gen_server入门

1）什么是gen_server？
gen_server是OTP（Open Telecom Platform）的一个组件，OTP是Erlang的应用程序框架，gen_server定义了自己的一套规范，用来写Erlang服务器程序
gen_server manual: http://www.erlang.org/doc/man/gen_server.html

2）使用gen_server程序的三个步骤：
1，为callback module起个名字
2，写接口function
3，在callback module里写6个必需的callback function

3）behaviour
关键字-behaviour供编译器使用，如果我们的gen_server程序没有定义合适的callback function则编译时会出错误和警告

4）gen_server模板

%%%-------------------------------------------------------------------
%%% File    : gen_server_template.full
%%% Author  : my name <yourname@localhost.localdomain>
%%% Description : 
%%%
%%% Created :  2 Mar 2007 by my name <yourname@localhost.localdomain>
%%%-------------------------------------------------------------------
-module().

-behaviour(gen_server).

%% API
-export([start_link/0]).

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

-record(state, {}).

%%====================================================================
%% API
%%====================================================================
%%--------------------------------------------------------------------
%% Function: start_link() -> {ok,Pid} | ignore | {error,Error}
%% Description: Starts the server
%%--------------------------------------------------------------------
start_link() ->
    gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).

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

%%--------------------------------------------------------------------
%% Function: init(Args) -> {ok, State} |
%%                         {ok, State, Timeout} |
%%                         ignore               |
%%                         {stop, Reason}
%% Description: Initiates the server
%%--------------------------------------------------------------------
init([]) ->
    {ok, #state{}}.

%%--------------------------------------------------------------------
%% Function: %% handle_call(Request, From, State) -> {reply, Reply, State} |
%%                                      {reply, Reply, State, Timeout} |
%%                                      {noreply, State} |
%%                                      {noreply, State, Timeout} |
%%                                      {stop, Reason, Reply, State} |
%%                                      {stop, Reason, State}
%% Description: Handling call messages
%%--------------------------------------------------------------------
handle_call(_Request, _From, State) ->
    Reply = ok,
    {reply, Reply, State}.

%%--------------------------------------------------------------------
%% Function: handle_cast(Msg, State) -> {noreply, State} |
%%                                      {noreply, State, Timeout} |
%%                                      {stop, Reason, State}
%% Description: Handling cast messages
%%--------------------------------------------------------------------
handle_cast(_Msg, State) ->
    {noreply, State}.

%%--------------------------------------------------------------------
%% Function: handle_info(Info, State) -> {noreply, State} |
%%                                       {noreply, State, Timeout} |
%%                                       {stop, Reason, State}
%% Description: Handling all non call/cast messages
%%--------------------------------------------------------------------
handle_info(_Info, State) ->
    {noreply, State}.

%%--------------------------------------------------------------------
%% Function: terminate(Reason, State) -> void()
%% Description: This function is called by a gen_server when it is about to
%% terminate. It should be the opposite of Module:init/1 and do any necessary
%% cleaning up. When it returns, the gen_server terminates with Reason.
%% The return value is ignored.
%%--------------------------------------------------------------------
terminate(_Reason, _State) ->
    ok.

%%--------------------------------------------------------------------
%% Func: code_change(OldVsn, State, Extra) -> {ok, NewState}
%% Description: Convert process state when code is changed
%%--------------------------------------------------------------------
code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

%%--------------------------------------------------------------------
%%% Internal functions
%%--------------------------------------------------------------------

gen_server:start_link(Name, Mod, InitArgs, Opts)创建一个名为Name的server，callback moudle为Mod
Mod:init(InitArgs)启动server
client端程序调用gen_server:call(Name, Request)来调用server，server处理逻辑为handle_call/3
gen_server:cast(Name, Name)调用callback handle_cast(_Msg, State)以改变server状态
handle_info(_Info, State)用来处理发给server的自发消息
terminate(_Reason, State)是server关闭时的callback
code_change是server热部署或代码升级时做callback修改进程状态

5）my_bank例子

%% ---
%%  Excerpted from "Programming Erlang",
%%  published by The Pragmatic Bookshelf.
%%  Copyrights apply to this code. It may not be used to create training material, 
%%  courses, books, articles, and the like. Contact us if you are in doubt.
%%  We make no guarantees that this code is fit for any purpose. 
%%  Visit http://www.pragmaticprogrammer.com/titles/jaerlang for more book information.
%%---
-module(my_bank).

-behaviour(gen_server).
-export([start/0]).
%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
	 terminate/2, code_change/3]).
-compile(export_all).


start() -> gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
stop()  -> gen_server:call(?MODULE, stop).

new_account(Who)      -> gen_server:call(?MODULE, {new, Who}).
deposit(Who, Amount)  -> gen_server:call(?MODULE, {add, Who, Amount}).
withdraw(Who, Amount) -> gen_server:call(?MODULE, {remove, Who, Amount}).



init([]) -> {ok, ets:new(?MODULE,[])}.

handle_call({new,Who}, _From, Tab) ->
    Reply = case ets:lookup(Tab, Who) of
		[]  -> ets:insert(Tab, {Who,0}), 
		       {welcome, Who};
		[_] -> {Who, you_already_are_a_customer}
	    end,
    {reply, Reply, Tab};
handle_call({add,Who,X}, _From, Tab) ->
    Reply = case ets:lookup(Tab, Who) of
		[]  -> not_a_customer;
		[{Who,Balance}] ->
		    NewBalance = Balance + X,
		    ets:insert(Tab, {Who, NewBalance}),
		    {thanks, Who, your_balance_is,  NewBalance}	
	    end,
    {reply, Reply, Tab};
handle_call({remove,Who, X}, _From, Tab) ->
    Reply = case ets:lookup(Tab, Who) of
		[]  -> not_a_customer;
		[{Who,Balance}] when X =< Balance ->
		    NewBalance = Balance - X,
		    ets:insert(Tab, {Who, NewBalance}),
		    {thanks, Who, your_balance_is,  NewBalance};	
		[{Who,Balance}] ->
		    {sorry,Who,you_only_have,Balance,in_the_bank}
	    end,
    {reply, Reply, Tab};
handle_call(stop, _From, Tab) ->
    {stop, normal, stopped, Tab}.

handle_cast(_Msg, State) -> {noreply, State}.
handle_info(_Info, State) -> {noreply, State}.
terminate(_Reason, _State) -> ok.
code_change(_OldVsn, State, Extra) -> {ok, State}.

6）编译运行my_bank:

Eshell > c(my_bank).
Eshell > my_bank:start().
Eshell > my_bank:new_account("hideto").
Eshell > my_bank:deposit("hideto", 100).
Eshell > my_bank:deposit("hideto", 200).
Eshell > my_bank:withdraw("hideto", 10).
Eshell > my_bank:withdraw("hideto", 10000).

gen_server可以在多个node上做分布计算，而ets（Erlang term storage）这种内存式BigTable可以负责高性能数据存取，所以gen_server+ets特别适合FreeWheel的forecasting模块:)