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Node Images

What are node-images?

Node-images are pre-configured operating system (OS) images for setting up nodes within a Kubernetes cluster. A Kubernetes cluster consists of multiple nodes that are physical or virtual machines. To run the necessary Kubernetes components for a fully functional cluster, each node runs an operating system that hosts these components. These OS images should be compatible with Kubernetes and ensure that the node has the required environment to join and perform in the cluster. The images often comes with necessary components that are pre-installed or can be easy installable facilitating a smooth setup process for Kubernetes nodes.

Node-images in CAPH

Node-image is necessary for using CAPH in production and as a user, there can be some specific needs as per your requirements that needs to be in the linux instance. The popular linux distributions might not contain all of those specifics. In such cases, the user need to build a node-image. These images can be uploaded to the Hetzner cloud as a snapshot and then the user can use these node-images for cluster creation.

Creating a Node Image

For using cluster-API with the bootstrap provider kubeadm, we need a server with all the necessary components for running Kubernetes. There are several ways to achieve this. In the quick-start guide, we use pre-kubeadm commands in the KubeadmControlPlane and KubeadmConfigTemplate objects. These are propagated from the bootstrap-provider-kubeadm and the control-plane-provider-kubeadm to the node as cloud-init commands. This way is usable universally also in other infrastructure providers.

For Hcloud, there is an alternative way of doing this using Packer. It creates a snapshot to boot from. This makes it easier to version the images, and creating new nodes using this image is faster. The same is possible for Hetzner BareMetal, as we could use installimage and a prepared tarball, which then gets installed as the OS for your nodes.

To use CAPH in production, it needs a node image. In Hetzner Cloud, it is not possible to upload your own images directly. However, a server can be created, configured, and then snapshotted. For this, Packer could be used, which already has support for Hetzner Cloud. In this repository, there is also an example Packer node-image. To use it, do the following:

export HCLOUD_TOKEN=<your-token>

## Only build
packer build templates/node-image/1.28.9-ubuntu-22-04-containerd/image.json

## Debug and ability to ssh into the created server
packer build --debug --on-error=abort templates/node-image/1.28.9-ubuntu-22-04-containerd/image.json

The first command is necessary so that Packer is able to create a server in hcloud. The second one creates the server with Packer. If you are developing your own packer image, the third command could be helpful to check what's going wrong.

It is essential to know that if you create your own packer image, you need to set a label so that CAPH can find the specified image name. We use for this label the following key: caph-image-name Please have a look at the image.json of the example node-image.

If you use your own node image, make sure also to use a cluster flavor that has packer in its name. The default one uses preKubeadm commands to install all necessary things. This is very helpful for testing but is not recommended in a production system.