KubeadmControlPlane should adopt existing controlplane Machines

[rudoi]

User Story

As a user I would like to be able to migrate from a v1alpha2 cluster to a v1alpha3 cluster that utilizes the KubeadmControlPlane object. To facilitate this, the KubeadmControlPlane object should be able to adopt existing v1alpha3 controlplane machines. To simplify implementation for now, this should only occur when:

• all Kubernetes version fields involved are equal
• Machine's bootstrap ref is to a KubeadmConfig

/kind feature

[ncdc]

Also needs to make sure the bootstrap ref is to a KubeadmConfig.

[rudoi]

/cc @chuckha who definitely has ideas on this one

[detiber]

We should be able to automagically fill in the required kubeadm config info (at least the join bits, init/clusterconfig bits may be missing if the cluster was upgraded) using one of the referenced bootstrap configs, and even be able to automagically create the required infrastructure template using one of the referenced infra configs.

That said, anything we are likely to do here could potentially conflict with the validation we have on incoming KubeadmControlPlane resources.

I think the main thing we have to define is what do we expect the migration (from non-KCP to KCP) to look like during the adoption process. I think we have a few different choices here:

• User creates a KCP and dependent resources manually that will match the existing control plane machines
  • Error prone and not very friendly
  • Should work today
  • Might be difficult to separate adoption issues from management issues, potentially causing disruption to a running cluster.
  • Provides an opt-in experience
• User creates an empty KCP that is marked in some way to avoid defaulting/validation errors and triggers an internal process to pre-populate the KCP (or error if unable to)
  • Still requires some type of manual effort on the part of the user or higher level tooling
  • Should reduce the number of issues related to the creation process
  • Requires changes to defaulting/validation to accept empty KCP resources for the purpose of adoption.
  • Errors can be returned to the user through Events and the KCP resource, and adoption would be separate from reconciliation, so should isolate adoption issues and avoid adoption issues causing disruption to a running cluster.
  • Provides an opt-in experience.
• An internal process attempts to identify an adoptable set of Machines and automatically create a pre-populated KCP resource with the proper config for the adopted cluster
  • Might require us to provide an opt-out experience for folks that don't want adoption
  • Should allow us to avoid changes to defaulting/validation
  • Would have to figure out how to surface adoption errors
    • Expose through a KCP resource that has failurereason/failuremessage set
    • Expose through Events (ephemeral)
    • ???
  • Would likely require an additional process separate from the KCP controller

[rudoi]

Does the KCP today rely on some kind of selector that could be exploited for the purpose of option 3? I think it would be a viable option to mark existing controlplane Machines in a way that tells the KCP controller to adopt.

Curious about

Might require us to provide an opt-out experience for folks that don't want adoption

Wouldn't this imply that a user wants a controlplane that is partially managed by a KCP? Is that a supportable pattern?

[detiber]

Wouldn't this imply that a user wants a controlplane that is partially managed by a KCP? Is that a supportable pattern?

I'm thinking more along the lines of opting not to use KubeadmControlPlane initially (at some point, I expect we'll make control plane required, but we specifically stated that it would not be required for v1alpha3), So I would expect this to be something like a command line flag on the controller manager (or whatever implements the automated kcp creation/adoption) that would disable the creation/adoption workflow. If they choose to create a new cluster that uses KCP, it should still work, but any existing clusters would remain as they were.

[detiber]

Does the KCP today rely on some kind of selector that could be exploited for the purpose of option 3? I think it would be a viable option to mark existing controlplane Machines in a way that tells the KCP controller to adopt.

Yes, currently we use the same labels that we previously did manually:

cluster-api/controlplane/kubeadm/controllers/kubeadm_control_plane_controller.go

Lines 488 to 493 in 065eb53

	func generateKubeadmControlPlaneLabels(clusterName string) map[string]string {
	return map[string]string{
	clusterv1.ClusterLabelName: clusterName,
	clusterv1.MachineControlPlaneLabelName: "",
	}
	}

Once upgrade handling is in place (prior to v1alpha3 release), this will also include another label that will act as a configuration "hash" as well. This is something that we could add during the adoption process, though.

[rudoi]

I'm digging option two:

User creates an empty KCP that is marked in some way to avoid defaulting/validation errors and triggers an internal process to pre-populate the KCP (or error if unable to)

To be more specific about some potential gotchas:

• cluster could be in a state where there are no controlplane machines with clusterConfig / initConfig in their KubeadmConfigs (this doesn't really matter until these fields become mutable -fingers crossed-)
• you should not be able to make a populated KubeadmControlPlane for a cluster that already has controlplane machines?
• probably others

[detiber]

you should not be able to make a populated KubeadmControlPlane for a cluster that already has controlplane machines?

Anything we do here could potentially cause issues for pivoting workflows

[rudoi]

Anything we do here could potentially cause issues for pivoting workflows

Why's that? Naturally, I'm assuming all clusters are pivoted from my perspective. 🍕

[detiber]

Why's that? Naturally, I'm assuming all clusters are pivoted from my perspective.

Assuming resources are pivoted, the resources are created on the remote cluster without ordering guarantees. If we try to enforce this as part of a defaulting webhook, then the KCP resource creation might fail.

If trying to handle it in the controller, then we'd have to provide asynchronous feedback to the user that the adoption process failed and would need to differentiate the adoption workflow from the reconciliation workflow for populated resources.

We could potentially make the constraint that we do not accept populated KCP resources that have the label that signals the adoption workflow set. That would allow us to validate that KCP resources without it have the fields we expect and would allow us to block populated KCP resources that have it set relatively easily in the admission webhooks.

[rudoi]

Ah, yep, understood.

We could potentially make the constraint that we do not accept populated KCP resources that have the label that signals the adoption workflow set.

This makes sense to me. I think this label may have to be unset once ownership is granted to protect future "move" workflows?

Or, as part of a move, it may need to be added... 🤔 I need to diagram...

[rudoi]

@detiber what are the potential failure modes of purely relying on the label selector and pushing adoption logic to the top of the reconcile loop?

Looks like today we're only looking at ownerRefs:

cluster-api/controlplane/kubeadm/controllers/kubeadm_control_plane_controller.go

Lines 180 to 186 in 7cdea78

	// TODO: handle proper adoption of Machines
	allMachines, err := r.getMachines(ctx, types.NamespacedName{Namespace: cluster.Namespace, Name: cluster.Name})
	if err != nil {
	logger.Error(err, "Failed to get list of machines")
	return ctrl.Result{}, err
	}
	ownedMachines := r.filterOwnedMachines(kcp, allMachines)

What if, after checking ownerRefs, we just checked for controlplane machines that match the label selector before checking replica count and adopt those that match?

Assorted random thoughts:

• KubeadmConfigSpec isn't checked against existing hosts
• InfrastructureTemplate could be interesting: What happens when those don't line up? I don't think these are checked against existing machines.
• Kubernetes version definitely needs to line up

[detiber]

What if, after checking ownerRefs, we just checked for controlplane machines that match the label selector before checking replica count and adopt those that match?

• How do we handle the case that replicas do not match what we expect?
• How do we provide safety that the join configuration we are applying for new replicas are similar to that of existing replicas?

I think my biggest concern is that I would expect the adoption process to provide some assurances and halt reconciliation if it hits a scenario it's not expecting. The worst case scenario in my mind is that we attempt adoption/reconciliation against a working control plane causing that control plane to get into a bad state (loss of quorum or deletion) causing cluster downtime.

[rudoi]

Two potential paths may be:

• don't pay attention at all to existing kubeadmconfigs (except for kubernetesVersion?) and allow the KCP to specify a kubeadmConfigSpec that will then be used when new machines are added. this is good because it's simple enough to implement but bad because it doesn't provide any extra guarantees.
• deep compare the joinConfig of each kubeadmConfig referenced by an existing controlplane machine and only adopt when they are 1. equal to each other and 2. equal to the KCP's joinConfig? otherwise halt. i don't think this would be very easy to do, but we'd be ensured safety from a few potential failure modes. I also don't entirely think this is necessary, because we don't have this kind of protection today. I can join a bunch of controlplane nodes with totally different configurations for the most part.

as for the replicas, i was thinking the controller would aggregate all possible children (ownerRefs OR labels) and then determine whether a scale-up or scale-down is needed.