How to Trigger a Window in Flink When No Further Watermarks Are Received?

I'm working with Apache Flink for stream processing and I've encountered a scenario that I haven't been able to find a solution for in the documentation or online resources. My scenario involves processing data streams where each element is keyed and windowed for analysis. I'm using event time and tumbling windows for this purpose.

Here's a simplified version of my code setup:

DataStream<MyEvent> timestampedStream = myDataStream
        .assignTimestampsAndWatermarks(WatermarkStrategy
                .<MyEvent>forMonotonousTimestamps()
                .withTimestampAssigner((event, timestamp) -> event.getTimestamp()));

DataStream<MyResult> resultStream = timestampedStream
        .keyBy(MyEvent::getKey)
        .window(TumblingEventTimeWindows.of(Time.seconds(5)))
        .apply(new MyWindowFunction());

My issue arises when a key receives data for a window (e.g., 0-5s), but then receives no further data or watermarks. In such cases, it seems that the window never triggers, potentially leaving data unprocessed indefinitely.

What I'm looking for is a way to ensure that these windows can trigger and complete their computation, even if no new watermarks are generated for their specific key. This could involve setting some form of timeout or employing a custom trigger mechanism, but I haven't been able to figure out how to implement this in Flink.

Is there a recommended approach for handling such scenarios where a window needs to be triggered after a period of inactivity or no new watermarks for a specific key?

I've considered custom triggers but am unsure how to implement this specific requirement. Any suggestions or examples would be greatly appreciated. Thank you!

Solution

Watermarks are not generated on a per-key basis, but rather across all of the events in each parallel instance (sub-task) of the watermark operator -- or in the case of sources like Kafka, on a per-partition basis.

Nevertheless, it can happen that either a partition or sub-task becomes idle, in which case the current watermark will cease to advance, and windows will stall. The withIdleness method on the WatermarkStrategy is designed to help with this. So long as there are some events flowing for at least one key, then using withIdleness should solve your problem.

On the other hand, if there are no events at all, then in order to close the windows still waiting for watermarks, you could implement a custom Trigger that in addition to being triggered by watermarks, can also be triggered by a processing time timer.