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📝 WalkthroughWalkthroughFailover elections now track per-voter ACK and NACK epochs. Quorum bounds count each voter once, including voters that change to FAIL after responding. Tests cover duplicate responses, response ordering, fast-fail behavior, and election-local state. ChangesFailover election accounting
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~25 minutes Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant clusterProcessPacket
participant clusterProcessFailoverAuthAck
participant clusterProcessFailoverAuthNack
participant clusterState
clusterProcessPacket->>clusterProcessFailoverAuthAck: Dispatch ACK
clusterProcessPacket->>clusterProcessFailoverAuthNack: Dispatch NACK
clusterProcessFailoverAuthAck->>clusterState: Record ACK once for the election
clusterProcessFailoverAuthNack->>clusterState: Record NACK once for the election
clusterProcessFailoverAuthNack->>clusterState: Compute achievable quorum
clusterState-->>clusterProcessFailoverAuthNack: Reset only if quorum is unreachable
Merge Risk: 🟠 High · up to Delayed election traffic can produce a false quorum during failover, so inactive-election ACKs should be rejected before merge. 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
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In `@src/cluster_legacy.c`:
- Line 5932: Update clusterFailoverMaxPossibleAcks to initialize its maximum
possible ACK count from failover_auth_count, then add only current voting
members that have not already acknowledged. Preserve ACKs from voters who become
replicas during the same election, and add a regression test covering that
topology change.
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| void clusterProcessFailoverAuthAck(clusterNode *sender) { | ||
| /* One vote per voter per election: a repeated ACK for the same election | ||
| * carries no new information. */ | ||
| if (voterAckedThisElection(sender)) return; |
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This only rejects a duplicate ACK; it still accepts an ACK from a voter already stamped as NACKed in this election. That leaves both stamps set and increments failover_auth_count, while clusterFailoverMaxPossibleAcks() then counts the voter as an ACK. A malformed peer, delayed contradictory packet, or future retry behavior can therefore make one voter contribute both a NACK and an ACK and produce a false quorum. Make ACK/NACK response state mutually exclusive: ignore any second response once either stamp matches this election (and add the symmetric guard in the NACK handler).
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ACK and NACK are now mutually exclusive per voter per election via voterRespondedThisElection() — the first response wins and any later contradictory reply is ignored. Added AckAfterNackFromSameVoterIsIgnored and NackAfterAckFromSameVoterIsIgnored
…sive Address review feedback on valkey-io#4655. Start the max-possible-ACKs bound from failover_auth_count instead of re-deriving received votes by scanning current voting primaries, so a voter that ACKed and then lost voting status in the same election keeps its vote in the bound. Treat ACK and NACK as one response per voter per election: whichever arrives first counts and any later contradictory reply is ignored, so a voter can never contribute both a vote and a rejection. Add tests for the demoted-after-ACK voter and for both contradictory orderings. Signed-off-by: Dante Knowles <xdk@amazon.com>
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## unstable #4655 +/- ##
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- Coverage 80.83% 80.64% -0.20%
============================================
Files 192 192
Lines 100853 100869 +16
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- Hits 81523 81342 -181
- Misses 19330 19527 +197
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Inline comments:
In `@src/cluster_legacy.c`:
- Line 5935: Update clusterFailoverMaxPossibleAcks() so max_possible_acks starts
with failover_auth_count, then adds only currently eligible voters that have not
ACKed; preserve the existing eligibility checks to avoid counting newly
ineligible voters.
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Inline comments:
In `@src/cluster_legacy.c`:
- Line 5968: Update the ACK dispatch path around voterRespondedThisElection to
reject acknowledgments unless a failover election request is currently active,
validating failover_auth_time and failover_auth_sent before reaching
clusterProcessFailoverAuthAck. Preserve normal sender and epoch checks, and add
a regression test covering a delayed ACK during the retry delay so it cannot
affect the next election’s quorum.
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The Repro at 3f9062e
The two ACKs land 2ms after One gap left after this PR and #4654: This was generated by AI but verified, with love, by a human. |
enjoy-binbin
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I haven't had enough time to think everything through yet, so I'm posting part of the content for now.
