The OpenShift Container Platform 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 OpenShift Container Platform 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 OpenShift Container Platform cluster.
The OpenShift Container Platform 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 Red Hat Enterprise Linux CoreOS (RHCOS) as the operating
system. RHCOS is the immutable container host version of
Red Hat Enterprise Linux (RHEL) and features a RHEL 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 OpenShift Container Platform 4.2 cluster must use RHCOS, 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 OpenShift Container Platform 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 RHCOS 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.
In OpenShift Container Platform 4.2, you can install a cluster that uses installer-provisioned infrastructure on the following platforms:
Amazon Web Services (AWS)
Google Cloud Platform (GCP)
Red Hat OpenStack Platform version 13 and 14
For these clusters, all machines, including the computer that you run the installation process on, must have direct internet access to pull images for platform containers and provide telemetry data to Red Hat.
In OpenShift Container Platform 4.2, you can install a cluster that uses user-provisioned infrastructure on the following platforms:
With installations on user-provisioned infrastructure, each machine can have full internet access, you can place your cluster behind a proxy, or you can perform a restricted network installation. In a restricted network installation, you can download the images that are required to install a cluster, place them in a mirror registry, and use that data to install your cluster. While you require internet access to pull images for platform containers, with a restricted network installation on vSphere or bare metal infrastructure, your cluster machines do not require direct internet access.
The OpenShift Container Platform 4.x Tested Integrations page contains details about integration testing for different platforms.
When you install an OpenShift Container Platform cluster, you download the installation program from the appropriate Infrastructure Provider page on the Red Hat OpenShift Cluster Manager site. This site manages:
REST API for accounts
Registry tokens, which are the pull secrets that you use to obtain the required components
Cluster registration, which associates the cluster identity to your Red Hat account to facilitate the gathering of usage metrics
In OpenShift Container Platform 4.2, 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 cannot use the advanced machine management and scaling capabilities that an installer-provisioned infrastructure cluster offers.
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 RHCOS 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, OpenShift Container Platform 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 OpenShift Container Platform 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 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 RHEL worker machines to your cluster.
Because each machine in the cluster requires information about the cluster when it is provisioned, OpenShift Container Platform 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 master machines that make up the control plane. The control plane machines then create the compute, or worker, machines. The following figure illustrates this process:
After the cluster machines initialize, the bootstrap machine is destroyed. All clusters use the bootstrap process to initialize the cluster, but if you provision the infrastructure for your cluster, you must complete many of the steps manually.
The Ignition config files that the installation program generates contain certificates that expire after 24 hours. You must complete your cluster installation and keep the cluster running for 24 hours in a non-degraded state to ensure that the first certificate rotation has finished.
Bootstrapping a cluster involves the following steps:
The bootstrap machine boots and starts hosting the remote resources required for the master machines to boot. (Requires manual intervention if you provision the infrastructure)
The master machines fetch the remote resources from the bootstrap machine and finish booting. (Requires manual intervention if you provision the infrastructure)
The master machines use the bootstrap machine to form an etcd cluster.
The bootstrap machine starts a temporary Kubernetes control plane using the new etcd cluster.
The temporary control plane schedules the production control plane to the master machines.
The temporary control plane shuts down and passes control to the production control plane.
The bootstrap machine injects OpenShift Container Platform components into the production control plane.
The installation program shuts down the bootstrap machine. (Requires manual intervention if you provision the infrastructure)
The control plane sets up the worker nodes.
The control plane installs additional services in the form of a set of Operators.
The result of this bootstrapping process is a fully running OpenShift Container Platform cluster. The cluster then downloads and configures remaining components needed for the day-to-day operation, including the creation of worker machines in supported environments.
The scope of the OpenShift Container Platform installation program is intentionally narrow. It is designed for simplicity and ensured success. You can complete many more configuration tasks after installation completes.
The OpenShift Container Platform update service is the hosted service that provides over-the-air updates to both OpenShift Container Platform and Red Hat Enterprise Linux CoreOS (RHCOS). It provides a graph, or diagram that contain vertices and the edges that connect them, of component Operators. The edges in the graph show which versions you can safely update to, and the vertices are update payloads that specify the intended state of the managed cluster components.
The Cluster Version Operator (CVO) in your cluster checks with the OpenShift Container Platform update service to see the valid updates and update paths based on current component versions and information in the graph. When you request an update, the OpenShift Container Platform CVO uses the release image for that update to upgrade your cluster. The release artifacts are hosted in Quay as container images.
To allow the OpenShift Container Platform update service to provide only compatible updates, a release verification pipeline exists to drive automation. Each release artifact is verified for compatibility with supported cloud platforms and system architectures as well as other component packages. After the pipeline confirms the suitability of a release, the OpenShift Container Platform update service notifies you that it is available.
Because the update service displays all valid updates, you must not force an update to a version that the update service does not display.
During continuous update mode, two controllers run. One continuously updates the payload manifests, applies them to the cluster, and outputs the status of the controlled rollout of the Operators, whether they are available, upgrading, or failed. The second controller polls the OpenShift Container Platform update service to determine if updates are available.
Reverting your cluster to a previous version, or a rollback, is not supported. Only upgrading to a newer version is supported.