otp深入浅出之：gen_server

what is the gen_server ?

man 中 gen_server定义是：

引用

A behaviour module for implementing the server of a client-server relation. A generic server process (gen_server) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree.

一个简单的echo gen_server

-module(echo).

-behaviour(gen_server).


%% API
-export([start_link/0]).
-export([echo/1]).
%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
	 terminate/2, code_change/3]).

-record(state, {}).

echo(Msg)->
	gen_server:call(?MODULE,{echo , Msg} ).
start_link() ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
init([]) ->
    {ok, #state{}}.

handle_call({echo , Msg}, _From, State) ->
	{reply , Msg , State };
handle_call(_Request, _From, State) ->
    Reply = ok,
    {reply, Reply, State}.
%%其他自动生成部分省略

echo gen_server代码的主干可以由模板生成，具体请参google。


运行并测试echo gen_server

         echo:start_link(),
         echo:echo("hello world").

how to start a echo gen_server ?


如上图，在主进程中的调用顺序为:echo:start_link->gen_server:start_link->gen:start->proc_lib:start_link

proc_lib：start_link中通过erlang:spawn_link创建echo进程，新进程中依次回调
gen:init_it->gen_server:init_it->echo:init，从而注入echo自己的初始化逻辑。

在gen_server：init_it中分析回调echo:init的返回值，初始化成功以后则进入循环。

init_it(Starter, Parent, Name, Mod, Args, Options) ->
    Debug = debug_options(Name, Options),
    case catch Mod:init(Args) of
        {ok, State} ->
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(Parent, Name, State, Mod, infinity, Debug);
        {ok, State, Timeout} ->
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(Parent, Name, State, Mod, Timeout, Debug);
        {stop, Reason} ->
            proc_lib:init_ack(Starter, {error, Reason}),
            exit(Reason);
        ignore ->
            proc_lib:init_ack(Starter, ignore),
            exit(normal);
        {'EXIT', Reason} ->
            proc_lib:init_ack(Starter, {error, Reason}),
            exit(Reason);
        Else ->
            Error = {bad_return_value, Else},
            proc_lib:init_ack(Starter, {error, Error}),
            exit(Error)
    end.

how to deal with a call ?

     gen_server:call({local,echo},｛echo,"hello,world"}).

gen_server:call调用gen:call发起一个同步的消息请求，使用标签"$gen_call"作为表示，表示这是应用的消息，不是系统的消息。

call(Name, Request) ->
    case catch gen:call(Name, '$gen_call', Request) of
        {ok,Res} ->
            Res;
        {'EXIT',Reason} ->
            exit({Reason, {?MODULE, call, [Name, Request]}})
    end.

在主循环gen_server:loop中从消息队列中依次取出消息并处理，注意这是串行的， 在一条消息处理完毕之前，不会处理下一条消息。

loop(Parent, Name, State, Mod, Time, Debug) ->
    Msg = receive
              Input ->
                    Input
          after Time ->
                  timeout
          end,
    decode_msg(Msg, Parent, Name, State, Mod, Time, Debug, false).

gen_server:decode_msg区分不同消息类型，分别处理

decode_msg(Msg, Parent, Name, State, Mod, Time, Debug, Hib) ->
    case Msg of
        {system, From, Req} ->
            sys:handle_system_msg(Req, From, Parent, ?MODULE, Debug,
                                  [Name, State, Mod, Time], Hib);
        {'EXIT', Parent, Reason} ->
            terminate(Reason, Name, Msg, Mod, State, Debug);
        _Msg when Debug =:= [] ->
            handle_msg(Msg, Parent, Name, State, Mod);
        _Msg ->
            Debug1 = sys:handle_debug(Debug, {?MODULE, print_event},
                                      Name, {in, Msg}),
            handle_msg(Msg, Parent, Name, State, Mod, Debug1)
    end.

gen_server:handle_msg回调echo:handle_call处理应用信息，处理成功则返回loop方法。

handle_msg({'$gen_call', From, Msg}, Parent, Name, State, Mod) ->
    case catch Mod:handle_call(Msg, From, State) of
        {reply, Reply, NState} ->
            reply(From, Reply),
            loop(Parent, Name, NState, Mod, infinity, []);
        {reply, Reply, NState, Time1} ->
            reply(From, Reply),
            loop(Parent, Name, NState, Mod, Time1, []);
        {noreply, NState} ->
            loop(Parent, Name, NState, Mod, infinity, []);
        {noreply, NState, Time1} ->
            loop(Parent, Name, NState, Mod, Time1, []);
        {stop, Reason, Reply, NState} ->
            {'EXIT', R} =
                (catch terminate(Reason, Name, Msg, Mod, NState, [])),
            reply(From, Reply),
            exit(R);
        Other -> handle_common_reply(Other, Parent, Name, Msg, Mod, State)
    end;

how to deal with a cast ?

how to deal with a info ?


要注意的地方