Status

Current state: Under Discussion Accepted

Discussion thread: https://lists.apache.org/thread.html/600996d83d485f2b8daf45037de64a60cebdfac9b234bf3449b6b753@%3Cdev.kafka.apache.org%3E

JIRA: https://issues.apache.org/jira/browse/

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

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public interface ConnectedStoreProvider {
    default Collection<StoreBuilder>Set<StoreBuilder> stores() {
        return null;
    }
}

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The process method would add the StoreBuilders to the topology using builder.addStateStore() and connect the store to that processor, rather than requiring the user to do it themselves.  In order to solve the problem of addStateStore potentially being called twice for the same store (because more than one Supplier specifies it), the check for duplicate stores in addStateStores will be relaxed to allow for duplicates if the same StoreBuilder instance for the same store namename (compared by referenced, not equals() ).

Compatibility, Deprecation, and Migration Plan

Because the added interface methods are default with a reasonable default, those additions are backwards compatible.  However, given that now there would be two ways

A user may continue to "connect" state stores to a low level processor, we would have to specify how they behave together.  I believe the correct decision is to enforce that for a given call of processor by passing stateStoreNames when calling stream.process/transform(...)  or stream.transform(...) it should only be possible to specify state stores one of the two ways, either through the stateStoreNames  argument or by implementing the stores method of ConnectedStoreProvider .  Specifically: if ConnectedStoreProvider::stores returns non-null and stateStoreNames is passed, an exception will be thrown making it clear that the user should choose one way or the other.  This may be used in combination with a Supplier  that provides its own state stores by implementing ConnectedStoreProvider::stores() .

If a StoreBuilder  that was manually added is also returned by a ConnectedStoreProvider , there is no issue since adding a state store will now be idempotent.

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