Status

Current state: Under Discussion

Discussion thread: https://lists.apache.org/thread/kc87jkgyvf9x7nwmgwrhx6fs6w0tqymj

JIRA: KAFKA-20395 - Getting issue details... STATUS

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Motivation

Kafka cluster membership is managed by the metadata layer, and consist mainly of two things: broker and controller registrations. Registrations are sent by brokers and controllers to the active controller via the BrokerRegistrationRequest  and ControllerRegistrationRequest RPCs, and are persisted to the metadata log via the BrokerRegistrationRecord and ControllerRegistrationRecord. 

The active controller uses registrations to determine the current members of the Kafka cluster, and is aware of each members' advertised listeners and supported feature ranges. Registration information is used for determining if all cluster members support a feature level during upgrades.

However, there is currently no way to unregister a controller, like there is for brokers via UnregisterBrokerRequest and UnregisterBrokerRecord. This means a stale controller registration from a controller that no longer exists can block feature upgrades. This KIP proposes adding support for operators to manually unregister controllers like they can with brokers.

Public Interfaces

New RPC 

UnregisterControllerRequest

{
  "apiKey": 93,
  "type": "request",
  "listeners": ["broker", "controller"],
  "name": "UnregisterControllerRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ControllerId", "type": "int32", "versions": "0+",
      "about": "The controller ID to unregister." }
  ]
}

UnregisterControllerResponse

{
  "apiKey": 93,
  "type": "response",
  "name": "UnregisterControllerResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "Duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
    { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The top-level error message, or `null` if there was no top-level error." }
  ]
}

New Metadata Record

UnregisterControllerRecord

{
  "apiKey": 29,
  "type": "metadata",
  "name": "UnregisterControllerRecord",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ControllerId", "type": "int32", "versions": "0+",
      "about": "The controller id." }
  ]
}

This KIP will also introduce a new MetadataVersion to support this new metadata record.

CLI changes

kafka-cluster 

Add a unregister-controller  for manually unregistering controllers. This command would be similar to how the unregister command works for brokers. Below is an example invocation.

kafka-cluster unregister-controller --controller-id 9990

kafka-metadata-quorum 

Add the --unregister flag to the kafka-metadata-quorum remove-controller command. When this flag is set, invoking this command will remove the controller as a KRaft voter and unregister it. Below is an example invocation.

kafka-metadata-quorum remove-controller --controller-id 9990 --controller-directory-id EXAMPLE_UUID --unregister

Proposed Changes

Controller Changes

The active controller will handle unregistering a controller in the same way it handles unregistering brokers. After a broker/controller is unregistered, it will no longer be part of the metadata image, so its registration will no longer be a part of subsequent snapshots. 

The registration manager of an unregistered controller already attempts to re-register with the active controller. This is to prevent accidental unregistrations. The intention for unregistration is for it to occur after the operator decommissions a controller node. This means the node is no longer part of the kafka cluster, and the controller process associated with this node is not expected to come back. 

Metadata Quorum Tool Changes

This new flag means that the remove-controller command could send two RPCs to the active controller, one to remove the node from the KRaft voter set, and another to remove the node's registration. The implementation of this command should be able to handle cases where:

• The node is part of the voter set and is registered
• The node is part of the voter set and is not registered
• The node is not part of the voter set and is registered
• The node is not part of the voter set and is not registered

Compatibility, Deprecation, and Migration Plan

Because this KIP is introducing a new metadata record alongside a new MetadataVersion, it means that existing clusters who have a stale controller registration will not be able to unregister it, and unblock feature upgrades thereafter. 

One approach is for the controller to allow unregistering controllers non-durably if the metadata version does not support the UnregisterControllerRecord. This gives operators a path to migrate existing clusters onto a MV that supports this record, where they can persist the unregister record. One issue with this approach is that the UX is not good, requiring the user to unregister the controller twice.

Another approach is to allow the active controller to unregister observer controllers that the user specifies as part of a MV upgrade to support UnregisterControllerRecord. The main issue with this approach is that it makes the UX for kafka-features upgrade pretty complex. Additionally, this approach may be overkill in non-managed deployments, where operators can simply provision a controller to update its stale registration.

Test Plan

Add an integration test for unregistering a controller in both static quorum and dynamic quorum clusters. 

Rejected Alternatives

Use KRaft to manage controller registration

Although KRaft has access to the necessary information to manage controller registrations (i.e. advertised endpoints, and feature versions which are negotiated as part of establishing a connection with another node), this kind of design poses a couple of issues, mainly stemming from the fact that logical cluster membership of brokers and controllers is really a metadata layer responsibility of the active controller. Managing members of the larger kafka cluster is not the responsibility of the KRaft leader. The KRaft leader is responsible for replicating a log, and thus should not be aware of metadata level feature records or registration records. This approach would require leaking metadata layer state + records such as feature versions (outside of the kraft.version).

Brokers and controllers are concepts of the metadata layer, whereas KRaft has the concepts of leader, voters, and observers. This distinction is noteworthy because both brokers and controllers can be observers in KRaft, but broker registrations are not something that should be managed by the KRaft layer.

Do not durably persist observer controller registrations

Although this feature would be nice, since we would couple controller registration with being a KRaft voter, the main issue with this is that it will prevent the proposed improvements from KIP-1141: Simplifying Add/Remove Voter in MetadataQuorumCommand from being implemented. The proposed changes from KIP-1141 improve the UX around adding a controller to the KRaft voter set, but they rely on observer controllers having persisted a registration to retrieve their endpoints.

Use UnregisterBrokerRecord to unregister controllers

This approach would be one way existing clusters could support unregistering stale controller registrations without updating the MV. However, the main issue with this approach is that it is unsafe. A user who unregisters a controller before updating the software versions on all controllers to support this feature would crash the controllers with an older software version.