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https://issues.apache.org/jira/browse/KAFKA-10383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17285225#comment-17285225
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Marco Lotz commented on KAFKA-10383:
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[~vvcephei] I have sent the email to Kafka's dev list requesting privileges to
create KIP last Thursday. It is still not granted yet. Was there the right
place to request the access to create the KIP?
> KTable Join on Foreign key is opinionated
> ------------------------------------------
>
> Key: KAFKA-10383
> URL: https://issues.apache.org/jira/browse/KAFKA-10383
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 2.4.1
> Reporter: Marco Lotz
> Assignee: Marco Lotz
> Priority: Major
> Labels: needs-kip
>
> *Status Quo:*
> The current implementation of [KIP-213
> |[https://cwiki.apache.org/confluence/display/KAFKA/KIP-213+Support+non-key+joining+in+KTable]]
> of Foreign Key Join between two KTables is _opinionated_ in terms of storage
> layer.
> Independently of the Materialization method provided in the method argument,
> it generates an intermediary RocksDB state store. Thus, even when the
> Materialization method provided is "in memory", it will use RocksDB
> under-the-hood for this internal state-store.
>
> *Related problems:*
> * IT Tests: Having an implicit materialization method for state-store
> affects tests using foreign key state-stores. [On windows based systems
> |[https://stackoverflow.com/questions/50602512/failed-to-delete-the-state-directory-in-ide-for-kafka-stream-application]],
> that are affected by the RocksDB filesystem removal problem, an approach to
> avoid the bug is to use in-memory state-stores (rather than exception
> swallowing). Having the intermediate RocksDB storage being created
> disregarding materialization method forces any IT test to necessarily use the
> manual FS deletion with exception swallowing hack.
> * Short lived Streams: Ktables can be short lived in a way that neither
> persistent storage nor change-logs creation are desired. The current
> implementation prevents this.
> *Suggestion:*
> One possible solution is to use a similar materialization method (to the one
> provided in the argument) when creating the intermediary Foreign Key
> state-store. If the Materialization is in memory and without changelog, the
> same happens in the intermediate state-sore.
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