Updating a cluster between minor versions

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.

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.

During the upgrade process, the Machine Config Operator (MCO) applies the new configuration to your cluster machines. It cordons the number of nodes that is specified by the maxUnavailable field on the machine configuration pool and marks them as unavailable. By default, this value is set to 1. It then applies the new configuration and reboots the machine. If you use Red Hat Enterprise Linux (RHEL) machines as workers, the MCO does not update the kubelet on these machines because you must update the OpenShift API on them first. Because the specification for the new version is applied to the old kubelet, the RHEL machine cannot return to the Ready state. You cannot complete the update until the machines are available. However, the maximum number of nodes that are unavailable is set to ensure that normal cluster operations are likely to continue with that number of machines out of service.

In OpenShift Container Platform 4.1, Red Hat introduced the concept of channels for recommending the appropriate release versions for cluster upgrade. By controlling the pace of upgrades, these upgrade channels allow you to choose an upgrade strategy. Upgrade channels are tied to a minor version of OpenShift Container Platform. For instance, OpenShift Container Platform 4.3 upgrade channels will never include an upgrade to a 4.4 release. This strategy ensures that administrators explicitly decide to upgrade to the next minor version of OpenShift Container Platform. Upgrade channels control only release selection and do not impact the version of the cluster that you install; the openshift-install binary file for a specific version of OpenShift Container Platform always installs that version.

OpenShift Container Platform 4.3 offers the following upgrade channels:

The candidate-4.3 channel contains candidate builds for a z-stream (4.3.z) release. Release candidates contain all the features of the product but are not supported. Use release candidate versions to test feature acceptance and assist in qualifying the next version of OpenShift Container Platform. A release candidate is any build that is available in the candidate channel, including ones that do not contain -rc in their names. After a version is available in the candidate channel, it goes through more quality checks. If it meets the quality standard, it is promoted to the fast-4.3 or stable-4.3 channels. Because of this strategy, if a specific release is available in both the candidate-4.3 channel and in the fast-4.3 or stable-4.3 channels, it is a Red Hat supported version. The candidate-4.3 channel can include release versions from which there are no recommended updates in any channel.

You can use the candidate-4.3 channel to upgrade from a previous minor version of OpenShift Container Platform.

The fast-4.3 channel is updated with new 4.3 versions as soon as Red Hat declares the given version as a general availability release. As such, these releases are fully supported, are production quality, and have performed well while available as a release candidate in the candidate-4.3 channel from where they were promoted. Some time after a release appears in the fast-4.3 channel, it is added to the stable-4.3 channel. Releases never appear in the stable-4.3 channel before they appear in the fast-4.3 channel.

You can use the fast-4.3 channel to upgrade from a previous minor version of OpenShift Container Platform.

While the fast-4.3 channel contains releases as soon as their errata are published, releases are added to the stable-4.3 channel after a delay. During this delay, data is collected from Red Hat SRE teams, Red Hat support services, and pre-production and production environments that participate in connected customer program about the stability of the release.

You can use the stable-4.3 channel to upgrade from a previous minor version of OpenShift Container Platform.

OpenShift Container Platform maintains an upgrade recommendation service that understands the version of OpenShift Container Platform you have installed as well as the path to take within the channel you choose to get you to the next release. You can imagine seeing the following in the fast-4.3 channel:

The service recommends only upgrades that have been tested and have no serious issues. If your cluster is on 4.3.1 and OpenShift Container Platform suggests 4.3.4, then it is safe for you to update from 4.3.1 to 4.3.4. Do not rely on consecutive patch numbers. In this example, 4.3.2 is not, and never was, available in the channel. The update service will not suggest updating to a version of OpenShift Container Platform that contains known vulnerabilities.

Update stability depends on your channel. The presence of an update recommendation in the candidate-4.3 channel does not imply that the update is supported. It means that no serious issues have been found with the update yet, but there might not be significant traffic through the update to suggest stability. The presence of an update recommendation in the fast-4.3 or stable-4.3 channels is a declaration that the update is fully supported while it is in the channel. While releases will never be removed from a channel, update recommendations that exhibit serious issues will be removed from all channels. Updates initiated after the update recommendation has been removed might not be supported.

Red Hat will eventually provide supported update paths from any supported release in the fast-4.3 or stable-4.3 channels to the latest release in 4.3.z, although there can be delays while safe paths away from troubled releases are constructed and verified.

The fast-4.3 and stable-4.3 channels present a choice between receiving general availability releases as soon as they are available or allowing Red Hat to control the rollout of those updates. If issues are detected during rollout or at a later time, upgrades to that version might be blocked in both the fast-4.3 and stable-4.3 channels, and a new version might be introduced that becomes the new preferred upgrade target.

Customers can improve this process by configuring pre-production systems on the fast-4.3 channel, configuring production systems on the stable-4.3 channel, and participating in Red Hat’s connected customer program. Red Hat uses this program to observe the impact of updates on your specific hardware and software configurations. Future releases might improve or alter the pace at which updates move from the fast-4.3 to the stable-4.3 channel.

If you manage the container images for your OpenShift Container Platform clusters yourself, you must consult the Red Hat errata that is associated with product releases and note any comments that impact upgrades. During upgrade, the user interface might warn you about switching between these versions, so you must ensure that you selected an appropriate version before you bypass those warnings.

Your cluster is still supported if you change from the stable-4.3 channel to the fast-4.3 channel. Although you can switch to the candidate-4.3 channel at any time, some releases in that channel might be unsupported release candidates. You can switch from the candidate-4.3 channel to the fast-4.3 channel if your current release is a general availability release. You can always switch from the fast-4.3 channel to the stable-4.3 channel, although if the current release was recently promoted to fast-4.3 there can be a delay of up to a day for the release to be promoted to stable-4.3. If you change to a channel that does not include your current release, an alert displays and no updates can be recommended, but you can safely change back to your original channel at any point.

If updates are available, you can update your cluster from the web console.

You can find information about available OpenShift Container Platform advisories and updates in the errata section of the Customer Portal.