Status

Current state: Under Discussion

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-1 to your own ticket]

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

This KIP will go over scenarios where we might expect disruptive servers and discuss how Pre-Vote (as originally detailed in the Raft paper and in KIP-650) along with Followers rejecting Pre-Vote Requests can ensure correctness when it comes to network partitions (as well as quorum reconfiguration and failed disk scenarios).  

Pre-Vote is the idea of “canvasing” the cluster to check if it would receive a majority of votes - if yes it increases its epoch and sends a disruptive vote request. If not, it does not increase its epoch and does not send a vote request. 

Followers rejecting Pre-Vote Requests entails servers rejecting any pre-vote requests received prior to their own fetch timeout expiring. The idea here is if we've recently heard from a leader, we should not attempt to elect a new one just yet.

Throughout this KIP, we will differentiate between Pre-Vote and the original Vote request behavior with "Pre-Vote" and "standard Vote".

Disruptive server scenarios

Network Partition

When a follower becomes partitioned from the rest of the quorum, it will continuously increase its epoch to start elections until it is able to regain connection to the leader/rest of the quorum. When the server regains connectivity, it will disturb the rest of the quorum as they will be forced to participate in an unnecessary election. While this situation only results in one leader stepping down, as we start supporting larger quorums these events may occur more frequently per quorum.

For instance, here's a great example from https://decentralizedthoughts.github.io/2020-12-12-raft-liveness-full-omission/ which demonstrates a scenario where we could have flip-flopping leadership changes.

Let's say server 3 is the current leader. Server 4 will eventually start an election because it is unable to find the leader, causing Server 2 to transition to Unattached. Server 4 will not be able to receive enough votes to become leader, but Server 2 will be able to once its election timer expires. As Server 5 is also unable to communicate to Server 2, this kicks off a back and forth of leadership transition between Servers 2 and 3.

While network partitions may be the main issue we expect to encounter/mitigate impact for, it’s possible that bugs and user error create similar effects to network partitions that we need to guard against. For instance, a bug which causes fetch requests to periodically timeout or setting controller.quorum.fetch.timeout.ms  and other related configs too low.

Public Interfaces

We will add a new field PreVote to VoteRequests and VoteResponses to signal whether the requests and responses are for Pre-Votes. The server does not increase its epoch prior to sending a Pre-Vote request, but will still report [epoch + 1]

{
  "apiKey": 52,
  "type": "request",
  "listeners": ["controller"],
  "name": "VoteRequest",
  "validVersions": "0-1",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterId", "type": "string", "versions": "0+",
      "nullableVersions": "0+", "default": "null"},
    { "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": "CandidateEpoch", "type": "int32", "versions": "0+",
          "about": "The bumped epoch of the candidate sending the request"},
        { "name": "CandidateId", "type": "int32", "versions": "0+", "entityType": "brokerId",
          "about": "The ID of the voter sending the request"},
        { "name": "LastOffsetEpoch", "type": "int32", "versions": "0+",
          "about": "The epoch of the last record written to the metadata log"},
        { "name": "LastOffset", "type": "int64", "versions": "0+",
          "about": "The offset of the last record written to the metadata log"},
        { "name": "PreVote", "type": "boolean", "versions": "1+", 
          "about": "Whether the request is a PreVote request (no epoch increase) or not."}
...
}

{
  "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."}

...
}

Proposed Changes

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

  * Unattached|Resigned transitions to:
  *    Unattached: After learning of a new election with a higher epoch
+ *    Voted: After granting a standard vote to a candidate
+ *    Prospective: After expiration of the election timeout
- *    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
+ *    Prospective: After expiration of the election timeout
- *    Candidate: After expiration of the election timeout
+ *
+ * Prospective transitions to:
+ *    Unattached: After learning of a new election with a higher epoch
+ *    Candidate: After receiving a majority of pre-votes
  *
  * Candidate transitions to:
  *    Unattached: After learning of a new election with a higher epoch
+ *    Prospective: After expiration of the election timeout
+ *    Leader: After receiving a majority of standard votes
- *    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
  *
  * Follower transitions to:
  *    Unattached: After learning of a new election with a higher epoch
+ *    Prospective: After expiration of the election timeout
- *    Candidate: After expiration of the fetch 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.

When servers receive VoteRequests with the PreVote field set to true, they will respond with VoteGranted set to

• true if the epoch and offsets in the Pre-Vote request satisfy the same conditions as a standard vote
• false if otherwise

When a server receives VoteResponses, it will follow it up with another VoteRequest with PreVote set to either true (send another Pre-Vote) or false (send a standard vote)

• false (standard vote) if the server has received [majority - 1] VoteResponses with VoteGranted set to true within [election.timeout.ms + a little randomness]
• true (another Pre-Vote) if the server receives [majority] VoteResponse with VoteGranted set to false within [election.timeout.ms + a little randomness]
• true if the server receives less than [majority] VoteResponse with VoteGranted set to false within [election.timeout.ms + a little randomness] and the first bullet point does not apply
  • Explanation for why we don't send a standard vote at this point is explained in rejected alternatives. 

