DRAFT

Current state: Under Discussion

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

JIRA:

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 makes sense to still accept a `null`, and add the left-hand record with an empty right-hand-side to the result.
Similarily, for outer-joins it makes sense to keep the record.
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:

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

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 left value.
• left join KStream-Ktable/GlobalTable: emits record even when left key is null.

Compatibility, Deprecation, and Migration Plan

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

E.g.

//left join for when old behavior is needed
leftStream.filter((key, value) -> key != null)
          .leftJoin(rightStream, (lv, rv) -> join(lv, rv), windows);


Why not make this change opt in?
By default we would like the behavior a stream defined via DSL to be consistent with the defined semantics.

Part of this KIP is also to make a Section in the upgrade guide to further elaborate on how the old behavior can be achieved if needed.