Rely on API Priority and Fairness (APF) instead of client-side rate limiting

[Sijoma]

When relying on controller-runtime defaults, it's difficult to spot whether a controller is rate-limited on the client-side. This can have a negative impact on controller performance as the reconcile loop will be slowed down.

The recommendation from slack discussions seems to be to always:

1. set QPS to -1 everywhere and rely on APF
2. use the cache wherever possible

One of the concerns was that this might not be safe default when users are relying on older Kubernetes versions. I'm not sure how those older Kubernetes versions would work with an updated controller-runtime in any case? Isn't the client too new then?

Discussion links:

lavalamp: even if it works the right way you should still turn it off and rely on APF

Link to sig-apimachinery recommendation: https://kubernetes.slack.com/archives/C0EG7JC6T/p1680796612287719?thread_ts=1680791299.631439&cid=C0EG7JC6T

Controller Runtime discussion - https://kubernetes.slack.com/archives/C02MRBMN00Z/p1724224928349419

[sbueringer]

I'm not sure how those older Kubernetes versions would work with an updated controller-runtime in any case? Isn't the client too new then?

Controller-runtime in general is compatible with a wide-range of Kubernetes versions on the server-side. Otherwise folks would have to make sure the controller they use use exactly the same Kubernetes version as the server. An example is Cluster API which usually is compatible with ~ 6-7 Kubernetes versions (https://cluster-api.sigs.k8s.io/reference/versions#core-provider-cluster-api-controller).

Of course folks have to be careful about which features of the kube-apiserver and builtin APIs they rely on, but controller-runtime itself tries to not depend on any specific new kube-apiserver feature.

[Sijoma]

Seems like Crossplane is also running into issues around these settings:

https://github.com/crossplane/crossplane/pull/5742/files

[nathanperkins]

Thanks for bringing up this discussion! We were having problems with backed up workqueues in some of our controllers and disabling the client-side rate limiter in favor of APF took our latency from O(hours) to O(seconds).

If it makes sense, we would love to see related options in controller-runtime client/manager rather than having to modify the *rest.Config before creating the client, but the current solution is workable for us :)

[vincepri]

This is generally a good idea, if it has to be in controller runtime, which we could support, needs to be able to determine if APF is enabled on the server, or fallback gracefully.

[sbueringer]

Not sure if we really should add additional options to Client / Manager that would overwrite the corresponding options of the rest.Config