这两天在研究 erlang 如何构建 TCP 服务器，看到一篇文章，基于Erlang OTP构建一个TCP服务器，里面讲述了两种混合型Socket的实现方法，着实让人欢欣鼓舞。对比老外写的Building a Non-blocking TCP server using OTP principles，作者写的那个有点简单。本文将结合这两篇文章，继续讨论Erlang/OTP 构建TCP服务器的具体实现，以示例演示如何如何使用标准Erlang/OTP行为创建一个简单的无阻塞的TCP服务器。
TCP Socket模式
主动模式{active, true}，非阻塞方式接收消息，但在系统无法应对超大流量请求时，客户端发送的数据过快，而且超过服务器可以处理的速度，那么，系统就可能会造成消息缓冲区被塞满，出现持续繁忙的流量的极端情况，系统因请求过多而溢出，造成Erlang虚拟机内存不足而崩溃。 被动模式{active, false}，阻塞方式接收消息，底层的TCP缓冲区可用于抑制请求，并拒绝客户端的消息，在接收数据的地方都会调用gen_tcp:recv，造成阻塞（单进程模式下就只能消极等待某一个具体的客户端Socket ，很危险）。需要注意的是，操作系统可能还会做一些缓存允许客户端机器继续发送少量数据，然后才将其阻塞，但这个时候Erlang还没有调用recv函数。 混合型模式（半阻塞，{active, once}），主动套接字仅针对一条消息，在控制进程发送完一个消息数据后，必须显式地调用inet:setopts(Socket, [{active, once}]) 重新激活以便接受下一个消息（在此之前，系统处于阻塞状态）。可见，混合型模式综合了主动模式和被动模式的两者优势，可实现流量控制，防止服务器被过多消息淹没。 所以如果想构建TCP服务器，比较合理的是建立在TCP Socket 混合型模式（半阻塞）基础上。
TCP服务器设计
这个TCP服务器的设计包含了主应用程序 tcp_server_app 和监督者 tcp_server_sup 进程，监督者进程拥有 tcp_server_listener 和 tcp_client_sup 两个子进程。tcp_server_listener 负责处理客户端的连接请求，并通知 tcp_client_sup 启动一个 tcp_server_handler 实例进程来处理一条客户端的请求，然后由该实例进程负责处理服务器与客户端的交互数据。
应用程序和监督行为
为了构建一个 Erlang/OTP 应用程序，我们需要构建一些模块来实现应用程序和监督行为。当应用程序启动时，tcp_server_app:start/2 会调用 tcp_server_sup:start_link/1 来创建主监督进程。该监督进程通过回调 tcp_server_sup:init/1 来实例化子工作进程 tcp_server_listener 和子监督进程 tcp_client_sup。该子监督进程回调 tcp_server_sup:init/1 来实例化负责处理客户端连接的工作进程 tcp_server_handler。
TCP服务器应用程序 （tcp_server_app.erl）
- -module(tcp_server_app).
- -behaviour(application).
- -export([start/2, stop/1]).
- -define(PORT, 2222).
- start(_Type, _Args) ->
- io:format("tcp app start~n"),
- case tcp_server_sup:start_link(?PORT) of
- {ok, Pid} ->
- {ok, Pid};
- Other ->
- {error, Other}
- end.
- stop(_S) ->
- ok.
TCP服务器监督者进程（tcp_server_sup.erl）
- -module(tcp_server_sup).
- -behaviour(supervisor).
- -export([start_link/1, start_child/1]).
- -export([init/1]).
- start_link(Port) ->
- io:format("tcp sup start link~n"),
- supervisor:start_link({local, ?MODULE}, ?MODULE, [Port]).
- start_child(LSock) ->
- io:format("tcp sup start child~n"),
- supervisor:start_child(tcp_client_sup, [LSock]).
