What is the default WindowBytesStoreSupplier for stream-stream join in Kafka Streams?

Question

The new API has a signature of:

join(KStream<K,VO> otherStream, ValueJoiner<? super V,? super VO,? extends VR> joiner, JoinWindows windows, StreamJoined<K,V,VO> streamJoined)

If I only set store name using StreamJoined<K,V,VO> streamJoined parameter, what would be the default configuration of WindowBytesStoreSupplier in terms of retentionPeriod , windowSize and retainDuplicates ? It's not clear from documentation. Is this an in memory state store? Or default one configured by Kafka Streams?

Answer 1

If you only set the store name with StreamJoined then:

1. The windowSize comes from what you provide the JoinWindows configuration object. Even if you use a custom WindowBytesStoreSupplier , Kafka Streams validates that the supplier window settings match the settings of the provided JoinWindows object. So a JoinWindows.of(Duration.ofSeconds(30)) would have a windowSize equal to 30000 ms.
2. The retentionPeriod is the window size + the grace period. The default grace period is 24 hours.
3. The retainDuplicates configuration is true . But even when providing a custom StoreSupplier , the retainDuplicates field must be set to true .
4. The store type is a persistent (RocksDB) store, configured by Kafka Streams, as described above.

Also, note that with StreamJoined , you can now provide your own StoreSupplier for both sides of the join, so it's possible to have in-memory stores.

HTH,

Bill