Add circuit breaker variable

[rleungx]

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• Other reference link(s): Implement Circuit Breaker Pattern to Protect PD Leader tikv/pd#8678, Support setting circuit breaker to avoid PD overloaded tidb#58780

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[rleungx]

/hold

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[benmeadowcroft]

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.

[ti-chi-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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@benmeadowcroft: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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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