Page History

...

For stream-table joins, the join key is the stream record's key, so the processor works correctly. For stream-globalTable joins, the join key is computed from the stream record via a user-supplied KeyValueMapper and is generally different from the stream record's key. However, the current KStreamKTableJoinProcessor#doJoin implementation passes the stream record's key into ValueJoinerWithKey.apply(), not the mapped join key.

g lThe The discrepancy is not surfaced at runtime: the DSL signature pins the joiner's first type parameter to the stream key type K, and the implementation passes passes record.key()(also K), so types align, and no error or warning is raised. The join still matches records correctly because matching is done by the KeyValueMapper, not the joiner. However, KIP-149 specified readOnlyKey would be the join key, which for stream–globalTable joins is the mapped key, not the stream key. Any joiner that uses readOnlyKey will silently produce output values that encode the stream key where it is expected to be the join key, with no error or warning surfaced.

...