Spark Executor原理

Master发指令给Worker启动Executor。

Worker接收到Master发送来的指令通过ExecutorRunner启动另外一个进程来启动Executor。

CoarseGrainedExecutorBackend通过发送RegisterExecutor向Driver注册，Driver在Executor注册成功后会返回RegisterExecutor信息给CoarseGrainedExecutorBackend。

一：Spark Executor工作原理 
1. 需要特别注意是在CoarseGrainedExecutorBackend启动时向Driver注册Executor其实质是注册ExecutorBackend实例，和Executor无关。 
2．CoarseGrainedExecutorBackend是Executor运行所在的进程名称，CoarseGrainedExecutorBackend本身并不会完成具体任务的计算，Executor才会完成计算。Executor才是处理Task的对象，Executor内部是通过线程池的方式来完成Task的计算的； 
3．CoarseGrainedExecutorBackend是一个进程，里面有一个Executor对象，CoarseGrainedExecutorBackend和Executor是一一对应的； 
4．CoarseGrainedExecutorBackend是一个消息通信体(其实现了ThreadSafeRpcEndPoint)。可以发送消息给Driver并可以接受Driver中发过来的指令，例如启动Task等； 
5. 在Driver进程有两个至关重要的Endpoint: 
a) ClientEndpoint: 主要负责向Master注册当前的程序,是AppClient的内部成员； 
b) DriverEndpoint:这是整个程序运行时候的驱动器,例如接收CoarseGrainedExecutorBackend的注册，是CoarseGrainedExecutorBackend的内部成员。

Executor内幕原理和运行流程解密

1. CoarseGrainedExecutorBackend通过onStart()发送RegisterExecutor向Driver注册。这里面的ref也就相当于Driver。

override def onStart() {
  logInfo("Connecting to driver: " + driverUrl)
  rpcEnv.asyncSetupEndpointRefByURI(driverUrl).flatMap { ref =>
    // This is a very fast action so we can use "ThreadUtils.sameThread"
    driver = Some(ref)
    ref.ask[RegisterExecutorResponse](
      RegisterExecutor(executorId, self, hostPort, cores, extractLogUrls))
 
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RegisterExecutor此时的命名是有问题的，因为向Driver端注册并不是注册Executor，而是注册ExecutorBackend. 
2. Driver要接受ExecutorBackend的注册，也就是接受CoarseGrainedExecutorBackend发来的请求。在DriverEndpoint中会接收到RegisterExecutor信息并完成在Driver的注册。

override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {

  case RegisterExecutor(executorId, executorRef, hostPort, cores, logUrls) =>
    if (executorDataMap.contains(executorId)) {
      context.reply(RegisterExecutorFailed("Duplicate executor ID: " + executorId))
 
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接收到请求之后Driver是怎么办的？ 
在Driver中通过ExecutorData封装并注册ExecutorBackend的信息到Driver的内存数据结构executorMapData中；

private val executorDataMap = new HashMap[String, ExecutorData]
 
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为啥是ExecutorBackend？ 
因为CoarseGrainedExecutorBackend是继承ThreadSafeRpcEndpoint，而Executor没有继承ThreadSafeRpcEndpoint，因此在Driver接收参数里面有RpcEndpoint，肯定是CoarseGrainedExecutorBackend发过来的。与Driver通信请求注册的是CoarseGrainedExecutorBackend，并不是Executor。

override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
  case RegisterExecutor(executorId, executorRef, hostPort, cores, logUrls) =>
    if (executorDataMap.contains(executorId)) {
      context.reply(RegisterExecutorFailed("Duplicate executor ID: " + executorId)) //ID不可重复注册
    } else {
      // If the executor's rpc env is not listening for incoming connections, `hostPort`
      // will be null, and the client connection should be used to contact the executor.
      val executorAddress = if (executorRef.address != null) {
          executorRef.address
        } else {
          context.senderAddress
        }
 
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实际在执行的时候DriverEndpoint会把信息吸入CoarseGrainedSchedulerBackend的内存数据结构executorMapData，所以说最终注册给了CoarseGrainedSchedulerBackend，也就是说，CoarseGrainedSchedulerBackend掌握了当前程序分配的所有的ExecutorBackend进程，而在每个ExecutorBackend进行实例中会通过Executor对象来负责具体Task的运行。在运行的时候使用synchronized关键字来保证executorMapData安全的并发写操作。

