spark内核揭秘-07-DAGScheduler源码解读初体验

当构建完TaskScheduler之后，我们需要构建DAGScheduler这个核心对象：

进入其构造函数中：

可以看出构建DAGScheduler实例的时候需要把TaskScheduler实例对象作为参数传入。

LiveListenerBus：

MapOutputTrackerMaster：

BlockManagerMaster：

通过阅读代码，我们可以发现DAGScheduler实例化的时候，调用了initializeEventProcessActor()方法

private def initializeEventProcessActor() {
  // blocking the thread until supervisor is started, which ensures eventProcessActor is
  // not null before any job is submitted
  // 阻塞当前线程，等待supervisor启动，这样可以确保Job提交时，eventProcessActor not null
  implicit val timeout = Timeout(30 seconds)
  val initEventActorReply =
    dagSchedulerActorSupervisor ? Props(new DAGSchedulerEventProcessActor(this))
  eventProcessActor = Await.result(initEventActorReply, timeout.duration).
    asInstanceOf[ActorRef]
}

initializeEventProcessActor()

DAGSchedulerEventProcessActor：

private[scheduler] class DAGSchedulerEventProcessActor(dagScheduler: DAGScheduler)
  extends Actor with Logging {

  override def preStart() {
    // set DAGScheduler for taskScheduler to ensure eventProcessActor is always
    // valid when the messages arrive
    // 设置taskScheduler对DAGScheduler的引用句柄。在此处设置保证了Job提交时候
    // eventProcessActor已经准备就绪
    dagScheduler.taskScheduler.setDAGScheduler(dagScheduler)
  }

  /**
   * The main event loop of the DAG scheduler.
   */
  def receive = {
    case JobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite, listener, properties) =>
      dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, allowLocal, callSite,
        listener, properties)

    case StageCancelled(stageId) =>
      dagScheduler.handleStageCancellation(stageId)

    case JobCancelled(jobId) =>
      dagScheduler.handleJobCancellation(jobId)

    case JobGroupCancelled(groupId) =>
      dagScheduler.handleJobGroupCancelled(groupId)

    case AllJobsCancelled =>
      dagScheduler.doCancelAllJobs()

    case ExecutorAdded(execId, host) =>
      dagScheduler.handleExecutorAdded(execId, host)

    case ExecutorLost(execId) =>
      dagScheduler.handleExecutorLost(execId, fetchFailed = false)

    case BeginEvent(task, taskInfo) =>
      dagScheduler.handleBeginEvent(task, taskInfo)

    case GettingResultEvent(taskInfo) =>
      dagScheduler.handleGetTaskResult(taskInfo)

    case completion @ CompletionEvent(task, reason, _, _, taskInfo, taskMetrics) =>
      dagScheduler.handleTaskCompletion(completion)

    case TaskSetFailed(taskSet, reason) =>
      dagScheduler.handleTaskSetFailed(taskSet, reason)

    case ResubmitFailedStages =>
      dagScheduler.resubmitFailedStages()
  }

  override def postStop() {
    // Cancel any active jobs in postStop hook
    dagScheduler.cleanUpAfterSchedulerStop()
  }
}

可以看出核心在于实例化eventProcessActor对象，eventProcessActor会负责接收和发送DAGScheduler的消息，是DAGScheduler的通信载体。

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