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很久没有做技术方面的分享了，今天闲来有空写一篇关于Kafka通信方面的文章与大家共同学习。

一、Kafka通信机制的整体结构

这个图采用的就是我们之前提到的SEDA多线程模型，链接如下：
http://www.jianshu.com/p/e184fdc0ade4
1、对于broker来说，客户端连接数量有限，不会频繁新建大量连接。因此一个Acceptor thread线程处理新建连接绰绰有余。
2、Kafka高吐吞量，则要求broker接收和发送数据必须快速，因此用proccssor thread线程池处理，并把读取客户端数据转交给缓冲区，不会导致客户端请求大量堆积。
3、Kafka磁盘操作比较频繁会且有io阻塞或等待，IO Thread线程数量一般设置为proccssor thread num两倍，可以根据运行环境需要进行调节。

二、SocketServer整体设计时序图

Kafka 通信时序图.jpg

说明:

Kafka SocketServer是基于Java NIO来开发的，采用了Reactor的模式，其中包含了1个Acceptor负责接受客户端请求，N个Processor线程负责读写数据，M个Handler来处理业务逻辑。在Acceptor和Processor，Processor和Handler之间都有队列来缓冲请求。

下面我们就针对以上整体设计思路分开讲解各个不同部分的源代码。

2.1 启动初始化工作

def startup() {
    val quotas = new ConnectionQuotas(maxConnectionsPerIp, maxConnectionsPerIpOverrides)
    for(i <- 0 until numProcessorThreads) {
      processors(i) = new Processor(i, 
                                    time, 
                                    maxRequestSize, 
                                    aggregateIdleMeter,
                                    newMeter("IdlePercent", "percent", TimeUnit.NANOSECONDS, Map("networkProcessor" -> i.toString)),
                                    numProcessorThreads, 
                                    requestChannel,
                                    quotas,
                                    connectionsMaxIdleMs)
      Utils.newThread("kafka-network-thread-%d-%d".format(port, i), processors(i), false).start()
    }

    newGauge("ResponsesBeingSent", new Gauge[Int] {
      def value = processors.foldLeft(0) { (total, p) => total + p.countInterestOps(SelectionKey.OP_WRITE) }
    })

    // register the processor threads for notification of responses
    requestChannel.addResponseListener((id:Int) => processors(id).wakeup())

    // start accepting connections
    this.acceptor = new Acceptor(host, port, processors, sendBufferSize, recvBufferSize, quotas)
    Utils.newThread("kafka-socket-acceptor", acceptor, false).start()
    acceptor.awaitStartup
    info("Started")
  }

说明：

ConnectionQuotas对象负责管理连接数/IP， 创建一个Acceptor侦听者线程，初始化N个Processor线程，processors是一个线程数组，可以作为线程池使用，默认是三个，Acceptor线程和N个Processor线程中每个线程都独立创建Selector.open()多路复用器，相关代码在下面：

val numNetworkThreads = props.getIntInRange("num.network.threads", 3, (1, Int.MaxValue))；

val serverChannel = openServerSocket(host, port)；

范围可以设定从1到Int的最大值。

2.2 Acceptor线程

def run() {
    serverChannel.register(selector, SelectionKey.OP_ACCEPT);
    startupComplete()
    var currentProcessor = 0
    while(isRunning) {
      val ready = selector.select(500)
      if(ready > 0) {
        val keys = selector.selectedKeys()
        val iter = keys.iterator()
        while(iter.hasNext && isRunning) {
          var key: SelectionKey = null
          try {
            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)
          }
        }
      }
    }
    debug("Closing server socket and selector.")
    swallowError(serverChannel.close())
    swallowError(selector.close())
    shutdownComplete()
  }

2.1.1 注册OP_ACCEPT事件

serverChannel.register(selector, SelectionKey.OP_ACCEPT);

