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

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.

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.