Concurrent computing is a form of computing in which programs are designed as collections of interacting computational processes that may be executed in parallel...
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
and runtime system. The sequential subset of Erlang is a functional language, with strict evaluation, single assignment, and dynamic typing. For concurrency it follows the Actor model
Actor model
In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and...
Ericsson , one of Sweden's largest companies, is a provider of telecommunication and data communication systems, and related services, covering a range of technologies, including especially mobile networks...
to support distributed, fault-tolerant, soft-real-time, non-stop applications. It supports hot swapping
Hot swapping
Hot swapping and hot plugging are terms used to describe the functions of replacing computer system components without shutting down the system...
, so that code can be changed without stopping a system.
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
s are considered to be complicated and error-prone topic in most languages, Erlang provides language-level features for creating and managing processes with the aim of simplifying concurrent programming. Though all concurrency is explicit in Erlang, processes communicate using message passing
Message passing
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
instead of shared variables, which removes the need for locks
Lock (computer science)
In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...
.
The first version was developed by Joe Armstrong in 1986. It was originally a proprietary language within Ericsson, but was released as open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
in 1998.
History
The name "Erlang", attributed to Bjarne Däcker, has been understood either as a reference to Danish mathematician and engineer Agner Krarup Erlang
Agner Krarup Erlang
Agner Krarup Erlang was a Danish mathematician, statistician and engineer, who invented the fields of traffic engineering and queueing theory....
, or alternatively, as an abbreviation of "Ericsson Language".
Erlang was designed with the aim of improving the development of telephony applications. The initial version of Erlang was implemented in Prolog
Prolog
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
.
In 1998, the Ericsson AXD301 switch was announced, containing over a million lines of Erlang, and reported to achieve a reliability of nine "9"s
Nines (engineering)
The colloquial term nines is used in engineering to indicate reliability or purity . It is preceded by a number indicating the degree of such reliability or purity. For example, 0.999 pure silver would be 3 nines pure...
. Shortly thereafter, Erlang was banned within Ericsson Radio Systems for new products, citing a preference for non-proprietary languages. The ban caused Armstrong and others to leave Ericsson. The implementation was open sourced at the end of the year. The ban at Ericsson was eventually lifted, and Armstrong was re-hired by Ericsson in 2004.
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
support was added to the runtime system and virtual machine.
Philosophy
The philosophy used to develop Erlang fits equally well with the development of Erlang-based
systems. Quoting Mike Williams, one of the three inventors of Erlang:
Find the right methods—Design by Prototyping.
It is not good enough to have ideas, you must also be able to implement them and know they work.
Make mistakes on a small scale, not in a production project.
In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n...
algorithm implemented in Erlang:
-module(fact). % This is the file 'fact.erl', the module and the filename must match
-export([fac/1]). % This exports the function 'fac' of arity 1 (1 parameter, no type, no name)
fac(0) -> 1; % If 0, then return 1, otherwise (note the semicolon ; meaning 'else')
fac(N) when N > 0, is_integer(N) -> N * fac(N-1).
% Recursively determine, then return the result
% (note the period . meaning 'endif' or 'function end')
A sorting algorithm (similar to quicksort):
%% qsort:qsort(List)
%% Sort a list of items
-module(qsort). % This is the file 'qsort.erl'
-export([qsort/1]). % A function 'qsort' with 1 parameter is exported (no type, no name)
qsort([]) -> []; % If the list [] is empty, return an empty list (nothing to sort)
qsort([Pivot|Rest]) ->
% Compose recursively a list with 'Front' for all elements that should be before 'Pivot'
% then 'Pivot' then 'Back' for all elements that should be after 'Pivot'
qsort([Front || Front <- Rest, Front < Pivot])
++ [Pivot] ++
qsort([Back || Back <- Rest, Back >= Pivot]).
The above example recursively invokes the function qsort until nothing remains to be sorted. The expression [Front || Front <- Rest, Front < Pivot] is a list comprehension, meaning “Construct a list of elements Front such that Front is a member of Rest, and Front is less than Pivot.” ++ is the list concatenation operator.
A comparison function can be used for more complicated structures for the sake of readability.
The following code would sort lists according to length:
% This is file 'listsort.erl' (the compiler is made this way)
-module(listsort).
