
pensive and communication relatively slow (for example, communication
using mobile devices and GPRS) such a reduction in data volume would
have considerable beneﬁt.

9. Future work

It is easy
Our system of contracts uses only a very simple type system.
to envisage extensions to allow more complex types and extensions to
describe non-functional properties of the system.

266

APPENDIX C. UBF

The non-functional properties of the system are of particular interest.
An example of these might be to add simple timing constraints, allowing
rules such as:

+STATE S1

T1 => T2 & S2 before Time1
| T3 & S3 after Time2

...

meaning that if a component is in state S1 and receives a message of
type T1 then it might respond with a message of type T2 and change to
state S2 within Time1 or, respond with a message of type T3 and change
state to a state S3 ader a time Time2.

Stricter contracts with timing constraints could be very useful in design-

ing real-time systems of interacting components.

Other extensions could be imagined which would allow us to deﬁne
contracts like subroutines - so that one contract could use a sub-contract to
perform a speciﬁc task.

10. Running the system

Since our system essentially exchanges characters, we can use telnet to
observe a session and test the behaviour of the system. Here is an example
of commands issued in a telnet session where the client is talking directly
to the ﬁle server speciﬁed by the file_server contract given above:

’info’$
{"I am a mini file server",’start’}$

Recall that the system starts in the state start and that the contract
says that the info command can be sent in any state. The response
should be a string, and the new state (in this case start since the state is
not changed by an ANYSTATE rule).

The application returns a two tuple, containing a descriptive string and
the new state. This is converted by the application driver to the UBF tuple
{"I am ... ", ’start’}$.

267

’ls’$
{{’files’,

#
"ubf.erl"&
"client.erl"&
"Makefile"& ...}

’start’}$

Here the client sends a message of type ls() - the server responds
with tuple {{’files’,#..., ’start’}$ message. This ﬁrst element in
the tuple is of type files().

Finally we ask the system to describe itself:

’contract’$
{’contract’,

{{’name’,"file_server"},

{’info’,"I am a mini file server"},
{’description’,"

Commands:

’ls’$ List files
{’get’ File} => Length ~ ... ~

| noSuchFile

"},

{’services’,#},
{’states’,

#{’start’,

#{’input’,{’tuple’,#{’prim’,’file’}&

{’constant’,’get’}&},

#{’output’,{’constant’,’noSuchFile’},’stop’}&

{’output’,{’prim’,’binary’},’start’}&}&

{’input’,{’constant’,’ls’},
#{’output’,

{’tuple’,

268

APPENDIX C. UBF

#{’list’,{’prim’,’string’}}&

{’constant’,’files’}&},’start’}&}&}&},

{’types’,

#{’file’,{’prim’,’string’}}&}}}$

The system responds to a message of type info() with a parse tree

representing the contract itself.

In the contract itself we used the generic type term() to describe the
contract. The contract itself is a well typed term in UBF, but a discussion
of the abstracted form of the contract itself is not relevant to this paper.

The example is given to illustrate the introspective power of the system.
Not only can we run the system, we can also ask the system to describe it-
self. We believe this to be a desirable property of a distributed component
in a system of communicating components.

11. A larger contract

Our previous examples showed the basic syntax of a contract. We ﬁnish
with a more complex example. The contract below describes an IRC[46]
like protocol.

+NAME("irc").

+VSN("ubf1.0").

+TYPES

info()
description() = description;
contract()

= contract;

= info;

bool()
nick()
oldnick()

= true | false;
= string();
= string();

269

= string();
= string();
= logon;
= {ok, nick()}
= groups;
= [group()];
= {join, group()}
= {leave, group()};
= ok;
= {nick, nick()}

newnick()
group()
logon()
proceed()
listGroups()
groups()
joinGroup()
leaveGroup()
ok()
changeNick()
%%
msg()
msgEvent()
joinEvent()
leaveEvent()
changeNameEvent() = {changesName,

send a message to a group";

= {msg, group(), string()}
= {msg, nick(), group(), string()};
= {joins, nick(), group()};
= {leaves, nick(), group()};

oldnick(),newnick(), group()}.

%% I am assigned an initial (random) nick

+STATE start logon() => proceed() & active.

+STATE active

listGroups() => groups() & active;
joinGroup() => ok() & active;
leaveGroup() => ok() & active;
changeNick() => bool() & active;

%%

false if not in group

msg()

=> bool() & active;

% Message from group
EVENT => msgEvent();
% Nick joins group
EVENT => joinEvent();
EVENT => leaveEvent();
% Nick leaves group
EVENT => changeNameEvent(). % Nick changes name

270

+ANYSTATE

APPENDIX C. UBF

=> string();
info()
description() => string();
contract()

=> term().

This example introduces a new keyword EVENT. The syntax:

+STATE S1
...
EVENT => T2;
...

means that the server can spontaneously send a message of type T2
to the client. Normally, messages are sent to the client in response to
requests, EVENT is used for asynchronous single messages from the server
to the client. Since the server cannot be sure that the client has received
such a message no change of state in the server is allowed.

12. Experience

The initial version of UBF was completed in about three weeks of in-
tensive programming - the system design changed many times and was
re-designed, implemented and re-implemented several times.

Once the basic infrastructure was running, a simple interface to Oz was
implemented - and following this an interface to Java. The Oz and Java
implementation only concerned UBF(A) and not the contract language or
checker.

The ﬁrst non-toy application (IRC) was implemented to test the system
on a non-trivial example. I started by writing the contract and then made
an Erlang client and server which followed the contract.

Interestingly the contract checker proved extremely helpful in develop-
ing the IRC system - I oden develop systems by writing a client and server
in the same time frame, shiding attention between the client and server as
necessary. Using the contract checker proved helpful in rapidly identifying

271

which of the two components was in error in the event of an error. Also,
since the intermediate communication has a fairly readable ASCII subset I
was able to test the server by typing simple text queries in a telnet session
- in this way I was able to immediately test the server (and the interaction
between the client and server) using telnet, rather than my Erlang code
(which at some stages was only partially complete).

Interestingly the contract checker oden complained about contract vi-
olations that I did not believe, so I erroneously assumed that the code
for checking the contracts was incorrect. Almost invariably the contract
checker was right and I was wrong. I think we have a tendency to believe
what we had expected to see - and not that which was actually present -
the contract checker had no such biases.

Concentration on the contact itself caused an interesting psychological
shid of perspective and forced me to think about the system in meta-
level terms considering the client and server as only stupid black boxes
which did what they were told. Trying to write the contracts in a clear
manner was also an exercise which resulted in a clearer understanding of
the problem by forcing me to think in terms of what messages are sent
between the client and server - and nothing else.

The contract proved also a valuable and easy-to-understand speciﬁca-
tion of the problem. Having implemented an Erlang client and server and
a graphic based Erlang client we decided to add a Java client.

The Java client was developed independently by Luke Gorrie using
only the UBF speciﬁcation and the irc contract. When it came to testing
the contract checker could provide extremely precise error diagnostics - of
the form:

I was in state S and I expected you to send me a message of
type T but you sent me the message M which is wrong.

Armed with such precise diagnostics it was easy to debug the Java
program. Needless to say when the Java client talked to the Erlang server
the system worked ﬁrst time. Testing both the Java client and the Erlang
server could be done independently using only a modiﬁed form of the
contract checker and the contract concerned.

272

APPENDIX C. UBF

Having developed the system we have a high degree of conﬁdence in
its correctness - and if it should fail we’ll immediately know which compo-
nent is broken.

13. Acknowledgments

I would like to thank Seif Haridi, Per Brand, Thomas Arts, and Luke
Gorrie for their helpful discussions - particular thanks go to Luke for im-
plementing the Java client.

