Commentary on “A History of Erlang”
Some time ago I found a fascinating paper on the history of Erlang, written by one of its main creators:
It showed up on programming.reddit for about a day, and then disappeared right back into the deep waters - which is too bad, because the paper is illuminating.
Erlang is a language with strong built-in support for concurrency based on message passing. As the author describes it:
Erlang was designed for writing concurrent programs that “run forever.” Erlang uses concurrent processes to structure the program. These processes have no shared memory and communicate by asynchronous message passing. Erlang processes are lightweight and belong to the language, not the operating system.
Reading the paper, I had several delightful “aha!” moments, where the details of Erlang design just clicked together. And I don’t mean the mundane issues, like single-assignment variables (hello, Prolog!), or interactively modifiable code (hello, Lisp!). Rather, the history put some architectural bits in perspective.
At a certain level, I always knew that Erlang was designed for programming telephone exchanges. But this paper really conveys the feel for what that means: huge collections of loosely coupled finite-state machines, all running in parallel, doing little bits of protocol validation here and there, but mostly staying dormant.
Typically, the software for call control is modeled using finite state machines that undergo state transitions in response to protocol messages. From the software point of view, the system behaves as a very large collection of parallel processes. At any point in time, most of the processes are waiting for an event caused by the reception of a message or the triggering of a timer. When an event occurs, the process does a small amount of computation, changes state, possibly sends messages to other processes and then waits for the next event. The amount of computation involved is very small.
Which definitely explains a number of their implementation choices which, without this motivation, look highly unusual. Let me highlight just three of them.