How to send final kafka-streams aggregation result of a time windowed KTable?

Question

What I'd like to do is this:

1. Consume records from a numbers topic (Long's)
2. Aggregate (count) the values for each 5 sec window
3. Send the FINAL aggregation result to another topic

My code looks like this:

KStream<String, Long> longs = builder.stream(             Serdes.String(), Serdes.Long(), "longs");  // In one ktable, count by key, on a five second tumbling window. KTable<Windowed<String>, Long> longCounts =              longs.countByKey(TimeWindows.of("longCounts", 5000L));  // Finally, sink to the long-avgs topic. longCounts.toStream((wk, v) -> wk.key())           .to("long-counts"); 

It looks like everything works as expected, but the aggregations are sent to the destination topic for each incoming record. My question is how can I send only the final aggregation result of each window?

Answer 1

In Kafka Streams there is no such thing as a "final aggregation". Windows are kept open all the time to handle out-of-order records that arrive after the window end-time passed. However, windows are not kept forever. They get discarded once their retention time expires. There is no special action as to when a window gets discarded.

See Confluent documentation for more details: http://docs.confluent.io/current/streams/

Thus, for each update to an aggregation, a result record is produced (because Kafka Streams also update the aggregation result on out-of-order records). Your "final result" would be the latest result record (before a window gets discarded). Depending on your use case, manual de-duplication would be a way to resolve the issue (using lower lever API, transform() or process())

This blog post might help, too: https://timothyrenner.github.io/engineering/2016/08/11/kafka-streams-not-looking-at-facebook.html

Another blog post addressing this issue without using punctuations: http://blog.inovatrend.com/2018/03/making-of-message-gateway-with-kafka.html

Update

With KIP-328, a KTable#suppress() operator is added, that will allow to suppress consecutive updates in a strict manner and to emit a single result record per window; the tradeoff is an increase latency.

Answer 2

From Kafka Streams version 2.1, you can achieve this using suppress.

There is an example from the mentioned apache Kafka Streams documentation that sends an alert when a user has less than three events in an hour:

KGroupedStream<UserId, Event> grouped = ...; grouped   .windowedBy(TimeWindows.of(Duration.ofHours(1)).grace(ofMinutes(10)))   .count()   .suppress(Suppressed.untilWindowCloses(unbounded()))   .filter((windowedUserId, count) -> count < 3)   .toStream()   .foreach((windowedUserId, count) -> sendAlert(windowedUserId.window(), windowedUserId.key(), count)); 

As mentioned in the update of this answer, you should be aware of the tradeoff. Moreover, note that suppress() is based on event-time.