...

public class Grouped<K, V> {
 	protected final Serde<K> keySerde;
 	protected final Serde<V> valueSerde;
 	protected final String name;
	protected final Integer numPartitions; // new

	public static <K, V> Grouped<K, V> with(final String name,
    	                                    final Serde<K> keySerde,
        	                                final Serde<V> valueSerde,
											final Integer numPartitions);
}

Proposed Changes

When configured number of partitions,  KStream   KStream application will first issue the topic lookup request and check whether the target topic is already up and running. If the target topic already exists, KStream application will log the error for topic creation failure, and still continue the initialization. For grouping operation, repartition topic gets a hint on how many partitions it should be created with. If repartition topic is not created yet, create one with specified numPartitions; otherwise use the upstream topic partition size as the new topic number of partitions. One open question is whether we should prevent destructive operation such as changing the repartition topic size if current topic numPartitions != configured numPartitions, since there could be potential data loss.

Backward Compatibility

This is a pure KStream library change that shouldn't affect previously setup applications. . If numPartitions is not configured by user, it will use a default value -1 which won't affect existing topology initialization logic.

Rejected Alternatives

N/A