Cluster API - Control Plane Bootstrap provider

k0smotron can act as a control plane bootstrap provider via usage of K0sControlPlane CRDs.

When creating a Cluster with Cluster API you typically need to create a Cluster object. With k0smotron there needs to be a link to the control plane provider K0sControlPlane:

apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: cp-test
spec:
  clusterNetwork:
    pods:
      cidrBlocks:
        - 10.244.0.0/16
    services:
      cidrBlocks:
        - 10.96.0.0/12
  controlPlaneRef:
    apiVersion: controlplane.cluster.x-k8s.io/v1beta1
    kind: K0sControlPlane
    name: cp-test

Next we need to provide the configuration for the actual K0sControlPlane and machineTemplate:

---
apiVersion: controlplane.cluster.x-k8s.io/v1beta1
kind: K0sControlPlane
metadata:
  name: cp-test
spec:
  replicas: 3
  k0sConfigSpec:
    k0s:
      apiVersion: k0s.k0sproject.io/v1beta1
      kind: ClusterConfig
      metadata:
        name: k0s
      spec:
        api:
          extraArgs:
            anonymous-auth: "true" # anonymous-auth=true is needed for k0s to allow unauthorized health-checks on /healthz 
  machineTemplate:
    infrastructureRef:
      apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
      kind: DockerMachineTemplate
      name: cp-test-machine-template
      namespace: default
---
apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
kind: DockerMachineTemplate
metadata:
  name: cp-test-machine-template
  namespace: default
spec:
  template:
    spec: {}

By applying this yaml, k0smotron will create 3 machines based on the MachineTemplate configuration, installs k0s with the role controller on each machine and bootstraps the k0s control plane.

For a full reference on K0sControlPlane configurability see the reference docs.

Downscaling the control plane

WARNING: Downscaling is a dangerous operation and should only be done if you know what you are doing.

Kubernetes using etcd as its backing store. It's crucial to have a quorum of etcd nodes available at all times. Always run etcd as a cluster of odd members.

When downscaling the control plane, you need firstly to deregister the node from the etcd cluster. This can be done by running the following command on the node that will be removed:

k0s etcd leave

NOTE: k0smotron gives node names sequentially and on downscaling it will remove the "latest" nodes. For instance, if you have k0smotron-test cluster of 5 nodes and you downscale to 3 nodes, the nodes k0smotron-test-3 and k0smotron-test-4 will be removed.

After removing members from etcd cluster, you can simply edit the K0sControlPlane object and change the spec.replicas field to the desired number of replicas. k0smotron will then automatically scale down the control plane to the desired number of replicas.