1.概述

kafka启动时做很多初始化运行环境工作，具体请参考：apache kafka系列之源码分析走读-kafka内部模块分析

其中SockeServer类启动时，首先初始化NIO网络环境、启动监听、创建主线程、工作线程池、设置参数等等。

从上图1中可以看到整个交互过程中，kafka的所有逻辑处理和交互实际是交给KafkaApi类来处理的。

通过请求的类型，把具体的request路由到对应的handler处理。目前kafka并没有把handler抽象出来，

而是每个handler都是一个函数，混在KafkaApi类中。

2. Request请求类别

kafka-0.8.1版本中定义了10种类型请求，请求类型说明如下：

参数	说明(解释)	请求二进制数据解码类
RequestKeys.ProduceKey	producer请求	ProducerRequest
RequestKeys.FetchKey	consumer请求	FetchRequest
RequestKeys.OffsetsKey	topic的offset请求	OffsetRequest
RequestKeys.MetadataKey	topic元数据请求	TopicMetadataRequest
RequestKeys.LeaderAndIsrKey	leader和isr信息更新请求	LeaderAndIsrRequest
RequestKeys.StopReplicaKey	停止replica请求	StopReplicaRequest
RequestKeys.UpdateMetadataKey	更新元数据请求	UpdateMetadataRequest
RequestKeys.ControlledShutdownKey	controlledShutdown请求	ControlledShutdownRequest
RequestKeys.OffsetCommitKey	commitOffset请求	OffsetCommitRequest
RequestKeys.OffsetFetchKey	consumer的offset请求	OffsetFetchRequest

下面是KafkaApi中handle方法代码：

  def handle(request: RequestChannel.Request) {
    try{
      trace("Handling request: " + request.requestObj + " from client: " + request.remoteAddress)
      request.requestId match {
        case RequestKeys.ProduceKey => handleProducerRequest(request)  // producer
        case RequestKeys.FetchKey => handleFetchRequest(request)       // consumer
        case RequestKeys.OffsetsKey => handleOffsetRequest(request)
        case RequestKeys.MetadataKey => handleTopicMetadataRequest(request)
        case RequestKeys.LeaderAndIsrKey => handleLeaderAndIsrRequest(request) //成为leader或follower设置同步副本组信息
        case RequestKeys.StopReplicaKey => handleStopReplicaRequest(request)
        case RequestKeys.UpdateMetadataKey => handleUpdateMetadataRequest(request)
        case RequestKeys.ControlledShutdownKey => handleControlledShutdownRequest(request)  //shutdown broker
        case RequestKeys.OffsetCommitKey => handleOffsetCommitRequest(request)
        case RequestKeys.OffsetFetchKey => handleOffsetFetchRequest(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
  }