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背景

当我们想要实现提前触发计算的触发器时，我们可以使用ContinuousEventTimeTrigger/ContinuousProcessingTimeTrigger作为触发器达到比如几分钟触发一次计算并发送计算结果的类，我们本文就从代码角度解析下实现自定义触发器的一些注意事项

ContinuousEventTimeTrigger源码解析

@PublicEvolving
public class ContinuousEventTimeTrigger<W extends Window> extends Trigger<Object, W> {private static final long serialVersionUID = 1L;private final long interval;/** When merging we take the lowest of all fire timestamps as the new fire timestamp. */private final ReducingStateDescriptor<Long> stateDesc =new ReducingStateDescriptor<>("fire-time", new Min(), LongSerializer.INSTANCE);private ContinuousEventTimeTrigger(long interval) {this.interval = interval;}@Overridepublic TriggerResult onElement(Object element, long timestamp, W window, TriggerContext ctx)throws Exception {if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {// if the watermark is already past the window fire immediately// 这里只需要fire触发计算，为什么不是FIRE_AND_PURGE?return TriggerResult.FIRE;} else {// 这里注册一个结束窗口的计时器是否必要？ctx.registerEventTimeTimer(window.maxTimestamp());}ReducingState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);if (fireTimestamp.get() == null) {long start = timestamp - (timestamp % interval);long nextFireTimestamp = start + interval;ctx.registerEventTimeTimer(nextFireTimestamp);fireTimestamp.add(nextFireTimestamp);}return TriggerResult.CONTINUE;}@Overridepublic TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception {//为什么是FIRE而不是FIRE_AND_PURGEif (time == window.maxTimestamp()) {return TriggerResult.FIRE;}ReducingState<Long> fireTimestampState = ctx.getPartitionedState(stateDesc);Long fireTimestamp = fireTimestampState.get();if (fireTimestamp != null && fireTimestamp == time) {fireTimestampState.clear();fireTimestampState.add(time + interval);ctx.registerEventTimeTimer(time + interval);return TriggerResult.FIRE;}return TriggerResult.CONTINUE;}@Overridepublic TriggerResult onProcessingTime(long time, W window, TriggerContext ctx)throws Exception {return TriggerResult.CONTINUE;}@Overridepublic void clear(W window, TriggerContext ctx) throws Exception {// 清除计时器ReducingState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);Long timestamp = fireTimestamp.get();if (timestamp != null) {ctx.deleteEventTimeTimer(timestamp);fireTimestamp.clear();}}@Overridepublic boolean canMerge() {return true;}@Overridepublic void onMerge(W window, OnMergeContext ctx) throws Exception {ctx.mergePartitionedState(stateDesc);Long nextFireTimestamp = ctx.getPartitionedState(stateDesc).get();if (nextFireTimestamp != null) {ctx.registerEventTimeTimer(nextFireTimestamp);}}@Overridepublic String toString() {return "ContinuousEventTimeTrigger(" + interval + ")";}@VisibleForTestingpublic long getInterval() {return interval;}/*** Creates a trigger that continuously fires based on the given interval.** @param interval The time interval at which to fire.* @param <W> The type of {@link Window Windows} on which this trigger can operate.*/public static <W extends Window> ContinuousEventTimeTrigger<W> of(Time interval) {return new ContinuousEventTimeTrigger<>(interval.toMilliseconds());}private static class Min implements ReduceFunction<Long> {private static final long serialVersionUID = 1L;@Overridepublic Long reduce(Long value1, Long value2) throws Exception {return Math.min(value1, value2);}}
}

