java - Kafka Streams API pushes old values to a topic only after a new event arrives

Question

I have surfed the internet for some time before writing the question, and it seems that this problem is not so much popular (not a mention of it anywhere). I use Kafka Streams API in my project for a calculation of something. I won't go into details of the algorithm, because it is not important here at all, what I have a problem with is that old values that are to be pushed to the topic are not pushed there until new values arrive. I guess it is because I use windowing and suspend the window, but I would like to somehow solve the problem (not with hacky solutions like pushing empty events). In this particular case, I need to wait till the end of the window before publishing the value to the output topic. The topology looks as follows:

firstStream.leftJoin(secondStream, ...)
    .leftJoin(thirdStream, ...)
    .filter(...)
    .groupByKey(...)
    .windowedBy(TimeWindows.of(Duration.ofSeconds(windowSeconds)).grace(Duration.ofSeconds(graceSeconds)))
    .aggregate(...)
    .suppress(Suppressed.untilWindowCloses(Suppressed.BufferConfig.unbounded()))
    .toStream()
    .to(outputTopic)

So, whenever I push an event to the firstStream, I should receive an output of the transformations in the outputTopic after the window closes, but actually, I receive the output only when I push a new event after the window closes. So, my question is whether there is a way to push output after the window closes without sending a new event?

question from:https://stackoverflow.com/questions/65917910/kafka-streams-api-pushes-old-values-to-a-topic-only-after-a-new-event-arrives

Answer 1

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