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This KIP provides a simple solution to an availability issue exposed by the auto-join feature proposed in KIP-853 and how AddRaftVoterRequest  RPC is currently handled. The problem is because the active controller does not send a response to complete the AddRaftVoterRequest until after the new voter set is committed, and that KRaft (and Kafka in general) only support one in-flight request to a node. Consider the following scenario:

Some controller A that is automatically joining by sending the AddRaftVoterRequest  RPC is the same controller who's Fetch is needed to commit the new voter set. A clear example of this is when bootstrapping with --standalone and having controllers auto-join, as the first controller to auto-join will increase the voter set size from 1 to 2. The active controller needs controller A to complete a Fetch  RPC to complete the AddRaftVoterRequest  RPC. However, this case also applies when going from a voter set of size X to size X + 1, where a minority of X + 1 nodes from the old new voter set are unavailable.

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Introduce a new version 1 to the AddRaftVoterRequest  RPC:

{
  "apiKey": 76,
  "type": "request",
  "listeners": ["controller", "broker"],
  "name": "AddVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterId", "type": "string", "versions": "0+" },
    { "name": "TimeoutMs", "type": "int32", "versions": "0+" },
    { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
      "about": "The name of the topic" },
    { "name": "TopicId", "type": "uuid", "versions": "0+",
      "about": "The unique topic ID" },
    { "name": "Partition", "type": "int32", "versions": "0+",
      "about": "The partition index" },
    { "name": "VoterId", "type": "int32", "versions": "0+",
      "about": "The replica id of the voter getting added to the topic partition" },
    { "name": "VoterDirectoryId", "type": "uuid", "versions": "0+",
      "about": "The directory id of the voter getting added to the topic partition" },
    { "name": "Listeners", "type": "[]Listener", "versions": "0+",
      "about": "The endpoints that can be used to communicate with the voter", "fields": [
      { "name": "Name", "type": "string", "versions": "0+", "mapKey": true,
        "about": "The name of the endpoint" },
      { "name": "Host", "type": "string", "versions": "0+",
        "about": "The hostname" },
      { "name": "Port", "type": "uint16", "versions": "0+",
        "about": "The port" }
    ]}
  ]
}

Compatibility, Deprecation, and Migration Plan

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