1.概述

最近有同學在學習Kafka的網路通訊這塊內容時遇到一些疑問，關於網路模型和通訊流程的相關內容，這裡筆者將通過這篇部落格為大家來剖析一下這部分內容。

2.內容

Kafka系統作為一個Message Queue，涉及到的網路通訊主要包含以下兩個方面：

• Pull：Consumer從訊息佇列中拉取訊息資料；
• Push：Producer往訊息佇列中推送訊息資料。

要實現高效能的網路通訊，可以使用更加底層的TCP協議或者UDP協議來實現。Kafka在Producer、Broker、Consumer之間設計了一套基於TCP層的通訊協議，這套協議完全是為了Kafka系統自身需求而定製實現的。

提示：
這裡需要注意的是，由於UDP協議是一種不可靠的傳輸協議，所以Kafka系統採用TCP協議作為服務間的通訊協議。

2.1 基本資料型別

通訊協議中的基本資料型別分為以下幾種：

• 定長資料型別：例如，int8、int16、int32和、int64，對應到Java語言中，分別是byte、short、int和long
• 可變資料型別：例如，Java語言中Map、List等
• 陣列：例如，Java語言中的int[]、String[]等

2.2 通訊模型

Kafka系統採用的是Reactor多執行緒模型，即通過一個Acceptor執行緒處理所有的新連線，通過多個Processor執行緒對請求進行處理（比如解析協議、封裝請求、、轉發等）。

提示：
Reactor是一種事件模型，可以將請求提交到一個或者多個服務程式中進行處理。
當收到Client的請求後，Server處理程式使用多路分發策略，由一個非阻塞的執行緒來接收所有的請求，然後將這些請求轉發到對應的工作執行緒中進行處理。

之後，在Kafka的版本迭代中，新增了一個Handler模組，它通過指定的執行緒數對請求進行處理。Handler和Processor之間通過一個Block Queue進行連線。如下圖所示：

這裡 Acceptor是一個繼承於AbstractServerThread的執行緒類，Acceptor的主要目的是監聽並且接收Client的請求，同時，建立資料傳輸通道（SocketChannel），然後通過輪詢的方式交給一個Processor處理。其核心程式碼在Acceptor的run方法中，程式碼如下：

def run() {
    serverChannel.register(nioSelector, SelectionKey.OP_ACCEPT)
    startupComplete()
    try {
      var currentProcessor = 0
      while (isRunning) {
        try {
          val ready = nioSelector.select(500)
          if (ready > 0) {
            val keys = nioSelector.selectedKeys()
            val iter = keys.iterator()
            while (iter.hasNext && isRunning) {
              try {
                val key = iter.next
                iter.remove()
                if (key.isAcceptable)
                  accept(key, processors(currentProcessor))
                else
                  throw new IllegalStateException("Unrecognized key state for acceptor thread.")

                // round robin to the next processor thread
                currentProcessor = (currentProcessor + 1) % processors.length
              } catch {
                case e: Throwable => error("Error while accepting connection", e)
              }
            }
          }
        }
        catch {
          // We catch all the throwables to prevent the acceptor thread from exiting on exceptions due
          // to a select operation on a specific channel or a bad request. We don't want
          // the broker to stop responding to requests from other clients in these scenarios.
          case e: ControlThrowable => throw e
          case e: Throwable => error("Error occurred", e)
        }
      }
    } finally {
      debug("Closing server socket and selector.")
      swallowError(serverChannel.close())
      swallowError(nioSelector.close())
      shutdownComplete()
    }
  }

這裡還有一個塊通道（BlockingChannel），用於連線Processor和Handler，其程式碼如下所示：

class BlockingChannel( val host: String, 
                       val port: Int, 
                       val readBufferSize: Int, 
                       val writeBufferSize: Int, 
                       val readTimeoutMs: Int ) extends Logging {
  private var connected = false
  private var channel: SocketChannel = null
  private var readChannel: ReadableByteChannel = null
  private var writeChannel: GatheringByteChannel = null
  private val lock = new Object()
  private val connectTimeoutMs = readTimeoutMs
  private var connectionId: String = ""

