nsq消息的生产与消费

Topic与Channel

Topic与Channel是NSQ中重要的两个概念。
生产者将消息写到Topic中，一个Topic下可以有多个Channel，每个Channel都是Topic的完整副本。
消费者从Channel处订阅消息，如果有多个消费者订阅同一个Channel，Channel中的消息将被传递到一个随机的消费者。

代码只抽取部分关键内容进行解析，省略一些非核心代码。
Topic相关函数

type Topic struct {
    //一个Topic可以拥有多个Channel
    channelMap        map[string]*Channel
}

// Topic constructor
func NewTopic(topicName string, ctx *context, deleteCallback func(*Topic)) *Topic {
    t := &Topic{
        name:              topicName,
        channelMap:        make(map[string]*Channel),
        memoryMsgChan:     make(chan *Message, ctx.nsqd.getOpts().MemQueueSize),

    }
    //开启一个goroutine负责监听写到该topic的Msg
    t.waitGroup.Wrap(t.messagePump)

    return t
}
// messagePump selects over the in-memory and backend queue and
// writes messages to every channel for this topic
func (t *Topic) messagePump() {
    var msg *Message
    var buf []byte
    var err error
    var chans []*Channel
    var memoryMsgChan chan *Message
    var backendChan chan []byte

    t.RLock()
    //取出Topic的所有Channel
    for _, c := range t.channelMap {
        chans = append(chans, c)
    }
    t.RUnlock()
    if len(chans) > 0 && !t.IsPaused() {
        memoryMsgChan = t.memoryMsgChan
        backendChan = t.backend.ReadChan()
    }

    // main message loop
    for {
        select {
        //从memoryMsgChan读取一个Msg
        case msg = <-memoryMsgChan:
        case <-t.exitChan:
            goto exit
        }
        //对每个Channel写入Msg
        for i, channel := range chans {
            chanMsg := msg
            // copy the message because each channel
            // needs a unique instance but...
            // fastpath to avoid copy if its the first channel
            // (the topic already created the first copy)
            if i > 0 {
                chanMsg = NewMessage(msg.ID, msg.Body)
                chanMsg.Timestamp = msg.Timestamp
                chanMsg.deferred = msg.deferred
            }
            if chanMsg.deferred != 0 {
                channel.PutMessageDeferred(chanMsg, chanMsg.deferred)
                continue
            }
            //把Msg传入channel的PutMessage
            err := channel.PutMessage(chanMsg)
            if err != nil {
                t.ctx.nsqd.logf(LOG_ERROR,
                    "TOPIC(%s) ERROR: failed to put msg(%s) to channel(%s) - %s",
                    t.name, msg.ID, channel.name, err)
            }
        }
    }

exit:
    t.ctx.nsqd.logf(LOG_INFO, "TOPIC(%s): closing ... messagePump", t.name)
}

总结上面部分，nsq很好的利用golang的特性-goroutine之间通过chan来进行通信，如果要对一个Topic写入Msg，只要往memoryMsgChan写入Msg。
每个Topic在NewTopic()创建之初都会开启一个goroutine负责监听该Topic的memoryMsgChan，一旦有消息就有复制N份Msg写入下面的Channel。
这里可以看到，通过select的方式来读取消息，NSQ的消息是无序的。

消息的产生

消息是由client主动进行pub指令进行发布的，我们看到主要的IOLoop()函数里，Exec会包含所有指令对应的函数。pub是生产者产生消息的指令，sub是消费者订阅消息的指令。

func (p *protocolV2) IOLoop(conn net.Conn) error {
    // ...
    response, err = p.Exec(client, params)
    // ...
}

func (p *protocolV2) Exec(client *clientV2, params [][]byte) ([]byte, error) {
    // ...
    case bytes.Equal(params[0], []byte("PUB")):
        return p.PUB(client, params)
    case bytes.Equal(params[0], []byte("SUB")):
        return p.SUB(client, params)
    // ...
}

func (p *protocolV2) PUB(client *clientV2, params [][]byte) ([]byte, error) {
    // ...
    topic := p.ctx.nsqd.GetTopic(topicName)
    msg := NewMessage(topic.GenerateID(), messageBody)
    err = topic.PutMessage(msg)
    // ...
}

func (t *Topic) PutMessage(m *Message) error {
    //  ...
    err := t.put(m)
    //  ...
    return nil
}

func (t *Topic) put(m *Message) error {
    select {
    case t.memoryMsgChan <- m:
    default:
        b := bufferPoolGet()
        err := writeMessageToBackend(b, m, t.backend)
        bufferPoolPut(b)
        t.ctx.nsqd.SetHealth(err)
        if err != nil {
            t.ctx.nsqd.logf(LOG_ERROR,
                "TOPIC(%s) ERROR: failed to write message to backend - %s",
                t.name, err)
            return err
        }
    }
    return nil
}

