Page History

...

With this KIP, the default value of partition.assignment.strategy changes from "RangeAssignor"  to "CooperativeStickyAssignor, RangeAssignor". Since the assignor list is ordered by preference, this means that the "CooperativeStickyAssignor" will become the new default partition assignor (once all members have been upgraded/use used the new default).

However this in itself would not effect the rebalancing protocol, as this is chosen during startup as "the highest protocol which is commonly supported by all assignors". So in addition to changing the default assignor, this static rebalance protocol selection will be replaced by a dynamic protocol that depends on the actual assignor that was selected for use in the rebalance. This means that cooperative rebalancing will also become enabled by default.

...

No special upgrade path is necessary, and this change should be transparent to the user – they will just see that cooperative rebalancing is enabled.

...