SocketServer主要用于接收外部的网络请求，并把请求添加到请求队列中。

一、入口

在KafkaServer.scala中的start方法中，有这样的入口：

socketServer = new SocketServer(config, metrics, kafkaMetricsTime)
socketServer.startup()

这块就是启动了一个SocketServer，我们具体看一下。

二、构造方法

我们看下SocketServer里面包含的参数：

private val endpoints = config.listeners
private val numProcessorThreads = config.numNetworkThreads
private val maxQueuedRequests = config.queuedMaxRequests
private val totalProcessorThreads = numProcessorThreads * endpoints.siz
private val maxConnectionsPerIp = config.maxConnectionsPerIp
private val maxConnectionsPerIpOverrides config.maxConnectionsPerIpOverride
this.logIdent = "[Socket Server on Broker " + config.brokerId + "], "

val requestChannel = new RequestChannel(totalProcessorThreadsmaxQueuedRequests)
private val processors = new Array[Processor](totalProcessorThreads)

private[network] val acceptors = mutable.Map[EndPoint, Acceptor]()
private var connectionQuotas: ConnectionQuotas = _

这里面涉及几个配置内容：

• listeners：默认是PLAINTEXT://:port,前面部分是协议,可配置为PLAINTEXT,SSL,SASL_PLAINTEXT,SASL_SSL
• num.network.threads：处理网络请求的线程个数配置,默认是3
• queued.max.requests：请求队列的最大个数,默认500
• max.connections.per.ip：单机IP的最大连接个数的配置,默认不限制
• max.connections.per.ip.overrides：针对某个特别的IP的连接个数限制的重新设置值.多个IP配置间使用逗号分开,如:host1:500,host2:600

三、启动SocketServer

启动的代码如下：

/**
 * Start the socket server
 */
def startup() {
    this.synchronized {

    //每个ip的连接数限制
    connectionQuotas = new ConnectionQuotas(maxConnectionsPerIp, maxConnectionsPerIpOverrides)

    val sendBufferSize = config.socketSendBufferBytes
    val recvBufferSize = config.socketReceiveBufferBytes
    val brokerId = config.brokerId

    //这里根据每一个endpoint(也就是配置的listener的协议与端口),生成处理的网络线程Processor与Acceptor实例.并启动endpoint对应的Acceptor实例.在生成Acceptor的实例时,会同时启动此实例中对应的线程处理实例数组Processor.
    var processorBeginIndex = 0
    endpoints.values.foreach { endpoint =>
        val protocol = endpoint.protocolType
        val processorEndIndex = processorBeginIndex + numProcessorThreads

        for (i <- processorBeginIndex until processorEndIndex)
          processors(i) = newProcessor(i, connectionQuotas, protocol)

        val acceptor = new Acceptor(endpoint, sendBufferSize, recvBufferSize, brokerId,
          processors.slice(processorBeginIndex, processorEndIndex), connectionQuotas)
        acceptors.put(endpoint, acceptor)
        Utils.newThread("kafka-socket-acceptor-%s-%d".format(protocol.toString, endpoint.port), acceptor, false).start()
        acceptor.awaitStartup()

        processorBeginIndex = processorEndIndex
      }
    }

    newGauge("NetworkProcessorAvgIdlePercent",
      new Gauge[Double] {
        def value = allMetricNames.map( metricName =>
          metrics.metrics().get(metricName).value()).sum / totalProcessorThreads
      }
    )

    info("Started " + acceptors.size + " acceptor threads")
}

这块涉及到几个配置项，主要用于生成socket中的SO_SNDBUF和SO_RCVBUF。

• socket.send.buffer.bytes：默认值100kb,这个用于SOCKET发送数据的缓冲区大小
• socket.receive.buffer.bytes：默认值100kb,这个用于SOCKET的接收数据的缓冲区大小
• broker.id

3.1 newProcessor

我们先看下这个简单的赋值。

protected[network] def newProcessor(id: Int, connectionQuotas: ConnectionQuotas, protocol: SecurityProtocol): Processor = {
    new Processor(id,
      time,
      config.socketRequestMaxBytes,
      requestChannel,
      connectionQuotas,
      config.connectionsMaxIdleMs,
      protocol,
      config.values,
      metrics
    )
  }

