一个完整的Erlang应用

这里介绍构建一个完整的Erlang/OTP应用的例子，最后还给出了一个在实际生成环境中，如何启动应用的才能方便运维人员维护和部署的实例。例子摘自 《Manning.Erlang.and.OTP.in.Action》（国内目前已经有中文版发行）。

关于该应用例子中，代码和配置文件中的基本的概念的解释我这里就省略了。

一个完整而简单的基于tcp的rpc 应用为例，一个应用包括：

1、元数据文件 tcp_rpc.app

{application,tcp_rpc,
 [{description,"RPC Server for Erlang and OTP in action"},
  {vsn, "0.1.0"},
  {modules,[tr_app,
            tr_sup,
            tr_server]},
  {registered, [tr_sup]},
  {applications, [kernel, stdlib]},
  {mod, {tr_app, []}}
]}.

2、应用启动模块 tr_app.erl

-module(tr_app).

-behaviour(application).
%% --------------------------------------------------------------------
%% Include files
%% --------------------------------------------------------------------

%% --------------------------------------------------------------------
%% Behavioural exports
%% --------------------------------------------------------------------
-export([
     start/2,
     stop/1
        ]).

%% --------------------------------------------------------------------
%% Internal exports
%% --------------------------------------------------------------------
%% -export([]).

%% ====================================================================!
%% External functions
%% ====================================================================!
%% --------------------------------------------------------------------
%% Func: start/2
%% Returns: {ok, Pid}        |
%%          {ok, Pid, State} |
%%          {error, Reason}
%% --------------------------------------------------------------------

start(_Type, _StartArgs) ->
    case tr_sup:start_link() of
    {ok, Pid} ->
        {ok, Pid};
    Error ->
        {error,Error}
    end.

%% --------------------------------------------------------------------
%% Func: stop/1
%% Returns: any
%% --------------------------------------------------------------------
stop(_State) ->
    ok.

3、顶层监督模块 tr_sup.erl

-module(tr_sup).

-behaviour(supervisor).
-export([start_link/0]).
-export([
     init/1
        ]).
-define(SERVER, ?MODULE).
start_link() ->
    supervisor:start_link({local, ?SERVER}, ?MODULE, []).
init([]) ->
    Server = {tr_server,
              {tr_server,start_link,[]},
             permanent, 2000, worker, [tr_server]},
    {ok,{
         {one_for_one,0,1},
         [Server]
        }
    }.

4、服务器模块 tr_server.erl

%%%-------------------------------------------------------------------
%%% @author  <龙二>
%%% @copyright (C) 2012,
%%% @doc  RPC over TCP server. This module defines a server process that
%%%       listens for incoming TCP connections and allows the user to
%%%       execute RPC commands via that TCP stream.
%%% @end
%%% Created :  2 Jul 2012 by  <emacs>
%%%-------------------------------------------------------------------
-module(tr_server).
-include_lib("eunit/include/eunit.hrl").  %% Eunit单元测试
-behaviour(gen_server).

%% 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()) -> {ok,Pid}
%% where
%%  Pid = pid()
%% @end
%%----------------------------------------------------
start_link(Port) ->
    gen_server:start_link({local,?SERVER},?MODULE,[Port],[]).

%% @spec start_link() -> {ok,Pid}
%% @doc Calls 'start_link(Port)' using the default port
start_link() ->
    start_link(?DEFAULT_PORT).

%%----------------------------------------------------
%% @doc Fetches the number of requests made to this server
%% @spec get_count() -> {ok,Count}
%% where
%%  Count = integer()
%% @end
%%----------------------------------------------------
get_count() ->
    gen_server:call(?SERVER,get_count).

%%----------------------------------------------------
%% @doc Stops the server
%% @spec stop() -> ok
%% @end
%%----------------------------------------------------
stop() ->
    gen_server:cast(?SERVER,stop).

%%%===================================================
%%% gen_server callbacks
%%%===================================================

init([Port]) ->
    {ok,LSock} = gen_tcp:listen(Port,[{active,true}]),
    %% 0 is timeout value
    %% a atom msg 'timeout' will be generate
    %% handle_info/2 will handle this msg immediately when init/1 finished
    {ok,#state{port = Port,lsock = LSock},0}.

handle_call(get_count,_From,State) ->
    {reply,{ok,State#state.request_count},State}.

handle_cast(stop,State) ->
    {stop,normal,State}.

%% handle_info/2: handle tcp,port,timeout msg
handle_info({tcp,Socket,RawData},State) ->
    do_rpc(Socket,RawData),
    RequestCount = State#state.request_count,
    {noreply,State#state{request_count = RequestCount + 1}};
handle_info(timeout,#state{lsock = LSock} = State) ->
    {ok,_Sock} = gen_tcp:accept(LSock),
    {noreply,State}.

terminate(_Reason,_State) ->
    ok.

code_change(_OldVsn,State,_Extra) ->
    {ok,State}.


%%%===================================================
%%% Internal functions
%%%===================================================

do_rpc(Socket,RawData) ->
    try
        {M,F,A} = split_out_mfa(RawData),
        Request = apply(M,F,A),
        gen_tcp:send(Socket,io_lib:fwrite("~p~n",[Request]))
    catch
        _Class:Err ->
            gen_tcp:send(Socket,io_lib:fwrite("~p~n",[Err]))
    end.

split_out_mfa(RawData) ->
     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) ->
    {ok,Toks,_Line} = erl_scan:string("[" ++ RawArgs ++ "]. ",1),
    {ok,Args} = erl_parse:parse_term(Toks),
    Args.

5、启动整个应用的开启模块 tr.erl

-module(tr).

-export([start/0]).

%%
%% API Functions
%%
%% 启动应用
start() ->
    case application:start(tcp_rpc) of
        ok ->
            ok;
        Msg ->
            {failur, Msg}
    end.

%% 关闭应用

stop() ->
case application:stop(tcp_rpc) of
ok ->
ok;
Msg ->
{failure, Msg}
end.

6、启动脚本 start_tr.sh (windows下为start_tr.bat)

erl  -pa ebin -name tr@127.0.0.1 -setcookie tr -s tr start
chmod +x start_tr.sh
windows下的话，需要将erl这个程序加入到环境变量，否则就要使用绝对路径来应用erl程序

 

7、编译代码可以通过:

erlc -o ebin src/*.erl

或者利用Emakefile

{ ["src/*"]
    , [
        {outdir, "./ebin"}]
}.

配合 erl -make 来编译这个应用。

8、启动。因为我们写了启动脚本和启动模块，所以不需要进入到erlang shell 里面启动这个应用，可以直接通过执行 start_tr.sh 来启动。

这里给出一个完整又简单的例子，希望可以对刚接触OTP知识的人有帮助。