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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 record.key()  (also K) — so , 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 specified readOnlyKey would would be the join key, which for stream–GlobalKTable joins stream–GlobalKTable 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.

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