  def connect() = lock synchronized  {
    if(!connected) {
      try {
        channel = SocketChannel.open()
        if(readBufferSize > 0)
          channel.socket.setReceiveBufferSize(readBufferSize)
        if(writeBufferSize > 0)
          channel.socket.setSendBufferSize(writeBufferSize)
        channel.configureBlocking(true)
        channel.socket.setSoTimeout(readTimeoutMs)
        channel.socket.setKeepAlive(true)
        channel.socket.setTcpNoDelay(true)
        channel.socket.connect(new InetSocketAddress(host, port), connectTimeoutMs)

        writeChannel = channel
        // Need to create a new ReadableByteChannel from input stream because SocketChannel doesn't implement read with timeout
        // See: http://stackoverflow.com/questions/2866557/timeout-for-socketchannel-doesnt-work
        readChannel = Channels.newChannel(channel.socket().getInputStream)
        connected = true
        val localHost = channel.socket.getLocalAddress.getHostAddress
        val localPort = channel.socket.getLocalPort
        val remoteHost = channel.socket.getInetAddress.getHostAddress
        val remotePort = channel.socket.getPort
        connectionId = localHost + ":" + localPort + "-" + remoteHost + ":" + remotePort
        // settings may not match what we requested above
        val msg = "Created socket with SO_TIMEOUT = %d (requested %d), SO_RCVBUF = %d (requested %d), SO_SNDBUF = %d (requested %d), connectTimeoutMs = %d."
        debug(msg.format(channel.socket.getSoTimeout,
                         readTimeoutMs,
                         channel.socket.getReceiveBufferSize, 
                         readBufferSize,
                         channel.socket.getSendBufferSize,
                         writeBufferSize,
                         connectTimeoutMs))

      } catch {
        case _: Throwable => disconnect()
      }
    }
  }
  
  def disconnect() = lock synchronized {
    if(channel != null) {
      swallow(channel.close())
      swallow(channel.socket.close())
      channel = null
      writeChannel = null
    }
    // closing the main socket channel *should* close the read channel
    // but let's do it to be sure.
    if(readChannel != null) {
      swallow(readChannel.close())
      readChannel = null
    }
    connected = false
  }

  def isConnected = connected

  def send(request: RequestOrResponse): Long = {
    if(!connected)
      throw new ClosedChannelException()

    val send = new RequestOrResponseSend(connectionId, request)
    send.writeCompletely(writeChannel)
  }
  
  def receive(): NetworkReceive = {
    if(!connected)
      throw new ClosedChannelException()

    val response = readCompletely(readChannel)
    response.payload().rewind()

    response
  }

  private def readCompletely(channel: ReadableByteChannel): NetworkReceive = {
    val response = new NetworkReceive
    while (!response.complete())
      response.readFromReadableChannel(channel)
    response
  }

}

3.通訊過程

Kafka系統的通訊框架也是經過了不同的版本迭代的。例如，在Kafka老的版本中，以NIO作為網路通訊的基礎，通過將多個Socket連線註冊到一個Selector上進行監聽，只用一個執行緒就能管理多個連線，這極大的節省了多執行緒的資源開銷。

在Kafka之後的新版本中，依然以NIO作為網路通訊的基礎，也使用了Reactor多執行緒模型，不同的是，新版本將具體的業務處理模組（Handler模組）獨立出去了，並用單獨的執行緒池進行控制。如下圖所示：

 通過上圖，我們可以總結一下Kafka的通訊流程：

• Client向Server傳送請求時，Acceptor負責接收TCP請求，連線成功後傳遞給Processor執行緒；
• Processor執行緒接收到新的連線後，將其註冊到自身的Selector中，並監聽READ事件
• 當Client在當前連線物件上寫入資料時，會觸發READ事件，根據TCP協議呼叫Handler進行處理
• Handler處理完成後，可能會有返回值給Client，並將Handler返回的結果繫結Response端進行傳送

通過總結和分析，我們可以知道Kafka新版中獨立Handler模組，用這樣以下幾點優勢：

• 能夠單獨指定Handler的執行緒數，便於調優和管理
• 防止一個過大的請求阻塞一個Processor執行緒
• Request、Handler、Response之間都是通過佇列來進行連線的，這樣它們彼此之間不存在耦合現象，對提升Kafka系統的效能很有幫助

