Overview

The OTP design principles define how to structure Erlang code in terms of processes, modules, and directories.

Behaviours

In a supervision tree, many of the processes have similar structures, they follow similar patterns. For example, the supervisors are similar in structure. The only difference between them is which child processes they supervise. Many of the workers are servers in a server-client relation, finite-state machines, or event handlers such as error loggers.

Behaviours are formalizations of these common patterns. The idea is to divide the code for a process in a generic part (a behaviour module) and a specific part (a callback module).

The behaviour module is part of Erlang/OTP. To implement a process such as a supervisor, the user only has to implement the callback module which is to export a pre-defined set of functions, the callback functions.

The following example illustrate how code can be divided into a generic and a specific part. Consider the following code (written in plain Erlang) for a simple server, which keeps track of a number of "channels". Other processes can allocate and free the channels by calling the functions alloc/0 and free/1, respectively.


-module(ch1).
-export([start/0]).
-export([alloc/0, free/1]).
-export([init/0]).

start() ->
    spawn(ch1, init, []).

alloc() ->
    ch1 ! {self(), alloc},
    receive
        {ch1, Res} ->
            Res
    end.

free(Ch) ->
    ch1 ! {free, Ch},
    ok.

init() ->
    register(ch1, self()),
    Chs = channels(),
    loop(Chs).

loop(Chs) ->
    receive
        {From, alloc} ->
            {Ch, Chs2} = alloc(Chs),
            From ! {ch1, Ch},
            loop(Chs2);
        {free, Ch} ->
            Chs2 = free(Ch, Chs),
            loop(Chs2)
    end.

The code for the server can be rewritten into a generic part server.erl:


-module(server).
-export([start/1]).
-export([call/2, cast/2]).
-export([init/1]).

start(Mod) ->
    spawn(server, init, [Mod]).

call(Name, Req) ->
    Name ! {call, self(), Req},
    receive
        {Name, Res} ->
            Res
    end.

cast(Name, Req) ->
    Name ! {cast, Req},
    ok.

init(Mod) ->
    register(Mod, self()),
    State = Mod:init(),
    loop(Mod, State).

loop(Mod, State) ->
    receive
        {call, From, Req} ->
            {Res, State2} = Mod:handle_call(Req, State),
            From ! {Mod, Res},
            loop(Mod, State2);
        {cast, Req} ->
            State2 = Mod:handle_cast(Req, State),
            loop(Mod, State2)
    end.

And a callback module ch2.erl:


-module(ch2).
-export([start/0]).
-export([alloc/0, free/1]).
-export([init/0, handle_call/2, handle_cast/2]).

start() ->
    server:start(ch2).

alloc() ->
    server:call(ch2, alloc).

free(Ch) ->
    server:cast(ch2, {free, Ch}).

init() ->
    channels().

handle_call(alloc, Chs) ->
    alloc(Chs). % => {Ch,Chs2}

handle_cast({free, Ch}, Chs) ->
    free(Ch, Chs). % => Chs2

Notice the following:

The code in server can be reused to build many different servers. The server name, in this example the atom ch2, is hidden from the users of the client functions. This means that the name can be changed without affecting them. The protocol (messages sent to and received from the server) is also hidden. This is good programming practice and allows one to change the protocol without changing the code using the interface functions. The functionality of server can be extended without having to change ch2 or any other callback module.

In ch1.erl and ch2.erl above, the implementation of channels/0, alloc/1, and free/2 has been intentionally left out, as it is not relevant to the example. For completeness, one way to write these functions are given below. This is an example only, a realistic implementation must be able to handle situations like running out of channels to allocate, and so on.


channels() ->
   {_Allocated = [], _Free = lists:seq(1,100)}.

alloc({Allocated, [H|T] = _Free}) ->
   {H, {[H|Allocated], T}}.

free(Ch, {Alloc, Free} = Channels) ->
   case lists:member(Ch, Alloc) of
      true ->
         {lists:delete(Ch, Alloc), [Ch|Free]};
      false ->
         Channels
   end.

Code written without using behaviours can be more efficient, but the increased efficiency is at the expense of generality. The ability to manage all applications in the system in a consistent manner is important.

Using behaviours also makes it easier to read and understand code written by other programmers. Improvised programming structures, while possibly more efficient, are always more difficult to understand.

The server module corresponds, greatly simplified, to the Erlang/OTP behaviour gen_server.

The standard Erlang/OTP behaviours are:

gen_server

For implementing the server of a client-server relation

gen_statem

For implementing state machines

gen_event

For implementing event handling functionality

supervisor

For implementing a supervisor in a supervision tree

The compiler understands the module attribute -behaviour(Behaviour) and issues warnings about missing callback functions, for example:


-module(chs3).
-behaviour(gen_server).
...

3> c(chs3).
./chs3.erl:10: Warning: undefined call-back function handle_call/3
{ok,chs3}