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• The query and discovery APIs will not affect existing users.
• However, exposing state store names to the API will affect existing users, since the current interfaces will change. The new interfaces will not be backwards compatible.
• As we now need to handle concurrent access to the State Stores, this may incur some overhead on streams applications while querying is on-going. We will measure the overhead in benchmarks.
  
  

Rejected Alternatives

Querying directly from Kafka topic. We considered allowing users to query directly from a Kafka topic, instead of a state store. The namespace would be global in this case, but instead of worrying about the StateStore namespace, we would be interested in the topic names instead (each state store is often backed into a Kafka topic). A user would create a RocksDb StateStore and instruct it to cache data from the desired topic. From then on, queries are performed on that local state store using the query method of direct calls, just like the above KIP proposal. Discovery with this option is implicitly done by the consumer that reads from the topic.

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