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{
  "apiKey": 52,
  "type": "response",
  "name": "VoteResponse",
  "validVersions": "0-1",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top level error code."},
    { "name": "Topics", "type": "[]TopicData",
      "versions": "0+", "fields": [
      { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]PartitionData",
        "versions": "0+", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+",
          "about": "The partition index." },
        { "name": "ErrorCode", "type": "int16", "versions": "0+"},
        { "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
          "about": "The ID of the current leader or -1 if the leader is unknown."},
        { "name": "LeaderEpoch", "type": "int32", "versions": "0+",
          "about": "The latest known leader epoch"},
        { "name": "VoteGranted", "type": "bool", "versions": "0+",
          "about": "True if the vote was granted and false otherwise"},
        { "name": "PreVote", "type": "boolean", "versions": "1+", 
          "about": "Whether the response is a PreVote response or not."}

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}

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Proposed Changes


Section
bordertrue

We add a new state Prospective for servers which are sending Pre-Vote requests as well as new state transitions

...

. The original (left) and new states (right) are below for comparison.

Column
width50%
Code Block
* Unattached|Resigned transitions to:

 *  
Unattached
 Unattached: After learning of a new election with a higher epoch
- 

 *  
Voted
 Voted: After granting a vote to a candidate
+

 *    Candidate: After expiration of the election timeout
 *    Follower: After discovering a leader with an equal or larger epoch
 *
 * Voted transitions to:
 *    Unattached: After learning of a new election with a higher epoch
 *    Candidate: After expiration of the election timeout
 *
 * Candidate transitions to:
 *    Unattached: After learning of a new election with a higher epoch
 *    Candidate: After expiration of the election timeout
 *    Leader: After receiving a majority of votes
 *
 * Leader transitions to:
 *    Unattached: After learning of a new election with a higher epoch
 *    Resigned: When shutting down gracefully
 *
 * Follower transitions to:
 *    Unattached: After learning of a new election with a higher epoch
 *    Candidate: After expiration of the fetch timeout
 *    Follower: After discovering a leader with a larger epoch


Column
width50%
 * Unattached|Resigned transitions to:
 *    Unattached: After learning of a candidate with a higher epoch (clarifying language)
 *    Voted: After granting a standard vote to a candidate
-
* Candidate: After expiration of the election timeout +
(clarifying language)
 *    Prospective: After expiration of the election timeout

 *    Follower: After discovering a leader with an equal or larger epoch

 *

 * Voted transitions to:

 
*    Unattached: After learning of a
new election
candidate with a higher epoch
- * Candidate: After expiration of the election
timeout
+
 *    Prospective: After expiration of the election timeout
+
 *    Follower: After discovering a leader with an equal or larger epoch (
already a valid transition, just 
missed in original docs)  
+
 *
+
 * Prospective transitions to:
+
 
 *    Unattached: After learning of a
new election
candidate with a higher epoch
+
   
 *    Candidate: After receiving a majority of pre-votes
+
 *    Follower: After discovering a leader with an equal or larger epoch
 
*
 
* Candidate transitions to:
    
 *    Unattached: After learning of a
new
candidate
election 
with a higher epoch
- * Candidate: After expiration of the election timeout + * Prospective:
After
expiration of the election
timeout
-
 *
Leader
Prospective: After
receiving
expiration
a
of
majority
the
of
election
votes
timeout
+
 *    Leader: After receiving a majority of standard votes
+
 *    Follower: After discovering a leader with an equal or larger epoch (
already a valid transition, just 
missed in original docs)

 *
 
* Leader transitions to:

 
*    Unattached: After learning of a
new election
candidate with a higher epoch
 
 *    

 * Follower transitions to:

 *    Unattached: After learning of a
new
candidate
election 
with a higher epoch
- * Candidate: After expiration
of
the fetch timeout
+
 *    Prospective: After expiration of the election timeout

 *    Follower: After discovering a leader with a larger epoch


A candidate will now send a VoteRequest with the PreVote field set to true and CandidateEpoch set to its [epoch + 1] when its election timeout expires. If [majority - 1] of VoteResponse grant the vote, the candidate will then bump its epoch up and send a VoteRequest with PreVote set to false which is our standard vote that will cause state changes for servers receiving the request.

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