If a server happens to receive multiple VoteResponses from another server for a particular VoteRequest, it can take the first and ignore the rest. We could also choose to take the last, but taking the first is simpler. A server does not need to worry about persisting its election state for a Pre-Vote response like we currently do for VoteResponses because the direct result of the Pre-Vote phase does not elect leaders. 

How does this prevent unnecessary elections when it comes to network partitions?

When a partitioned server rejoins and forces the cluster to participate in an election, all servers reject the pre-vote request from the disruptive follower since they've recently heard from the active leader. The disruptive server continuously kicks off elections but is unable to be elected. It should rejoin the quorum when it discovers the higher epoch on the next valid election by another server (todo: check this for accuracy, there should be other ways for the server to become follower earlier)

Can this prevent necessary elections?

Yes. If a leader is unable to receive fetch responses from a majority of servers, it can impede followers that are able to communicate with it from voting in an eligible leader that can communicate with a majority of the cluster. This is the reason why an additional "Check Quorum" safeguard is needed which is what KAFKA-15489 implements. Check Quorum ensures a leader steps down if it is unable to receive fetch responses from a majority of servers.

Do we still need to reject VoteRequests received within fetch timeout if we have implemented Pre-Vote and Check Quorum?

Yes. Specifically we would be rejecting Pre-Vote requests received within fetch timeout. We need to avoid bumping epochs without a new leader being elected else the server requesting the election(s) will be unable to rejoin the quorum because its epoch is greater than everyone else's while its log continues to fall behind.

The following are two scenarios where having just Pre-Vote and Check Quorum is not enough.

• Scenario A: A server in an old configuration (e.g. S1 in the below diagram pg. 41) starts a “pre-vote” when the leader is temporarily unavailable, and is elected because it is as up-to-date as the majority of the quorum. The Raft paper argues we can not rely on the original leader replicating fast enough to get past this scenario, however unlikely that it is. We can imagine some bug/limitation with quorum reconfiguration causes S1 to continuously try to reconnect with the quorum (i.e. start elections) when the leader is trying to remove it from the quorum.

• Scenario B: We can also imagine a non-reconfiguration scenario where two servers, one of which is the leader, are simply unable to communicate with each other. Since the non-leader server is unable to find a leader, it will start an election and may get elected. Since the prior leader is now unable to find the new leader, it will start an election and may get elected. This could continue in a cycle.

Rejecting Pre-Vote requests received within fetch timeout

As mentioned in the prior section, a server should reject Pre-Vote requests received from other servers if its own fetch timeout has not expired yet. The logic now looks like the following for servers receiving VoteRequests with PreVote  set to true 

When servers receive VoteRequests with the PreVote field set to true, they will respond with VoteGranted set to

• true if they haven't heard from a leader in fetch.timeout.ms and all conditions that normally need to be met for VoteRequests are satisfied
• false if they have heard from a leader in fetch.timeout.ms (could help cover Scenario A & B from above) or conditions that normally need to be met for VoteRequests are not satisfied
• (Not in scope) To address the disk loss and 'Servers in new configuration' scenario, one option would be to have servers respond false to vote requests from servers that have a new disk and haven't caught up on replication 

Compatibility, Deprecation, and Migration Plan

We can gate Pre-Vote with a new VoteRequest and VoteResponse version. , and gate rejecting Pre-Vote requests with the same metadata version bump. (todo)

Raft equiv for metadata version?

If X servers are upgraded and start ignoring vote requests and Y servers continue to reply and transition to “Unattached” could we run into a scenario where we violate raft invariants (more than one leader) or cause extended quorum unavailability?

• We can never have more than one leader - if Y servers are enough to vote in a new leader, the resulting state change they undergo (e.g. to Voted, to Follower) will cause maybeFireLeaderChange to update the current leader’s state (e.g. to Resigned) as needed.

• Proving that unavailability would remain the same or better w/ a mish-mash of upgraded servers is a bit harder.

  • What happens if the leader is part of the Y servers that will continue to respond to unnecessary vote requests?

    • If Y is a minority of the cluster, the current leader and minority of cluster may vote for this disruptive server to be the new leader but the server will never win the election. The disruptive server will keep trying with exponential backoff. At some point a majority of the cluster will not have heard from a leader for fetch.timeout.ms and will start participating in voting. A leader can be elected at this point. In the worst case this would extend the time we are leader-less by the length of the fetch timeout.

    • If Y is a majority of the cluster, we have enough servers to potentially elect the disruptive server. (This is the same as the old behavior.) If a majority of votes were not received by the server, it will keep trying with exponential backoff. At some point the rest of the cluster will not have heard from a leader for fetch.timeout.ms and will start participating in voting. In the worst case this would extend the time we are leader-less by the length of the fetch timeout.

