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With the new config, the leader election decision will be made in the following order when the current leader is fenced or the last ELR member registers with unclean shutdown:

1. If there are other ISR members, choose an ISR member.

2. If there are unfenced ELR members, choose an ELR member.

3. If there are fenced ELR members

   1. If the unclean.recovery.strategy=Proactive, then an unclean recovery will happen.

   2. Otherwise, we will wait for the fenced ELR members to be unfenced.

4. If there are no ELR members.

   1. If the unclean.recovery.strategy=balanced or ProactiveProactive, the controller will do the unclean recovery.

   2. If the unclean.recovery.strategy=Balanced, the controller will do the unclean recovery when all the LastKnownELR are unfenced. See the following section for the explanations.

   3. Otherwise, unclean.recovery.strategy=Manual, the controller will not attempt to elect a leader. Waiting for the user operations.

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The URM manages the recovery process for a leaderless partition. This new unclean recovery process takes the place of the unclean leader election. Instead of electing a random unfenced replica as the leader, the URM will query the log end offset and the leader epoch from each replica. The one with the highest leader epoch and the longest log end offset will be the new leader.The controller will trigger the unclean recovery when the leader is fenced and there is no other good candidate.

Workflow

The URM takes the topic partition info to initiate start an unclean recovery task. The fields used in the recovery:

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Replica IDs

Next, the URM will initiate the log query requests with a new component BrokerRequestSender(BRS) which handles the RPC request asynchronously. Then the query requests will be sent in a new GetReplicaLogInfo API. The response should include the following information for each partition:

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1. In Balance mode, all the LastKnownELR members have replied, plus the replicas replied within the timeout. Due to this requirement, the controller will only start the recovery if the LastKnownELR members are all unfenced.

2. In Proactive mode, any replicas replied within a fixed amount of time OR the first response received after the timeout. We don’t want to make a separate config for it so just make the fixed time of 5 seconds.

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