% Export 'by_length' with 1 parameter (don't care of the type and name)
-export([by_length/1]).
by_length(Lists) -> % Use 'qsort/2' and provides an anonymous function as a parameter
qsort(Lists, fun(A,B) -> A < B end).
qsort([], _)-> []; % If list is empty, return an empty list (ignore the second parameter)
qsort([Pivot|Rest], Smaller) ->
% Partition list with 'Smaller' elements in front of 'Pivot' and not-'Smaller' elements
% after 'Pivot' and sort the sublists.
qsort([X || X <- Rest, Smaller(X,Pivot)], Smaller)
++ [Pivot] ++
qsort([Y || Y <- Rest, not(Smaller(Y, Pivot))], Smaller).
Here again, a Pivot is taken from the first parameter given to qsort and the rest of Lists is named Rest. Note that the expression
[X || X <- Rest, Smaller(X,Pivot)]
is no different in form from
[Front || Front <- Rest, Front < Pivot]
(in the previous example) except for the use of a comparison function in the last part, saying “Construct a list of elements X such that X is a member of Rest, and Smaller is true", with Smaller being defined earlier as
In programming language theory, an anonymous function is a function defined, and possibly called, without being bound to an identifier. Anonymous functions are convenient to pass as an argument to a higher-order function and are ubiquitous in languages with first-class functions such as Haskell...
is named Smaller in the parameter list of the second definition of qsort so that it can be referenced by that name within that function. It is not named in the first definition of qsort, which deals with the base case of an empty list and thus has no need of this function, let alone a name for it.
Data structures
Erlang has eight primitive data types:
Integers: integers are written as sequences of decimal digits, for example, 12, 12375 and -23427 are integers. Integer arithmetic is exact and only limited by available memory on the machine. (This is called Arbitrary-precision arithmetic
Arbitrary-precision arithmetic
In computer science, arbitrary-precision arithmetic indicates that calculations are performed on numbers whose digits of precision are limited only by the available memory of the host system. This contrasts with the faster fixed-precision arithmetic found in most ALU hardware, which typically...
.)
Atoms: atoms are used within a program to denote distinguished values. They are written as strings of consecutive alphanumeric characters, the first character being a small letter. Atoms can contain 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 use the IEEE 754 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
representation. (Range: ±10308.)
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-efficient way of storing binary data. Erlang primitives exist for composing and decomposing binaries and for efficient input/output of binaries.
Pids: Pid is short for Process Identifier—a Pid is created by the Erlang primitive spawn(...) Pids are references to Erlang processes.
Ports: ports are used to communicate with the external world. Ports are created with the built-in function
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
(BIF) open_port. Messages can be sent to and received from ports, but these messages must obey the so-called "port protocol."
Funs : Funs are function closures. Funs are created by expressions of the form: fun(...) -> ... end.
And two compound data types:
Tuples : tuples are containers for a fixed number of Erlang data types. The syntax {D1,D2,...,Dn} denotes a tuple whose arguments 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.
Syntactic sugar is a computer science term that refers to syntax within a programming language that is designed to make things easier to read or to express....
are provided:
Strings : strings are written as doubly quoted lists of characters, this is syntactic sugar for a list of the integer ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
codes for the characters in the string, thus for example, the string "cat" is shorthand for [99,97,116]. It has partial support for unicode strings
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.
It has no possibility of defining classes althought it has some external libraries for doing that
In computer science, concurrency is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other...
. It has a small but powerful set of primitives to create processes and communicate among them. Processes are the primary means to structure an Erlang application. Erlang processes loosely follow the communicating sequential processes
Communicating sequential processes
In computer science, Communicating Sequential Processes is a formal language for describing patterns of interaction in concurrent systems. It is a member of the family of mathematical theories of concurrency known as process algebras, or process calculi...
(CSP) model. They are neither operating system processes
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
, but lightweight processes somewhat similar to Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
In computer programming, green threads are threads that are scheduled by a virtual machine instead of natively by the underlying operating system...
”. Like operating system processes (and unlike green threads and operating system threads) they have no shared state between them. The estimated minimal overhead for each is 300 words; thus many of them can be created without degrading performance: a benchmark with 20 million processes has been successfully performed. Erlang has supported symmetric multiprocessing
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
Asynchronous method dispatch is a data communication method used when there is a need for the server side to handle a large number of long lasting client requests...
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
system: every process has a “mailbox”, a queue of messages that have been sent by other processes and not yet consumed. A process uses the receive primitive to retrieve messages that match desired patterns. A message-handling routine tests messages in turn against each pattern, until one of them matches. When the message is consumed and removed from the mailbox the process resumes execution. A message may comprise any Erlang structure, including primitives (integers, floats, characters, atoms), tuples, lists, and functions.
