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https://issues.apache.org/jira/browse/KAFKA-14202?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17602107#comment-17602107
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Guozhang Wang commented on KAFKA-14202:
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Hey [~vvcephei] are you considering the scope for store key schemas only, or 
both keys and values?

> IQv2: Expose binary store schema to store implementations
> ---------------------------------------------------------
>
>                 Key: KAFKA-14202
>                 URL: https://issues.apache.org/jira/browse/KAFKA-14202
>             Project: Kafka
>          Issue Type: Improvement
>          Components: streams
>            Reporter: John Roesler
>            Priority: Minor
>
> One feature of IQv2 is that store implementations can handle custom queries. 
> Many custom query handlers will need to process the key or value bytes, for 
> example deserializing them to implement some filter or aggregations, or even 
> performing binary operations on them.
> For the most part, this should be straightforward for users, since they 
> provide Streams with the serdes, the store implementation, and the custom 
> queries.
> However, Streams will sometimes pack extra data around the data produced by 
> the user-provided serdes. For example, the Timestamped store wrappers add a 
> timestamp on the beginning of the value byte array. And in Windowed stores, 
> we add window timestamps to the key bytes.
> It would be nice to have some generic mechanism to communicate those schemas 
> to the user-provided inner store layers to support users who need to write 
> custom queries. For example, perhaps we can add an extractor class to the 
> state store context



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