Consumer groups and offsets

The Kafka consumer works by issuing fetch requests to the brokers leading the partitions it wants to consume, specifying its offset in the log with each request and receiving back a chunk of log beginning at that position. A topic is divided into totally ordered partitions, each consumed by exactly one consumer within each subscribing consumer group at any given time, so the position of a consumer in each partition is just a single integer — the offset of the next message to consume — which can be periodically checkpointed. This makes the equivalent of message acknowledgements very cheap compared with broker-side per-message state. Because consumers control their position, they can deliberately rewind to an old offset and re-consume data, which is essential when a consumer bug is discovered after messages were consumed. For applications built on the group rebalance protocol, static membership lets group members provide persistent entity ids so restarts and code deploys do not trigger rebalances; using it requires upgrading both the broker cluster and client apps to 2.3 or beyond. Scope: Apache Kafka. Reviewed October 4, 2026.