The code example below shows the built-in support for distributed processes:
% Create a process and invoke the function web:start_server(Port, MaxConnections)
ServerProcess = spawn(web, start_server, [Port, MaxConnections]),
% Create a remote process and invoke the function
% web:start_server(Port, MaxConnections) on machine RemoteNode
RemoteProcess = spawn(RemoteNode, web, start_server, [Port, MaxConnections]),
% Send a message to ServerProcess (asynchronously). The message consists of a tuple
% with the atom "pause" and the number "10".
ServerProcess ! {pause, 10},
% Receive messages sent to this process
receive
a_message -> do_something;
{data, DataContent} -> handle(DataContent);
{hello, Text} -> io:format("Got hello message: ~s", [Text]);
{goodbye, Text} -> io:format("Got goodbye message: ~s", [Text])
end.
As the example shows, processes may be created on remote nodes, and communication with them is transparent in the sense that communication with remote processes works exactly as communication with local processes.
Concurrency supports the primary method of error-handling in Erlang. When a process crashes, it neatly exits and sends a message to the controlling process which can take action. This way of error handling increases maintainability and reduces complexity of code.
Implementation
The Ericsson Erlang implementation loads virtual machine bytecode
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...
In computer science, the term threaded code refers to a compiler implementation technique where the generated code has a form that essentially consists entirely of calls to subroutines...
at load time. It also includes a native code compiler on most platforms, developed by the High Performance Erlang Project (HiPE) at Uppsala University
Uppsala University
Uppsala University is a research university in Uppsala, Sweden, and is the oldest university in Scandinavia, founded in 1477. It consistently ranks among the best universities in Northern Europe in international rankings and is generally considered one of the most prestigious institutions of...
. Since October 2001 the HiPE system is fully integrated in Ericsson's Open Source Erlang/OTP system. It also supports interpreting, directly from source code via abstract syntax tree
Abstract syntax tree
In computer science, an abstract syntax tree , or just syntax tree, is a tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code. The syntax is 'abstract' in the sense that it...
, via script as of R11B-5.
Hot code loading and modules
Code is loaded and managed as "module" units; the module is a compilation unit. The system can keep two versions of a module in memory at the same time, and processes can concurrently run code from each. The versions are referred to as the "new" and the "old" version. A process will not move into the new version until it makes an external call to its module.
An example of the mechanism of hot code loading:
%% A process whose only job is to keep a counter.
%% First version
-module(counter).
-export([start/0, codeswitch/1]).
start -> loop(0).
loop(Sum) ->
receive
{increment, Count} ->
loop(Sum+Count);
{counter, Pid} ->
Pid ! {counter, Sum},
loop(Sum);
code_switch ->
?MODULE:codeswitch(Sum)
% Force the use of 'codeswitch/1' from the latest MODULE version
end.
codeswitch(Sum) -> loop(Sum).
For the second version, we add the possibility to reset the count to zero.
%% Second version
-module(counter).
-export([start/0, codeswitch/1]).
Only when receiving a message consisting of the atom 'code_switch' will the loop execute an external call to codeswitch/1 (?MODULE is a preprocessor macro for the current module). If there is a new version of the "counter" module in memory, then its codeswitch/1 function will be called. The practice of having a specific entry-point into a new version allows the programmer to transform state to what is required in the newer version. In our example we keep the state as an integer.
In practice, systems are built up using design principles from the Open Telecom Platform which leads to more code upgradable designs. Successful hot code loading is a tricky subject; code needs to be written to make use of Erlang's facilities.
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
to ensure its independence from a single vendor and to increase awareness of the language. Erlang, together with libraries and the real-time distributed database Mnesia
Mnesia
Mnesia is a distributed, soft real-time database management system written in the Erlang programming language.- Purpose of Mnesia :As with Erlang, Mnesia was developed by Ericsson for soft real-time distributed and high-availability computing work related to telecoms. It was not intended as a...
, forms the Open Telecom Platform (OTP) collection of libraries. Ericsson and a few other companies offer commercial support for Erlang.
Since the open source release, Erlang has been used by several firms worldwide, including Nortel and T-Mobile
T-Mobile
T-Mobile International AG is a German-based holding company for Deutsche Telekom AG's various mobile communications subsidiaries outside Germany. Based in Bonn, Germany, its subsidiaries operate GSM and UMTS-based cellular networks in Europe, the United States, Puerto Rico and the US Virgin Islands...
. Although Erlang was designed to fill a niche and has remained an obscure language for most of its existence, its popularity is growing due to demand for concurrent services.