其实就是Processor的实例生成，主要涉及几个配置项：

• socket.request.max.bytes：设置每次请求的数据大小.默认值,100MB
• connections.max.idle.ms：默认为10分钟,用于设置每个连接最大的空闲回收时间

3.2 Acceptor

每个endPoint对应一个Acceptor，也就是每个listener对应一个Acceptor。Acceptor主要用于接收网络请求，将请求分发到processor处理。我们来看下Acceptor的run方法：

def run() {
    //将channel注册到selector上
    serverChannel.register(nioSelector, SelectionKey.OP_ACCEPT)
    startupComplete()
    try {
      var currentProcessor = 0
      while (isRunning) {
        try {
          //这里进行堵塞接收,最多等500ms,如果ready返回的值是0表示还没有准备好,否则表示准备就绪.表示有通道已经被注册
          val ready = nioSelector.select(500)
          if (ready > 0) {
            //这里得到已经准备好的网络通道的key的集合
            val keys = nioSelector.selectedKeys()
            val iter = keys.iterator()
            while (iter.hasNext && isRunning) {
              try {
                val key = iter.next
                iter.remove()
                //如果selectkey已经注册到accept事件,通过accept函数与对应的线程Processor进行处理.这里表示这个socket的通道包含有一个client端的连接请求.
                if (key.isAcceptable)
                  accept(key, processors(currentProcessor))
                else
                  throw new IllegalStateException("Unrecognized key state for acceptor thread.")

                // round robin to the next processor thread
                //每次接收一个socket请求后,用于处理的线程进行轮询到一个线程中处理.
                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
          // 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()
    }
  }

下面我们看下accept方法：

  /*
   * Accept a new connection
   */
  def accept(key: SelectionKey, processor: Processor) {
    val serverSocketChannel = key.channel().asInstanceOf[ServerSocketChannel]
    //得到请求的socket通道
    val socketChannel = serverSocketChannel.accept()
    try {
      //这里检查当前的IP的连接数是否已经达到了最大的连接数,如果是,直接throw too many connect.
      connectionQuotas.inc(socketChannel.socket().getInetAddress)
      socketChannel.configureBlocking(false)
      socketChannel.socket().setTcpNoDelay(true)
      socketChannel.socket().setKeepAlive(true)
      socketChannel.socket().setSendBufferSize(sendBufferSize)

      debug("Accepted connection from %s on %s and assigned it to processor %d, sendBufferSize [actual|requested]: [%d|%d] recvBufferSize [actual|requested]: [%d|%d]"
            .format(socketChannel.socket.getRemoteSocketAddress, socketChannel.socket.getLocalSocketAddress, processor.id,
                  socketChannel.socket.getSendBufferSize, sendBufferSize,
                  socketChannel.socket.getReceiveBufferSize, recvBufferSize))

      //对应的processor处理socket通道
      processor.accept(socketChannel)
    } catch {
      case e: TooManyConnectionsException =>
        info("Rejected connection from %s, address already has the configured maximum of %d connections.".format(e.ip, e.count))
        close(socketChannel)
    }
  }

3.3 Processor

上面accept方法中，调用到了processor的accept方法，我们看下这个accept方法：

  /**
   * Queue up a new connection for reading
   */
  def accept(socketChannel: SocketChannel) {
    newConnections.add(socketChannel)
    wakeup()
  }

其实就是向队列中新增了一个socket通道，等待processor线程处理。下面我们看下processor是怎么处理的。

override def run() {
    startupComplete()
    while (isRunning) {
      try {
        // setup any new connections that have been queued up
        configureNewConnections()
        // register any new responses for writing
        processNewResponses()
        poll()
        processCompletedReceives()
        processCompletedSends()
        processDisconnected()
      } catch {
        // We catch all the throwables here to prevent the processor thread from exiting. We do this because
        // letting a processor exit might cause a bigger impact on the broker. Usually the exceptions thrown would
        // be either associated with a specific socket channel or a bad request. We just ignore the bad socket channel
        // or request. This behavior might need to be reviewed if we see an exception that need the entire broker to stop.
        case e: ControlThrowable => throw e
        case e: Throwable =>
          error("Processor got uncaught exception.", e)
      }
    }

    debug("Closing selector - processor " + id)
    swallowError(closeAll())
    shutdownComplete()
  }

