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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.spark.util
import java.util.concurrent._
import java.util.concurrent.atomic.AtomicBoolean
import org.apache.spark.SparkContext
/**
* Asynchronously passes events to registered listeners.
*
* 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.
*
* @param name name of the listener bus, will be the name of the listener thread.
* @tparam L type of listener
* @tparam E type of event
*/
private[spark] abstract class AsynchronousListenerBus[L <: AnyRef, E](name: String)
extends ListenerBus[L, E] {
self =>
private var sparkContext: SparkContext = null
/* 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 val EVENT_QUEUE_CAPACITY = 10000
private val eventQueue = new LinkedBlockingQueue[E](EVENT_QUEUE_CAPACITY)
// Indicate if `start()` is called
private val started = new AtomicBoolean(false)
// Indicate if `stop()` is called
private val stopped = new AtomicBoolean(false)
// Indicate if we are processing some event
// Guarded by `self`
private var processingEvent = false
// 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) {
while (true) {
eventLock.acquire()
self.synchronized {
processingEvent = true
}
try {
if (stopped.get()) {
// Get out of the while loop and shutdown the daemon thread
return
}
val event = eventQueue.poll
assert(event != null, "event queue was empty but the listener bus was not stopped")
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.
*
* @param sc Used to stop the SparkContext in case the listener thread dies.
*/
def start(sc: SparkContext) {
if (started.compareAndSet(false, true)) {
sparkContext = sc
listenerThread.start()
} else {
throw new IllegalStateException(s"$name already started!")
}
}
def post(event: E) {
if (stopped.get) {
// Drop further events to make `listenerThread` exit ASAP
logError(s"$name has already stopped! Dropping event $event")
return
}
val eventAdded = eventQueue.offer(event)
if (eventAdded) {
eventLock.release()
} else {
onDropEvent(event)
}
}
/**
* For testing only. Wait until there are no more events in the queue, or until the specified
* time has elapsed. Throw `TimeoutException` if the specified time elapsed before the queue
* emptied.
* Exposed for testing.
*/
@throws(classOf[TimeoutException])
def waitUntilEmpty(timeoutMillis: Long): Unit = {
val finishTime = System.currentTimeMillis + timeoutMillis
while (!queueIsEmpty) {
if (System.currentTimeMillis > finishTime) {
throw new TimeoutException(
s"The event queue is not empty after $timeoutMillis milliseconds")
}
/* Sleep rather than using wait/notify, because this is used only for testing and
* wait/notify add overhead in the general case. */
Thread.sleep(10)
}
}
/**
* For testing only. Return whether the listener daemon thread is still alive.
* Exposed for testing.
*/
def listenerThreadIsAlive: Boolean = listenerThread.isAlive
/**
* Return whether the event queue is empty.
*
* The use of synchronized here guarantees that all events that once belonged to this queue
* have already been processed by all attached listeners, if this returns true.
*/
private def queueIsEmpty: Boolean = synchronized { eventQueue.isEmpty && !processingEvent }
/**
* Stop the listener bus. It will wait until the queued events have been processed, but drop the
* new events after stopping.
*/
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
}
}
/**
* If the event queue exceeds its capacity, the new events will be dropped. The subclasses will be
* notified with the dropped events.
*
* Note: `onDropEvent` can be called in any thread.
*/
def onDropEvent(event: E): Unit
}
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