Can we use state variables in Flink's trigger? - triggers

Question

I am using Apache Flink's sliding window with trigger. I would like to use a state variable in the trigger. However, I did not find any rich function corresponding to trigger function.
inputStream.keyBy(new KeySelector<Integer, String>() {
#Override
public String getKey(Integer i) throws Exception {
return i;
}
}).window(SlidingEventTimeWindows.of(Time.seconds(windowSize), Time.seconds(slideStep)))
.trigger(new Trigger<String, TimeWindow> {
#Override
public TriggerResult onElement(String str, long l, TimeWindow timeWindow, TriggerContext triggerContext) throws Exception {
return TriggerResult.CONTINUE;
}
#Override
public TriggerResult onProcessingTime(long l, TimeWindow timeWindow, TriggerContext triggerContext) throws Exception {
return TriggerResult.FIRE;
}
#Override
public TriggerResult onEventTime(long l, TimeWindow timeWindow, TriggerContext triggerContext) throws Exception {
return TriggerResult.FIRE;
}
#Override
public void clear(TimeWindow timeWindow, TriggerContext triggerContext) throws Exception {
})
.apply(new RichWindowFunction<String, String, String, TimeWindow>() {...}
So my question is, can we use state variables inside trigger block?

Answer 1

Yes, Triggers can be stateful. Look at CountTrigger for an example. You'll see that it keeps a count of how many elements have been added to the window in
private final ReducingStateDescriptor<Long> stateDesc =
new ReducingStateDescriptor<>("count", new Sum(), LongSerializer.INSTANCE);
It then uses that state in its onElement method
public TriggerResult onElement(Object element, long timestamp, W window, TriggerContext ctx)
throws Exception {
ReducingState<Long> count = ctx.getPartitionedState(stateDesc);
count.add(1L);
if (count.get() >= maxCount) {
count.clear();
return TriggerResult.FIRE;
}
return TriggerResult.CONTINUE;
}
and clears it in its clear method
public void clear(W window, TriggerContext ctx) throws Exception {
ctx.getPartitionedState(stateDesc).clear();
}
Keep in mind that if you are implementing a mergeable window type, then your Trigger will have to use state that can be merged, and then merge it when windows are merged. (Since you are using some flavor of sliding window, you can probably ignore this.)