kafka 请求处理与RPC原理解析(四)

偏执的太偏执、 2022-05-18 11:07 400阅读 0赞

### 文章目录 ###

*   *  kafka请求处理架构图
     *  SocketServer的启动
     *  Acceptor启动监听相关代码
     *  KafkaRequestHandler 线程相关代码
     *  KafkaApis 相关代码

kafka server启动后，会监听一些端口，然后开始接收请求进行日常的工作。  
与请求处理相关的组件有 SocketServer、KafkaApis、KafkaRequestHandlerPool。这些都是在kafka server启动时初始化并开始运行的。SocketServer是一个NIO服务，基于N+M的线程模型，由N个Acceptor线程以及M个Processor线程组成，和netty的网络模型有点像。N个Acceptor线程专门用于监听连接事件，连接建立后将连接交给其他M个Processor线程继续监听读事件，这样的线程模型使kafka可以很轻松的处理高并发的场景。

## kafka请求处理架构图 ##

![kakfa server socket架构图.png][kakfa server socket_.png]

1.  `kakfa server`在启动时调用`SocketServer#startup()`方法，这个方法内会初始化N个Acceptor开始监听`OP_ACCEPT`事件，等待客户端连接。初始化的Acceptor数量取决于用户配置的listeners有几个。在初始化每个Acceptor的同时，还会初始化M个Processor，并分配给Acceptor用于监听连接事件。Processor的数量取决于`num.network.threads`配置，该配置默认值是3，表示每个Acceptor分配3个Processor。
2.  `Acceptor`接收到一个新的连接时，会将这个请求以轮询的方式分配给它管理的其中一个Processor处理
3.  `Processor`收到一个连接时，便开始监听它的`OP_READ`事件
4.  如果Processor发现有请求发过来，就将这个请求放入Request队列中，等待处理。该Request队列的容量由配置`queued.max.requests`决定，改配置默认值是500.
5.  kakfa server在启动时会初始化KafkaRequestHandlerPool类，该类在初始化时会构造一些的KafkaRequestHandler线程并启动，构造的KafkaRequestHandler线程数量取决于配置`num.io.threads`的值，该配置默认值是8.。
6.  KafkaRequestHandler线程启动后，会不断自旋，从`request queue`中获取请求，然后交给KafkaApis进行处理。KafkaApis根据请求的类型进行不同的业务处理
7.  KafkaApis组件处理完后，会将结果放入`对应的Processor的response queue`中,等待Processor处理
8.  Processor也是一个不断自旋的线程，在自旋的过程中，Processor会检查自己的response queue中是否有新的结果，如果有新的结果就将其从队列中取出，准备发回给客户端
9.  Processor通过NioChannel将结果写回客户端，自此一个通信流程结束

## SocketServer的启动 ##

def startup() {
        this.synchronized {
          connectionQuotas = new ConnectionQuotas(maxConnectionsPerIp, maxConnectionsPerIpOverrides)
    
          val sendBufferSize = config.socketSendBufferBytes
          val recvBufferSize = config.socketReceiveBufferBytes
          val brokerId = config.brokerId
    
          var processorBeginIndex = 0
          config.listeners.foreach { endpoint =>
            val listenerName = endpoint.listenerName
            val securityProtocol = endpoint.securityProtocol
            val processorEndIndex = processorBeginIndex + numProcessorThreads
    
            for (i <- processorBeginIndex until processorEndIndex)
              processors(i) = newProcessor(i, connectionQuotas, listenerName, securityProtocol)
    
            val acceptor = new Acceptor(endpoint, sendBufferSize, recvBufferSize, brokerId,
              processors.slice(processorBeginIndex, processorEndIndex), connectionQuotas)
            acceptors.put(endpoint, acceptor)
            Utils.newThread(s"kafka-socket-acceptor-$listenerName-$securityProtocol-${endpoint.port}", acceptor, false).start()
            acceptor.awaitStartup()
    
            processorBeginIndex = processorEndIndex
          }
        }
    
        info("Started " + acceptors.size + " acceptor threads")
      }

socketServer启动时，会初始化N个Acceptor，并为其分配好对应数量的Processor，然后启动Acceptor线程。

## Acceptor启动监听相关代码 ##

def run() {
      //往selector监听OP_ACCEPT事件
      serverChannel.register(nioSelector, SelectionKey.OP_ACCEPT)
      startupComplete()
      try {
        var currentProcessor = 0
        while (isRunning) {
          try {
    		//开始轮询
            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()
    			  //如果有连接进来,就将它交给指定的Processor处理
                  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 {
            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()
      }
    }
    def accept(key: SelectionKey, processor: Processor) {
      //获取对应channel
      val serverSocketChannel = key.channel().asInstanceOf[ServerSocketChannel]
      val socketChannel = serverSocketChannel.accept()
      try {
        connectionQuotas.inc(socketChannel.socket().getInetAddress)
        socketChannel.configureBlocking(false)
        socketChannel.socket().setTcpNoDelay(true)
        socketChannel.socket().setKeepAlive(true)
        if (sendBufferSize != Selectable.USE_DEFAULT_BUFFER_SIZE)
          socketChannel.socket().setSendBufferSize(sendBufferSize)
    	//Processor接收channel
        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)
      }
    }

