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Removing a node without unregistering causes various issues, such as a newly created partition will still get assigned to the removed replicas, or preventing metadata version updates after an upgrade. If this happens, the cluster admin/operator needs to get the node id to unregister it. However, we admin cannot list the removed node using the describeQuorum API to unregister it because the describeQuorum response excludes observer nodes that haven't sent a heartbeat within the 5-minute observer session timeout.

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• Older versions of AdminClient will not include inactive observers in the QuorumInfo. Consequently, the new field for inactive observers returned by newer controllers will be ignoreduse the older version of the RPC, therefore will not see the field.
• When a newer AdminClient connects to older controllers, it will use the older protocol version and will not receive the field for inactive observers. If includeInactiveObserver  is set to true when communicating with older controllers, the field value set to null.

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