The OKD installation program offers you flexibility. You can use the installation program to deploy a cluster on infrastructure that the installation program provisions and the cluster maintains or deploy a cluster on infrastructure that you prepare and maintain.
These two basic types of OKD clusters are frequently called installer-provisioned infrastructure clusters and user-provisioned infrastructure clusters.
Both types of clusters have the following characteristics:
Highly available infrastructure with no single points of failure is available by default
Administrators maintain control over what updates are applied and when
You use the same installation program to deploy both types of clusters. The main assets generated by the installation program are the Ignition config files for the bootstrap, master, and worker machines. With these three configurations and correctly configured infrastructure, you can start an OKD cluster.
The OKD installation program uses a set of targets and dependencies to manage cluster installation. The installation program has a set of targets that it must achieve, and each target has a set of dependencies. Because each target is only concerned with its own dependencies, the installation program can act to achieve multiple targets in parallel. The ultimate target is a running cluster. By meeting dependencies instead of running commands, the installation program is able to recognize and use existing components instead of running the commands to create them again.
The following diagram shows a subset of the installation targets and dependencies:
After installation, each cluster machine uses Fedora CoreOS (FCOS) as the operating system. FCOS is the immutable container host version of Fedora and features a Fedora kernel with SELinux enabled by default. It includes the
kubelet, which is the Kubernetes node agent, and the CRI-O container runtime, which is optimized for Kubernetes.
Every control plane machine in an OKD 4.10 cluster must use FCOS, which includes a critical first-boot provisioning tool called Ignition. This tool enables the cluster to configure the machines. Operating system updates are delivered as an Atomic OSTree repository that is embedded in a container image that is rolled out across the cluster by an Operator. Actual operating system changes are made in-place on each machine as an atomic operation by using rpm-ostree. Together, these technologies enable OKD to manage the operating system like it manages any other application on the cluster, via in-place upgrades that keep the entire platform up-to-date. These in-place updates can reduce the burden on operations teams.
If you use FCOS as the operating system for all cluster machines, the cluster manages all aspects of its components and machines, including the operating system. Because of this, only the installation program and the Machine Config Operator can change machines. The installation program uses Ignition config files to set the exact state of each machine, and the Machine Config Operator completes more changes to the machines, such as the application of new certificates or keys, after installation.
When you install an OKD cluster, you download the installation program from
In OKD 4.10, the installation program is a Go binary file that performs a series of file transformations on a set of assets. The way you interact with the installation program differs depending on your installation type.
For clusters with installer-provisioned infrastructure, you delegate the infrastructure bootstrapping and provisioning to the installation program instead of doing it yourself. The installation program creates all of the networking, machines, and operating systems that are required to support the cluster.
If you provision and manage the infrastructure for your cluster, you must provide all of the cluster infrastructure and resources, including the bootstrap machine, networking, load balancing, storage, and individual cluster machines.
You use three sets of files during installation: an installation configuration file that is named
install-config.yaml, Kubernetes manifests, and Ignition config files for your machine types.
It is possible to modify Kubernetes and the Ignition config files that control the underlying FCOS operating system during installation. However, no validation is available to confirm the suitability of any modifications that you make to these objects. If you modify these objects, you might render your cluster non-functional. Because of this risk, modifying Kubernetes and Ignition config files is not supported unless you are following documented procedures or are instructed to do so by Red Hat support.
The installation configuration file is transformed into Kubernetes manifests, and then the manifests are wrapped into Ignition config files. The installation program uses these Ignition config files to create the cluster.
The installation configuration files are all pruned when you run the installation program, so be sure to back up all configuration files that you want to use again.
You cannot modify the parameters that you set during installation, but you can modify many cluster attributes after installation.
The default installation type uses installer-provisioned infrastructure. By default, the installation program acts as an installation wizard, prompting you for values that it cannot determine on its own and providing reasonable default values for the remaining parameters. You can also customize the installation process to support advanced infrastructure scenarios. The installation program provisions the underlying infrastructure for the cluster.
You can install either a standard cluster or a customized cluster. With a standard cluster, you provide minimum details that are required to install the cluster. With a customized cluster, you can specify more details about the platform, such as the number of machines that the control plane uses, the type of virtual machine that the cluster deploys, or the CIDR range for the Kubernetes service network.
If possible, use this feature to avoid having to provision and maintain the cluster infrastructure. In all other environments, you use the installation program to generate the assets that you require to provision your cluster infrastructure.
With installer-provisioned infrastructure clusters, OKD manages all aspects of the cluster, including the operating system itself. Each machine boots with a configuration that references resources hosted in the cluster that it joins. This configuration allows the cluster to manage itself as updates are applied.
You can also install OKD on infrastructure that you provide. You use the installation program to generate the assets that you require to provision the cluster infrastructure, create the cluster infrastructure, and then deploy the cluster to the infrastructure that you provided.
If you do not use infrastructure that the installation program provisioned, you must manage and maintain the cluster resources yourself, including:
The underlying infrastructure for the control plane and compute machines that make up the cluster
Cluster networking, including the DNS records and required subnets
Storage for the cluster infrastructure and applications
If your cluster uses user-provisioned infrastructure, you have the option of adding Fedora compute machines to your cluster.
Because each machine in the cluster requires information about the cluster when it is provisioned, OKD uses a temporary bootstrap machine during initial configuration to provide the required information to the permanent control plane. It boots by using an Ignition config file that describes how to create the cluster. The bootstrap machine creates the control plane machines that make up the control plane. The control plane machines then create the compute machines, which are also known as worker machines. The following figure illustrates this process: