Introduce distributionVersion field for improved Kubernetes Distribution version handling

[rccrdpccl]

What would you like to be added (User Story)?

As a user I would like to have a distributionVersion field to better handle versioning of the kubernetes distribution being installed

Detailed Description

Currently, the handling of Kubernetes versions in the version field is problematic when installing or upgrading a distribution of Kubernetes that utilizes its own version scheme and lifecycle. A distribution can include more software components than just Kubernetes, and/or a distinct support lifecycle that includes the release of fixes and other changes on its own timeline. Those characteristics can necessitate the use of an independent version scheme.
The version field, present in multiple resources: ControlPlane, MachineSpec, Cluster Topology, specifically represents the Kubernetes version. In order to control the cluster’s distribution version, we need to specify its value for the ControlPlane and MachineSet/MachineDeployment objects involved. This is problematic when a user wants to deploy a specific version of a kubernetes distribution, they cannot specify the version, but it should be calculated based on the related kubernetes version (which is not always possible in any case).

To address this, we propose introducing a new field, spec.distributionVersion in the following resources:

• ControlPlane (ControlPlane.Spec.Version)
• MachineSpec (Machine.Spec.Version /MachineSet.Spec.Template.Spec.Version/MachineDeploymen.Spec.Template.Spec.Version/MachinePool.Spec.Template.Spec.Version)
• Topology (Cluster.Topology.Version)

The new field and the current spec.version would be mutually exclusive.

This would be an optional field for both the ControlPlane contract and MachineSpec. Its value would be the distribution version (e.g. for OpenShift it should be something like v4.17.0). No version semantics will be imposed on the field.

If distributionVersion is present, the status.version should be populated with the related kubernetes version (e.g. for OpenShift with distributionVersion with value 4.17.0, status.version should be 1.30.4).

All logic that makes a decision based on evaluating the kubernetes version should rely on the status field instead of the spec field.

Related:

#11564

/cc @fabriziopandini @enxebre @sbueringer

Anything else you would like to add?

No response

Label(s) to be applied

/kind feature
/area api

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[fabriziopandini]

Thanks for filing this issue as a follow up of the discussion on the PR!

Somehow related, we should consider also KEP-4330: Compatibility Versions in Kubernetes which is introducing emulated-versions and min compatibility version as a key info to influence the cluster behaviour (see user stories in the KEP)

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[rccrdpccl]

/remove-lifecycle stale

[fabriziopandini]

Discussed in the 22nd of May office hours

The new field and the current spec.version would be mutually exclusive.

Unfortunately, the idea of leaving the spec.version field under some circumstances is going to create many problems in CAPI.

We need a plan (possibily not invasive) to address the fact that CAPI is full of code paths where the code assumes that version is set and represents a K8s version, and this is at the core of fundationational construct like the entire upgrade process and all the relelated test machinery.

Hopefully previous comments on the same topic might help in starting the work on this plan

• 📖 clarify semantic meaning of the Version fields #11564 (comment)
• 📖 clarify semantic meaning of the Version fields #11564 (comment)
• 📖 clarify semantic meaning of the Version fields #11564 (comment)

Also, worth to recall #11816 (comment) from above
and to bring up possible impacts on the ongoing discussion in #12199

[rccrdpccl]

Unfortunately, the idea of leaving the spec.version field under some circumstances is going to create many problems in CAPI.

Would moving it to status (mirror it when spec is defined, and derive from distributionVersion when not) be too disruptive?
I totally understand that K8s version should always be available, that was one of the target of this initial proposal.

In any case I'll take all this into account and get back with a possibly improved and more detailed proposal, thanks for the feedback - much appreciated 🙏

[JoelSpeed]

If distributionVersion is present, the status.version should be populated with the related kubernetes version (e.g. for OpenShift with distributionVersion with value 4.17.0, status.version should be 1.30.4).

What component would be responsible for this?

Would moving it to status (mirror it when spec is defined, and derive from distributionVersion when not) be too disruptive?
I totally understand that K8s version should always be available, that was one of the target of this initial proposal.

This could be beneficial for more than just the proposal here. Do we do any validation of upgrades at the moment? If we had the option of spec (I want this) and status (controller has verified through some rules that the upgrade is permitted), then that could unlock more in the way of pre-flight checks couldn't it?

I know within OpenShift for example we have similar patterns in other places where a user requests "I want this" and we validate that transition before accepting, and then the controllers observe the status where we've said "yes, we verify this transition is acceptable"

[rccrdpccl]

What component would be responsible for this?

I think for each CR it's owner should be responsible for updating this field. However we need a component that would resolve the k8s version for each distribution version. I don't think this task is fitting particularly in any currently defined component.
If we want to discuss implementation, what I had in mind is to have a thin controller (optional in general, but mandatory for distributionVersion usage) maintaining a new CRD DistributionVersionSet which would have a list of distributionVersion/k8s version pairs, and each controller could watch that resource to then update its status field - something like the following:

apiVersion: cluster.x-k8s.io/v1beta1
kind: DistributionVersionSet
metadata:
   name: ocp-distribution-versions
spec:
  versions:
  - distributionVersion: 4.18.0
    version: 1.31.1
  - distributionVersion: 4.19.4
    version: 1.33.4

This way Cluster, MachineDeployment, Controlplane, etc controllers could just look up the values and fill the status of their own resources.