How does kafka streams compute watermarks?
Kafka Streams does not use watermarks internally, but a new feature in 2.1.0 lets you observe the result of a window when it closed. It's called
Suppressed, and you can read about it in the docs: Window Final Results:
KGroupedStream<UserId, Event> grouped = ...; grouped .windowedBy(TimeWindows.of(Duration.ofHours(1)).grace(ofMinutes(10))) .count() .suppress(Suppressed.untilWindowCloses(unbounded()))
Does Kafka Streams internally compute watermarks?
No. Kafka Streams follows a continuous update processing model that does not require watermarks. You can find more details about this online:
Is it possible to observe the result of a window (only) when it completes (i.e. when watermark passes end of window)?
You can observe of result of a window at any point in time. Either via subscribing to the result changelog stream via for example
KTable#toStream()#foreach() (ie, a push based approach), or via Interactive Queries that let you query the result window actively (ie, a pull based approach).
As mentioned by @Dmitry, for the push based approach, you can also use
suppress() operator if you are only interested in a window's final result.