Current state: Under Discussion

Discussion thread: here

JIRA: here


The KStream API provides merge(KStream) to merge another KStream with this. Sometimes, it may be useful to merge more than 2 KStreams together. Currently, the best way to do this is using Java's Stream.reduce:

List<KStream<K, V>> streams ...;, right) -> left.merge(right));

This creates a merge node in the process graph for every KStream in the collection being merged.

Complex process graphs can make understanding an application and debugging more difficult.

Public Interfaces

Two new methods will be added to KStream:

KStream<K, V> merge(Collection<KStream<K, V>> streams);
KStream<K, V> merge(Collection<K, V> streams, Named named);

Two static methods will also be added to KStream:

static <K, V> KStream<K, V> merged(Collection<KStream<K, V>> streams);
static <K, V> KStream<K, V> merged(Collection<KStream<K, V>> streams, Named named);

These are utility methods for merging a collection of KStreams when the user doesn't already have a KStream to invoke merge upon.

Proposed Changes

KStream will be updated with the above API, and KStreamImpl will have its implementation updated to match.

As with the existing implementation, if any of the input KStreams need to be repartitioned, the entire merged KStream will be repartitioned. If users only wish to repartition the sub-pairs of KStreams that need to be, they can fall-back to the previous strategy of iteratively merging pairs of KStream down using Stream#reduce 

The KStream that merge is called on is treated no differently in the merge than the KStreams in the Collection being merged. This is the same behaviour we have today, where the order of the KStreams being merged has no effect.

Compatibility, Deprecation, and Migration Plan

  • Due to ambiguity in the type, passing the null literal as the first parameter to merge  will no longer be possible without explicitly casting to either KStream<K, V>  or Collection<KStream<K, V>> first. Since passing null  for this argument will always produce a runtime error anyway, passing a null literal was never intended in the original API.
  • No other modifications, deprecations or removals are being made to the existing API.

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