Playing with Cluster API helm addon

In the previous post, we continued the journey of playing with Cluster API and tried to cover how to get things onto a workload cluster — see the post here. We went through ClusterResourceSet and HelmChartProxy. In this post we will dig deeper into the capabilities of HelmChartProxy, with a focus on templating and propagating cluster infrastructure-specific details.

Why is this important?

When bootstrapping controllers and other components onto a workload cluster, we might need to provide the controllers with information that is not known upfront when provisioning the cluster. VPC ID or OIDC ARN are good examples of this. This means we may need to retrofit this information into a Helm chart afterwards.

Helm addon valuesTemplete

It turns out that the Helm addon for Cluster API has templating capabilities built in for fetching data from the spec of Cluster and ControlPlane resources. Let's see an example of this using our favorite metrics-server chart as a naive example:

apiVersion: addons.cluster.x-k8s.io/v1alpha1
kind: HelmChartProxy
metadata:
  name: metrics-server
  namespace: default
spec:
  clusterSelector:
    matchLabels:
      helm-addon: enabled
  repoURL: "https://kubernetes-sigs.github.io/metrics-server/"
  chartName: metrics-server
  options:
    waitForJobs: true
    wait: true
    timeout: 5m
    install:
      createNamespace: true
  valuesTemplate: |
    commonLabels:
      test: im-a-static-label # static label
      vpcId: {{ .ControlPlane.spec.network.vpc.id }} # label read from control plane resource
      host: "{{ .Cluster.spec.controlPlaneEndpoint.port }}" # label read from cluster resource

We can verify that the metrics-server Deployment in the target cluster will have labels containing the VPC ID and control plane endpoint port, read from the management cluster's knowledge of the workload cluster.

This is a significant advantage the Helm add-on has over ClusterResourceSet and can be very valuable.