- init([tcp_client_sup]) ->
- io:format("tcp sup init client~n"),
- {ok,
- { {simple_one_for_one, 0, 1},
- [
- { tcp_server_handler,
- {tcp_server_handler, start_link, []},
- temporary,
- brutal_kill,
- worker,
- [tcp_server_handler]
- }
- ]
- }
- };
- init([Port]) ->
- io:format("tcp sup init~n"),
- {ok,
- { {one_for_one, 5, 60},
- [
- % client supervisor
- { tcp_client_sup,
- {supervisor, start_link, [{local, tcp_client_sup}, ?MODULE, [tcp_client_sup]]},
- permanent,
- 2000,
- supervisor,
- [tcp_server_listener]
- },
- % tcp listener
- { tcp_server_listener,
- {tcp_server_listener, start_link, [Port]},
- permanent,
- 2000,
- worker,
- [tcp_server_listener]
- }
- ]
- }
- }.
- -module(tcp_server_listener).
- -behaviour(gen_server).
- -export([start_link/1]).
- -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
- terminate/2, code_change/3]).
- -record(state, {lsock}).
- start_link(Port) ->
- io:format("tcp server listener start ~n"),
- gen_server:start_link({local, ?MODULE}, ?MODULE, [Port], []).
- init([Port]) ->
- process_flag(trap_exit, true),
- Opts = [binary, {packet, 0}, {reuseaddr, true},
- {keepalive, true}, {backlog, 30}, {active, false}],
- State =
- case gen_tcp:listen(Port, Opts) of
- {ok, LSock} ->
- start_server_listener(LSock),
- #state{lsock = LSock};
- _Other ->
- throw({error, {could_not_listen_on_port, Port}}),
- #state{}
- end,
- {ok, State}.
- handle_call(_Request, _From, State) ->
- io:format("tcp server listener call ~p~n", [_Request]),
- {reply, ok, State}.
- handle_cast({tcp_accept, Pid}, State) ->
- io:format("tcp server listener cast ~p~n", [tcp_accept]),
- start_server_listener(State, Pid),
- {noreply, State};
- handle_cast(_Msg, State) ->
- io:format("tcp server listener cast ~p~n", [_Msg]),
- {noreply, State}.
- handle_info({'EXIT', Pid, _}, State) ->
- io:format("tcp server listener info exit ~p~n", [Pid]),
- start_server_listener(State, Pid),
- {noreply, State};
- handle_info(_Info, State) ->
- io:format("tcp server listener info ~p~n", [_Info]),
- {noreply, State}.
- terminate(_Reason, _State) ->
- io:format("tcp server listener terminate ~p~n", [_Reason]),
- ok.
- code_change(_OldVsn, State, _Extra) ->
- {ok, State}.
- start_server_listener(State, Pid) ->
- unlink(Pid),
- start_server_listener(State#state.lsock).
- start_server_listener(Lsock) ->
- case tcp_server_sup:start_child(Lsock) of
- {ok, Pid} ->
- link(Pid);
- _Other ->
- do_log
- end.
- -module(tcp_server_handler).
- -behaviour(gen_server).
- -export([start_link/1]).
- -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
- terminate/2, code_change/3]).
- -record(state, {lsock, socket, addr}).
- -define(Timeout, 120*1000).
- start_link(LSock) ->
- io:format("tcp handler start link~n"),
- gen_server:start_link(?MODULE, [LSock], []).
- init([LSock]) ->
- io:format("tcp handler init ~n"),
- inet:setopts(LSock, [{active, once}]),
- gen_server:cast(self(), tcp_accept),
- {ok, #state{lsock = LSock}}.
- handle_call(Msg, _From, State) ->
- io:format("tcp handler call ~p~n", [Msg]),
- {reply, {ok, Msg}, State}.
- handle_cast(tcp_accept, #state{lsock = LSock} = State) ->
- {ok, CSock} = gen_tcp:accept(LSock),
- io:format("tcp handler info accept client ~p~n", [CSock]),
- {ok, {IP, _Port}} = inet:peername(CSock),
- start_server_listener(self()),
- {noreply, State#state{socket=CSock, addr=IP}, ?Timeout};
- handle_cast(stop, State) ->
- {stop, normal, State}.
