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• As per KIP-866, a separate Controller quorum is setup first, and only then the existing brokers are reconfigured and upgraded.
• When configured for the migration and while still in ZK mode, brokers will:
  • update meta.properties to generate and include directory.id  and directory.ids;
  • send BrokerRegistrationRequest including the log directory UUIDs;
  • notify the controller of log directory failures via BrokerHeartbeatRequest.
• During the migration, the controller:
  • persists log directories indicated in broker registration requests in the cluster metadata;
  • relies on heartbeat requests to detect log directory failure instead of monitoring the ZK znode for notifications;
  • still uses full LeaderAndIsr requests to process log directory failures for any brokers still running in ZK mode.
• The brokers restarting into KRaft mode will want to stay fenced until their log directory assignments for all hosted partitions are persisted in the cluster metadata.
• The active controller will also check ensure that any given broker stay stays fenced until it learns of all partition to log directory assignments in that specific broker via the new AssignReplicasToDirs RPC.
• During the migration, replicas are assumed and assigned to log directory Uuid.ZERO until the actual log directory is learnt by the active controller from a broker running in KRaft mode.

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