KIP-77 — Improve Kafka Streams Join Semantics

Accepted Kafka 1.0 Streams

Revises Kafka Streams KStream-KStream, KStream-KTable, and KTable-KTable join semantics to be more predictable: specifying that KTable-KTable joins produce results only when both sides have a value for a key, and clarifying windowed stream-stream join behavior for out-of-order records. The original join semantics produced counterintuitive results (e.g., joins triggered on tombstones, asymmetric left-join behavior) that diverged from user expectations rooted in SQL semantics.

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