spark ListenerBus 监听器

Spark 源码中对livelistenerBus进行了这样的注释：

即所有spark消息SparkListenerEvents 被异步的发送给已经注册过的SparkListeners. 

在SparkContext中, 首先会创建LiveListenerBus实例,这个类主要功能如下:

• 保存有消息队列,负责消息的缓存
• 保存有注册过的listener,负责消息的分发

private val EVENT_QUEUE_CAPACITY = 10000
  private val eventQueue = new LinkedBlockingQueue[E](EVENT_QUEUE_CAPACITY)

private[spark] class LiveListenerBus
  extends AsynchronousListenerBus[SparkListener, SparkListenerEvent]("SparkListenerBus")
  with SparkListenerBus {

  private val logDroppedEvent = new AtomicBoolean(false)

  override def onDropEvent(event: SparkListenerEvent): Unit = {
    if (logDroppedEvent.compareAndSet(false, true)) {
      // Only log the following message once to avoid duplicated annoying logs.
      logError("Dropping SparkListenerEvent because no remaining room in event queue. " +
        "This likely means one of the SparkListeners is too slow and cannot keep up with " +
        "the rate at which tasks are being started by the scheduler.")
    }
  }

}

private val listenerThread = new Thread(name) {
    setDaemon(true)
    override def run(): Unit = Utils.tryOrStopSparkContext(sparkContext) {
      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
          }
        }
      }
    }
  }

整个思想就是典型的生产者消费者思想.为了保证生产者和消费者对消息队列的并发访问,在每次需要获取消息的时候,调用eventLock.acquire()来获取信号量, 信号量的值就是当前队列中所含有的事件数量.如果正常获取到事件,就调用postToAll将事件分发给所有listener, 继续下一次循环. 如果获取到null值, 则有下面两种情况:

1. 整个application正常结束, 此时stopped值已经被设置为true
2. 系统发生了错误, 立即终止运行

下面来看看生产者,代码如下:

def post(event: E) {
    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)
    }
  }

该函数用来将事件放入到消息队列中,每成功放入一个事件,就调用eventLock.release()来增加信号量额值.以供消费者线程来进行消费. 如果队列满了,就调用onDropEvent来处理, 该函数已经在上面列出,此处不再赘述.

真正的消息路由是由SparkListenerBus的onPostEvent函数完成的:

上面的代码很直观,根据不同的消息类型,调用listener对应的方法来进行处理. 
下面来看看listener的定义, 所有的listener都混入了SparkListener特质.该特质定义了针对所有消息的处理函数, 定义全部为空:

对于特定的listener,在混入SparkListener特质之后,只需要重写相应的处理函数即可. 为了方便在进行消息路由时进行模式匹配,所有的具体的listener类均被定义为样本类.

对于spark中的事件来说,思想与listener类似,只是混入的特质不同而已, 事件混入的特质为SparkListenerEvent.

消息队列建立及发送流程介绍:

具体的消息发送流程如下所示: 

在SparkContext中,会

1. 创建LiveListenerBus类类型的成员变量listenerBus
2. 创建各种listener,并加入到listenerBus中
3. post一些事件到listenerBus中
4. 调用listenerBus.start() 来启动事件处理程序

这里有一点需要注意的是, 在listenerBus.start() 调用之前, 可以向其中post消息, 这些消息会被缓存起来,等start函数调用之后, 消费者线程会分发这些缓存的消息. listenerBus.start()是在SparkContext中的setupAndStartListenerBus函数中被调用的, 下面来看看该函数的实现:

    // Use reflection to instantiate listeners specified via `spark.extraListeners`
    try {
      val listenerClassNames: Seq[String] =
        conf.get("spark.extraListeners", "").split(',').map(_.trim).filter(_ != "")
      for (className <- listenerClassNames) {
        // Use reflection to find the right constructor
        val constructors = {
          val listenerClass = Utils.classForName(className)
          listenerClass.getConstructors.asInstanceOf[Array[Constructor[_ <: SparkListener]]]
        }
        val constructorTakingSparkConf = constructors.find { c =>
          c.getParameterTypes.sameElements(Array(classOf[SparkConf]))
        }
        lazy val zeroArgumentConstructor = constructors.find { c =>
          c.getParameterTypes.isEmpty
        }
        val listener: SparkListener = {
          if (constructorTakingSparkConf.isDefined) {
            constructorTakingSparkConf.get.newInstance(conf)
          } else if (zeroArgumentConstructor.isDefined) {
            zeroArgumentConstructor.get.newInstance()
          } else {
            throw new SparkException(
              s"$className did not have a zero-argument constructor or a" +
                " single-argument constructor that accepts SparkConf. Note: if the class is" +
                " defined inside of another Scala class, then its constructors may accept an" +
                " implicit parameter that references the enclosing class; in this case, you must" +
                " define the listener as a top-level class in order to prevent this extra" +
                " parameter from breaking Spark's ability to find a valid constructor.")
          }
        }
        listenerBus.addListener(listener)
        logInfo(s"Registered listener $className")
      }
    } catch {
      case e: Exception =>
        try {
          stop()
        } finally {
          throw new SparkException(s"Exception when registering SparkListener", e)
        }
    }

    listenerBus.start(this)
    _listenerBusStarted = true
  }

A comma-separated list of classes that implement SparkListener; when initializing SparkContext, instances of these classes will be created and registered with Spark’s listener bus. If a class has a single-argument constructor that accepts a SparkConf, that constructor will be called; otherwise, a zero-argument constructor will be called. If no valid constructor can be found, the SparkContext creation will fail with an exception.

大意为:该设置制定的listener会在初始化SparkContext时被创建注册,然而对于listener的构造函数时有要求的:

• 有一个单参数构造函数且参数为SparkConf类型, 则该构造函数被调用
• 否则,如果有无参构造函数, 则将被调用
• 如果没有定义构造函数,则程序异常结束

当extraListeners被构造并注册之后, listenerBus.start被调用:

def start(sc: SparkContext) {
    if (started.compareAndSet(false, true)) {
      sparkContext = sc
      listenerThread.start()
    } else {
      throw new IllegalStateException(s"$name already started!")
    }
  }

与此同时,启动消费者线程listenerThread, 开始进行消息路由.

当程序运行结束后,会调用stop函数:

def stop() {
    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
    }
  }

这里可以看到,在stop函数中调用了eventLock.release()来增加信号量的值. 然而并未向消息队列中加入新的消息,这就导致在消费者线程listenerThread读取队列时会返回null值,进而达到结束listenerThread线程的目的.

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