Spark源码学习笔记3-LiveListenerBus

继2-JobProgressListener之后，我们再来看看SparkContext构造函数中的listenerBus: LiveListenerBus，JobProgressListener在SparkContext构造函数new出来之后就添加到listenerBus中了，代码如下：

_jobProgressListener = new JobProgressListener(_conf)
    listenerBus.addListener(jobProgressListener)

• LiveListenerBus.scala所在package位置：
  package org.apache.spark.scheduler

/**
 * Asynchronously passes SparkListenerEvents to registered SparkListeners.
 *
 * Until `start()` is called, all posted events are only buffered. Only after this listener bus
 * has started will events be actually propagated to all attached listeners. This listener bus
 * is stopped when `stop()` is called, and it will drop further events after stopping.
 */
private[spark] class LiveListenerBus(val sparkContext: SparkContext) extends SparkListenerBus {

  self =>

  import LiveListenerBus._

  // Cap the capacity of the event queue so we get an explicit error (rather than
  // an OOM exception) if it's perpetually being added to more quickly than it's being drained.
  private lazy val EVENT_QUEUE_CAPACITY = validateAndGetQueueSize()
  private lazy val eventQueue = new LinkedBlockingQueue[SparkListenerEvent](EVENT_QUEUE_CAPACITY)

......

  // A counter that represents the number of events produced and consumed in the queue
  private val eventLock = new Semaphore(0)

  private val listenerThread = new Thread(name) {
    setDaemon(true)
    override def run(): Unit = Utils.tryOrStopSparkContext(sparkContext) {
      LiveListenerBus.withinListenerThread.withValue(true) {
        while (true) {
          eventLock.acquire()
          self.synchronized {
            processingEvent = true
          }
          try {
            val event = eventQueue.poll
            if (event == null) {
              // Get out of the while loop and shutdown the daemon thread
              if (!stopped.get) {
                throw new IllegalStateException("Polling `null` from eventQueue means" +
                  " the listener bus has been stopped. So `stopped` must be true")
              }
              return
            }
            postToAll(event)
          } finally {
            self.synchronized {
              processingEvent = false
            }
          }
        }
      }
    }
  }

  /**
   * Start sending events to attached listeners.
   *
   * This first sends out all buffered events posted before this listener bus has started, then
   * listens for any additional events asynchronously while the listener bus is still running.
   * This should only be called once.
   *
   */
  def start(): Unit = {
    if (started.compareAndSet(false, true)) {
      listenerThread.start()
    } else {
      throw new IllegalStateException(s"$name already started!")
    }
  }

  def post(event: SparkListenerEvent): Unit = {
    if (stopped.get) {
      // Drop further events to make `listenerThread` exit ASAP
      logError(s"$name has already stopped! Dropping event $event")
      return
    }
    val eventAdded = eventQueue.offer(event)
    if (eventAdded) {
      eventLock.release()
    } else {
      onDropEvent(event)
      droppedEventsCounter.incrementAndGet()
    }

    val droppedEvents = droppedEventsCounter.get
    if (droppedEvents > 0) {
      // Don't log too frequently
      if (System.currentTimeMillis() - lastReportTimestamp >= 60 * 1000) {
        // There may be multiple threads trying to decrease droppedEventsCounter.
        // Use "compareAndSet" to make sure only one thread can win.
        // And if another thread is increasing droppedEventsCounter, "compareAndSet" will fail and
        // then that thread will update it.
        if (droppedEventsCounter.compareAndSet(droppedEvents, 0)) {
          val prevLastReportTimestamp = lastReportTimestamp
          lastReportTimestamp = System.currentTimeMillis()
          logWarning(s"Dropped $droppedEvents SparkListenerEvents since " +
            new java.util.Date(prevLastReportTimestamp))
        }
      }
    }
  }

  /**
   * Stop the listener bus. It will wait until the queued events have been processed, but drop the
   * new events after stopping.
   */
  def stop(): Unit = {
    if (!started.get()) {
      throw new IllegalStateException(s"Attempted to stop $name that has not yet started!")
    }
    if (stopped.compareAndSet(false, true)) {
      // Call eventLock.release() so that listenerThread will poll `null` from `eventQueue` and know
      // `stop` is called.
      eventLock.release()
      listenerThread.join()
    } else {
      // Keep quiet
    }
  }
}

private[spark] object LiveListenerBus {
  // Allows for Context to check whether stop() call is made within listener thread
  val withinListenerThread: DynamicVariable[Boolean] = new DynamicVariable[Boolean](false)

  /** The thread name of Spark listener bus */
  val name = "SparkListenerBus"
}

LiveListenerBus的主要作用是异步的把SparkListenerEvents传递给已向它注册的SparkListeners。它是伴生类，具有伴生对象object LiveListenerBus。