I had considered the issue of a node suddenly dying after ACK or NACK, but I didn't view it as a critical problem before. For instance, after a node sends an ACK or NACK, it takes a full cluster-node-timeout period before it is declared dead; clearly, a normal election process wouldn't take that long. And if a node is marked as "fast-fail" due to a double count after the timeout expires, well, that seems acceptable (or OK) to me. My reasoning was that since a node is only declared dead after the cluster-node-timeout elapses, the scenario isn't particularly severe. Is there a way to reproduce this issue within the current test suite, rather than relying on the C++ tests?
| * twice, resetting an election that is still winnable. */ | ||
| int needed_quorum = (server.cluster->size / 2) + 1; | ||
| int max_possible_acks = server.cluster->size - server.cluster->size_fail - server.cluster->failover_auth_nack_count; | ||
| int max_possible_acks = clusterFailoverMaxPossibleAcks(); |
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For the record: I introduced size_fail specifically to avoid iterating through the cluster dictionary to tally up every ACK/NACK. If it is indeed problematic, we should remove the concept of size_fail.
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I think we can avoid the loop over all nodes whenever we receive a vote and instead keep the simple equation
int max_possible_acks = server.cluster->size - server.cluster->size_fail - server.cluster->failover_auth_nack_count;... if we just make sure to update server.cluster->size_fail appropriately when a node is marked as FAIL.
Idea: When we mark a node as FAIL and it has not yet voted, increment server.cluster->size_fail. If it has already voted, don't increment it. When a node is marked as not-FAIL and has not yet voted, decrement server.cluster->size_fail, otherwise don't decrement it. Will that be correct?
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size_fail isn't maintained incrementally today since clusterUpdateState recounts it from scratch every run (and that's queued after every FAIL mark/clear). So bumping it at mark/clear would just get overwritten. Going fully incremental also gets complex around election boundaries, e.g. a voter that ACKed last election and fails before the next one.
I put your rule in the existing recount instead: a FAIL voter only counts toward size_fail if it hasn't responded to the current election. The bound stays size - size_fail - nack_count, the per-vote loop is gone, NACK-then-FAIL is subtracted once and ACK-then-FAIL keeps its vote.
One pre-existing gap: a voter that ACKs and then stops being a voting primary mid-election shrinks size and the bound loses that vote. Only costs a retry, and fixing it needs the loop back, so I left it.
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@zuiderkwast Do you also want to take a look. i am suck at math. |
| uint64_t failover_auth_acked_epoch; /* Election epoch in which this voter's ACK was counted, 0 if never. */ | ||
| uint64_t failover_auth_nacked_epoch; /* Election epoch in which this voter's NACK was counted, 0 if never. */ |
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Do we need to store both? If we want to save some bits, we can store less:
uint64_t failover_voted_epoch;
bool failover_voted_ack; /* true=ACK, false=NACK */There was a problem hiding this comment.
Or actually, we just need to store that it has voted in our current election epoch (server.cluster->failover_auth_epoch). Enough, right? Two bits:
uint8_t failover_auth_acked_in_current_election : 1;
uint8_t failover_auth_nacked_in_current_election : 1;There was a problem hiding this comment.
Does this mean we need to add clusterResetFailoverAuthResponses, which will iterates through the cluster dict?
Counting a node twice is indeed a match problem if it happens. 😆 But:
I think the C code changes are OK, and I agree with @enjoy-binbin's comments:
We can consider it a code simplification to make it clearer the four sets are disjoint (NACKed, ACKed, FAILed without voting, can still vote). This clarity makes it easier to reason about the votes and idempotency of votes. |
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It is a problem in the sense that it can delay promotion and reduce the chance that the rank 0 replica wins, but its not a serious problem.
It seems rare. A voter goes FAIL inside the election window, so correlated failures, the primary we're failing over and a voter dying together (AZ/rack loss, bad rolling restart) where the voter can still answer us but the others already have PFAIL reports on it and the FAIL message lands mid-election. The window isn't tiny though, it stays open until quorum or the auth timeout (2x node-timeout, min 2s), so a slow or split election overlaps with node-timeout-driven FAIL marking.
Both done.