为什么说注册信息其实给了CoarseGrainedSchedulerBackend？ 
executorMapData属于CoarseGrainedSchedulerBackend的成员，在注册的时候，把信息注册给CoarseGrainedSchedulerBakend的成员的数据结构中。

synchronized关键字？ 
因为集群中会有很多ExecutorBackend向Driver注册，为了避免写冲突，所以用同步代码块。

// This must be synchronized because variables mutated
// in this block are read when requesting executors
CoarseGrainedSchedulerBackend.this.synchronized {
  executorDataMap.put(executorId, data)
  if (numPendingExecutors > 0) {
    numPendingExecutors -= 1
    logDebug(s"Decremented number of pending executors ($numPendingExecutors left)")
  }
}
 
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3 Driver会在Executor注册成功后，返回RegisteredExecutor信息给 
CoarseGrainedExecutorBackend

// Note: some tests expect the reply to come after we put the executor in the map
  context.reply(RegisteredExecutor(executorAddress.host))
  listenerBus.post(
    SparkListenerExecutorAdded(System.currentTimeMillis(), executorId, data))
  makeOffers()
}
 
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4 CoarseGrainedExecutorBackend收到DriverEndpoint发送过来的RegisteredExecutor消息后会启动Executor实例对象，而Executor实例对象是事实上负责真正Task计算的；

override def receive: PartialFunction[Any, Unit] = {
  case RegisteredExecutor(hostname) =>
    logInfo("Successfully registered with driver")
    executor = new  Executor(executorId, hostname, env, userClassPath, isLocal = false)
 
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其在实例化的时候会实例化一个线程池来准备Task的计算。

// Start worker thread pool
private val threadPool = ThreadUtils.newDaemonCachedThreadPool("Executor task launch worker")
private val executorSource = new ExecutorSource(threadPool, executorId)
 
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5 创建的threadPool中以多线程并发执行和线程复用的方式来高效的执行Spark发过来的Task。

public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>(),
                                  threadFactory);
}
 
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现在线程池准备好了，那么就等任务了。 
6. 当Driver发送过来Task的时候，其实是发送给了CoarseGrainedExecutorBackend这个RpcEndpoint,而不是直接发送给了Executor(Executor由于不是消息消息循环体，所以永远也无法直接接受远程发过来的信息)；attemptNumber executor可以重试的次数.

case LaunchTask(data) =>
  if (executor == null) {
    logError("Received LaunchTask command but executor was null")
    System.exit(1)
  } else {
    val taskDesc = ser.deserialize[TaskDescription](data.value) //反序列化
    logInfo("Got assigned task " + taskDesc.taskId)
    executor.launchTask(this, taskId = taskDesc.taskId, attemptNumber = taskDesc.attemptNumber,
      taskDesc.name, taskDesc.serializedTask)
  }
 
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ExecutorBackend在收到Driver中发送过来的消息后会提供调用LaunchTask来交给Executor去执行；

LaunchTask来执行任务，接收到Task执行的命令之后，会首先把Task封装在TaskRunner里面，然后交给线程池中的线程处理。

def launchTask(
    context: ExecutorBackend,
    taskId: Long,
    attemptNumber: Int,
    taskName: String,
    serializedTask: ByteBuffer): Unit = {
  val tr = new TaskRunner(context, taskId = taskId, attemptNumber = attemptNumber, taskName,
 
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7 TaskRunner是Java中的Runnable的接口的具体实现，在真正工作的时候会交给线程池中的线程去运行，此时会调用Run方法来执行Task。

class TaskRunner(
    execBackend: ExecutorBackend,
    val taskId: Long,
    val attemptNumber: Int,
    taskName: String,
    serializedTask: ByteBuffer)
  extends Runnable {
 
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8 TaskRunner在调用run方法的时候会调用Task的run方法，而Task的run方法会调用runTask。

val res = task.run(
  taskAttemptId = taskId,
  attemptNumber = attemptNumber,
  metricsSystem = env.metricsSystem)
threwException = false
res
 
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Task的run方法：

final def run(
  taskAttemptId: Long,
  attemptNumber: Int,
  metricsSystem: MetricsSystem)

 
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try {
  (runTask(context), context.collectAccumulators())
 
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总结流程图如下：