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Chapter 2. Understanding OpenShift Container Platform updates

With OpenShift Container Platform 4, you can update a OpenShift Container Platform cluster with a single operation by using the web console or the OpenShift CLI (oc). Platform administrators are automatically notified when an update is available for their cluster.

The OpenShift Update Service (OSUS) builds a graph of update possibilities based on release images in the registry. The graph is based on recommended, tested update paths from a specific version. OpenShift Container Platform clusters connect to the Red Hat Hybrid Cloud servers and identify which clusters the user is running, along with the version information. OSUS responds with information about known update targets. Either a cluster administrator or an automatic update controller edits the custom resource (CR) of the Cluster Version Operator (CVO) with the new version to update to. After the CVO receives the update image from the registry, the CVO then applies the changes.

2.1. Common terms

Control plane
The control plane, which is composed of control plane machines, manages the OpenShift Container Platform cluster. The control plane machines manage workloads on the compute machines, which are also known as worker machines.
Cluster Version Operator
The Cluster Version Operator (CVO) starts the update process for the cluster. It checks with OSUS based on the current cluster version and retrieves the graph which contains available or possible update paths.
Machine Config Operator
The Machine Config Operator (MCO) is a cluster-level Operator that manages the operating system and machine configurations. Through the MCO, platform administrators can configure and update systemd, CRI-O and Kubelet, the kernel, NetworkManager, and other system features on the worker nodes.
OpenShift Update Service
The OpenShift Update Service (OSUS) provides over-the-air updates to OpenShift Container Platform, including to Red Hat Enterprise Linux CoreOS (RHCOS). It provides a graph, or diagram, that contains the vertices of component Operators and the edges that connect them.
Channels
Channels declare an update strategy tied to minor versions of OpenShift Container Platform. The OSUS uses this configured strategy to recommend update edges consistent with that strategy.
Recommended update edge
A recommended update edge is a recommended update between OpenShift Container Platform releases. Whether a given update is recommended can depend on the cluster’s configured channel, current version, known bugs, and other information. OSUS communicates the recommended edges to the CVO, which runs in every cluster.
Extended Update Support

All post-4.7 even-numbered minor releases are labeled as Extended Update Support (EUS) releases. These releases introduce a verified update path between EUS releases, permitting customers to streamline updates of worker worker nodes and formulate update strategies of EUS-to-EUS OpenShift Container Platform releases that will cause fewer reboots of worker nodes.

For more information, see Red Hat OpenShift Extended Update Support (EUS) Overview.