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The new configs are introduced for ELR

1. eligible.leader.replicas.enabled. It controls whether the controller will record the ELR-related metadata and whether ISR can be empty. False is the default value. It will turn true in the future.

The new configs are introduced for Unclean Recovery.

1. unclean.recovery.strategy. Described in the above section. Balanced is the default value. 
2. unclean.recovery.manager.enabled. True for using the unclean recovery manager to perform an unclean recovery. False means the random election will be used in the unclean leader election. False is the default value.
3. unclean.recovery.timeout.ms. The time limits of waiting for the replicas' response during the Unclean Recovery. 5 min is the default value.

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It will be guarded by a new metadata version and the eligible.leader.replicas.enabled. So it is not enabled during the rolling upgrade.

After the controller picked up the new MV and eligible.leader.replicas.enabled is true,  when it loads the partition states, it will populate the ELR as empty if the PartitionChangeRecord uses an old version. In the next partition update, the controller will record the current ELR. Note, the leader can only enable empty ISR after the new metadata version.

MV downgrade: Once the MV version is downgraded, all the ELR related fields will be removed on the next partition change. The controller will also ignore the ELR fields.

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