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Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

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• Existing users with the optimization config turned on, will see their topologies failing. They will need to restart their applications to create the new topologies. This will not affect users that don't use optimizations.

Rejected Alternatives

Add to the DSL the operator selfJoin. We did not go with this approach as we prefer to push the complexity of the optimization to streams instead of to the user.

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1. This will make backwards-compatibility an a non-issue as a user that upgrades from an older version does not have access to this DSL. If they want to use a self-join, they have to manually make the change in their code.
2. The code for a join can be chained like: builder.stream("topic").selfJoin().map(...)... If we don't have the operator, the code for a self-join needs to be broken into two parts: create the stream and get a reference, use the reference in a join like: stream.join(stream)
3. Implementation is straight-forward as we don't need to implement an optimization rule to do the rewriting

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