LiveListenerBus内部维持着一个eventQueue: LinkedBlockingQueue[SparkListenerEvent]，即基于链表的阻塞队列，用于存放SparkListenerEvent。同时LiveListenerBus内部维持一个线程listenerThread: Thread，在调用LiveListenerBus的start和stop时，实际调用的是这个线程的start和join（Waits for this thread to die.）。对事件队列存取使用了一个eventLock: Semaphore

// A counter that represents the number of events produced and consumed in the queue
  private val eventLock = new Semaphore(0)

当成功放入一个事件到事件队列时，就调用eventLock.release()，信号量值加1，listener线程取事件之前通过eventLock.acquire()，阻塞直到有一个信号（信号量大于0），然后信号量减1，取出一个事件处理。另外，LiveListenerBus的stop函数里面也调用了eventLock.release()，意思就是通过让listener线程最后可以多取一个空事件，通过对空事件的处理，可以进行相关的停止操作：

// Call eventLock.release() so that listenerThread will poll `null` from `eventQueue` and know
      // `stop` is called.
      eventLock.release()

LiveListenerBus继承自trait SparkListenerBus。

• SparkListenerBus如下：

package org.apache.spark.scheduler

import org.apache.spark.util.ListenerBus

private[spark] trait SparkListenerBus
  extends ListenerBus[SparkListenerInterface, SparkListenerEvent] {

  protected override def doPostEvent(
      listener: SparkListenerInterface,
      event: SparkListenerEvent): Unit = {
    event match {
      case stageSubmitted: SparkListenerStageSubmitted =>
        listener.onStageSubmitted(stageSubmitted)
      case stageCompleted: SparkListenerStageCompleted =>
        listener.onStageCompleted(stageCompleted)
      case jobStart: SparkListenerJobStart =>
        listener.onJobStart(jobStart)
      case jobEnd: SparkListenerJobEnd =>
        listener.onJobEnd(jobEnd)
      case taskStart: SparkListenerTaskStart =>
        listener.onTaskStart(taskStart)
      ......
      case _ => listener.onOtherEvent(event)
    }
  }

trait SparkListenerBus只实现了一个方法doPostEvent，传递的参数为具体的listerner和event，运用模式匹配对各种事件调用该listener的事件处理函数（trait SparkListenerInterface的接口实现）

protected override def doPostEvent(
      listener: SparkListenerInterface,
      event: SparkListenerEvent): Unit

trait SparkListenerBus继承ListenerBus[SparkListenerInterface, SparkListenerEvent]

• trait ListenerBus:

package org.apache.spark.util
......
/**
 * An event bus which posts events to its listeners.
 */
private[spark] trait ListenerBus[L <: AnyRef, E] extends Logging {

  // Marked `private[spark]` for access in tests.
  private[spark] val listeners = new CopyOnWriteArrayList[L]

  /**
   * Add a listener to listen events. This method is thread-safe and can be called in any thread.
   */
  final def addListener(listener: L): Unit = {
    listeners.add(listener)
  }

  /**
   * Remove a listener and it won't receive any events. This method is thread-safe and can be called
   * in any thread.
   */
  final def removeListener(listener: L): Unit = {
    listeners.remove(listener)
  }

  /**
   * Post the event to all registered listeners. The `postToAll` caller should guarantee calling
   * `postToAll` in the same thread for all events.
   */
  final def postToAll(event: E): Unit = {
    // JavaConverters can create a JIterableWrapper if we use asScala.
    // However, this method will be called frequently. To avoid the wrapper cost, here we use
    // Java Iterator directly.
    val iter = listeners.iterator
    while (iter.hasNext) {
      val listener = iter.next()
      try {
        doPostEvent(listener, event)
      } catch {
        ......
      }
    }
  }

  /**
   * Post an event to the specified listener. `onPostEvent` is guaranteed to be called in the same
   * thread for all listeners.
   */
  protected def doPostEvent(listener: L, event: E): Unit

  private[spark] def findListenersByClass[T <: L : ClassTag](): Seq[T] = {
    ......
  }

}

ListenerBus内部维持着一个私有的CopyOnWriteArrayList[L]对象listeners，对ListenerBus的addListener和removeListener都是调用listeners的add和remove操作。这些操作都是线程安全的，CopyOnWriteArrayList内部实现对具有修改的操作都加锁了。

• CopyOnWriteArrayList就简单摘个描述吧：

package java.util.concurrent;
......
/**
 * A thread-safe variant of {@link java.util.ArrayList} in which all mutative
 * operations ({@code add}, {@code set}, and so on) are implemented by
 * making a fresh copy of the underlying array.
 * ......
 * /
 public class CopyOnWriteArrayList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8673264195747942595L;

    /** The lock protecting all mutators */
    final transient ReentrantLock lock = new ReentrantLock();
    ......

本次学习中的LinkedBlockingQueue及offer(in)和poll(out)操作，内部使用了重入锁ReentrantLock。暂且标记，后续继续了解。

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