这块其实是个门面模式，里面调用的内容比较多，我们一一看一下。

3.3.1 configureNewConnections

这块是从队列中取一个连接，并注册到selector上。

  /**
   * Register any new connections that have been queued up
   */
  private def configureNewConnections() {
    while (!newConnections.isEmpty) {
      val channel = newConnections.poll()
      try {
        debug(s"Processor $id listening to new connection from ${channel.socket.getRemoteSocketAddress}")
        val localHost = channel.socket().getLocalAddress.getHostAddress
        val localPort = channel.socket().getLocalPort
        val remoteHost = channel.socket().getInetAddress.getHostAddress
        val remotePort = channel.socket().getPort
        val connectionId = ConnectionId(localHost, localPort, remoteHost, remotePort).toString
        selector.register(connectionId, channel)
      } catch {
        // We explicitly catch all non fatal exceptions and close the socket to avoid a socket leak. The other
        // throwables will be caught in processor and logged as uncaught exceptions.
        case NonFatal(e) =>
          // need to close the channel here to avoid a socket leak.
          close(channel)
          error(s"Processor $id closed connection from ${channel.getRemoteAddress}", e)
      }
    }
  }

3.3.2 processNewResponses

private def processNewResponses() {
    var curr = requestChannel.receiveResponse(id)
    while (curr != null) {
      try {
        curr.responseAction match {
          case RequestChannel.NoOpAction =>
            // There is no response to send to the client, we need to read more pipelined requests
            // that are sitting in the server's socket buffer
            curr.request.updateRequestMetrics
            trace("Socket server received empty response to send, registering for read: " + curr)
            selector.unmute(curr.request.connectionId)
          case RequestChannel.SendAction =>
            sendResponse(curr)
          case RequestChannel.CloseConnectionAction =>
            curr.request.updateRequestMetrics
            trace("Closing socket connection actively according to the response code.")
            close(selector, curr.request.connectionId)
        }
      } finally {
        curr = requestChannel.receiveResponse(id)
      }
    }
  }

3.3.3 poll

  private def poll() {
    try selector.poll(300)
    catch {
      case e @ (_: IllegalStateException | _: IOException) =>
        error(s"Closing processor $id due to illegal state or IO exception")
        swallow(closeAll())
        shutdownComplete()
        throw e
    }
  }

    @Override
    public void poll(long timeout) throws IOException {
        if (timeout < 0)
            throw new IllegalArgumentException("timeout should be >= 0");

        clear();

        if (hasStagedReceives() || !immediatelyConnectedKeys.isEmpty())
            timeout = 0;

        /* check ready keys */
        long startSelect = time.nanoseconds();
        int readyKeys = select(timeout);
        long endSelect = time.nanoseconds();
        currentTimeNanos = endSelect;
        this.sensors.selectTime.record(endSelect - startSelect, time.milliseconds());

        if (readyKeys > 0 || !immediatelyConnectedKeys.isEmpty()) {
            pollSelectionKeys(this.nioSelector.selectedKeys(), false);
            pollSelectionKeys(immediatelyConnectedKeys, true);
        }

        addToCompletedReceives();

        long endIo = time.nanoseconds();
        this.sensors.ioTime.record(endIo - endSelect, time.milliseconds());
        maybeCloseOldestConnection();
    }

这块主要看一下pollSelectionKeys方法：

private void pollSelectionKeys(Iterable<SelectionKey> selectionKeys, boolean isImmediatelyConnected) {
        Iterator<SelectionKey> iterator = selectionKeys.iterator();
        while (iterator.hasNext()) {
            SelectionKey key = iterator.next();
            iterator.remove();
            KafkaChannel channel = channel(key);

            // register all per-connection metrics at once
            sensors.maybeRegisterConnectionMetrics(channel.id());
            lruConnections.put(channel.id(), currentTimeNanos);

            try {

                /* complete any connections that have finished their handshake (either normally or immediately) */
                if (isImmediatelyConnected || key.isConnectable()) {
                    if (channel.finishConnect()) {
                        this.connected.add(channel.id());
                        this.sensors.connectionCreated.record();
                    } else
                        continue;
                }

