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POErlang/OTP behaviors for beginner
 

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    <p>As I understood from the "Erlang and OTP in action" book, the word behavior refers to:</p> <ul> <li>the behaviour interface, which is a set of functions;</li> <li>the behaviour implementation, which is the application-specific code (a callback module);</li> <li>the behaviour container, which is a process.</li> </ul> <p><strong>Question:</strong></p> <p><em>What an Erlang/OTP beginner should know about behaviours? Is it possible to describe and understand the notion of OTP behaviour in a nutshell?</em></p> <p><em>What 'callback function' does actually mean in the context of Elang/OTP?</em></p> <p>Can we consider the callbacks in a behaviour implemenation as methods overriden in Java?</p> <p>The book says that the associated callback function for the library function 'gen_server:start_link/4' in the following code is 'Module:init/1'.</p> <p>Does that mean that with init/1 we call the gen_server:start_link/4 library function? Or does that mean anything else?</p> <pre><code>-module(tr_server). -behaviour(gen_server). -include_lib("eunit/include/eunit.hrl"). %% API -export([ start_link/1, start_link/0, get_count/0, stop/0 ]). %% gen_server callbacks -export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]). -define(SERVER, ?MODULE). -define(DEFAULT_PORT, 1055). -record(state, {port, lsock, request_count = 0}). %%%=================================================================== %%% API %%%=================================================================== %%-------------------------------------------------------------------- %% @doc Starts the server. %% %% @spec start_link(Port::integer()) -&gt; {ok, Pid} %% where %% Pid = pid() %% @end %%-------------------------------------------------------------------- start_link(Port) -&gt; gen_server:start_link({local, ?SERVER}, ?MODULE, [Port], []). %% @spec start_link() -&gt; {ok, Pid} %% @doc Calls `start_link(Port)' using the default port. s tart_link() -&gt; start_link(?DEFAULT_PORT). %%-------------------------------------------------------------------- %% @doc Fetches the number of requests made to this server. %% @spec get_count() -&gt; {ok, Count} %% where %% Count = integer() %% @end %%-------------------------------------------------------------------- get_count() -&gt; gen_server:call(?SERVER, get_count). %%-------------------------------------------------------------------- %% @doc Stops the server. %% @spec stop() -&gt; ok %% @end %%-------------------------------------------------------------------- stop() -&gt; gen_server:cast(?SERVER, stop). %%%=================================================================== %%% gen_server callbacks %%%=================================================================== init([Port]) -&gt; {ok, LSock} = gen_tcp:listen(Port, [{active, true}]), {ok, #state{port = Port, lsock = LSock}, 0}. handle_call(get_count, _From, State) -&gt; {reply, {ok, State#state.request_count}, State}. handle_cast(stop, State) -&gt; {stop, normal, State}. handle_info({tcp, Socket, RawData}, State) -&gt; do_rpc(Socket, RawData), RequestCount = State#state.request_count, {noreply, State#state{request_count = RequestCount + 1}}; handle_info(timeout, #state{lsock = LSock} = State) -&gt; {ok, _Sock} = gen_tcp:accept(LSock), {noreply, State}. terminate(_Reason, _State) -&gt; ok. code_change(_OldVsn, State, _Extra) -&gt; {ok, State}. %%%=================================================================== %%% Internal functions %%%=================================================================== do_rpc(Socket, RawData) -&gt; try {M, F, A} = split_out_mfa(RawData), Result = apply(M, F, A), gen_tcp:send(Socket, io_lib:fwrite("~p~n", [Result])) catch _Class:Err -&gt; gen_tcp:send(Socket, io_lib:fwrite("~p~n", [Err])) end. split_out_mfa(RawData) -&gt; MFA = re:replace(RawData, "\r\n$", "", [{return, list}]), {match, [M, F, A]} = re:run(MFA, "(.*):(.*)\s*\\((.*)\s*\\)\s*.\s*$", [{capture, [1,2,3], list}, ungreedy]), {list_to_atom(M), list_to_atom(F), args_to_terms(A)}. args_to_terms(RawArgs) -&gt; {ok, Toks, _Line} = erl_scan:string("[" ++ RawArgs ++ "]. ", 1), {ok, Args} = erl_parse:parse_term(Toks), Args. %% test start_test() -&gt; {ok, _} = tr_server:start_link(1055). </code></pre>
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