ContinuousEventTimeTrigger触发细节探究

背景

现有业务需要在同一个页面展示全天人次和分时人次，全天和分时对应两个任务，消费同一个topic，两者都使用ContinuousEventTimeTrigger，每隔30s trigger window的计算，两任务operator的背压都是0，那么现在问题来了，如果两个任务间隔10s先后启动，那么之后这两个任务的trigger的时间点是相同的吗？

个人误解

误认为任务触发的时间为任务启动时间+n*trigger interval，即任务的触发时间和任务的启动时间有关，如下图

源码正解

ReducingState

ContinuousEventTimeTrigger类使用ReducingState来保存要trigger的时间戳（long型）

private final ReducingStateDescriptor<Long> stateDesc =
			new ReducingStateDescriptor<>("fire-time", new Min(), LongSerializer.INSTANCE);

ReducingState中reduce function为Min，功能为返回较小的时间戳，flink对此注释为
/** When merging we take the lowest of all fire timestamps as the new fire timestamp. */

private static class Min implements ReduceFunction<Long> {
     
		private static final long serialVersionUID = 1L;

		@Override
		public Long reduce(Long value1, Long value2) throws Exception {
     
			return Math.min(value1, value2);
		}
	}

定义好了state后，来看下操作state的两个方法

onElement

@Override
public TriggerResult onElement(Object element, long timestamp, W window, TriggerContext ctx) throws Exception {
     
        //如果window的最大时间戳<当前watermark,则fire,否则将window的最大时间戳注册为timer
		if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {
     
			// if the watermark is already past the window fire immediately
			return TriggerResult.FIRE;
		} else {
     
			ctx.registerEventTimeTimer(window.maxTimestamp());
		}
        //获取ReducingState,如果ReducingState.get为null,则使用根据element的timestamp注册一个timer
		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;
	}

注意以上代码的细节：

long start = timestamp - (timestamp % interval);
long nextFireTimestamp = start + interval;
ctx.registerEventTimeTimer(nextFireTimestamp);
fireTimestamp.add(nextFireTimestamp);

根据element的timestamp注册的timer为1970-01-01 08:00:00+N*trigger_interval,与任务启动时间无关

onEventTime

@Override
public TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception {
     

		if (time == window.maxTimestamp()){
     
			return TriggerResult.FIRE;
		}

		ReducingState<Long> fireTimestampState = ctx.getPartitionedState(stateDesc);

		Long fireTimestamp = fireTimestampState.get();

        //如果fireTimestamp==event_time，则触发计算
        //触发计算的第一步就是clear ReducingState,一以便注册下一个trigger时间
		if (fireTimestamp != null && fireTimestamp == time) {
     
			fireTimestampState.clear();
			fireTimestampState.add(time + interval);
			ctx.registerEventTimeTimer(time + interval);
			return TriggerResult.FIRE;
		}

		return TriggerResult.CONTINUE;
}