What is the purpose of a changelog topic in Kafka Streams?

• A. It caches recently accessed keys.
• B. It records changes to a state store so the state can be restored after a failure.
• C. It stores consumer group offsets.
• D. It holds broker health metrics.

Answer: (B)

A changelog topic is used to record changes to a state store so the state can be restored after a failure. In Kafka Streams, each state store gets its own changelog topic where every update to the store is written as a Kafka record. If a processor or a whole task crashes or the application restarts after a rebalance, the store can be rebuilt by replaying this changelog from the beginning or from the point of failure. These topics are typically configured as compacted, so the latest value for each key is retained, making restoration efficient. Standby stores also use changelog topics to recover quickly. This mechanism is about fault tolerance and state recovery, not about caching data, offsets, or broker metrics.