2.1.2 内部逻辑

此处采用的是同步非阻塞逻辑，每隔500MS轮询一次，关于同步非阻塞的知识点在http://www.jianshu.com/p/e9c6690c0737
当有请求到来的时候采用轮询的方式获取一个Processor线程处理请求，代码如下：

currentProcessor = (currentProcessor + 1) % processors.length

之后将代码添加到newConnections队列之后返回，代码如下：

def accept(socketChannel: SocketChannel) {  newConnections.add(socketChannel)  wakeup()}

//newConnections是一个线程安全的队列，存放SocketChannel通道
private val newConnections = new ConcurrentLinkedQueue[SocketChannel]()

2.3 kafka.net.Processor

override def run() {
    startupComplete()
    while(isRunning) {
      // setup any new connections that have been queued up
      configureNewConnections()
      // register any new responses for writing
      processNewResponses()
      val startSelectTime = SystemTime.nanoseconds
      val ready = selector.select(300)
      currentTimeNanos = SystemTime.nanoseconds
      val idleTime = currentTimeNanos - startSelectTime
      idleMeter.mark(idleTime)
      // We use a single meter for aggregate idle percentage for the thread pool.
      // Since meter is calculated as total_recorded_value / time_window and
      // time_window is independent of the number of threads, each recorded idle
      // time should be discounted by # threads.
      aggregateIdleMeter.mark(idleTime / totalProcessorThreads)

      trace("Processor id " + id + " selection time = " + idleTime + " ns")
      if(ready > 0) {
        val keys = selector.selectedKeys()
        val iter = keys.iterator()
        while(iter.hasNext && isRunning) {
          var key: SelectionKey = null
          try {
            key = iter.next
            iter.remove()
            if(key.isReadable)
              read(key)
            else if(key.isWritable)
              write(key)
            else if(!key.isValid)
              close(key)
            else
              throw new IllegalStateException("Unrecognized key state for processor thread.")
          } catch {
            case e: EOFException => {
              info("Closing socket connection to %s.".format(channelFor(key).socket.getInetAddress))
              close(key)
            } case e: InvalidRequestException => {
              info("Closing socket connection to %s due to invalid request: %s".format(channelFor(key).socket.getInetAddress, e.getMessage))
              close(key)
            } case e: Throwable => {
              error("Closing socket for " + channelFor(key).socket.getInetAddress + " because of error", e)
              close(key)
            }
          }
        }
      }
      maybeCloseOldestConnection
    }
    debug("Closing selector.")
    closeAll()
    swallowError(selector.close())
    shutdownComplete()
  }

先来重点看一下configureNewConnections这个方法：

private def configureNewConnections() {
    while(newConnections.size() > 0) {
      val channel = newConnections.poll()
      debug("Processor " + id + " listening to new connection from " + channel.socket.getRemoteSocketAddress)
      channel.register(selector, SelectionKey.OP_READ)
    }
  }

循环判断NewConnections的大小，如果有值则弹出，并且注册为OP_READ读事件。
再回到主逻辑看一下read方法。

def read(key: SelectionKey) {
    lruConnections.put(key, currentTimeNanos)
    val socketChannel = channelFor(key)
    var receive = key.attachment.asInstanceOf[Receive]
    if(key.attachment == null) {
      receive = new BoundedByteBufferReceive(maxRequestSize)
      key.attach(receive)
    }
    val read = receive.readFrom(socketChannel)
    val address = socketChannel.socket.getRemoteSocketAddress();
    trace(read + " bytes read from " + address)
    if(read < 0) {
      close(key)
    } else if(receive.complete) {
      val req = RequestChannel.Request(processor = id, requestKey = key, buffer = receive.buffer, startTimeMs = time.milliseconds, remoteAddress = address)
      requestChannel.sendRequest(req)
      key.attach(null)
      // explicitly reset interest ops to not READ, no need to wake up the selector just yet
      key.interestOps(key.interestOps & (~SelectionKey.OP_READ))
    } else {
      // more reading to be done
      trace("Did not finish reading, registering for read again on connection " + socketChannel.socket.getRemoteSocketAddress())
      key.interestOps(SelectionKey.OP_READ)
      wakeup()
    }
  }