                /* if channel is not ready finish prepare */
                if (channel.isConnected() && !channel.ready())
                    channel.prepare();

                /* if channel is ready read from any connections that have readable data */
                if (channel.ready() && key.isReadable() && !hasStagedReceive(channel)) {
                    NetworkReceive networkReceive;
                    while ((networkReceive = channel.read()) != null)
                        addToStagedReceives(channel, networkReceive);
                }

                /* if channel is ready write to any sockets that have space in their buffer and for which we have data */
                if (channel.ready() && key.isWritable()) {
                    Send send = channel.write();
                    if (send != null) {
                        this.completedSends.add(send);
                        this.sensors.recordBytesSent(channel.id(), send.size());
                    }
                }

                /* cancel any defunct sockets */
                if (!key.isValid()) {
                    close(channel);
                    this.disconnected.add(channel.id());
                }

            } catch (Exception e) {
                String desc = channel.socketDescription();
                if (e instanceof IOException)
                    log.debug("Connection with {} disconnected", desc, e);
                else
                    log.warn("Unexpected error from {}; closing connection", desc, e);
                close(channel);
                this.disconnected.add(channel.id());
            }
        }
    }

这里开始处理socket通道中的请求,根据如下几个流程进行处理:

• 如果请求中包含有一个isConnectable操作,把这个连接缓存起来.
• 如果请求中包含有isReadable操作.表示这个client的管道中包含有数据,需要读取,接收数据.
• 如果包含有isWriteable的操作,表示需要向client端进行写操作.
• 最后检查是否有connect被关闭的请求或connect连接空闲过期

3.3.4 processCompletedReceives

得到对应的请求的Request的实例,并把这个Request通过SocketServer中的RequestChannel的sendRequest的函数,把请求添加到请求的队列中.等待KafkaApis来进行处理.

private def processCompletedReceives() {
    selector.completedReceives.asScala.foreach { receive =>
      try {
        val channel = selector.channel(receive.source)
        val session = RequestChannel.Session(new KafkaPrincipal(KafkaPrincipal.USER_TYPE, channel.principal.getName),
          channel.socketAddress)
        val req = RequestChannel.Request(processor = id, connectionId = receive.source, session = session, buffer = receive.payload, startTimeMs = time.milliseconds, securityProtocol = protocol)
        //这是重点！！！可以看下KafkaApis对消息的处理，后续会分析到
        requestChannel.sendRequest(req)
        selector.mute(receive.source)
      } catch {
        case e @ (_: InvalidRequestException | _: SchemaException) =>
          // note that even though we got an exception, we can assume that receive.source is valid. Issues with constructing a valid receive object were handled earlier
          error(s"Closing socket for ${receive.source} because of error", e)
          close(selector, receive.source)
      }
    }
  }

3.3.5 processCompletedSends

这里的send完成表示有向client端进行响应的写操作处理完成

  private def processCompletedSends() {
    selector.completedSends.asScala.foreach { send =>
      val resp = inflightResponses.remove(send.destination).getOrElse {
        throw new IllegalStateException(s"Send for ${send.destination} completed, but not in `inflightResponses`")
      }
      resp.request.updateRequestMetrics()
      selector.unmute(send.destination)
    }
  }

3.3.6 processDisconnected

如果socket server中包含有已经关闭的连接,减少这个quotas中对此ip的连接数的值.
这个情况包含connect处理超时或者说有connect的消息处理错误被发起了close的请求后的处理成功的消息.

  private def processDisconnected() {
    selector.disconnected.asScala.foreach { connectionId =>
      val remoteHost = ConnectionId.fromString(connectionId).getOrElse {
        throw new IllegalStateException(s"connectionId has unexpected format: $connectionId")
      }.remoteHost
      inflightResponses.remove(connectionId).foreach(_.request.updateRequestMetrics())
      // the channel has been closed by the selector but the quotas still need to be updated
      connectionQuotas.dec(InetAddress.getByName(remoteHost))
    }
  }