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"kafka streams" is an add on product to kafka

"Built-in" is a odd word choice there. Kafka Streams is a framework for building stream-processing applications on top of Kafka topics.

Kafka Streams is a streaming framework that uses Kafka's already existing features to implement itself - so resilience and parallelisation is implemented using consumer groups, exactly once using Kafka's transactions and idempotence, topics are used (as well as RocksDB) to store state for stateful aggregations etc. etc.

So unlike Flink, Storm, Spark, Heron etc. it's only useful with Kafka.