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• If the partition is not assigned to a log directory (refers to Uuid.ZERO)
  • If the partition already exists, the broker uses the new RPC — AssignReplicasToDirs — to notify the controller to change the metadata assignment to the actual log directory.
  • If the partition does not exist, the broker selects a log directory and uses the new RPC — AssignReplicasToDirs — to notify the controller to create the metadata assignment to the actual log directory.
• If the partition is assigned to an online log directory
  • If the partition does not exist it is created in the indicated log directory.
  • If the partition already exists in the indicated log directory and no future replica exists, then no action is taken.
  • If the partition already exists in the indicated log directory, and there is a future replica in another log directory, then the broker starts the process to replicate the current replica to the future replica.
  • If the partition already exists in another online log directory and is a future replica in the log directory indicated by the metadata, the broker will replace the current replica with the future replica after making sure that the future replica is fully caught up with the current replica.
  • If the partition already exists in another online log directory, the broker uses the new RPC — AssignReplicasToDirs — to the controller to change the metadata assignment to the actual log directory. The partition might have been moved to a different log directory whilst the broker was offline. 
• If the partition is assigned to an unknown log directory
  • If there are offline log
  directory
  • directories, no action is taken — the
  controller is already aware of this, and
  • assignment refers to a a log directory which may be offline, we don't want to fill the remaining online log directories with replicas that existed in the offline ones.
  • If
  the partition is assigned to an unknown log directory
  • there are no offline directories, no action is taken
  — the controller is already aware of this and
  • either — the assignment refers to a log directory which was removed from configuration. The Controller will reassign the replica to
  one of the online log directories in a future metadata update. 
  • UUID.Zero, so that a new log directory may be chosen.

If the broker is configured with multiple log directories it remains FENCED until it can verify that all partitions are assigned to the correct log directories in the cluster metadata. This excludes the log directory that hosts the cluster metadata topic, if it is configured separately to a different path — using metadata.log.dir.

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