Acceptor线程启动后，就开始监听端口看有没有新的连接进来。这里使用nio实现无阻塞的监听请求。收到请求后就分发给它管理的其中一个Processor线程处理。

def accept(socketChannel: SocketChannel) {
      //接收一个新的连接,newConnections集合表示当前Processor管理的连接
      newConnections.add(socketChannel)
      wakeup()
    }
    override def run() {
      startupComplete()
      //Processor线程不断自旋
      while (isRunning) {
        try {
          //把新接收的连接拿到并注册OP_READ事件
          configureNewConnections()
          //从对应的response队列中获取response,然后进行相应的操作.这里不一定是将响应发送给客户端，可能不用响应客户端，也可能关闭连接
    	  //另外，这个方法也不真正的发送响应，即使要发送响应给客户端，这个方法里面也只是往对应的连接注册OP_WRITE事件，然后等后面的poll()方法执行时才真正将响应发送出去
          processNewResponses()
    	  //select()阻塞等待OP_READ和OP_WRITE事件被触发，然后处理,最长阻塞时间是300ms
    	  //如果OP_READ事件就绪，说明有新的请求发送过来，这些请求的信息最终会被放入selector.completedReceives集合中，也就是List<NetworkReceive>
    	  //如果OP_WRITE事件就绪，说明有响应需要发送出去，这时候才会将响应发送给客户端。同时将这个连接放入completedSends表示该连接已经完成
          poll()
    	  //开始处理selector.completedReceives中的信息，最终会被封装成RequestChannel.Request后放入request队列中
          processCompletedReceives()
    	  //遍历completedSends集合，将已经完成的连接从inflightResponses集合中移除
          processCompletedSends()
    	  //将已经断开的连接从inflightResponses集合中移除
          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()
    }

Processor线程拿到Acceptor传过来的请求后开始监听该连接的读请求。同时还会做许多事情。比如发送响应、读取请求、关闭连接等等。

## KafkaRequestHandler 线程相关代码 ##

kakfa server在启动时会初始化KafkaRequestHandlerPool类，该类在初始化时会构造一些的KafkaRequestHandler线程并启动，构造的KafkaRequestHandler线程数量取决于配置`num.io.threads`的值，该配置默认值是8。

下面是KafkaRequestHandler线程的run方法

def run() {
      while(true) {
        try {
          var req : RequestChannel.Request = null
          while (req == null) {
            //从request队列中获取请求
            val startSelectTime = time.nanoseconds
            req = requestChannel.receiveRequest(300)
            val idleTime = time.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 = time.milliseconds
          trace("Kafka request handler %d on broker %d handling request %s".format(id, brokerId, req))
    	  //使用KafkaApis处理请求
          apis.handle(req)
        } catch {
          case e: Throwable => error("Exception when handling request", e)
        }
      }
    }

KafkaRequestHandler线程不断的从`请求队列`中取出请求处理。具体的请求最后交给KafkaApis处理。

## KafkaApis 相关代码 ##

def handle(request: RequestChannel.Request) {
      try {
        trace("Handling request:%s from connection %s;securityProtocol:%s,principal:%s".
          format(request.requestDesc(true), request.connectionId, request.securityProtocol, request.session.principal))
    	//根据请求的类型处理请求
        ApiKeys.forId(request.requestId) match {
          case ApiKeys.PRODUCE => handleProducerRequest(request)
          case ApiKeys.FETCH => handleFetchRequest(request)
          case ApiKeys.LIST_OFFSETS => handleOffsetRequest(request)
          case ApiKeys.METADATA => handleTopicMetadataRequest(request)
          case ApiKeys.LEADER_AND_ISR => handleLeaderAndIsrRequest(request)
          case ApiKeys.STOP_REPLICA => handleStopReplicaRequest(request)
          case ApiKeys.UPDATE_METADATA_KEY => handleUpdateMetadataRequest(request)
          case ApiKeys.CONTROLLED_SHUTDOWN_KEY => handleControlledShutdownRequest(request)
          case ApiKeys.OFFSET_COMMIT => handleOffsetCommitRequest(request)
          case ApiKeys.OFFSET_FETCH => handleOffsetFetchRequest(request)
          case ApiKeys.GROUP_COORDINATOR => handleGroupCoordinatorRequest(request)
          case ApiKeys.JOIN_GROUP => handleJoinGroupRequest(request)
          case ApiKeys.HEARTBEAT => handleHeartbeatRequest(request)
          case ApiKeys.LEAVE_GROUP => handleLeaveGroupRequest(request)
          case ApiKeys.SYNC_GROUP => handleSyncGroupRequest(request)
          case ApiKeys.DESCRIBE_GROUPS => handleDescribeGroupRequest(request)
          case ApiKeys.LIST_GROUPS => handleListGroupsRequest(request)
          case ApiKeys.SASL_HANDSHAKE => handleSaslHandshakeRequest(request)
          case ApiKeys.API_VERSIONS => handleApiVersionsRequest(request)
          case ApiKeys.CREATE_TOPICS => handleCreateTopicsRequest(request)
          case ApiKeys.DELETE_TOPICS => handleDeleteTopicsRequest(request)
          case requestId => throw new KafkaException("Unknown api code " + requestId)
        }
      } catch {
        case e: Throwable =>
          if (request.requestObj != null) {
            request.requestObj.handleError(e, requestChannel, request)
            error("Error when handling request %s".format(request.requestObj), e)
          } else {
            val response = request.body.getErrorResponse(e)
    
            /* If request doesn't have a default error response, we just close the connection.
               For example, when produce request has acks set to 0 */
            if (response == null)
              requestChannel.closeConnection(request.processor, request)
            else
              requestChannel.sendResponse(new Response(request, response))
    
            error("Error when handling request %s".format(request.body), e)
          }
      } finally
        request.apiLocalCompleteTimeMs = time.milliseconds
    }

kafkaApis根据请求的类型执行不同的操作来处理请求。  
在0.10.2版本中，kafkaApis可以处理21种类型的请求。

[kakfa server socket_.png]: http://on-img.com/chart_image/5b4348e1e4b0ade3e27398a8.png?_=1533214932518