Erlang has found some use in fielding MMORPG
MMORPG
Massively multiplayer online role-playing game is a genre of role-playing video games in which a very large number of players interact with one another within a virtual game world....
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
Apache CouchDB, commonly referred to as CouchDB, is an open source document-oriented database written mostly in the Erlang programming language. It is part of the NoSQL group of data stores and is designed for local replication and to scale horizontally across a wide range of devices...
MapReduce is a software framework introduced by Google in 2004 to support distributed computing on large data sets on clusters of computers. Parts of the framework are patented in some countries....
Membase is an Open Source distributed, key-value database management system optimized for storing data behind interactive web applications. These applications must service many concurrent users; creating, storing, retrieving, aggregating, manipulating and presenting data...
, database management system optimized for storing data behind interactive web applications.
Riak is a NoSQL database implementing the principles from Amazon's Dynamo paper.Riak has a pluggable backend for its core shard-partitioned storage, with the default storage backend being Bitcask as of the 0.12 release...
Amazon Web Services is a collection of remote computing services that together make up a cloud computing platform, offered over the Internet by Amazon.com...
ejabberd is an XMPP application server, written mainly in the Erlang programming language.It can run under several Unix-like operating systems such as Mac OS X, GNU/Linux, FreeBSD, NetBSD, OpenBSD and OpenSolaris...
Extensible Messaging and Presence Protocol is an open-standard communications protocol for message-oriented middleware based on XML . The protocol was originally named Jabber, and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging , presence...
Zotonic is an Open Source CMS and Web Framework built in Erlang and released under the Apache 2.0 license. The CMS part of Zotonic is designed to be user friendly and hides development tasks from the user. One of Zotonic's primary goals is speed, and it claims to be up to ten times faster than PHP...
RabbitMQ is an open source message broker software , using the standard Advanced Message Queuing Protocol . The RabbitMQ server is written in Erlang and is built on the Open Telecom Platform framework for clustering and failover. Rabbit Technologies Ltd., acquired in April 2010 by VMware, develops...
The Advanced Message Queuing Protocol is an open standard application layer protocol for message-oriented middleware. The defining features of AMQP are message orientation, queuing, routing , reliability and security.AMQP mandates the behaviour of the messaging provider and client to the extent...
Wings 3D is a free, open source, subdivision modeler inspired by Nendo and Mirai from Izware. Wings 3D is named after the winged-edge data structure it uses internally to store coordinate and adjacency data, and is commonly referred to by its users simply as Wings.Wings 3D is available for most...
WhatsApp Messenger is a proprietary, cross-platform instant messaging application for smartphones. In addition to basic messaging WhatsApp Messenger users can send each other images, video and audio media messages....
GitHub is a web-based hosting service for software development projects that use the Git revision control system. GitHub offers both commercial plans and free accounts for open source projects...
egitd, a replacement for stock git-daemon that ships with Git
Git (software)
Git is a distributed revision control system with an emphasis on speed. Git was initially designed and developed by Linus Torvalds for Linux kernel development. Every Git working directory is a full-fledged repository with complete history and full revision tracking capabilities, not dependent on...
Issuu is an online service that allows for realistic and customizable viewing of digitally uploaded material, such as portfolios, books, magazine issues, newspapers, and other print media. It integrates with social networking sites to promote uploaded material. Issuu's service is comparable to what...
Twitterfall is a UK-based website designed to allow users of the social networking site Twitter to view upcoming trends and patterns posted by users in the form of tweets...
, a service to view trends and patterns from Twitter
Twitter
Twitter is an online social networking and microblogging service that enables its users to send and read text-based posts of up to 140 characters, informally known as "tweets".Twitter was created in March 2006 by Jack Dorsey and launched that July...
The Goldman Sachs Group, Inc. is an American multinational bulge bracket investment banking and securities firm that engages in global investment banking, securities, investment management, and other financial services primarily with institutional clients...
Erlang may refer to:* Agner Krarup Erlang , a mathematician and engineer after whom several concepts are named** Erlang , a unit to measure traffic in telecommunications or other domains...
High-frequency trading is the use of sophisticated technological tools to trade securities like stocks or options, and is typically characterized by several distinguishing features:...
programs.
Clones
Erlang has inspired clones of its concurrency facilities for other languages:
Reia is a general-purpose concurrent object-oriented programming language for the Erlang virtual machine.Reia supports multiple programming paradigms including imperative, functional, declarative, object oriented, and concurrent. It uses the actor model for concurrency in a manner that works...