APPENDIX

UBF grammar

form -> ’+’ ’NAME ’(’ string ’)’ dot.
form -> ’+’ ’VSN’ ’(’ string ’)’ dot.
form -> ’+’ ’TYPES’ types dot.
form -> ’+’ ’STATE’ atom transitions dot.
form -> ’+’ ’ANYSTATE’anyrules dot.

types -> typeDef ’;’ types.
types -> typeDef.

typeDef -> atom ’(’ ’)’ ’=’ type annotation.

annotation -> string.
annotation -> ’$empty’.

type -> primType ’|’ type.
type -> primType.

primType -> ’int’
’(’ ’)’.
’(’ ’)’.
primType -> ’string’
primType -> ’constant’ ’(’ ’)’.
’(’ ’)’.
primType -> ’bin’

273

’(’ ’)’.

primType -> atom
primType -> ’{’ typeSeq ’}’.
primType -> ’[’ type ’]’.
primType -> atom.
primType -> integer.
primType -> integer ’.’ ’.’ integer.
primType -> string.

typeSeq -> type.
typeSeq -> type ’,’ typeSeq.

typeRef -> atom ’(’ ’)’.

transitions -> transition ’;’ transitions.
transitions -> transition.

transition -> typeRef ’=>’ outputs.
transition -> ’EVENT’ ’=>’ typeRef.

outputs -> responseAndState ’|’ outputs.
outputs -> responseAndState.

responseAndState -> typeRef ’&’ atom.

anyrules -> anyrule ’;’ anyrules.
anyrules -> anyrule.

anyrule -> typeRef ’=>’ typeRef.

strings -> string ’,’ strings.
strings -> string.
strings -> ’$empty’.

274

APPENDIX C. UBF

D Colophon

I t had been my intention to write a new typesetting system in order to

typeset this thesis—I therefore started work on Erlguten [7] which has
been described elsewhere.

Having read Knuth’s masterwork digital typography I realised that pro-
ducing a high quality type-setting system would delay the work on the the-
sis by several years (in my estimate about ﬁve years) and so I have decided,
reluctantly to use LATEX.

It turns out that with a little tweaking even pdfLATEX can be persuaded

to produce almost readable text.

This thesis was produced for A4 paper in 14 pt FSBaskerville, reduced
by 81% to ﬁt onto A5 paper. The drop capitals and chapter numbers are
set in Old English and all computer programs, shell dialogs, and references
to code in the text are set in Computer Modern.

John Baskerville (1706–1775) was an English calligrapher, stonecutter,
type designer, and printer. In 1750 he started a printing business, but being
a perfectionist his ﬁrst work was delayed until 1757. His work was much
admired by Fournier, Bodoni, and Benjamin Franklin.

I chose the Baskerville typeface because it has heavily weighted hair-
lines1 and thus is well suited for documents that are produced on a low
quality laser printer or for photocopying.

Text using typefaces (like Garamond) which have delicate serifs, and
lightly weighted hairlines tends to fade when printed with a poor quality
printer on poor quality paper. This fading reduces the legibility of the text.

1Hairlines are the thinnest lines used in a letterform.

275

276

APPENDIX D. COLOPHON

I have also tried using the micro-typographic extensions developed by
H`an Thˆe Th`anh in his doctoral thesis [64], which reintroduced the original
practice used in the Gutenberg 42 line bible of allowing punctuation at the
end of lines to slightly protrude into the right-hand margin. This practice
improves the optical alignment of the led-hand margin and produces better
line breaks.

Unfortunately I abandoned this approach, since the sodware for margin
kerning did not always work correctly with multiple typefaces on the same
line. For this reason alone further work on Erlguten seems motivated.

Finally, I would like to thank the small army of people who have read
and commented on this thesis. My wife, Helen, proof read the entire thesis
and found an embarrassing number of small mistakes in the text. Peter
Van Roy, and Ulf Wiger found some spelling mistakes that even Helen had
missed and Richard O’Keefe found some formatting errors that everybody
missed.

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