Add circuit breaker variable

[rleungx]

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• Other reference link(s): https://github.com/tikv/pd/issues/8678, https://github.com/pingcap/tidb/issues/58780

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[rleungx]

/hold

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[ti-chi-bot[bot]]

@benmeadowcroft: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Let's make it a bit clearer how this should be set and the meaning.

E.g. perhaps updating to:

This variable is used to control when TiDB triggers the circuit breaker. If set to 0 (the default) then the circuit breaker is disabled. If the variable is set to 1 to 100 then the circuit breaker is triggered if the error rate percentage, of the specific requests sent to PD, meets or exceeds the threshold.

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[benmeadowcroft]

I am also confused why we are using Range: [0, 100] for this variable instead of Range: [0.0, 1.0] as we have done for other ratio/percentage based thresholds in this configuration file.

[rleungx]

Maybe we can change it to [0, 1]? /cc @tema @niubell @okJiang

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In response to this:

Maybe we can change it to [0, 1]? /cc @tema @niubell @okJiang

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[okJiang]

Maybe we can change it to [0, 1]? /cc @Tema @niubell @okJiang

LGTM

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[Tema]

Maybe we can change it to [0, 1]? /cc @Tema @niubell @okJiang

@rleungx I don't mind the pct/ratio, but I wish you changed the name as well. The _pct suffix in the name meant to mean percentage. Now it is a bit confusing. cc: @benmeadowcroft

[rleungx]

AFAIK, we have another variable tidb_instance_plan_cache_reserved_percentage which uses the percentage as the suffix. And its range is [0, 1]. See https://docs.pingcap.com/tidb/stable/system-variables/#tidb_instance_plan_cache_reserved_percentage-new-in-v840. /cc @benmeadowcroft

[benmeadowcroft]

@rleungx my primary interest is consistency so my suggestion to address the feedback from @Tema would be to either (1) spell out the abbreviation fully, e.g. tidb_cb_pd_metadata_error_rate_threshold_percentage, or (2) use the suffix _ratio for this variable.

Looking over the set of variables provided it looks like there is 1 instance of *_percentage and 6 instances of *_ratio so my preference would be to use the latter.

[rleungx]

change to tidb_cb_pd_metadata_error_rate_threshold_ratio.

[rleungx]

/hold cancel

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-06-30 03:51:35.490446256 +0000 UTC m=+1280548.213625234: :ballot_box_with_check: agreed by hfxsd.
• 2025-10-09 10:28:15.922494706 +0000 UTC m=+349684.953594024: :ballot_box_with_check: agreed by lilin90.

[hfxsd]

/approve

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