Marco Lotz created KAFKA-10383:
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Summary: 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: core
Affects Versions: 2.4.1
Reporter: Marco Lotz
*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 state-store.
*Related problems:*
* **IT Test: 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 have the RocksDB filesystem removal problem, a solution to avoid the bug
is to use in-memory state-stores (rather than exception swallowing). Having the
RocksDB storage being forcely created makes that any IT test necessarily use
the manual FS deletion with exception swallow hack.
* Short lived Streams: Sometimes, Ktables are short lived in a way that
neither Persistance storage nor changelogs are desired. The current
implementation prevents this.
*Suggestion:*
One possible solution is to use the same materialization method that is
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 state-sore.
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