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Improved batching: We are also providing the opportunity for additional batching when updating Zookeeper. For example, ISR expansions following a rolling restart are often not time sensitive. In the future, the controller could delay the expansion in order to do multiple updates at once. Here we take a simpler approach: when the leader receives a Fetch request from a follower, some number of partitions will come into sync. The leader will send the AlterIsr for all of these partitions. The controller will respond as soon as it receives the request and apply the ISR updates. Once they are complete, it will send LeaderAndIsr updates for all affected partitions. This handles a common case today during a rolling restart when a broker comes back online and is added to many ISRs at once.

It can happen that a leader and ISR update fails after the AlterIsr was acknowledged. For example, if the controller fails before applying the changes, a new controller will be elected. It could also happen that there was already a pending change at the time the leader wanted to update ISR. In either case, the leader will receive a LeaderAndIsr update with the new version. The leader will check the status of current replicas at that time to see whether any additional changes are needed.

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