📖 Update autoscaling from zero enhancement proposal with support for platform-aware autoscale from zero

[aleskandro]

What this PR does / why we need it:

This PR updates the contract between the cluster-autoscaler Cluster API provider and the infrastructure provider's controllers that reconcile the Infrastructure Machine Template to support platform-aware autoscale from 0 in clusters consisting of nodes heterogeneous in CPU architecture and OS.

With this commit, the infrastructure providers implementing controllers to reconcile the status of their Infrastructure Machine Templates for supporting autoscale from 0 will be able to fill the status.nodeInfo stanza with additional information about the nodes.

The status.nodeInfo stanza has type corev1.NodeSystemInfo to reflect the same content, the rendered nodes' objects would store in their status field.

The cluster-autoscaler can use that information to build the node template labels kubernetes.io/arch and kubernetes.io/os if that information is present.

Suppose the pending pods that trigger the cluster autoscaler have a node selector or a requiredDuringSchedulingIgnoredDuringExecution node affinity concerning the architecture or operating system of the node where they can execute. In that case, the autoscaler will be able to filter the nodes groups options according to the architecture or operating system requested by the pod.

The users could already provide this information to the cluster autoscaler through the labels capacity annotation. However, there is no similar capability to support future labels/taints through information set by the reconcilers of the status of Infrastructure Machine Templates.

Which issue(s) this PR fixes

Related to #11961

/area provider/core

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[JoelSpeed]

Seems very straightforward, no objection from me

[elmiko]

this generally makes sense to me, and i don't have any objections.

i would like to get some attention on the API field change though, just to make sure folks agree about the name and placement.

[aleskandro]

Thanks @elmiko. To give more context about the choice of naming and structure for everyone to discuss alternatives.

I couldn't consider the architecture as part of the capacity field because the values of keys in that stanza are expected to be a Quantity. I tried to trace back the reasoning the community did about the capacity field and I noticed that the Node objects' status field has the same field - capacity - with the same content we set in the Infrastructure Machine Template objects.

The Node objects' status field also has a nodeInfo field with the same name and type that I'm now proposing for the Infrastructure Machine Template objects.

Among the alternatives I was evaluating, this seemed the most reasonable to me.

[elmiko]

makes sense to me @aleskandro , no objection from me.

[chrischdi]

/hold

Especially for getting consensus on and resolve #11962 (comment)

[sbueringer]

@JoelSpeed If you find some time, PTAL :)

[aleskandro]

/label tide/merge-method-squash

[elmiko]

i am happy with the state this is in.

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 1e88b9ce31402b9352a67b7a6637e488da7436ac

[aleskandro]

/unhold

[fabriziopandini]

Just a few nits from my side.
Considering the type of changes we are introducing, I think we should consider addressing #12033 as integral part of the implementation.

Last nit, could you kindly also add a new line in the Implementation History down below

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[aleskandro]

Just a few nits from my side. Considering the type of changes we are introducing, I think we should consider addressing #12033 as integral part of the implementation.

Hi @fabriziopandini , thanks for your review.

I could take #12033 and resolve it in a separate PR, if this sounds good to you.

Last nit, could you kindly also add a new line in the Implementation History down below

Done

[elmiko]

i think the updates are nice, and this is looking good to me in general.

i have one minor question about if we should put an example in here. i don't think it's needed, but might help to illustrate the override mechanism. wdyt?

[fabriziopandini]

Last comment from my side, otherwise lgtm

[JoelSpeed]

I assume we will define constants to match the available values?

[aleskandro]

Hi @JoelSpeed , we have them set in https://github.com/kubernetes-sigs/cluster-api/pull/11962/files#diff-4c8f17be01a9d7dac2f991e0d8a273e19345291dea411d5f773f49297ed64facR210-R214. Should I copy them here too?

[aleskandro]

(added too)