说明

1、把当前SelectionKey和事件循环时间放入LRU映射表中，将来检查时回收连接资源。
2、建立BoundedByteBufferReceive对象，具体读取操作由这个对象的readFrom方法负责进行，返回读取的字节大小。

• 如果读取完成，则修改状态为receive.complete，并通过requestChannel.sendRequest(req)将封装好的Request对象放到RequestQueue队列中。
• 如果没有读取完成，则让selector继续侦听OP_READ事件。

2.4 kafka.server.KafkaRequestHandler

def run() {
    while(true) {
      try {
        var req : RequestChannel.Request = null
        while (req == null) {
          // We use a single meter for aggregate idle percentage for the thread pool.
          // Since meter is calculated as total_recorded_value / time_window and
          // time_window is independent of the number of threads, each recorded idle
          // time should be discounted by # threads.
          val startSelectTime = SystemTime.nanoseconds
          req = requestChannel.receiveRequest(300)
          val idleTime = SystemTime.nanoseconds - startSelectTime
          aggregateIdleMeter.mark(idleTime / totalHandlerThreads)
        }

        if(req eq RequestChannel.AllDone) {
          debug("Kafka request handler %d on broker %d received shut down command".format(
            id, brokerId))
          return
        }
        req.requestDequeueTimeMs = SystemTime.milliseconds
        trace("Kafka request handler %d on broker %d handling request %s".format(id, brokerId, req))
        apis.handle(req)
      } catch {
        case e: Throwable => error("Exception when handling request", e)
      }
    }
  }

说明

KafkaRequestHandler也是一个事件处理线程，不断的循环读取requestQueue队列中的Request请求数据，其中超时时间设置为300MS，并将请求发送到apis.handle方法中处理，并将请求响应结果放到responseQueue队列中去。
代码如下：

try{
      trace("Handling request: " + request.requestObj + " from client: " + request.remoteAddress)
      request.requestId match {
        case RequestKeys.ProduceKey => handleProducerOrOffsetCommitRequest(request)
        case RequestKeys.FetchKey => handleFetchRequest(request)
        case RequestKeys.OffsetsKey => handleOffsetRequest(request)
        case RequestKeys.MetadataKey => handleTopicMetadataRequest(request)
        case RequestKeys.LeaderAndIsrKey => handleLeaderAndIsrRequest(request)
        case RequestKeys.StopReplicaKey => handleStopReplicaRequest(request)
        case RequestKeys.UpdateMetadataKey => handleUpdateMetadataRequest(request)
        case RequestKeys.ControlledShutdownKey => handleControlledShutdownRequest(request)
        case RequestKeys.OffsetCommitKey => handleOffsetCommitRequest(request)
        case RequestKeys.OffsetFetchKey => handleOffsetFetchRequest(request)
        case RequestKeys.ConsumerMetadataKey => handleConsumerMetadataRequest(request)
        case requestId => throw new KafkaException("Unknown api code " + requestId)
      }
    } catch {
      case e: Throwable =>
        request.requestObj.handleError(e, requestChannel, request)
        error("error when handling request %s".format(request.requestObj), e)
    } finally
      request.apiLocalCompleteTimeMs = SystemTime.milliseconds
  }

说明如下：

2.5 Processor响应数据处理

private def processNewResponses() {  
  var curr = requestChannel.receiveResponse(id)  
  while(curr != null) {  
    val key = curr.request.requestKey.asInstanceOf[SelectionKey]  
    curr.responseAction match {  
      case RequestChannel.SendAction => {  
        key.interestOps(SelectionKey.OP_WRITE)  
        key.attach(curr)  
      }  
    }  
  curr = requestChannel.receiveResponse(id)  
  }  
}

我们回到Processor线程类中，processNewRequest()方法是发送请求，那么会调用processNewResponses()来处理Handler提供给客户端的Response，把requestChannel中responseQueue的Response取出来，注册OP_WRITE事件，将数据返回给客户端。