Ejabberd网络结构
ejabberd作为otp应用，与其相关的套接字模块为
ejabberd_receiver
ejabberd_socket
ejabberd_listener
ejabberd是分为两步来完成此任务的：
1、ejabberd_listener首先根据配置文件，把相应的端口打开（处于监听状态），但是未接受连接
配置文件（可知监听三个端口）：
[{{5222,{0,0,0,0},tcp},
ejabberd_c2s,
[{access,c2s},{shaper,c2s_shaper},{max_stanza_size,65536}]},
{{5269,{0,0,0,0},tcp},ejabberd_s2s_in,[]},
{{5280,{0,0,0,0},tcp},
ejabberd_http,
[{captcha,true},
{http_bind,true},
{web_admin,true},
{request_handlers,[{<<"/websocket">>,ejabberd_http_ws}]}]}]
相应的代码为：
Sup = ejabberd_sup:start_link(), Listener = {ejabberd_listener, {ejabberd_listener, start_link, []}, permanent, infinity, supervisor, [ejabberd_listener]}, start_link() -> supervisor:start_link({local, ejabberd_listeners}, ?MODULE, []). init(_) -> ets:new(listen_sockets, [named_table, public]), bind_tcp_ports(), {ok, {{one_for_one, 10, 1}, []}}. bind_tcp_ports() -> case ejabberd_config:get_option(listen, fun validate_cfg/1) of undefined -> ignore; Ls -> lists:foreach( fun({Port, Module, Opts}) -> ModuleRaw = strip_frontend(Module), case ModuleRaw:socket_type() of independent -> ok; _ -> bind_tcp_port(Port, Module, Opts) end end, Ls) end. bind_tcp_port(PortIP, Module, RawOpts) -> try check_listener_options(RawOpts) of ok -> {Port, IPT, IPS, IPV, Proto, OptsClean} = parse_listener_portip(PortIP, RawOpts), {_Opts, SockOpts} = prepare_opts(IPT, IPV, OptsClean), case Proto of udp -> ok; _ -> ListenSocket = listen_tcp(PortIP, Module, SockOpts, Port, IPS), ets:insert(listen_sockets, {PortIP, ListenSocket}), ok end catch throw:{error, Error} -> ?ERROR_MSG(Error, []) end. listen_tcp(PortIP, Module, SockOpts, Port, IPS) -> case ets:lookup(listen_sockets, PortIP) of [{PortIP, ListenSocket}] -> ?INFO_MSG("Reusing listening port for ~p", [PortIP]), ets:delete(listen_sockets, PortIP), ListenSocket; _ -> Res = gen_tcp:listen(Port, [binary, {packet, 0}, {active, false}, {reuseaddr, true}, {nodelay, true}, {send_timeout, ?TCP_SEND_TIMEOUT}, {send_timeout_close, true}, {keepalive, true} | SockOpts]), case Res of {ok, ListenSocket} -> ListenSocket; {error, Reason} -> socket_error(Reason, PortIP, Module, SockOpts, Port, IPS) end end.
