Support scaling to zero about custom-pod-autoscaler

[jthomperoo]

Ah OK, thanks for the feedback, helpful to hear a use case. So the way the current code works is if the current replica count is 0 scaling is disabled on the resource, so a resource can be scaled down to 0 replicas, but it cannot be scaled again after it has been set to 0. In your use case you need to be able to scale down to 0 and then afterwards scale upwards from 0 - is that right?

It seems reasonable to me, I think the way to do this would be to have a boolean configuration option allowScaleFromZero, if this is true you can scale upwards from 0, if it is false you can't scale upwards from 0 - would that be suitable for you? If so I'll add in that flag and get it in a new release (v0.12.0), thanks for finding that!

[jjhuff]

Yup, that's exactly right. I'm curious about the use case of 0 being disabled? Seems like a label on the deployment might be cleaner to mark it as 'autoscale disabled' might be cleaner?

[jthomperoo]

Hmm that's a good point, I'm not sure if there actually is a use case for disabling autoscaling, I was basing it off the Horizontal Pod Autoscaler's behaviour:

https://github.com/kubernetes/kubernetes/blob/57108f6c3ea285284ef693be6d86d4bbd1d8a143/pkg/controller/podautoscaler/horizontal.go#L602-L607

But I think I implemented this incorrectly, autoscaling here is only disabled if minReplicas is > 0 so behaviour should be the same as you have laid out, if minReplicas is set to 0 scaling down to and up from 0 should be allowed - I think this is better.

[jjhuff]

I think the HPA behavior might have changed when they added the ability to scale to zero.
Thanks!

[jthomperoo]

This is now available in version v0.12.0, there's an example here and some documentation here.

Hope that helps with your use case!