nsq消息队列是怎么实现消费者接收消息的？

1.创建消费者

通过c:=nsq.NewConsumer(…)方式创建消费者

2.消费者注册消息监听

(1)给消费者c增加异步处理器handler:
c.AddHandler(nsq.HandlerFunc(func(msg *nsq.Message)))

(2)AddHandler方法中调用AddConcurrentHandlers()

(3)AddConcurrentHandlers()中开启一个goroutine，调用handlerLoop(handler),
handlerLoop中开启无限循环，通过无缓冲通道incomingMessages，阻塞监听最新消息，
获取到消息，传递给回调handler.HandleMessage(message)处理

func (r *Consumer) AddHandler(handler Handler) {
	r.AddConcurrentHandlers(handler, 1)
}

func (r *Consumer) AddConcurrentHandlers(handler Handler, concurrency int) {
	for i := 0; i < concurrency; i++ {
		go r.handlerLoop(handler)
	}
}

func (r *Consumer) handlerLoop(handler Handler) {

	for {
		message, ok := <-r.incomingMessages
		if !ok {
			goto exit
		}

		if r.shouldFailMessage(message, handler) {
			message.Finish()
			continue
		}

		err := handler.HandleMessage(message)
                ...
             }

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3.连接NSQLookupd

调用c.ConnectToNSQLookupd(v),该方法中会调用queryLookupd()->ConnectToNSQD()

func (r *Consumer) ConnectToNSQLookupd(addr string) error {
	
	...
	if numLookupd == 1 {
		r.queryLookupd()
		r.wg.Add(1)
		go r.lookupdLoop()
	}

	return nil
}

func (r *Consumer) queryLookupd() {
	..
	for _, addr := range nsqdAddrs {
		err = r.ConnectToNSQD(addr)
		...
	}
}
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在ConnectToNSQD()中我们可以看到，会通过conn.Connect()建立跟nsqd的TCP连接

func (r *Consumer) ConnectToNSQD(addr string) error {
	...
	resp, err := conn.Connect()
	...
	
}
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在Connect会开启一个goroutine，在readLoop方法中无限循环的监听消息的到来

func (c *Conn) Connect() (*IdentifyResponse, error) {
	dialer := &net.Dialer{
		LocalAddr: c.config.LocalAddr,
		Timeout:   c.config.DialTimeout,
	}

	conn, err := dialer.Dial("tcp", c.addr)
	if err != nil {
		return nil, err
	}
	c.conn = conn.(*net.TCPConn)
	...
	go c.readLoop()
	go c.writeLoop()
	return resp, nil
}
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当收到消息后，交给c.delegate.OnMessage()方法处理，在该方法中，会把消息发送给无缓冲消息通道incomingMessages，这样就完成了整个接收消息的逻辑.

func (c *Conn) readLoop() {
	delegate := &connMessageDelegate{c}
	for {
		...
		frameType, data, err := ReadUnpackedResponse(c)

		switch frameType {
		...
		case FrameTypeMessage:
			msg, err := DecodeMessage(data)
			...
			c.delegate.OnMessage(c, msg)
                        ...
                   }
		...
}

func (r *Consumer) onConnMessage(c *Conn, msg *Message) {
	...
	r.incomingMessages <- msg
	...
}
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