How to merge multiple streams with Kafka Streams

In this tutorial, we take a look at the case when you have multiple streams, but you'd like to merge them into one.
We'll use multiple streams of a music catalog to demonstrate.

Here's the first stream of rock music:

KStream<String, SongEvent> rockSongs = builder.stream(ROCK_MUSIC_INPUT, Consumed.with(stringSerde, songEventSerde));

And the stream of classical music:

 KStream<String, SongEvent> classicalSongs = builder.stream(CLASSICAL_MUSIC_INPUT, Consumed.with(stringSerde, songEventSerde));

To merge these two streams you'll use the KStream.merge operator:

KStream<String, SongEvent> allSongs = rockSongs.merge(classicalSongs);

The KStream.merge method does not guarantee any order of the merged record streams. The records maintain their ordering relative to the original source topic.