【转】Erlang 中的 half-sync/half-async 和 Leader/Followers 模式
原文地址：avindev.javaeye.com
原文作者：AvinDev
http://www.javaeye.com/article/60414 里面，谈到了半同步/半异步和领导者/追随者模式，在Erlang里面可以很简单得实现它
下面看看 half-sync/half-async 的例子
代码
- start() ->
- case gen_tcp:listen(80, [binary,
- {nodelay,true},
- {packet, 0},
- {reuseaddr, true},
- {active, false}]) of
- {ok, Listen} ->
- io:format(“Listning port 80 @ ~p~n”, [self()]),
- accept_loop(Listen);
- Error -> io:format(“Error occur: ~p~n”, [Error])
- end.
- accept_loop(Listen) ->
- case gen_tcp:accept(Listen) of
- {ok, Socket} ->
- io:format(“Socket ~p connected~n”, [Socket]),
- spawn(?MODULE, handler, [Socket]),
- accept_loop(Listen);
- Other -> exit(oops)
- end.
- handler(Socket) ->
- io:format(“Waiting incoming message @ ~p~n”, [self()]),
- inet:setopts(Socket, [{nodelay,true}, {active, false}]),
- case gen_tcp:recv(Socket, 0) of
- {ok, Packet} ->
- io:format(“Receive msg ~p~n”, [binary_to_list(Packet)]),
- gen_tcp:send(Socket, Packet),
- gen_tcp:close(Socket),
- handler(Socket);
- {error, Reason} ->
- io:format(“Socket ~p error ~p~n”, [Socket, Reason])
- end.
在许多“网络编程”的教程里面，都会给出类似这样的例子。首先当前线程先监听一个端口，然后accept外来Socket连接，并将Socket传 递到新的线程中处理。在C、Java这样的语言中，创建一个线程的开销是很大的，一般来说，会使用线程池来处理短连接，一些Web Server就是基于这样的模式。然而在Erlang里面，创建一个Proces的开销极少，进程间切换完全在用户态实现，这样减少了用户态和核心态间切 换的开销，使得这种模式成为了可能。
接下来是Leader/Followers模式
代码
- start2() ->
- case gen_tcp:listen(81, [binary,
- {nodelay,true},
- {packet, 0},
- {reuseaddr, true},
- {active, false}]) of
- {ok, Listen} ->
- handler2(Listen);
- Error -> io:format(“Error occur: ~p~n”, [Error])
- end.
- handler2(Listen) ->
- io:format(“Listning port 81 @ ~p~n”, [self()]),
- case gen_tcp:accept(Listen) of
- {ok, Socket} ->
- io:format(“Socket ~p connected~n”, [Socket]),
- inet:setopts(Socket, [{nodelay,true}, {active, true}]),
- spawn(?MODULE, handler2, [Listen]),
- io:format(“Waiting incoming message @ ~p~n”, [self()]),
- receive
- {tcp, Socket, Bin} ->
- io:format(“Receive msg ~p~n”, [binary_to_list(Bin)]),
- gen_tcp:send(Socket, Bin),
- gen_tcp:close(Socket);
- {tcp_closed, Socket} ->
- io:format(“Socket ~p closed ~n”, [Socket]);
- Any ->
- io:format(“~p~n”, [Any])
- end;
- Other ->
- io:format(“~p~n”, [Other]),
- exit(oops)
- end.
当一个Socket连接建立后，就会将监听gen_tcp:accept/1的调用权转移到新的Process当中进行处理。
在这里，Leader Process 设置了这样一个参数
inet:setopts(Socket, [{nodelay,true}, {active, true}]),
在gen_tcp模块的文档中，有一段如下的说明
引用
Packets can be sent to the returned socket Socket using send/2. Packets sent from the peer are delivered as messages:
{tcp, Socket, Data}unless {active, false} was specified in the option list for the listen socket, in which case packets are retrieved by calling recv/2.
{tcp, Socket, Data}unless {active, false} was specified in the option list for the listen socket, in which case packets are retrieved by calling recv/2.
当在gen_tcp:listen/2中使用了{active, true}参数，那么当接收到数据的时候，就会主动发送消息到调用gen_tcp:accept/1的Process中，否则需要使用 gen_tcp:recv 来阻塞获取。通过这样的方式，就可以灵活实现同步/异步接收Socket数据。
Erlang网络编程还有其他有趣的地方，接下来的笔记我会写写这方面的内容。
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