Changing Infrastructure Machine Templates

Several different components of Cluster API leverage infrastructure machine templates, including KubeadmControlPlane, MachineDeployment, and MachineSet. These MachineTemplate resources should be immutable, unless the infrastructure provider documentation indicates otherwise for certain fields (see below for more details).

The correct process for modifying an infrastructure machine template is as follows:

  1. Duplicate an existing template. Users can use kubectl get <MachineTemplateType> <name> -o yaml > file.yaml to retrieve a template configuration from a running cluster to serve as a starting point.
  2. Update the desired fields. Fields that might need to be modified could include the SSH key, the AWS instance type, or the Azure VM size. Refer to the provider-specific documentation for more details on the specific fields that each provider requires or accepts.
  3. Give the newly-modified template a new name by modifying the field (or by using metadata.generateName).
  4. Create the new infrastructure machine template on the API server using kubectl. (If the template was initially created using the command in step 1, be sure to clear out any extraneous metadata, including the resourceVersion field, before trying to send it to the API server.)

Once the new infrastructure machine template has been persisted, users may modify the object that was referencing the infrastructure machine template. For example, to modify the infrastructure machine template for the KubeadmControlPlane object, users would modify the field. For a MachineDeployment or MachineSet, users would need to modify the field. In all cases, the name field should be updated to point to the newly-modified infrastructure machine template. This will trigger a rolling update. (This same process is described in the documentation for upgrading the underlying machine image for KubeadmControlPlane in the “How to upgrade the underlying machine image” section.)

Some infrastructure providers may, at their discretion, choose to support in-place modifications of certain infrastructure machine template fields. This may be useful if an infrastructure provider is able to make changes to running instances/machines, such as updating allocated memory or CPU capacity. In such cases, however, Cluster API will not trigger a rolling update.