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Event Stream Merger

An Event Streaming Application may contain multiple Event Stream instances. But in some cases, it may make sense for the application to merge the separate Event Streams into a single Event Stream, without changing the individual Events. While this may seem logically related to a join, this merge is a completely different operation. A join produces results by combining Events with the same key to produce a new Event, possibly of a different type. A merge of Event Streams combines the Events from multiple Event Streams into a single Event Stream, but the individual Events are unchanged and remain independent of each other.


How can an application merge separate Event Streams?




For Apache Kafka®, the Kafka Streams client library provides a merge operator in its DSL. This operator merges two Event Streams into a single Event Stream. We can then take the merged stream and perform any number of operations on it.

KStream<String, Event> eventStream =;
KStream<String, Event> eventStreamII =;
KStream<String, Event> allEventsStream = eventStream.merge(eventStreamII);



  • Kafka Streams provides no guarantees on the processing order of records from the underlying Event Streams.
  • In order for multiple Event Streams to be merged, they must use the same key and value types.


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