emqtt 4 (我要publish消息了)

这次，分析处理publish msg的流程。

由protocol开始

publish 类型的packet的处理是：

process(Packet = ?PUBLISH_PACKET(_Qos, Topic, _PacketId, _Payload), State) ->
    %% ACL check
    ...
    publish(Packet, State);
    ...
publish(Packet = ?PUBLISH_PACKET(?QOS_0, _PacketId),
        #proto_state{client_id = ClientId, username = Username, session = Session}) ->
    %% 处理packet 获得msg
    Msg = emqttd_message:from_packet(Username, ClientId, Packet),
    %% 调用emqttd_session module的publish/2 函数
    %% subscribe的时候，也是由protocol 进入的emqttd_session module
    emqttd_session:publish(Session, Msg);

1、ACL 检查

2、处理packet 获得msg

3、调用session module进行处理

emqttd_session 模块处理

和subscribe的处理流程类似，emqttd_session:publish/2 也只是一个接口函数，该函数要根据QoS的不同，决定是

1. 自己调用后续函数完成处理
2. call session process 完成后续处理

%% @doc Publish message
-spec(publish(pid(), mqtt_message()) -> ok | {error, any()}).
publish(_SessPid, Msg = #mqtt_message{qos = ?QOS_0}) ->
    %% publish qos0 directly
    emqttd:publish(Msg);
publish(_SessPid, Msg = #mqtt_message{qos = ?QOS_1}) ->
    %% publish qos1 directly, and client will puback automatically
    emqttd:publish(Msg);
publish(SessPid, Msg = #mqtt_message{qos = ?QOS_2}) ->
    %% publish qos2 by session 
    gen_server2:call(SessPid, {publish, Msg}, ?PUBSUB_TIMEOUT). 

直接处理

如果是自己调用后续函数完成处理的话，则继续调用emqttd:publish/2，则在emqttd module 中继续调用emqttd_server:publish/1：

%% @doc Publish a Message
-spec(publish(Msg :: mqtt_message()) -> any()).
publish(Msg = #mqtt_message{from = From}) ->
    ...
    %% 处理topic
    ...
    %% pulish
    emqttd_pubsub:publish(Topic, Msg2),
    ...

还是subscribe处理套路：

emqttd_protocol ---> emqttd_session ---> emqttd ---> emqttd_server ---> emqttd_pubsub

在emqttd_pubsub module中的处理是：

%% @doc Publish message to Topic.
-spec(publish(binary(), any()) -> any()).
publish(Topic, Msg) ->
    lists:foreach(
        fun(#mqtt_route{topic = To, node = Node}) when Node =:= node() ->
            ?MODULE:dispatch(To, Msg);
           (#mqtt_route{topic = To, node = Node}) ->
            rpc:cast(Node, ?MODULE, dispatch, [To, Msg])
        end, emqttd_router:lookup(Topic)).

dispatch(Topic, Msg) ->
    case subscribers(Topic) of
        [] ->
            dropped(Topic);
        [SubPid] ->
            SubPid ! {dispatch, Topic, Msg};
        SubPids ->
            lists:foreach(fun(SubPid) ->
                SubPid ! {dispatch, Topic, Msg}
            end, SubPids)
    end.
%% @private
%% @doc Find all subscribers
subscribers(Topic) ->
    case ets:member(subscriber, Topic) of
        true -> %% faster then lookup?
            try ets:lookup_element(subscriber, Topic, 2) catch error:badarg -> [] end;
        false ->
            []
    end.

至此，msg 就已经以{dispatch, Topic, Msg}的形式发送给 clientid 对应的session process了。

那么，就需要在emqttd_session module中的handle_info callback 函数处进行处理：

%% Dispatch Message
handle_info({dispatch, Topic, Msg}, Session = #session{subscriptions = Subscriptions})
    when is_record(Msg, mqtt_message) ->
    dispatch(tune_qos(Topic, Msg, Subscriptions), Session);

%% Deliver qos0 message directly to client
dispatch(Msg = #mqtt_message{qos = ?QOS0}, Session = #session{client_pid = ClientPid}) ->
    ClientPid ! {deliver, Msg},
    hibernate(Session);
dispatch(Msg = #mqtt_message{qos = QoS}, Session = #session{message_queue = MsgQ})
    when QoS =:= ?QOS1 orelse QoS =:= ?QOS2 ->
    case check_inflight(Session) of
        true  ->
            noreply(deliver(Msg, Session));
        false ->
            hibernate(Session#session{message_queue = emqttd_mqueue:in(Msg, MsgQ)})
    end.

继而，将信息发送给socket controlling process，然后根据QoS的不同，判断是否需要等待ack。

总结

（流程示意图待补）