ContinuousProcessingTimeTrigger源码

@PublicEvolving
public class ContinuousProcessingTimeTrigger<W extends Window> extends Trigger<Object, W> {private static final long serialVersionUID = 1L;private final long interval;/** When merging we take the lowest of all fire timestamps as the new fire timestamp. */private final ReducingStateDescriptor<Long> stateDesc =new ReducingStateDescriptor<>("fire-time", new Min(), LongSerializer.INSTANCE);private ContinuousProcessingTimeTrigger(long interval) {this.interval = interval;}@Overridepublic TriggerResult onElement(Object element, long timestamp, W window, TriggerContext ctx)throws Exception {ReducingState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);timestamp = ctx.getCurrentProcessingTime();// 注册计时器，为什么这里不需要类似ContinuousEventTimeTrigger一样注册一个窗口结束时间的计时器？if (fireTimestamp.get() == null) {long start = timestamp - (timestamp % interval);long nextFireTimestamp = start + interval;ctx.registerProcessingTimeTimer(nextFireTimestamp);fireTimestamp.add(nextFireTimestamp);return TriggerResult.CONTINUE;}return TriggerResult.CONTINUE;}@Overridepublic TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception {return TriggerResult.CONTINUE;}@Overridepublic TriggerResult onProcessingTime(long time, W window, TriggerContext ctx)throws Exception {ReducingState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);if (fireTimestamp.get().equals(time)) {fireTimestamp.clear();fireTimestamp.add(time + interval);ctx.registerProcessingTimeTimer(time + interval);return TriggerResult.FIRE;}//为什么这里没有FIRE_AND_PURGE？状态是何时清理的？return TriggerResult.CONTINUE;}@Overridepublic void clear(W window, TriggerContext ctx) throws Exception {//清除计时器// State could be merged into new window.ReducingState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);Long timestamp = fireTimestamp.get();if (timestamp != null) {ctx.deleteProcessingTimeTimer(timestamp);fireTimestamp.clear();}}@Overridepublic boolean canMerge() {return true;}@Overridepublic void onMerge(W window, OnMergeContext ctx) throws Exception {// States for old windows will lose after the call.ctx.mergePartitionedState(stateDesc);// Register timer for this new window.Long nextFireTimestamp = ctx.getPartitionedState(stateDesc).get();if (nextFireTimestamp != null) {ctx.registerProcessingTimeTimer(nextFireTimestamp);}}@VisibleForTestingpublic long getInterval() {return interval;}@Overridepublic String toString() {return "ContinuousProcessingTimeTrigger(" + interval + ")";}/*** Creates a trigger that continuously fires based on the given interval.** @param interval The time interval at which to fire.* @param <W> The type of {@link Window Windows} on which this trigger can operate.*/public static <W extends Window> ContinuousProcessingTimeTrigger<W> of(Time interval) {return new ContinuousProcessingTimeTrigger<>(interval.toMilliseconds());}private static class Min implements ReduceFunction<Long> {private static final long serialVersionUID = 1L;@Overridepublic Long reduce(Long value1, Long value2) throws Exception {return Math.min(value1, value2);}}
}

疑问：

1.为什么ContinuousEventTimeTrigger需要注册一个窗口结束时间的计时器而ContinuousProcessingTimeTrigger不注册？
答案: 其实我们需要看下它注册后的目的作用是什么，ContinuousEventTimeTrigger的ontimer在处理窗口结束的触发器时会返回FIRE触发计算，那问题就来了，如果只是触发计算，那么如果没有，那么仅仅只是窗口结束的时候没有触发一次计算而已。所以这里不是必须的
2.为什么ContinuousEventTimeTrigger/ContinuousProcessingTimeTrigger返回的结果是FIRE而不是FIRE_AND_PURGE？状态是什么时候清理的？
答案： 首先要明确状态的清理这个逻辑，状态其实包括窗口的状态，触发器的状态等，返回FIRE仅仅是触发计算，而不会清理任何状态，而假设返回FIRE_AND_PURGE的作用是触发计算，并进行窗口状态的清理(注意这里是不包括触发器的状态的清理的)，其实状态的清理是由WindowOperator在清理时间到时进行的（对于事件时间是窗口结束时间+迟到容忍间隔时间，对于处理时间是窗口结束时间），所以不必要在窗口结束时间到的时候返回FIRE_AND_PURGE，可以统一由WindowOperator在清理时间到之后统一清理状态