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https://issues.apache.org/jira/browse/KAFKA-7497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16743468#comment-16743468
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Guozhang Wang commented on KAFKA-7497:
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>From the expressiveness of the operators, I think there are cases of stream 
>self-join that cannot be captured with stream aggregations still, since the 
>window is really "sliding" (but if we add a sliding window type aggregations, 
>it may equal to the semantics of streams self-join).

>From the API point of view, I think allowing stream self join even assuming 
>its use cases can be captured with sliding window aggregations still provides 
>programmability benefits. But the underlying implementation should be 
>different to any of our current internal impls. I think we can still have an 
>umbrella KIP that includes the following:

1. Add sliding window based aggregations.
2. Allow windowed stream self-join; and when detected it convert it to a 
sliding window based aggregation behind the scene for efficient 
implementations. 

> Kafka Streams should support self-join on streams
> -------------------------------------------------
>
>                 Key: KAFKA-7497
>                 URL: https://issues.apache.org/jira/browse/KAFKA-7497
>             Project: Kafka
>          Issue Type: New Feature
>          Components: streams
>            Reporter: Robin Moffatt
>            Priority: Major
>              Labels: needs-kip
>
> There are valid reasons to want to join a stream to itself, but Kafka Streams 
> does not currently support this ({{Invalid topology: Topic foo has already 
> been registered by another source.}}).  To perform the join requires creating 
> a second stream as a clone of the first, and then doing a join between the 
> two. This is a clunky workaround and results in unnecessary duplication of 
> data.



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