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