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Users of Streams have reported difficulty in achieving their desired results with the existing cache-based model. This is variously due to confusion (because the model yields complex interactions with the various operators) or because of actual bugs in the implementation (but the mentioned complexity makes it difficult to decide if observed behavior is a bug or not). Having a more explicit and semantically tighter controls will make improve both of these struggles.

"Final

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result" of a window aggregation

Windowed computations in particular present a unique opportunity. The output of a windowed computation is a KTable in which the keys are annotated with window ids. Because the window has a defined end, we know that the keys belonging to that window can never be modified again once that end time has passed, except by late-arriving events. If we bound the lateness of events, we can know the "final value" of the window. This is especially useful for taking irrevocable actions in response to window results, such as sending an email or alert when a windowed aggregation is below a certain threshold.

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