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The more intrusive version gives the user better clarity that his resulting KTable is not only keyed by B but its also keyed by A. So he will be less surprised that in a theoretical later aggregation he might find the same B-key twice with different A-keys joined to them. On the other hand the less intrusive method doesn't need to introduce this wrapper class but let the user handle the need of having A' and B's key present in the output key himself. This might lead to a deeper understanding for the user + and serdes might be able to pack the data denser. 

Proposed Changes

Goal

With the two relations A,B and there is one A for each B and there may be many B's for each A. We want to implement a Set of processors that allows a user to create a new KTable where A and B are joined based on the reference to A in B. A and B are represented as KTable B being partitioned by B's key and A being partitioned by A's key.

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