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This KIP won't change any public interfaces. However, It will change the behavior of the following operators:

• left KStream-Ktable/GlobalTablejoin Kstream-Kstream
• outer joinleft/outer Kstream-Kstream
• left-foreign-key join Ktable-Ktable - allow foreign key to be null.
• left  join KStream-Ktable/GlobalTable

Proposed Changes

• left join Kstream-Kstream: no longer drop left records with null-key and call ValueJoiner with 'null' for right value.
• outer join Kstream-Kstream: no longer drop left/right records with null-key and call ValueJoiner with 'null' for right/left value.
• left-foreign-key join Ktable-Ktable: no longer drop left records with null-foreign-key returned by the ForeignKeyExtractor and call ValueJoiner with 'null' for right value.
• left join KStream-Ktable/GlobalTable: no longer drop left records with null-key and call ValueJoiner with 'null' for right value.

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Users who want to keep the current behavior can prepend a .filter() operator to the aforementioned operators and filter accordingly.

E.g.

//

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new if old behavior is needed
leftStream.filter((key, value) -> key != null)

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          .leftJoin(rightStream, (lv, rv) -> join(lv, rv), windows);

Why not make this change opt in?

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