Agree that this is the correct model. I can look to follow up with a change to improve this or at least create an issue to track the work. |
Let's do this in this PR, and btw since 9.2 RC1 is out, so we have enouth time to improve it. BTW, please avoid force-push, you can just push the new commits, and we'll eventually squash merge them, so the number of commits doesn't matter. Incremental commits allow us to perform better incremental diff reviews. |
The election bound size - size_fail - nack_count subtracted a voter twice if it NACKed and was then marked FAIL, resetting elections that were still winnable. Remember per voter whether it ACKed or NACKed our current election, and only count a FAIL voter in size_fail if it has not responded, so each voter is subtracted once and a received vote is kept. Also only count an ACK while its request is outstanding, so a straggler from a reset election cannot carry a vote into the next one. Add a cluster bus test that reproduces the double-count with crafted NACK and FAIL packets. Fixes valkey-io#4626 Signed-off-by: Dante Knowles <xdk@amazon.com>
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ack on the force-push, my bad on the last one |
… by state Replace the per-node acked/nacked bits with one failover_auth_response field (NONE/ACK/NACK), so a voter is in exactly one response state. Add voterElectionState (CAN_RESPOND, ACKED, NACKED, FAILED_WITHOUT_RESPONSE) and clusterVoterElectionState() to classify a voting primary while a FAILOVER_AUTH_REQUEST is outstanding. clusterUpdateState counts the FAILED_WITHOUT_RESPONSE voters. Signed-off-by: Dante Knowles <xdk@amazon.com>
Done. I added an enum to track the 4 states (disjoint sets) per "(NACKed, ACKed, FAILed without voting, can still vote)." and renamed Also collapsed the ack/nack bits and added some macros to improve the readability there |
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Thanks, the 4 states makes it more clear. I think we can even store this enum directly in the clusterNode instead of the acked/nacked bits. No need to convert between the two representations.
| case CLUSTER_FAILOVER_AUTH_RESPONSE_NACK: return VOTER_NACKED; | ||
| default: return nodeFailed(voter) ? VOTER_FAILED_WITHOUT_RESPONSE : VOTER_CAN_RESPOND; | ||
| } | ||
| } |
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We can even store the voterElectionState enum field directly in the clusterNode. It's even clearer, avoids the questions about what happens if a node was failed and later isn't failing anymore.
An enum is a 32-bit int (I think), but that's OK.
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Done, I do think this makes it clearer.
Rather than tracking failover_auth_response and nodeFailed(sender) we simply track voter_election_state per node.
This requires updating voter_election_state to VOTER_FAILED_WITHOUT_RESPONSE when a node is marked as failed, and resetting the state when the node's failure is cleared.
| if (!server.cluster->failover_auth_time || !server.cluster->failover_auth_sent) return; | ||
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| sender->failover_auth_acked_in_current_election = 1; | ||
| sender->failover_auth_response = CLUSTER_FAILOVER_AUTH_RESPONSE_ACK; |
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Here (for ACK) we can accept the vote only if sender->voter_election_state == VOTER_CAN_RESPOND and ignore it otherwise. That would also prevent double voting in a clear way.
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replaced with voter_election_state per other comment and we modify the state to VOTER_ACKED
| /* Handle a FAILOVER_AUTH_NACK from a voter. */ | ||
| void clusterProcessFailoverAuthNack(clusterNode *sender, clusterMsg *request) { | ||
| /* Ignore NACKs from FAIL nodes to avoid double-counting: FAIL nodes are | ||
| * already accounted for in size_fail, and they will never ACK, so including | ||
| * their NACK would undercount achievable votes. */ | ||
| * already accounted for in failed_voters_without_response, and they will never | ||
| * ACK, so including their NACK would undercount achievable votes. */ | ||
| if (nodeFailed(sender)) { | ||
| return; | ||
| } | ||
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| sender->failover_auth_nacked_in_current_election = 1; | ||
| sender->failover_auth_response = CLUSTER_FAILOVER_AUTH_RESPONSE_NACK; |
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Here we can check if sender->voter_election_state == VOTER_CAN_RESPOND instead of nodeFailed(sender). It moves the state in a state machine way that prevents double voting and is resistant (I think) to a node changing its FAIL state in the middle of an election window.
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replaced with voter_election_state per other comment and we modify the state to VOTER_NACKED
… into the clusterNode struct directly. Update state when incrementing ack/nack counts and marking/unmarking nodes as failed. Signed-off-by: Dante Knowles <xdk@amazon.com>
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still working on this, so delay any review until I get another commit in. Currently trying to work through solving the problem where a node is marked failed and state changes to |
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The election bound subtracted a voter twice if it NACKed and was then marked FAIL, resetting elections that were still winnable. Track ACK/NACK per voter per election instead and add unit tests.
Fixes #4626