2、在ejabberd启动的最后开始接受连接
ejabberd_listener:start_listeners(), start_listeners() -> case ejabberd_config:get_option(listen, fun validate_cfg/1) of undefined -> ignore; Ls -> Ls2 = lists:map( fun({Port, Module, Opts}) -> case start_listener(Port, Module, Opts) of {ok, _Pid} = R -> R; {error, Error} -> throw(Error) end end, Ls), report_duplicated_portips(Ls), {ok, {{one_for_one, 10, 1}, Ls2}} end. start_listener(Port, Module, Opts) -> case start_listener2(Port, Module, Opts) of {ok, _Pid} = R -> R; {error, {{'EXIT', {undef, [{M, _F, _A}|_]}}, _} = Error} -> ?ERROR_MSG("Error starting the ejabberd listener: ~p.~n" "It could not be loaded or is not an ejabberd listener.~n" "Error: ~p~n", [Module, Error]), {error, {module_not_available, M}}; {error, {already_started, Pid}} -> {ok, Pid}; {error, Error} -> {error, Error} end. start_listener2(Port, Module, Opts) -> %% It is only required to start the supervisor in some cases. %% But it doesn't hurt to attempt to start it for any listener. %% So, it's normal (and harmless) that in most cases this call returns: {error, {already_started, pid()}} maybe_start_sip(Module), start_module_sup(Port, Module), start_listener_sup(Port, Module, Opts). start_module_sup(_Port, Module) -> Proc1 = gen_mod:get_module_proc(<<"sup">>, Module), ChildSpec1 = {Proc1, {ejabberd_tmp_sup, start_link, [Proc1, strip_frontend(Module)]}, permanent, infinity, supervisor, [ejabberd_tmp_sup]}, supervisor:start_child(ejabberd_sup, ChildSpec1). start_listener_sup(Port, Module, Opts) -> ChildSpec = {Port, {?MODULE, start, [Port, Module, Opts]}, transient, brutal_kill, worker, [?MODULE]}, supervisor:start_child(ejabberd_listeners, ChildSpec). start(Port, Module, Opts) -> %% Check if the module is an ejabberd listener or an independent listener ModuleRaw = strip_frontend(Module), case ModuleRaw:socket_type() of independent -> ModuleRaw:start_listener(Port, Opts); _ -> start_dependent(Port, Module, Opts) end. %% @spec(Port, Module, Opts) -> {ok, Pid} | {error, ErrorMessage} start_dependent(Port, Module, Opts) -> try check_listener_options(Opts) of ok -> proc_lib:start_link(?MODULE, init, [Port, Module, Opts]) catch throw:{error, Error} -> ?ERROR_MSG(Error, []), {error, Error} end. init(PortIP, Module, RawOpts) -> {Port, IPT, IPS, IPV, Proto, OptsClean} = parse_listener_portip(PortIP, RawOpts), {Opts, SockOpts} = prepare_opts(IPT, IPV, OptsClean), if Proto == udp -> init_udp(PortIP, Module, Opts, SockOpts, Port, IPS); true -> init_tcp(PortIP, Module, Opts, SockOpts, Port, IPS) end. init_tcp(PortIP, Module, Opts, SockOpts, Port, IPS) -> ListenSocket = listen_tcp(PortIP, Module, SockOpts, Port, IPS), %% Inform my parent that this port was opened succesfully proc_lib:init_ack({ok, self()}), case erlang:function_exported(Module, tcp_init, 2) of false -> accept(ListenSocket, Module, Opts); true -> case catch Module:tcp_init(ListenSocket, Opts) of {'EXIT', _} = Err -> ?ERROR_MSG("failed to process callback function " "~p:~s(~p, ~p): ~p", [Module, tcp_init, ListenSocket, Opts, Err]), accept(ListenSocket, Module, Opts); NewOpts -> accept(ListenSocket, Module, NewOpts) end end.
由此可知针对每一个要开启的server，系统分别开启了两个进程：
一个作为ejabberd_listeners的子进程，打开端口，接受外部的连接 start_listener_sup/3（最后走到init_tcp，利用accept接受连接）
一个作为supervisor开启，管理上面进程派生的子进程start_module_sup/2，因为start_listener_sup接受到一个连接后，就会开启一个进程处理接受到的连接，原进程继续处于监听状态
当一个外部连接到来时：
accept根据CallMod选取对应的模块：
例如 ejabberd_socket，进入到ejabberd_socket:start(strip_frontend(Module), gen_tcp, Socket, Opts)
ejabberd_socket为每个接受到的连接生成两个进程（以ejabberd_c2s为例）：
1、ejabberd_receiver处理接受到的XML Stanza信息（SockMod:controlling_process(Socket, Receiver)），并转发给业务逻辑模块
2、开启ejabberd_c2s（Module:start({?MODULE, SocketData}, Opts)），业务逻辑模块handler，handler决定如何处理接受到的信息，并可能使用ejabberd_socket发送响应数据
最后如果想扩展ejabberd使用其他的非xmpp协议：
则需要自定义receiver作为解析协议模块，handler作为业务逻辑模块