- handle_info({tcp, Socket, Data}, State) ->
- inet:setopts(Socket, [{active, once}]),
- io:format("tcp handler info ~p got message ~p~n", [self(), Data]),
- ok = gen_tcp:send(Socket, <<Data/binary>>),
- {noreply, State, ?Timeout};
- handle_info({tcp_closed, _Socket}, #state{addr=Addr} = State) ->
- io:format("tcp handler info ~p client ~p disconnected~n", [self(), Addr]),
- {stop, normal, State};
- handle_info(timeout, State) ->
- io:format("tcp handler info ~p client connection timeout~n", [self()]),
- {stop, normal, State};
- handle_info(_Info, State) ->
- io:format("tcp handler info ingore ~p~n", [_Info]),
- {noreply, State}.
- terminate(_Reason, #state{socket=Socket}) ->
- io:format("tcp handler terminate ~p~n", [_Reason]),
- (catch gen_tcp:close(Socket)),
- ok.
- code_change(_OldVsn, State, _Extra) ->
- {ok, State}.
- start_server_listener(Pid) ->
- gen_server:cast(tcp_server_listener, {tcp_accept, Pid}).
- {application,tcp_server,
- [{description,"TCP Server"},
- {vsn,"1.0.0"},
- {modules,[tcp_server,tcp_server_app,tcp_server_handler,
- tcp_server_listener,tcp_server_sup]},
- {registered,[]},
- {mod,{tcp_server_app,[]}},
- {env,[]},
- {applications,[kernel,stdlib]}]}.
编译程序
为应用程序创建如下的目录结构：
- ./tcp_server
- ./tcp_server/ebin/
- ./tcp_server/ebin/tcp_server.app
- ./tcp_server/src/tcp_server_app.erl
- ./tcp_server/src/tcp_server_sup.erl
- ./tcp_server/src/tcp_server_listener.erl
- ./tcp_server/src/tcp_server_handler.erl
Linux:
- $ cd tcp_server/src
- $ for f in tcp*.erl ; do erlc -o ../ebin $f
- cd tcp_server/src
- for %%i in (tcp*.erl) do erlc -o ../ebin %%i
运行程序
1、启动TCP服务器
- erl -pa ebin
- ...
- 1> application:start(tcp_server).
- tcp app start
- tcp sup start link
- tcp sup init
- tcp sup init client
- tcp server listener start
- tcp sup start child
- tcp handler start link
- tcp handler init
- ok
- 2> appmon:start().
- {ok,<0.41.0>}
2、创建一个客户端来请求TCP服务器：
- 3> f(S), {ok,S} = gen_tcp:connect({127,0,0,1},2222,[{packet,0}]).
- {ok,#Port<0.1859>}
- tcp handler info accept client #Port<0.1860>
- tcp server listener cast tcp_accept
- tcp sup start child
- tcp handler start link
- tcp handler init
3、使用该请求向服务端发送消息：
- 4> gen_tcp:send(S,<<"hello">>).
- ok
- tcp handler info <0.53.0> got message <<"hello">>
- 5> f(M), receive M -> M end.
- {tcp,#Port<0.1861>,"hello"}
- 6> gen_tcp:connect({127,0,0,1},2222,[{packet,0}]).
- ...
- 7> gen_tcp:connect({127,0,0,1},2222,[{packet,0}]).
- ...
- 8> gen_tcp:connect({127,0,0,1},2222,[{packet,0}]).
- ...
6、服务端程序写有超时功能，如果2分钟内没操作，连接将自动退出
- 9> tcp handler info <0.39.0> client connection timeout
- 9> tcp handler terminate normal
- 9> tcp handler info <0.52.0> client connection timeout
- 9> tcp handler terminate normal
- 9> tcp handler info <0.54.0> client connection timeout
- 9> tcp handler terminate normal
- 9> tcp handler info <0.56.0> client connection timeout
- 9> tcp handler terminate normal
7、下面我们简单演示一下服务器的监督行为：
- 9> exit(pid(0,58,0),kill).
- tcp server listener info exit <0.58.0>
- true
- tcp sup start child
- tcp handler start link
- tcp handler init
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