Can you explain what Kafka Streams is and how it is used for stream processing?

Kafka Streams is a library for stream processing built on top of Apache Kafka. It allows creating applications that read, process, and write data to Kafka in real-time.

Main features of Kafka Streams:

• Stream processing of data.
• Support for transformation, aggregation, filtering, and joining streams.
• High fault tolerance and scalability.
• Integration with Kafka without the need for a separate cluster.

Example usage: an application reads a stream of events from a Kafka topic, filters relevant messages, aggregates data, and writes the result to another topic.

StreamsBuilder builder = new StreamsBuilder();
KStream<String, String> source = builder.stream("input-topic");

KStream<String, String> filtered = source.filter((key, value) -> value.contains("important"));

filtered.to("output-topic");

KafkaStreams streams = new KafkaStreams(builder.build(), config);
streams.start();

Thus, Kafka Streams simplifies the creation of applications for stream data processing using Kafka.