practice

Doi: 10.1145/1467247.1467263

Designed for concurrency from the ground
up, the Erlang language can be a valuable tool
to help solve concurrent problems.
BY Jim LaRson
erlang for
concurrent
Programming
Erlang is a language developed to let mere mortals
write, test, deploy, and debug fault-tolerant concurrent
software. Developed at the Swedish telecom company
a

Ericsson in the late 1980s, it started as a platform for developing soft real-time software for managing phone switches. ¹ It has since been open sourced and ported to several common platforms, finding a natural fit not only in distributed Internet server applications, but also in graphical user interfaces and ordinary batch applications.

Erlang’s minimal set of concurrency primitives, together with its rich and well-used libraries, give guidance to anyone trying to design a concurrent program. Erlang provides an effective platform for concurrent programming for the following reasons:

˲ The language, the standard libraries (Open
Telecom Platform, or OTP), and the tools have been
designed from ground up for supporting concurrency.
˲ There are only a few concurrency primitives, so it’s

a See Erlang Web site www.erlang.org.

easy to reason about the behavior of programs (though there are limits to how easy this can ever be).

˲ The implementation makes the simple primitives fast and scalable, and makes effective use of modern multicore hardware, eliminating the need for more complex mechanisms.

˲ The execution model eliminates some classes of errors from unsynchronized access to shared state—or at least makes these errors more noticeable.

˲ The model of concurrency is natural to think about and requires no mathematical sophistication.

˲ The environment makes failures detectable and recoverable, making it possible to deploy a less-than-perfect system in the field that can nonetheless maintain high availability.

˲ The concurrency model maps naturally to distributed deployments.

This article introduces the Erlang language and shows how it can be used in practice to implement concurrent programs correctly and quickly.

sequential erlang

Erlang is built from a small number of sequential programming types and concepts, and an even smaller number of concurrent programming types and concepts. Those who want a full introduction can find several excellent tutorials on the Web, ^b but the following examples (required by functional programming union regulations) should convey the essentials.

As shown in Figure 1A, every file of Erlang code is a module. Declarations within the file name the module (which must match the filename) and declare which functions can be called from other modules. Comments run from the percent sign (%) to the end of the line.

Factorial is implemented by two functions. Both are named factorial, but they have different numbers of arguments; hence, they are distinct. The definition of factorial/2 (the

illustration By andy gilmore

b See Erlang course www.erlang.org/download/ armstrong_thesis_2003.pdf.