Kubernetes Cluster API (CAPI)

What is Kubernetes CAPI?

The Kubernetes Cluster API is an open-source project that extends the Kubernetes platform, it is a Kubernetes sub-project focused on providing declarative APIs and tooling to simplify provisioning, upgrading, and operating multiple Kubernetes clusters.

It provides a declarative way to create, manage, and destroy Kubernetes clusters and aims to simplify and automate the management of clusters across various cloud providers and on-premises environments.

It takes managing the infrastructure, cluster configuration and deploying workload and merges them into one single component, which eliminates the need of having to rely on multiple disconnected tools to achieve the same.

Why use it?

Kubernetes is nowadays the de facto standard for deploying, scaling, and managing applications. However, as the complexity of modern infrastructure environments increases, so does the need for efficient and scalable management solutions. 

Traditionally, provisioning and managing Kubernetes clusters involved using provider-specific tools or scripts. There are tens if not hundreds of different Kubernetes distributions and installers available today. Each one of these uses different methods for building the infrastructure and has different Kubernetes configurations. 

This approach often led to fragmentation, inconsistent behaviors, vendor lock-in, and limited interoperability between different infrastructure environments. 

The Kubernetes Cluster API addresses these challenges by treating clusters as Kubernetes custom resources, allowing operators to define cluster configurations using familiar Kubernetes constructs such as Custom Resource Definitions (CRDs) and YAML manifests.

With the Kubernetes Cluster API, managing clusters becomes more consistent, automated, and scalable. Instead of interacting with different provider APIs, administrators can use Kubernetes-native tools and workflows to define and manage clusters. The API acts as a control plane that handles the cluster provisioning, scaling, and lifecycle operations, while leveraging the powerful features of the core Kubernetes API and Controller framework.

Simply put, cluster API standardizes the process of building Kubernetes clusters across various infrastructure providers to generate consistent and reliable results irrespective of the infrastructure provider used.

How It Works (Architecture)

The Kubernetes Cluster API or simply CAPI is composed of several components that work together to enable the management of Kubernetes clusters as Kubernetes custom resources.

CAPI requires a working Kubectl accessible Kubernetes Cluster in order to host the CRDs and the controllers. A CAPI architecture typically consists of a management Kubernetes cluster where the CAPI components reside, plus one or more Workload Clusters that are deployed using it.

Let’s have a look at the key components:

Components

Management cluster
This is a Kubernetes cluster that manages the lifecycle of Workload Clusters. Here is where one or more providers run, and where resources such as Machines are stored. It can be any lightweight cluster such as a kind cluster or an already existing K8s cluster in your environment.


Workload cluster
A Kubernetes cluster whose lifecycle is managed by a Management Cluster. This is where the payload and containerized applications are intended to run.


Cluster API Controllers
The Cluster API Controllers are responsible for managing the lifecycle of the Kubernetes clusters created using the Cluster API. These controllers are responsible for provisioning, scaling, and deleting the clusters, and ensuring that the Workload clusters are in the desired state.

Cluster API Provider
The Cluster API Provider is responsible for translating the Kubernetes API objects into provider-specific resources for creating and managing the underlying infrastructure. For example, a Cluster API Provider for AWS will create and manage EC2 instances, security groups, and load balancers.


Cluster API Provider Components
These are the provider-specific components that interact with the infrastructure provider’s API to create and manage the underlying infrastructure resources. These components can include load balancers, virtual machines, security groups, and other infrastructure resources.


Cluster API Bootstrap
The Cluster API Bootstrap provides a way to bootstrap the Kubernetes control plane components on a newly created cluster. It is responsible for installing and configuring the Kubernetes control plane components such as etcd, kube-apiserver, kube-controller-manager, and kube-scheduler, generating the cluster certificates, if not otherwise specified as well as for Initializing the control plane (and gating the creation of other Workload nodes until it is finished).


Infrastructure provider
Is responsible for the provisioning of infrastructure/computational resources required by the Cluster or by Machines (e.g. VMs, networking, etc.). For example, cloud Infrastructure Providers include AWS, Azure, and Google, and bare metal Infrastructure Providers can include VMware, MAAS, metal3.io and so on.


When there is more than one way to obtain resources from the same Infrastructure Provider (such as AWS offering both EC2 and EKS), each way is referred to as a variant.


Control plane

The control plane is a set of components that serve the Kubernetes API and continuously reconcile desired state using control loops.

  • Self-provisioned: consists of pods or machines wholly managed by a single Cluster API deployment. E.g default kubeadm uses static pods for running components such as kube-apiserver, kube-controller-manager and kube-scheduler on control plane machines.
  • Pod-based: deployments require an external hosting cluster. The control plane components are deployed using standard Deployment and StatefulSet objects and the API is exposed using a Service.
  • External or Managed: control planes are offered and controlled by some system other than Cluster API, such as GKE, AKS, EKS, or IKS services offered by cloud providers.


The default provider uses kubeadm to bootstrap the control plane. As of v1alpha3, it exposes the configuration via the KubeadmControlPlane object. The controller, capi-kubeadm-control-plane-controller-manager, can then create Machine and BootstrapConfig objects based on the requested replicas in the KubeadmControlPlane object.


Cluster API Custom Resource Definitions (CRDs)
The Cluster API CRDs define the Kubernetes resources used by the Cluster API to create, manage, and delete clusters. These resources include Cluster, Machine, MachineSet, and MachineDeployment.


Cluster API Machine
The Cluster API Machine is responsible for defining the machine instances that will be used to run the Kubernetes control plane and worker nodes. It specifies the operating system, instance size, networking, and other details needed to create the machine instances.


Cluster API Webhooks
The Cluster API Webhooks provide a way to validate and mutate Kubernetes API objects before they are persisted to the Kubernetes API server. Webhooks can be used to perform validation checks, default values, and other customization on the Cluster API objects.


Together, these components enable the creation, management, and deletion of Kubernetes clusters as Kubernetes custom resources, providing a declarative way to manage clusters across multiple cloud providers and on-premises environments. The Cluster API is designed to be extensible, allowing providers to customize and extend the API to support new use cases and workflows.

Pros and Cons

Pros:

  • Simplified cluster management across multiple cloud providers and on-premises environments.
  • Infrastructure as code approach enhances consistency and reproducibility.
  • Seamless integration with existing Kubernetes tools and ecosystem.
  • Automation capabilities increase operational efficiency.
  • Improved portability for hybrid cloud environments.

 

Cons:

  •  Good knowledge of Kubernetes concepts and infrastructure management is required.
  • As an evolving project, the Kubernetes Cluster API may have certain limitations or occasional issues. API is not yet fully mature.
  • Managing infrastructure through custom resources may introduce additional complexity compared to some provider-specific interfaces.

Conclusion

As the demand for scalable and portable infrastructure management grows, the Kubernetes Cluster API provides a powerful, vanilla solution for simplifying the provisioning and lifecycle management of Kubernetes clusters. 

By abstracting away the complexities of various cloud providers and enabling best practices such as Infrastructure as Code, it empowers organizations to achieve consistency, automation, and scalability in their cluster operations.

While it requires some initial investment in learning and adapting existing workflows, the benefits of increased efficiency, portability, and integration with the Kubernetes ecosystem make the Kubernetes Cluster API a compelling choice for modern infrastructure management.

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