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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.{BlockingQueue, LinkedBlockingDeque}
import java.util.concurrent.atomic.AtomicBoolean
import scala.util.control.NonFatal
import org.apache.spark.internal.Logging
/**
* An event loop to receive events from the caller and process all events in the event thread. It
* will start an exclusive event thread to process all events.
*
* Note: The event queue will grow indefinitely. So subclasses should make sure `onReceive` can
* handle events in time to avoid the potential OOM.
*/
private[spark] abstract class EventLoop[E](name: String) extends Logging {
private val eventQueue: BlockingQueue[E] = new LinkedBlockingDeque[E]()
private val stopped = new AtomicBoolean(false)
private val eventThread = new Thread(name) {
setDaemon(true)
override def run(): Unit = {
try {
while (!stopped.get) {
val event = eventQueue.take()
try {
onReceive(event)
} catch {
case NonFatal(e) => {
try {
onError(e)
} catch {
case NonFatal(e) => logError("Unexpected error in " + name, e)
}
}
}
}
} catch {
case ie: InterruptedException => // exit even if eventQueue is not empty
case NonFatal(e) => logError("Unexpected error in " + name, e)
}
}
}
def start(): Unit = {
if (stopped.get) {
throw new IllegalStateException(name + " has already been stopped")
}
// Call onStart before starting the event thread to make sure it happens before onReceive
onStart()
eventThread.start()
}
def stop(): Unit = {
if (stopped.compareAndSet(false, true)) {
eventThread.interrupt()
var onStopCalled = false
try {
eventThread.join()
// Call onStop after the event thread exits to make sure onReceive happens before onStop
onStopCalled = true
onStop()
} catch {
case ie: InterruptedException =>
Thread.currentThread().interrupt()
if (!onStopCalled) {
// ie is thrown from `eventThread.join()`. Otherwise, we should not call `onStop` since
// it's already called.
onStop()
}
}
} else {
// Keep quiet to allow calling `stop` multiple times.
}
}
/**
* Put the event into the event queue. The event thread will process it later.
*/
def post(event: E): Unit = {
eventQueue.put(event)
}
/**
* Return if the event thread has already been started but not yet stopped.
*/
def isActive: Boolean = eventThread.isAlive
/**
* Invoked when `start()` is called but before the event thread starts.
*/
protected def onStart(): Unit = {}
/**
* Invoked when `stop()` is called and the event thread exits.
*/
protected def onStop(): Unit = {}
/**
* Invoked in the event thread when polling events from the event queue.
*
* Note: Should avoid calling blocking actions in `onReceive`, or the event thread will be blocked
* and cannot process events in time. If you want to call some blocking actions, run them in
* another thread.
*/
protected def onReceive(event: E): Unit
/**
* Invoked if `onReceive` throws any non fatal error. Any non fatal error thrown from `onError`
* will be ignored.
*/
protected def onError(e: Throwable): Unit
}
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