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Moreover, we will hint to the DSL users via Java docs that they have now have the option to distinguish between the following scenarios within a left join operator:
"null-key" v.s. "not-null-key but null-value" by passing a 'ValueJoinerWithKey' instead of 'ValueJoiner' to the left join operator.
The remark will be made in the Java docs of all left join methods with a 'ValueJoiner' as a parameter.

Furthermore, as part of this KIP the above information on how to keep the old behavior will also be documented here: https://kafka.apache.org/documentation/streams/upgrade-guide

Why not make this change opt in?

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