Only fast-fail on NACK reasons a new election epoch can fix - #4700
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FAILOVER_AUTH_NACK (valkey-io#3833) lets a candidate abandon an election as soon as the votes it can still collect drop below quorum, instead of waiting for auth_timeout. Every NACK is counted, but most rejection reasons (NO_PRIMARY, PRIMARY_UP, NOT_SAFE, REQ_IS_PRIMARY) describe state that a new epoch leaves unchanged: the voters answer the next request identically, so resetting only restarts the election at event-loop frequency, bumping the epoch and re-broadcasting a request that cannot win on every round. Count only the reasons that carry split-vote information, i.e. that a fresh epoch can actually change: ALREADY_VOTED (the vote is freed in the next epoch), REQ_EPOCH_OLD (the next request carries a higher epoch) and STALE_CONFIG (the voter sends an UPDATE just before the NACK). The rest are ignored, leaving those elections to expire and be rescheduled on the auth_retry_time cadence. Add a regression test where the voters CLUSTER FORGET the failed primary so that every request is answered with NO_PRIMARY: the candidate must keep one election per auth_retry_time instead of retrying once per NACK. Signed-off-by: Binbin <binloveplay1314@qq.com>
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Included review availability: Your plan provides up to 10 included reviews per hour; 8 remain after this review. 📝 WalkthroughWalkthroughFailover NACK handling now counts only reasons that can change in a new election round. Tests cover counted, ignored, stale, retry, and expiration cases. ChangesFailover NACK filtering
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: ⚪ Minimal · up to The failover retry behavior is covered for both fast-fail and ignored NACK reasons, with no actionable current-head risk identified. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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@xdk-amz Do you mind taking a look? Because you are fixing NACK-related issues. issue: madolson/valkey-agents#20 |
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
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- Hits 81523 81328 -195
- Misses 19330 19532 +202
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We can also just skip sending NACK in those other cases? Only split-vote NACK matters. The other ones can just be logging instead? |
| # auth_retry_time = 8000 ms, i.e. at most one attempt per 8s. | ||
| # Ensure that we time out rather than fail fast. | ||
| R 3 CONFIG SET cluster-replica-no-failover no | ||
| wait_for_log_messages -3 {"*Failover attempt expired*"} 0 2000 10 |
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The test proves the election eventually expires, but does not directly assert that rapid retries stopped. Could we also bound election or currentEpoch growth before auth_retry_time elapses?
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If we still have fast-fail, we won't have the expires here. Adding an assert for the epoch is also a great point, i am adding it.
| # that answer, so those NACKs must not be counted: the candidate stays on the | ||
| # auth_retry_time cadence instead of resetting the election once per NACK, | ||
| # which would retry at event-loop frequency. | ||
| start_cluster 3 1 {tags {external:skip cluster} overrides {cluster-node-timeout 2000 cluster-ping-interval 100 cluster-replica-validity-factor 0}} { |
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Should we add regression coverage for clusterNackIsPerRoundReason for each reason like STALE_CONFIG to confirm it still triggers immediate retry? If new reasons are introduced or the classification changes for reasons other than NO_PRIMARY we would not catch those in this test.
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Writing test cases for every single reason might seem like overkill, but since we have AI now, let me see if i can come out a good test case.
Yes, we can. Before, i figured that NACK packets aren't very large, and the receiver could log the reason upon receiving one (though i ended up removing that logging, think of it again, having the receiver log the reason should be useful). |
Signed-off-by: Binbin <binloveplay1314@qq.com>
Signed-off-by: Binbin <binloveplay1314@qq.com>
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Inline comments:
In `@tests/unit/cluster/failover-nack.tcl`:
- Around line 276-277: Split the two wait_for_log_messages invocations into
separate Tcl commands by removing the quote that spans the lines, and use
double-quoted or unquoted arguments so voter1_id and voter2_id are substituted.
Preserve the existing log pattern and timeout arguments for each voter.
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src/cluster_legacy.ctests/unit/cluster/failover-nack.tcl
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4 participants
FAILOVER_AUTH_NACK (#3833) lets a candidate abandon an election as soon as
the votes it can still collect drop below quorum, instead of waiting for
auth_timeout. Every NACK is counted, but most rejection reasons (NO_PRIMARY,
PRIMARY_UP, NOT_SAFE, REQ_IS_PRIMARY) describe state that a new epoch leaves
unchanged: the voters answer the next request identically, so resetting only
restarts the election at event-loop frequency, bumping the epoch and
re-broadcasting a request that cannot win on every round.
Count only the reasons that carry split-vote information, i.e. that a fresh
epoch can actually change: ALREADY_VOTED (the vote is freed in the next
epoch), REQ_EPOCH_OLD (the next request carries a higher epoch) and
STALE_CONFIG (the voter sends an UPDATE just before the NACK). The rest are
ignored, leaving those elections to expire and be rescheduled on the
auth_retry_time cadence.
Add a regression test where the voters CLUSTER FORGET the failed primary so
that every request is answered with NO_PRIMARY: the candidate must keep one
election per auth_retry_time instead of retrying once per NACK.