clusterctl Provider Contract

The clusterctl command is designed to work with all the providers compliant with the following rules.

Provider Repositories

Each provider MUST define a provider repository, that is a well-known place where the release assets for a provider are published.

The provider repository MUST contain the following files:

  • The metadata YAML
  • The components YAML

Additionally, the provider repository SHOULD contain the following files:

  • Workload cluster templates

Creating a provider repository on GitHub

You can use GitHub release to package your provider artifacts for other people to use.

A github release can be used as a provider repository if:

  • The release tag is a valid semantic version number
  • The components YAML, the metadata YAML and eventually the workload cluster templates are include into the release assets.

See the GitHub help for more information about how to create a release.

Creating a local provider repository

clusterctl supports reading from a repository defined on the local file system.

A local repository can be defined by creating a <provider-label> folder with a <version> sub-folder for each hosted release; the sub-folder name MUST be a valid semantic version number. e.g.

~/local-repository/infrastructure-aws/v0.5.2

Each version sub-folder MUST contain the corresponding components YAML, the metadata YAML and eventually the workload cluster templates.

Metadata YAML

The provider is required to generate a metadata YAML file and publish it to the provider’s repository.

The metadata YAML file documents the release series of each provider and maps each release series to an API Version of Cluster API (contract).

For example, for Cluster API:

apiVersion: clusterctl.cluster.x-k8s.io/v1alpha3
kind: Metadata
releaseSeries:
- major: 0
  minor: 3
  contract: v1alpha3
- major: 0
  minor: 2
  contract: v1alpha2

Components YAML

The provider is required to generate a components YAML file and publish it to the provider’s repository. This file is a single YAML with all the components required for installing the provider itself (CRDs, Controller, RBAC etc.).

The following rules apply:

Naming conventions

It is strongly recommended that:

  • Core provider release a file called core-components.yaml
  • Infrastructure providers release a file called infrastructure-components.yaml
  • Bootstrap providers release a file called bootstrap-components.yaml
  • Control plane providers release a file called control-plane-components.yaml

Shared and instance components

The objects contained in a component YAML file can be divided in two sets:

  • Instance specific objects, like the Deployment for the controller, the ServiceAccount used for running the controller and the related RBAC rules.
  • The objects that are shared among all the provider instances, like e.g. CRDs, ValidatingWebhookConfiguration or the Deployment implementing the web-hook servers and related Service and Certificates.

As per the Cluster API contract, all the shared objects are expected to be deployed in a namespace named capi-webhook-system (if applicable).

clusterctl implements a different lifecycle for shared resources e.g.

  • ensuring that the version of the shared objects for each provider matches the latest version installed in the cluster.
  • ensuring that deleting an instance of a provider does not destroy shared resources unless explicitly requested by the user.

Target namespace

The instance components should contain one Namespace object, which will be used as the default target namespace when creating the provider components.

All the objects in the components YAML MUST belong to the target namespace, with the exception of objects that are not namespaced, like ClusterRoles/ClusterRoleBinding and CRD objects.

Controllers & Watching namespace

Each provider is expected to deploy controllers using a Deployment.

While defining the Deployment Spec, the container that executes the controller binary MUST be called manager.

The manager MUST support a --namespace flag for specifying the namespace where the controller will look for objects to reconcile.

Variables

The components YAML can contain environment variables matching the regexp ${\s*([A-Z0-9_]+)\s*}; it is highly recommended to prefix the variable name with the provider name e.g. ${ AWS_CREDENTIALS }

Additionally, each provider should create user facing documentation with the list of required variables and with all the additional notes that are required to assist the user in defining the value for each variable.

Labels

The components YAML components should be labeled with cluster.x-k8s.io/provider and the name of the provider. This will enable an easier transition from kubectl apply to clusterctl.

As a reference you can consider the labels applied to the following providers.

Provider NameLabel
CAPIcluster.x-k8s.io/provider=cluster-api
CABPKcluster.x-k8s.io/provider=bootstrap-kubeadm
CACPKcluster.x-k8s.io/provider=control-plane-kubeadm
CAPAcluster.x-k8s.io/provider=infrastructure-aws
CAPVcluster.x-k8s.io/provider=infrastructure-vsphere
CAPDcluster.x-k8s.io/provider=infrastructure-docker
CAPM3cluster.x-k8s.io/provider=infrastructure-metal3
CAPPcluster.x-k8s.io/provider=infrastructure-packet
CAPZcluster.x-k8s.io/provider=infrastructure-azure

Workload cluster templates

An infrastructure provider could publish a cluster templates file to be used by clusterctl config cluster. This is single YAML with all the objects required to create a new workload cluster.

The following rules apply:

Naming conventions

Cluster templates MUST be stored in the same folder as the component YAML and follow this naming convention:

  1. The default cluster template should be named cluster-template.yaml.
  2. Additional cluster template should be named cluster-template-{flavor}.yaml. e.g cluster-template-prod.yaml

{flavor} is the name the user can pass to the clusterctl config cluster --flavor flag to identify the specific template to use.

Each provider SHOULD create user facing documentation with the list of available cluster templates.

Target namespace

The cluster template YAML MUST assume the target namespace already exists.

All the objects in the cluster template YAML MUST be deployed in the same namespace.

Variables

The cluster templates YAML can also contain environment variables (as can the components YAML).

Additionally, each provider should create user facing documentation with the list of required variables and with all the additional notes that are required to assist the user in defining the value for each variable.

Common variables

The clusterctl config cluster command allows user to set a small set of common variables via CLI flags or command arguments.

Templates writers should use the common variables to ensure consistency across providers and a simpler user experience (if compared to the usage of OS environment variables or the clusterctl config file).

CLI flagVariable nameNote
--target-namespace${ NAMESPACE }The namespace where the workload cluster should be deployed
--kubernetes-version${ KUBERNETES_VERSION }The Kubernetes version to use for the workload cluster
--controlplane-machine-count${ CONTROL_PLANE_MACHINE_COUNT }The number of control plane machines to be added to the workload cluster
--worker-machine-count${ WORKER_MACHINE_COUNT }The number of worker machines to be added to the workload cluster

Additionally, value of the command argument to clusterctl config cluster <cluster-name> (<cluster-name> in this case), will be applied to every occurrence of the ${ CLUSTER_NAME } variable.

OwnerReferences chain

Each provider is responsible to ensure that all the providers resources (like e.g. VSphereCluster, VSphereMachine, VSphereVM etc. for the vsphere provider) MUST have a Metadata.OwnerReferences entry that links directly or indirectly to a Cluster object.

Please note that all the provider specific resources that are referenced by the Cluster API core objects will get the OwnerReference sets by the Cluster API core controllers, e.g.:

  • The Cluster controller ensures that all the objects referenced in Cluster.Spec.InfrastructureRef get an OwnerReference that links directly to the corresponding Cluster.
  • The Machine controller ensures that all the objects referenced in Machine.Spec.InfrastructureRef get an OwnerReference that links to the corresponding Machine, and the Machine is linked to the Cluster through its own OwnerReference chain.

That means that, practically speaking, provider implementers are responsible for ensuring that the OwnerReferences are set only for objects that are not directly referenced by Cluster API core objects, e.g.:

  • All the VSphereVM instances should get an OwnerReference that links to the corresponding VSphereMachine, and the VSphereMachine is linked to the Cluster through its own OwnerReference chain.

Additional notes

Components YAML transformations

Provider authors should be aware of the following transformations that clusterctl applies during component installation:

  • Variable substitution;
  • Enforcement of target namespace:
    • The name of the namespace object is set;
    • The namespace field of all the objects is set (with exception of cluster wide objects like e.g. ClusterRoles);
    • ClusterRole and ClusterRoleBinding are renamed by adding a “${namespace}-“ prefix to the name; this change reduces the risks of conflicts between several instances of the same provider in case of multi tenancy;
  • Enforcement of watching namespace;
  • All components are labeled;

Cluster template transformations

Provider authors should be aware of the following transformations that clusterctl applies during components installation:

  • Variable substitution;
  • Enforcement of target namespace:
    • The namespace field of all the objects is set;

Links to external objects

The clusterctl command requires that both the components YAML and the cluster templates contain all the required objects.

If, for any reason, the provider authors/YAML designers decide not to comply with this recommendation and e.g. to

  • implement links to external objects from a component YAML (e.g. secrets, aggregated ClusterRoles NOT included in the component YAML)
  • implement link to external objects from a cluster template (e.g. secrets, configMaps NOT included in the cluster template)

The provider authors/YAML designers should be aware that it is their responsibility to ensure the proper functioning of all the clusterctl features both in single tenancy or multi-tenancy scenarios and/or document known limitations.

Move

Provider authors should be aware that clusterctl move command implements a discovery mechanism that considers:

  • All the objects of Kind defined in one of the CRDs installed by clusterctl using clusterctl init.
  • Secret and ConfigMap objects.
  • the OwnerReference chain of the above objects.

clusterctl move does NOT consider any objects:

  • Not included in the set of objects defined above.
  • Included in the set of objects defined above, but not directly or indirectly to a Cluster object through the OwnerReference chain.

If moving some of excluded object is required, the provider authors should create documentation describing the the exact move sequence to be executed by the user.

Additionally, provider authors should be aware that clusterctl move assumes all the provider’s Controllers respect the Cluster.Spec.Paused field introduced in the v1alpha3 Cluster API specification.