  • What happens if leader is part of the X servers which will reject unnecessary vote requests?

    • If Y is a minority of the cluster, the minority of cluster may vote for this disruptive server to be the new leader but the server will never win the election. The disruptive server will keep trying with exponential backoff, increasing its epoch. If the current leader ever resigns or a majority of the cluster hasn’t heard from a leader, there’s a potential we will now vote for the server - but an election would have been necessary in this case anyways

    • If Y is a majority of the cluster, we have enough servers to potentially elect the disruptive server. (This is the same as the old behavior.) If a majority of votes were not received by the server, it will keep trying with exponential backoff. The servers in Y will transition to unattached after receiving the first vote request and potentially start their own elections. The servers in X will still be receiving fetch responses from the current leader and will be rejecting all vote requests from servers in Y. Although there must be at least one server in Y that is eligible to become leader (otherwise, Y couldn’t have been a majority of the cluster), if we suffer enough server failures in Y where Y is no longer a majority of the cluster, we get stuck. X (a minority of the cluster, but now needed to form a majority) stays connected to the current leader and never votes for an eligible leader in Y. No new commits can be made to the log. Check quorum would solve this case (leader resigns since it senses it no longer has a majority).

Test Plan

This will be tested with unit tests, integration tests, system tests, and TLA+. (todo)

Rejected Alternatives

Rejecting VoteRequests received within fetch timeout (w/o Pre-Vote)

This was originally proposed in the Raft paper as a necessary safeguard to prevent Scenario A from occurring, but we can see how this could extend to cover all the other disruptive scenarios mentioned.

• When a partitioned server rejoins and forces the cluster to participate in an election …

  • if a majority of the cluster is not receiving fetch responses from the current leader, they consider the vote request and make the appropriate state transitions. An election would be needed in this case anyways.

  • if the rest of the cluster is still receiving fetch responses from the current leader, they reject the vote request from the disruptive follower. No one transitions to a new state (e.g. Unattached) as a result of the vote request, current leader is not disrupted.

• For a server in an old configuration (that’s not in the new configuration) …

  • if the current leader is still responding to fetch requests in a reasonable amount of time, the server is prevented from starting and winning elections, which would delay reconfiguration.

  • if the current leader is not responding to fetch requests, then the server could still win an election (this scenario calls for an election anyways). KIP-853 should cover preventing this case if necessary.

• For a server w/ new disk/data loss …

  • if the current leader is still responding to fetch requests in a reasonable amount of time, the server is prevented from starting and winning elections, which could lead to loss of committed data.

  • if the current leader is not responding to fetch requests, we can reject VoteRequests from the server w/ new disk/data loss if it isn't sufficiently caught up on replication. KIP-853 should cover this case w/ storage ids if necessary.

However, this would not be a good standalone alternative to Pre-Vote because once a server starts a disruptive election (disruptive in the sense that the current leader still has majority), its epoch may increase while none of the other servers' epochs do. The most likely way for the server to rejoin the quorum now with its inflated epoch would be to win an election. Since epochs are not increased with Pre-Vote requests, it is easier for a disruptive server to rejoin the quorum once it finds any of the servers in the cluster have been elected. (todo: can it join once it finds the current leader)

Separate RPC for Pre-Vote

This would be added toil with no real added benefits. Since a Pre-Vote and a standard Vote are similar in concept, it makes sense to cover both with the same RPC. We can add clear logging and metrics to easily differentiate between Pre-Vote and standard Vote requests.

Covering disk loss scenario in scope

This scenario shares similarities with adding new servers to the quorum, which KIP-853: KRaft Controller Membership Changes would handle. If a server loses its disk and fails to fully catch up to the leader prior to another server starting an election, it may vote for any server which is at least as caught up as itself (which might be less than the last leader). One way to handle this is to add logic preventing servers with new disks (determined via a unique storage id) from voting prior to sufficiently catching up on the log. Another way is to reject pre-vote requests from these servers. We leave this scenario to be covered by KIP-853 or future work because of the similarities with adding new servers.

Sending Standard Votes after failure to win Pre-Vote

When a server receives VoteResponses, it will follow it up with another VoteRequest with PreVote set to either true (send another Pre-Vote) or false (send a standard vote). If the server receives less than [majority] VoteResponse with VoteGranted set to false and has not received [majority - 1] VoteResponses with VoteGranted set to true within [election.timeout.ms + a little randomness], the server could send a standard vote at this point. An argument for doing this might be that it's safer to start sending non-Pre-Vote requests when we encounter unideal situations (e.g. unable to communicate with 1 server in a 3 server quorum) so we know we fallback to the original protocol. We can do this, but it should be safe for a server to continually send Pre-Vote requests if it does not receive a majority of VoteGranted responses - either the leader is able to communicate with the entire cluster or another server can start a Pre-Vote. If the cluster is truly partitioned and no server is able to communicate with a majority of the cluster, there is no point in servers sending out non-Pre-Vote requests anyways.