概述
在window执行过程篇也提到了定时器的注册，在flink中有很多定时器的使用，比如窗口trigger的触发、watermark的周期生成，其实定时器底层是依赖jdk的ScheduledThreadPoolExecutor来调度的。
入口
通过TimerService 接口注册和删除定时器。
public interface TimerService {
long currentProcessingTime();
long currentWatermark();
void registerProcessingTimeTimer(long time);
void registerEventTimeTimer(long time);
void deleteProcessingTimeTimer(long time);
void deleteEventTimeTimer(long time);
}
Flink 内部使用 InternalTimerService，可以设置 timer 关联的 namespace 和 key。 在 InternalTimeService 中注册的 timer 有两种类型，分别为基于系统时间的和基于事件时间的，它使用两个优先级队列分别保存这两种类型的 timer。Timer 则被抽象为接口 InternalTimer，每个 timer 有绑定的 key，namespace 和触发时间 timestamp，TimerHeapInternalTimer 是其具体实现。InternalTimerServiceImpl 内部的两个优先级队列会按照触发时间的大小进行排序。
Timer 只能在 KeyedStream 中使用，例如在KeyedProcessFunction中注册
ctx.timerService().registerEventTimeTimer(time);
ctx.timerService().registerProcessingTimeTimer(time);
执行过程
Processing time timer
public void registerProcessingTimeTimer(N namespace, long time) {
InternalTimer<K, N> oldHead = processingTimeTimersQueue.peek();
if (processingTimeTimersQueue.add(new TimerHeapInternalTimer<>(time, (K) keyContext.getCurrentKey(), namespace))) {
long nextTriggerTime = oldHead != null ? oldHead.getTimestamp() : Long.MAX_VALUE;
// check if we need to re-schedule our timer to earlier
if (time < nextTriggerTime) {
if (nextTimer != null) {
nextTimer.cancel(false);
}
nextTimer = processingTimeService.registerTimer(time, this::onProcessingTime);
}
}
}
当调用registerProcessingTimeTimer注册后，会尝试将当前time添加到优先级队列中，如果添加失败说明已经存在了同一个key同一个time的定时器已存在。如果当前time时间小于队列头的Timer时间，那么将当前时间注册为下一个待调度的定时器。 具体注册定时器的过程在SystemProcessingTimeService
/**
* Registers a task to be executed no sooner than time {@code timestamp}, but without strong
* guarantees of order.
*
* @param timestamp Time when the task is to be enabled (in processing time)
* @param callback The task to be executed
* @return The future that represents the scheduled task. This always returns some future,
* even if the timer was shut down
*/
@Override
public ScheduledFuture<?> registerTimer(long timestamp, ProcessingTimeCallback callback) {
// delay the firing of the timer by 1 ms to align the semantics with watermark. A watermark
// T says we won't see elements in the future with a timestamp smaller or equal to T.
// With processing time, we therefore need to delay firing the timer by one ms.
long delay = Math.max(timestamp - getCurrentProcessingTime(), 0) + 1;
// we directly try to register the timer and only react to the status on exception
// that way we save unnecessary volatile accesses for each timer
try {
return timerService.schedule(wrapOnTimerCallback(callback, timestamp), delay, TimeUnit.MILLISECONDS);
}
catch (RejectedExecutionException e) {
final int status = this.status.get();
if (status == STATUS_QUIESCED) {
return new NeverCompleteFuture(delay);
}
else if (status == STATUS_SHUTDOWN) {
throw new IllegalStateException("Timer service is shut down");
}
else {
// something else happened, so propagate the exception
throw e;
}
}
}
把callback回调函数封装为ScheduledTask放入线程池中等待调度。 当定时器触发时会回调onProcessingTime方法。触发Triggerable的onProcessingTime方法，执行所有满足条件的定时器，也就是用户实现的udf处理逻辑，并注册下一个定时器。
private void onProcessingTime(long time) throws Exception {
// null out the timer in case the Triggerable calls registerProcessingTimeTimer()
// inside the callback.
nextTimer = null;
InternalTimer<K, N> timer;
while ((timer = processingTimeTimersQueue.peek()) != null && timer.getTimestamp() <= time) {
processingTimeTimersQueue.poll();
keyContext.setCurrentKey(timer.getKey());
triggerTarget.onProcessingTime(timer);
}
if (timer != null && nextTimer == null) {
nextTimer = processingTimeService.registerTimer(timer.getTimestamp(), this::onProcessingTime);
}
}
Event time timer
Event time的定时器则依赖于水印的流动。
public void registerEventTimeTimer(N namespace, long time) {
eventTimeTimersQueue.add(new TimerHeapInternalTimer<>(time, (K) keyContext.getCurrentKey(), namespace));
}
注册后会添加到eventTime的优先级队列中，我们知道watermark是一直递增的，随着element在datastream中流动，当watermark被处理时
private void processElement(StreamElement recordOrMark, DataOutput<T> output) throws Exception {
if (recordOrMark.isRecord()){
output.emitRecord(recordOrMark.asRecord());
} else if (recordOrMark.isWatermark()) {
statusWatermarkValve.inputWatermark(recordOrMark.asWatermark(), lastChannel);
} else if (recordOrMark.isLatencyMarker()) {
output.emitLatencyMarker(recordOrMark.asLatencyMarker());
} else if (recordOrMark.isStreamStatus()) {
statusWatermarkValve.inputStreamStatus(recordOrMark.asStreamStatus(), lastChannel);
} else {
throw new UnsupportedOperationException("Unknown type of StreamElement");
}
}
之后会调用AbstractStreamOperator#processWatermark
public void processWatermark(Watermark mark) throws Exception {
if (timeServiceManager != null) {
timeServiceManager.advanceWatermark(mark);
}
output.emitWatermark(mark);
}
最终调用到InternalTimerServiceImpl#advanceWatermark
调用链路是StreamTaskNetworkInput#processElement→StatusWatermarkValve#inputWatermark→StatusWatermarkValve#findAndOutputNewMinWatermarkAcrossAlignedChannels→OneInputStreamTask#emitWatermark→ProcessOperator#processWatermark→AbstractStreamOperator#processWatermark→InternalTimeServiceManager#advanceWatermark→InternalTimerServiceImpl#advanceWatermark
public void advanceWatermark(long time) throws Exception {
currentWatermark = time;
InternalTimer<K, N> timer;
while ((timer = eventTimeTimersQueue.peek()) != null && timer.getTimestamp() <= time) {
eventTimeTimersQueue.poll();
keyContext.setCurrentKey(timer.getKey());
triggerTarget.onEventTime(timer);
}
}
这里就和processing time的类似，从优先级队列中取出满足触发条件的timer，调用udf的onEventTime执行具体逻辑。
总结
本文主要分析了定时器Timer在Flink内部的应用，分别分析了基于Processing time和Event time具体的执行逻辑。
