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PartitionRecord and PartitionChangeRecord will both have a new Assignment field which replaces the current Replicas field:
Directories field

{ "name": "Replicas", "type":  "[]int32", "versions":  "0", "entityType": "brokerId",
  "about": "The replicas of this partition, sorted by preferred order." },
(...)
{ "name": "Assignment", "type": "[]ReplicaAssignment", "versions": "1+",
  "about": "The replicas of this partition, sorted by preferred order.", "fields": [
  { "name": "Broker", "type": "int32", "versions": "1+", "entityType": "brokerId",
    "about": "The broker ID hosting the replica." },
  { "name": "DirectoryDirectories", "type": "[]uuid", "versions": "1+",
    "about": "The log directory hosting each replica, sorted in the same exact order as the replicaReplicas field." }]}

Although not explicitly specified in the schema, the default value for Directory is Uuid.UNASSIGNED_DIR (Uuid.ZERO), as that's the default default value for UUID types.

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Currently, Replicas are considered offline if the hosting broker is offline. Additionally, replicas will also be considered offline if the replica references a log directory UUID (in the new field partitionRecord.Assignment.DirectoryDirectories) that is not present in the hosting Broker's latest registration under LogDirs and either:

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When configured with multiple log.dirs, as the broker catches up with metadata, and sees the partitions which it should be hosting, it will check the associated log directory UUID for each partition (partitionRecord.Assignment.DirectoryDirectories).

• If the partition is not assigned to a log directory (refers to Uuid.UNASSIGNED_DIR)
  • 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 or refers to Uuid.LOST_DIR
  • If there are offline log directories, no action is taken — the 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 there are no offline directories, 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.

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