What is Kafka Streams?

Kafka Streams is a lightweight Java client library (not a separate cluster) for building streaming applications that read from, process, and write to Kafka. It provides a high-level DSL (filter, map, groupBy, aggregate, join) and a lower-level Processor API. State is stored in local RocksDB instances replicated back to Kafka for fault tolerance. It supports stateless and stateful processing, windowed aggregations, and exactly-once semantics. It's ideal for real-time data transformation pipelines.

Expected key concepts: client library, Java, stream processing, DSL, filter, map, groupBy, aggregate, join, RocksDB, stateful, windowed, exactly-once