KIP-904 — Kafka Streams - Guarantee subtractor is called before adder if

Accepted Kafka 3.5 Streams

KIP-904 changes `KTable.groupBy()` to emit a single combined event when the grouping key is unchanged (carrying both the old and new value together) instead of two separate tombstone/insert events, guaranteeing the subtractor is applied before the adder in the downstream aggregate. With the two-event approach, a stream-table join could observe the intermediate state after the subtractor fires but before the adder fires, producing a transiently inconsistent aggregate value that is visible to downstream consumers.

Details