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1. Limited Accessibility: The node executing the tool must have direct access to the metadata path of the node being added or removed. This restricts the ability to use node A to manage node B, as node A may not have access to the metadata folder on node B.

2. Dependency on Node Configuration: The tool requires access to the configuration of the node being managed.

However, the essential information for these operations — the directory UUID and endpoints — is already available from the active controller’s in-memory state and the ClusterImage.

Leveraging these sources allows us to simplify voter addition and removal, enabling the command to run without direct access to the target node’s metadata directory.

Public Interfaces

CLI

Adding a controller

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Proposed Changes

Server side changes

• During  AddRaftVoterRequest handling, if api version >= 2,
  • the voter directory id is derived from in-memory LeaderState when the value is Uuid.ZERO_UUID,
  • the controller endpoints are derived from ClusterImage if endpoint set is empty, note that the ClusterImage may lag behind actual state, so endpoints are not strictly idempotent.

• During AddRaftVoterRequest handing, if multiple observers share the same node ID, reject with IllegalStateException indicating the duplicate node ID and instruct the user to resolve the conflict.

• During RemoveRaftVoterRequest handing, if api version >=1, the voter directory id is derived from in-memory LeaderState when the value is Uuid.ZERO_UUID.

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• Two convenience methods for adding and removing controllers have been introduced in Admin.java, addRaftVoter documented with Javadoc warnings about idempotency risks, and are intended for use only when the user understands and accepts those risks.

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Integration tests will be added for the two new methods in Admin.java.

Rejected Alternatives

• Deprecate —-command-config option in add-controller and --controller-directory-id option in remove-controller.

  The main reason not to deprecate these two parameters is that they were only just introduced in 4.0, so deprecating them in a 4.x release feels a bit too soon. Also, the --command-config can be used in a different user scenario, where the user can still provide the configuration file to add-controller if they already have it locally.

• Using admin APIs to retrieve directory UUID and controller endpoints, but this brings extra network communication overhead.

  1. The Admin#describeMetadataQuorum method can provide the directory UUID.

  2. The Admin#describeConfigs method, utilizing the bootstrap.controller address, can be used to retrieve the necessary endpoints.