Seems like there are many questions on SO and related about "how do I know when my windowed aggregation is 'done'?" The answer has always been "It's never done. You're thinking about it the wrong way."
I propose a new function for KStream: finiteAggregateByKey(Initializer, Aggregator, Windows, Timeout) where 'Timeout' would be a struct of how many seconds after the window is finished and a topic to dump the resultant values to. EG I have a topic 'bar' with an 'rolling' aggregator of 20 minutes / 1 minute. During the 20 minutes it's building a list of things from the 'bar' topic. I tell it to terminate 5 minutes after the initial 20 have elapsed and write out the final values from 'T <https://kafka.apache.org/0100/javadoc/org/apache/kafka/streams/kstream/Aggregator.html> aggregate' to a topic 'allBar'. This would allow me to put a consumer on that topic and watch for new entries. Subsequent values that would have matched said aggregator-window above are ignored.