這裡需要注意的是，在Kafka的網路通訊中，RequestChannel為Processor執行緒與Handler執行緒之間資料交換提供了一個緩衝區，是通訊中Request和Response快取的地方。因此，其作用就是在通訊中起到了一個數據緩衝佇列的作用。Processor執行緒將讀取到的請求新增至RequestChannel的全域性佇列（requestQueue）中，Handler執行緒從請求佇列中獲取並處理，處理完成後將Response新增至RequestChannel的響應佇列（responseQueues）中，通過responseListeners喚醒對應的Processor執行緒，最後Processor執行緒從響應佇列中取出後傳送到Client。實現程式碼如下：

class RequestChannel(val numProcessors: Int, val queueSize: Int) extends KafkaMetricsGroup {
  private var responseListeners: List[(Int) => Unit] = Nil
  private val requestQueue = new ArrayBlockingQueue[RequestChannel.Request](queueSize)
  private val responseQueues = new Array[BlockingQueue[RequestChannel.Response]](numProcessors)
  for(i <- 0 until numProcessors)
    responseQueues(i) = new LinkedBlockingQueue[RequestChannel.Response]()

  newGauge(
    "RequestQueueSize",
    new Gauge[Int] {
      def value = requestQueue.size
    }
  )

  newGauge("ResponseQueueSize", new Gauge[Int]{
    def value = responseQueues.foldLeft(0) {(total, q) => total + q.size()}
  })

  for (i <- 0 until numProcessors) {
    newGauge("ResponseQueueSize",
      new Gauge[Int] {
        def value = responseQueues(i).size()
      },
      Map("processor" -> i.toString)
    )
  }

  /** Send a request to be handled, potentially blocking until there is room in the queue for the request */
  def sendRequest(request: RequestChannel.Request) {
    requestQueue.put(request)
  }

  /** Send a response back to the socket server to be sent over the network */
  def sendResponse(response: RequestChannel.Response) {
    responseQueues(response.processor).put(response)
    for(onResponse <- responseListeners)
      onResponse(response.processor)
  }

  /** No operation to take for the request, need to read more over the network */
  def noOperation(processor: Int, request: RequestChannel.Request) {
    responseQueues(processor).put(RequestChannel.Response(processor, request, null, RequestChannel.NoOpAction))
    for(onResponse <- responseListeners)
      onResponse(processor)
  }

  /** Close the connection for the request */
  def closeConnection(processor: Int, request: RequestChannel.Request) {
    responseQueues(processor).put(RequestChannel.Response(processor, request, null, RequestChannel.CloseConnectionAction))
    for(onResponse <- responseListeners)
      onResponse(processor)
  }

  /** Get the next request or block until specified time has elapsed */
  def receiveRequest(timeout: Long): RequestChannel.Request =
    requestQueue.poll(timeout, TimeUnit.MILLISECONDS)

  /** Get the next request or block until there is one */
  def receiveRequest(): RequestChannel.Request =
    requestQueue.take()

  /** Get a response for the given processor if there is one */
  def receiveResponse(processor: Int): RequestChannel.Response = {
    val response = responseQueues(processor).poll()
    if (response != null)
      response.request.responseDequeueTimeMs = Time.SYSTEM.milliseconds
    response
  }

  def addResponseListener(onResponse: Int => Unit) {
    responseListeners ::= onResponse
  }

  def shutdown() {
    requestQueue.clear()
  }
}

4.總結

通過認真閱讀和分析Kafka的網路通訊層程式碼，可以收穫不少關於NIO的網路通訊知識。通過對Kafka的原始碼進行閱讀和學習，這對大規模Kafka叢集效能的調優和問題定位排查是很有幫助的。

5.結束語

這篇部落格就和大家分享到這裡，如果大家在研究學習的過程當中有什麼問題，可以加群進行討論或傳送郵件給我，我會盡我所能為您解答，與君共勉！

另外，博主出書了《Kafka並不難學》和《Hadoop大資料探勘從入門到進階實戰》，喜歡的朋友或同學， 可以在公告欄那裡點選購買連結購買博主的書進行學習，在此感謝大家的支援。關注下面公眾號，根據提示，可免費獲取書籍的教學視訊。&n