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https://issues.apache.org/jira/browse/KAFKA-9923?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sophie Blee-Goldman resolved KAFKA-9923.
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Resolution: Not A Problem
[~cadonna] pointed out that this actually ins't a problem/has been fixed by
KAFKA-5804. We could certainly stand to do some cleaning up around the
duplicates handling but at least we aren't losing data!
> Join window store duplicates can be compacted in changelog
> -----------------------------------------------------------
>
> Key: KAFKA-9923
> URL: https://issues.apache.org/jira/browse/KAFKA-9923
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Reporter: Sophie Blee-Goldman
> Assignee: Bruno Cadonna
> Priority: Blocker
> Fix For: 2.6.0
>
>
> Stream-stream joins use the regular `WindowStore` implementation but with
> `retainDuplicates` set to true. To allow for duplicates while using the same
> unique-key underlying stores we just wrap the key with an incrementing
> sequence number before inserting it.
> This wrapping occurs at the innermost layer of the store hierarchy, which
> means the duplicates must first pass through the changelogging layer. At this
> point the keys are still identical. So, we end up sending the records to the
> changelog without distinct keys and therefore may lose the older of the
> duplicates during compaction.
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