MachinePool remains in WaitingForReplicasReady because CAPA does not reconcile node references after instance refresh

[AndiDog]

/kind bug

What steps did you take and what happened:

Related to kubernetes-sigs/cluster-api#8858, #4071

CAPA's AWSMachinePool reconciler unconditionally returns return ctrl.Result{}, r.reconcileNormal(ctx, machinePoolScope, infraScope, infraScope), i.e. does not schedule reconciliation of the ASG's EC2 instances into .Status.Instances at regular intervals.

I made a change where CAPA triggers an instance refresh (e.g. change of AMI IDs), rolling out new EC2 instances. The parent MachinePool object remained in non-ready state with reason WaitingForReplicasReady, with CAPI continuously logging NodeRefs != ReadyReplicas messages. Only the next, random reconciliation of my AWSMachinePool object solves this by checking which instances exist in the ASG.

What did you expect to happen:

CAPA should regularly reconcile in order to check the ASG for a changed set of instances. Particularly if it's expected because CAPA triggered an instance refresh.

Environment:

• Cluster-api-provider-aws version: v2.2.4 + preliminary fixes from KubeadmConfig changes should be reconciled for machine pools, triggering instance recreation cluster-api#8858 which trigger instance refresh on change of the bootstrap config reference
• Kubernetes version: (use kubectl version): v1.24.14
• OS (e.g. from /etc/os-release): Flatcar Linux

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[cnmcavoy]

In your opinion, is this resolved by the MachinePools Machines implementation in CAPA? If not, what is missing that would need to be added?

#4527

[AndiDog]

@cnmcavoy I think your PR is separate from this issue. CAPI reconciles based on AWSMachinePool.Spec.ProviderIDList (unless you tell me that will change once infra providers create <Infra>Machine objects for machine pools?). That field is already correctly updated in awsmachinepool_controller.go, but CAPA does not regularly update it when the ASG (or an explicit instance refresh) creates/rolls instances.

[cnmcavoy]

@cnmcavoy I think your PR is separate from this issue. CAPI reconciles based on AWSMachinePool.Spec.ProviderIDList (unless you tell me that will change once infra providers create <Infra>Machine objects for machine pools?). That field is already correctly updated in awsmachinepool_controller.go, but CAPA does not regularly update it when the ASG (or an explicit instance refresh) creates/rolls instances.

Correct... I agree that this isn't solved by #4527.

My understanding is that the solution requires a way to detect any change in the status of an ASGs instances and trigger a new reconcile of the AWSMachinePool. One approach would be to implement this ontop of the work in #4527 and have the AWSMachine enqueue their AWSMachinePool when their status changes.

Alternatively, another approach would be to use the AWS events and set up the resources to receive those. I believe there is a way to have AWS send something when the ASG changes.

[AndiDog]

A bulletproof solution would be to reconcile every 1-5 minutes (configurable?!) for AWSMachinePool. No matter if using events or not, since they may not arrive correctly if the controller or network is misconfigured (assuming this feature were implemented).

There's Amazon EventBridge, but it can mainly perform actions in other AWS services, so I'm not sure if it could trigger a call to a controller webhook in order for it to reconcile.

I like the idea of observing the AWSMachine state and bubbling that up to AWSMachinePool. An ASG instance refresh would include node termination after some minutes, so there's an event on Node (which we don't watch or finalize, I assume?), or a Kubernetes Event (which we don't watch). An ASG scale-up (instance added) might – in the success case – have a "node added" event in Kubernetes. Did you have something in mind how the event observation could technically work?

If we don't have a clear idea, should we first fix the low-hanging fruit and use a regular reconciliation interval (RequeueAfter)?

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[AndiDog]

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