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Add a way to know if DemotePrimary is blocked and send it in the health stream #17289

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Add a way to know if DemotePrimary is blocked and send it in the health stream #17289

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@GuptaManan100 GuptaManan100 commented Nov 27, 2024 •
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Description

This PR adds the feature requested in #17288.

We spawn a new goroutine when we start DemotePrimary and when enough time has passed such that DemotePrimary hasn't finished despite context cancellation, we deem it to be blocked. In this case we send an error in the health stream that the users can track and use to restart MySQL and vttablet.

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  • "Backport to:" labels have been added if this change should be back-ported to release branches
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vitess-bot bot commented Nov 27, 2024

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

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  • Ensure that the Pull Request has a descriptive title.
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@vitess-bot vitess-bot bot added NeedsBackportReason If backport labels have been applied to a PR, a justification is required NeedsDescriptionUpdate The description is not clear or comprehensive enough, and needs work NeedsIssue A linked issue is missing for this Pull Request NeedsWebsiteDocsUpdate What it says labels Nov 27, 2024
@github-actions github-actions bot added this to the v22.0.0 milestone Nov 27, 2024
@GuptaManan100 GuptaManan100 removed NeedsDescriptionUpdate The description is not clear or comprehensive enough, and needs work NeedsWebsiteDocsUpdate What it says NeedsIssue A linked issue is missing for this Pull Request NeedsBackportReason If backport labels have been applied to a PR, a justification is required labels Nov 27, 2024
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codecov bot commented Nov 27, 2024 •
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Codecov Report

Attention: Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.62%. Comparing base (8648264) to head (a7cbc4a).
Report is 59 commits behind head on main.

Files with missing lines Patch % Lines
go/vt/vttablet/tabletserver/tabletserver.go 0.00% 5 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_replication.go 87.50% 2 Missing ⚠️
go/vt/vttablet/tabletservermock/controller.go 0.00% 2 Missing ⚠️
Additional details and impacted files 
@@            Coverage Diff             @@
##             main   #17289      +/-   ##
==========================================
+ Coverage   67.40%   67.62%   +0.21%     
==========================================
  Files        1574     1583       +9     
  Lines      253205   254067     +862     
==========================================
+ Hits       170676   171807    +1131     
+ Misses      82529    82260     -269

☔ View full report in Codecov by Sentry.
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deepthi
deepthi reviewed Nov 27, 2024
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go/vt/vttablet/tabletmanager/rpc_replication.go Outdated Show resolved Hide resolved
go/vt/vttablet/tabletserver/state_manager.go Outdated Show resolved Hide resolved
go/vt/vttablet/tabletserver/state_manager.go Outdated Show resolved Hide resolved
@GuptaManan100 GuptaManan100 force-pushed the demote-primary-blocked branch from 806c8e6 to 0c20bed Compare December 5, 2024 05:58
deepthi
deepthi reviewed Dec 9, 2024
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go/vt/vttablet/tabletmanager/rpc_replication.go
// We waited for over 10 times of remote operation timeout, but DemotePrimary is still not done.
// Collect more information and signal demote primary is indefinitely stalled.
log.Errorf("DemotePrimary seems to be stalled. Collecting more information.")
tm.QueryServiceControl.SetDemotePrimaryStalled()
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We don't seem to ever reset this. Is that because once it is stalled the only solution is to restart the tablet?

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Yes, exactly!

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if we read the end of demotePrimary and we have called SetDemotePrimaryStalled, what is the correct course of action? it seems like we're assuming this will never happen. should we do something like block forever without returning, to ensure that the tablet doesn't accidentally make forward progress or re-enter the set of serving tablets?

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Yes, there is an inherent race between the finishCtx completing (DemotePrimary finishing) and the timeout triggering. For that matter, DemotePrimary can unblock and finish, after we've marked the tablet as Stalled. If it is successful, even then I don't really see an issue with the tablet rejoining the serving tablets, until it is eventually restarted by the operator.

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@maxenglander maxenglander Dec 16, 2024 •
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even then I don't really see an issue with the tablet rejoining the serving tablets, until it is eventually restarted by the operator.

hm I think that is potentially a problem. because if the operator gets notified that a tablet is stalled, it's going to forcefully throw that tablet away with the assumption that (a) there is another tablet that is the real primary and (b) the stalled primary is not serving any traffic. if the stalled primary is able to rejoin the set of serving tablets, both of those assumptions go out the window, and it is unsafe for the operator to safely throw it away.

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Yes, that is true, this would trigger an ERS. Let me see if we can make the tablet not become serving ever again if it is stalled.

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Okay! I added few more safeties to ensure nothing goes wrong -

  1. After we set DemotePrimaryStalled we immediately trigger a health check update, which would make vtgate mark this tablet not-serving and not send it any requests ever again, because we never clear the field.
  2. For any requests already sent, if DemotePrimaryStalled is set, we won't process it on vttablet and instead just return an error.

I think with these safeguards we can be sure htat a vttablet is not going to accept any new writes once we mark it as stalled.

WDYT @maxenglander? Let's also wait for @deepthi to be able to take a look.

@deepthi deepthi requested a review from maxenglander December 10, 2024 00:33
@vitess-bot vitess-bot bot mentioned this pull request Dec 16, 2024
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@GuptaManan100
test: fix test expectation
3dd0376 
Signed-off-by: Manan Gupta <[email protected]>
maxenglander
maxenglander approved these changes Dec 19, 2024
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looks good!

@GuptaManan100
test: fix data race
a7cbc4a 
Signed-off-by: Manan Gupta <[email protected]>
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@deepthi deepthi deepthi approved these changes

@notfelineit notfelineit notfelineit approved these changes

@maxenglander maxenglander maxenglander approved these changes

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Feature Request: Way to detect DemotePrimary is blocked
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