KIP-1104 — Allow Foreign Key Extraction from Both Key and Value in KTable Joins

Accepted Kafka 4.1 Streams

Adds a BiFunction<K, V, KO> foreignKeyExtractor overload to KTable.join() and related DSL methods so that the foreign key can be derived from both the record key and value, not just the value. The existing Function<V, KO> extractor forced users to duplicate key fields into the record value whenever the foreign key depended on the message key, creating data redundancy and potential inconsistency.

Details