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  • KIP 155 - Add range scan for windowed state stores
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Status

Current stateAccepted

Discussion thread: here

JIRA: KAFKA-5192 - Range Scan for Windowed State Stores Resolved

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

Motivation

Unlike for key-value state stores, Kafka Streams currently does not provide a way to query the range of keys available in a windowed state store.

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 given key, requiring the user to maintain the set of queryable keys separately. 

Public Interfaces

This KIP would add the following methods:

ReadOnlyWindowStore interface

The time range would follow the existing ReadOnlyWindowStore.fetch(K key, long timeFrom, long timeTo) behavior for the time range.

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

 

ReadOnlySessionStore interface

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

 

SessionStore interface

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

Proposed Changes

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

Compatibility, Deprecation, and Migration Plan

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

Rejected Alternatives

The existing return type in ReadOnlyWindowStore.fetch((K key, long timeFrom, long timeTo) is not ideal, since WindowStoreIterator is a KeyValueIterator<Long, V>, which abuses the key as a timestamp and the value as the object of interest. However we want to be able to return the keys as part of range scans, so we considered using WindowStoreIterator<KeyValue<K, V>> as the return type for windowed stores for consistency between single key and range scan methods.

It was pointed out that doing so would limit the usefulness of peekNextKey() on the iterator, while also being somewhat confusing, since the iterator key doesn't actually contain the keys. As a result, it seemed simpler to follow the model already used in ReadOnlySessionStore to return use a KeyValueIterator with a Windowed<K> as the key.

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