PUB函数中，client会将Msg放入对应Topic的memoryMsgChan中，如果被阻塞，将会写入Backend中，Backend是磁盘存储，然后就是上一节所说的每个Topic一直有一个goroutine将Msg复制分发到所有的Channel中，Channel中的Msg等待client消费者去获取。

消息的分发

消费者发送sub指令后，订阅指定topic下的指定channel，获得SubEventChan。每个client连接后，IOLoop还会开启一个messagePump去把channel中的Msg发送给client。

func (p *protocolV2) SUB(client *clientV2, params [][]byte) ([]byte, error) {
    var channel *Channel
    for {
        topic := p.ctx.nsqd.GetTopic(topicName)
        // 获取指定Topic下的指定channel，如果没有则创建
        channel = topic.GetChannel(channelName)
        channel.AddClient(client.ID, client)

        if (channel.ephemeral && channel.Exiting()) || (topic.ephemeral && topic.Exiting()) {
            channel.RemoveClient(client.ID)
            time.Sleep(1 * time.Millisecond)
            continue
        }
        break
    }
    atomic.StoreInt32(&client.State, stateSubscribed)
    client.Channel = channel
    // 把这个channel传给消费者的SubEventChan，该值会在IOLoop的第二个goroutine的messagePump中读取
    client.SubEventChan <- channel        
}

func (p *protocolV2) IOLoop(conn net.Conn) error {
    clientID := atomic.AddInt64(&p.ctx.nsqd.clientIDSequence, 1)
    client := newClientV2(clientID, conn, p.ctx)
    p.ctx.nsqd.AddClient(client.ID, client)
    
    // synchronize the startup of messagePump in order
    // to guarantee that it gets a chance to initialize
    // goroutine local state derived from client attributes
    // and avoid a potential race with IDENTIFY (where a client
    // could have changed or disabled said attributes)
    // 相当于标识，下面会阻塞该channel来保证goroutine的初始化的完成
    messagePumpStartedChan := make(chan bool)
    // 如果client订阅了topic，messagePump会将Msg最终分发到client
    go p.messagePump(client, messagePumpStartedChan)
    <-messagePumpStartedChan
    
｝

func (p *protocolV2) messagePump(client *clientV2, startedChan chan bool) {
    var err error
    var memoryMsgChan chan *Message
    var backendMsgChan chan []byte
    var subChannel *Channel

    subEventChan := client.SubEventChan
    identifyEventChan := client.IdentifyEventChan
    outputBufferTicker := time.NewTicker(client.OutputBufferTimeout)
    heartbeatTicker := time.NewTicker(client.HeartbeatInterval)
    heartbeatChan := heartbeatTicker.C
    msgTimeout := client.MsgTimeout

    // signal to the goroutine that started the messagePump
    // that we've started up
    close(startedChan)

    for {
        if subChannel == nil || !client.IsReadyForMessages() {
            // ...
        } else {
            // we're buffered (if there isn't any more data we should flush)...
            // select on the flusher ticker channel, too
            memoryMsgChan = subChannel.memoryMsgChan
            backendMsgChan = subChannel.backend.ReadChan()
            flusherChan = outputBufferTicker.C
        }

        select {
        case <-client.ReadyStateChan:
        // sub订阅之后可以获取subChannel
        case subChannel = <-subEventChan:
            // 每个client只能订阅一个channel
            subEventChan = nil
        case <-heartbeatChan:
            err = p.Send(client, frameTypeResponse, heartbeatBytes)
            if err != nil {
                goto exit
            }
        case msg := <-memoryMsgChan:
            if sampleRate > 0 && rand.Int31n(100) > sampleRate {
                continue
            }
            msg.Attempts++
            // inflight 队列用来实现“至少投递一次消息”
            subChannel.StartInFlightTimeout(msg, client.ID, msgTimeout)
            client.SendingMessage()
            err = p.SendMessage(client, msg)
            if err != nil {
                goto exit
            }
            flushed = false
        case <-client.ExitChan:
            goto exit
        }
    }

}

整理整个流程，Client连接的是Channel，Topic在接收到消息后会分发到左右的Channel，如果多个Client连接同一个Channel，那么从实现上来看，每个消息在由Channel分发到Client的时候实现了负载均衡。每个消息在多个Client中，只会有一个接收到。这么回头看上一篇的消息传递图，就很明了了。

image.png