ClusterAPI (CAPI) is an open-source Kubernetes project that provides a declarative API for cluster creation, configuration, and management. It is designed to automate the creation and management of Kubernetes clusters in various environments, including on-premises data centers, public clouds, and edge devices.
CAPI abstracts away the details of infrastructure provisioning, networking, and other low-level tasks, allowing users to define their desired cluster configuration using simple YAML manifests. This makes it easier to create and manage clusters in a repeatable and consistent manner, regardless of the underlying infrastructure. In this way a wide range of infrastructure providers has been made available, including but not limited to Amazon Web Services (AWS), Microsoft Azure, Google Cloud Platform (GCP), and OpenStack.
CAPI also abstracts the provisioning and management of Kubernetes clusters allowing for a variety of Kubernetes distributions to be delivered in all of the supported infrastructure providers. MicroK8s is one such Kubernetes distribution that seamlessly integrates with Cluster API.
With MicroK8s CAPI you can:
- provision a cluster with:
- Kubernetes version 1.24 onwards
- risk level of the track you want to follow (stable, candidate, beta, edge)
- integration to a snapstore proxy
- deploy behind proxies
- upgrade clusters with no downtime:
- rolling upgrades for HA clusters and worker nodes
- in-place upgrades for non-HA control planes
- initilize clusters with workloads:
- enable addons during bootstrap
Please refer to the “HowTo” section for concrete examples on CAPI deployments:
Being a cloud-native framework, CAPI implements all its components as controllers that run within a Kubernetes cluster. There is a separate controller, called a ‘provider’, for each supported infrastructure substrate. The infrastructure providers are responsible for provisioning physical or virtual nodes and setting up networking elements such as load balancers and virtual networks. In a similar way, each Kubernetes distribution that integrates with ClusterAPI is managed by two providers: the control plane provider and the bootstrap provider. The bootstrap provider is responsible for delivering and managing Kubernetes on the nodes, while the control plane provider handles the control plane’s specific lifecycle.
The CAPI providers operate within a Kubernetes cluster known as the management cluster. The administrator is responsible for selecting the desired combination of infrastructure and Kubernetes distribution by instantiating the respective infrastructure, bootstrap, and control plane providers on the management cluster.
The management cluster functions as the control plane for the ClusterAPI operator, which is responsible for provisioning and managing the infrastructure resources necessary for creating and managing additional Kubernetes clusters. It is important to note that the management cluster is not intended to support any other workload, as the workloads are expected to run on the provisioned clusters. As a result, the provisioned clusters are referred to as workload clusters.
Typically, the management cluster runs in a separate environment from the clusters it manages, such as a public cloud or an on-premises data center. It serves as a centralized location for managing the configuration, policies, and security of multiple managed clusters. By leveraging the management cluster, users can easily create and manage a fleet of Kubernetes clusters in a consistent and repeatable manner.
The MicroK8s team maintains the two providers required for integrating with CAPI:
The Cluster API Bootstrap Provider MicroK8s (CABPM) responsible for provisioning the nodes in the cluster and preparing them to be joined to the Kubernetes control plane. When you use the CABPM you define a Kubernetes Cluster object that describes the desired state of the new cluster and includes the number and type of nodes in the cluster, as well as any additional configuration settings. The Bootstrap Provider then creates the necessary resources in the Kubernetes API server to bring the cluster up to the desired state. Under the hood, the Bootstrap Provider uses cloud-init to configure the nodes in the cluster. This includes setting up SSH keys, configuring the network, and installing necessary software packages.
The Cluster API Control Plane Provider MicroK8s (CACPM) enables the creation and management of Kubernetes control planes using MicroK8s as the underlying Kubernetes distribution. Its main tasks are to update the machine state and to generate the kubeconfig file used for accessing the cluster. The kubeconfig file is stored as a secret which the user can then retrieve using the
- Cluster API docs: Introduction - The Cluster API Book
- Cluster API control plane provider MicroK8s docs in Cluster API docs:
MicroK8s based control plane management - The Cluster API Book
- Cluster API bootstrap provider MicroK8s docs in Cluster API docs: MicroK8s based bootstrap - The Cluster API Book
- GitHub repo for CACPM: GitHub - canonical/cluster-api-control-plane-provider-microk8s: This project offers a cluster API control plane controller that manages the control plane of a MicroK8s cluster. It is expected to be used along with the respective MicroK8s specific machine bootstrap provider.
- GitHub repo for CABPM: GitHub - canonical/cluster-api-bootstrap-provider-microk8s: This project offers a cluster API bootstrap provider controller that manages the node provision of a MicroK8s cluster.