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docs: do not recommend combining pull-based autoscaling with webhook-based autoscaling (#2051) 

Ref https://github.com/actions-runner-controller/actions-runner-controller/issues/1962
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Yusuke Kuoka 2022-11-28 04:31:12 +09:00 committed by GitHub
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@ -916,10 +916,9 @@ The main use case for scaling from 0 is with the `HorizontalRunnerAutoscaler` ki
- `TotalNumberOfQueuedAndInProgressWorkflowRuns`
- `PercentageRunnersBusy` + `TotalNumberOfQueuedAndInProgressWorkflowRuns`
- `PercentageRunnersBusy` + Webhook-based autoscaling
- Webhook-based autoscaling only
- Webhook-based autoscaling
`PercentageRunnersBusy` can't be used alone as, by its definition, it needs one or more GitHub runners to become `busy` to be able to scale. If there isn't a runner to pick up a job and enter a `busy` state then the controller will never know to provision a runner to begin with as this metric has no knowledge of the job queue and is relying on using the number of busy runners as a means for calculating the desired replica count.
`PercentageRunnersBusy` can't be used alone for scale-from-zero as, by its definition, it needs one or more GitHub runners to become `busy` to be able to scale. If there isn't a runner to pick up a job and enter a `busy` state then the controller will never know to provision a runner to begin with as this metric has no knowledge of the job queue and is relying on using the number of busy runners as a means for calculating the desired replica count.
If a HorizontalRunnerAutoscaler is configured with a secondary metric of `TotalNumberOfQueuedAndInProgressWorkflowRuns` then be aware that the controller will check the primary metric of `PercentageRunnersBusy` first and will only use the secondary metric to calculate the desired replica count if the primary metric returns 0 desired replicas.