gen_server入门
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 <[email protected]> %%% Description : %%% %%% Created : 2 Mar 2007 by my name <[email protected]> %%%------------------------------------------------------------------- -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).