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  • ISR Shrink: it is not safe for the leader to assume a smaller ISR until the update has been successfully written. If we allow the high watermark to advance and the ISR shrink ultimately fails, then we risk violating replication semantics if the replica being evicted becomes leader. Therefore the leader will continue relying on the previous ISR until the LeaderAndIsr with the successful change has been received.
  • ISR Expand: it is always safe to let high watermark advance according to a superset of the ISR. When expanding the ISR, the leader will assume the larger set immediately and delay high watermark advancement.

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