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As part of the propagation of KRaft metadata back to ZooKeeper while in dual-write mode, we need to keep track of what has been synchronized. A new ZNode will be introduced to keep track of which KRaft record offset has been written back to ZK. This will be used to recover the synchronization state following a KRaft controller failover. 

ZNode /migration

{
  "lastOffset": 100,
  "lastTimestamp": "2022-01-01T00:00:00.000Z",
  "kraftControllerId": 3000,
  "kraftControllerEpoch": 1
}

By using conditional updates on this ZNode, will can fence old KRaft controllers from synchronizing data to ZooKeeper if there has been a new election.

Controller ZNodes

The two controller ZNodes "/controller" and "/controller_epoch" will be managed by the KRaft quorum during the migration. Rather than using ephemeral ZNodes, the KRaft controller will use a persistent ZNode for "/controller" to prevent ZK brokers from attempting to become the active controller. The "/controller_epoch" ZNode will be managed by the active KRaft controller and incremented anytime a new KRaft controller is elected.

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