Rebalance

Rebalance - a Kafka consumer rebalance is the group reassigning partitions when membership changes: a consumer joins, leaves, crashes, or just misses a heartbeat, and the group coordinator hands the partitions out again. Under the classic eager protocol every consumer first revokes everything it owns, so the whole group stops consuming until the new assignment settles: a stop-the-world pause triggered by any one member. The cooperative protocol softens this by moving only the partitions that actually change hands, but the failure mode everyone learns first is the rebalance storm: a consumer pauses too long (a GC, a slow poll loop), gets ejected, rejoins, and triggers the next rebalance, in a loop.

To be fair, this is the honest price of dynamic membership. If any consumer can die and any can join, someone has to redeal the cards.

Do you need it? Only if you run a consumer group. A single ubik process reads every partition of its topic itself, with no group membership to negotiate, so there is no rebalance to trigger and no pause to sit through. One reader, one checkpoint.