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Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-1 to your own ticket]

Motivation

Kafka Streams enforces a strict non-null-key policy in the DSL across all key-dependent operations (like aggregations and joins).
For left-joins, it might make sense to still accept a `null`, and add the left-hand record with an empty right-hand-side to the result.
The defined semantics for left/outer join are: keep the stream record if no matching join record was found.
Thus, Kstreams todays behavior is incosistent with the defined semantics.

Public Interfaces

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

• streamleft KStream-table/globalTable
• left & /outer join stream-table left & outer joinstream
• left-foreign-key table-table left join: - allow foreign key may to be null.

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.Make the above operators emit

Compatibility, Deprecation, and Migration Plan

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