Kafka Streams: Add Cogroup in the DSL

When multiple streams aggregate together to form a single larger object (e.g. a shopping website may have a cart stream, a wish list stream, and a purchases stream. Together they make up a Customer), it is very difficult to accommodate this in the Kafka-Streams DSL: it generally requires you to group and aggregate all of the streams to KTables then make multiple outer join calls to end up with a KTable with your desired object. This will create a state store for each stream and a long chain of ValueJoiners that each new record must go through to get to the final object.

Creating a cogroup method where you use a single state store will:

• Reduce the number of gets from state stores. With the multiple joins when a new value comes into any of the streams a chain reaction happens where the join processor keep calling ValueGetters until we have accessed all state stores.

• Slight performance increase. As described above all ValueGetters are called also causing all ValueJoiners to be called forcing a recalculation of the current joined value of all other streams, impacting performance.