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The only available alternative is to aggregate all the distinct keys into a single key for a given window, or to implement your own windowed state store.

Similarly, session stores currently only offer querying by a single key,  

Public Interfaces

This KIP would add the following method to the methods:

ReadOnlyWindowStore interface

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Key range behavior would be consistent with the existing ReadOnlyKeyValueStore.range(K from, K to) behavior.

ReadOnlySessionStore interface

KeyValueIterator<Windowed<K>, AGG> fetch(final K from, final K to)

Key range behavior would be consistent with the existing ReadOnlyKeyValueStore.range(K from, K to) behavior.

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This KIP proposes to add the interface described above and implement range scan returning all the entries in the given key range that also match the given time interval.

Compatibility, Deprecation, and Migration Plan

• Users implementing their own windowed state stores would be affected by the interface changes.

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Arguably, the return type in ReadOnlyWindowStore is not ideal, since WindowStoreIterator is already a KeyValueIterator which abuses the key as a timestamp and the value as the object of interest. However, given that the existing API already abuses the iterator in a similar way, and barring more invasive changes to the API, or the introduction of type that embeds all of timestamp, key, and value – which would probably be a much larger discussion in itself – it seemed simpler to follow the existing model.