Beginner6 min
Consumer Group Rebalance
Watch partition assignments change when consumers join or leave a group.
A rebalance reassigns partitions among the members of a consumer group, and each partition goes to exactly one consumer in the group. With the classic protocol and an eager assignor, every member gives up all of its partitions before the new assignment is made; the CooperativeStickyAssignor revokes only the partitions that must move. The default partition.assignment.strategy is RangeAssignor,CooperativeStickyAssignor, which uses the RangeAssignor's eager behaviour until you remove RangeAssignor from the list. The simulator starts with the classic protocol; the Consumer (KIP-848) switch on the Consumer tab shows the newer one.
The steps use the simulator and name the control to use. The outcomes they describe are what the simulator models; the sources below are what Apache documents.
Steps
0 of 6 steps completed
Verification
However many consumers a group has, the three partitions are shared out among them with each partition read by exactly one consumer; consumers beyond three have no partition.
The interview angle
Each partition is consumed by exactly one consumer in a group, so the partition count limits how many consumers can work in parallel. Static membership (group.instance.id with a larger session.timeout.ms) avoids rebalances when members restart briefly.
Sources
- Consumer configs (4.2 documentation) (opens in a new tab)
- KafkaConsumer Javadoc (4.2) (opens in a new tab)
- Design (4.2 documentation) (opens in a new tab)
- KIP-345: Static membership protocol (opens in a new tab)
Checked against Kafka 4.2 documentation on 11 October 2026. Draft: not yet reviewed by a Kafka specialist, and releases move on, so confirm against the current documentation before you act on it.