Chapter 2. Understanding OpenShift updates

There are several factors that affect if and when an update is made available to an OpenShift Container Platform cluster. The following list provides common questions regarding the availability of an update:

What are the differences between each of the update channels?

Is a release on the stable channel safer or more supported than a release on the fast channel?

What does it mean if an update is supported but not recommended?

What if I see that an update to a particular release is no longer recommended?

How long until the next z-stream release is made available on the fast and stable channels?

The OpenShift Update Service (OSUS) provides update recommendations to OpenShift Container Platform, including 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. The edges in the graph show which versions you can safely update to. 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 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 CVO uses the corresponding release image to update your cluster. The release artifacts are hosted in Quay as container images.

To allow the OpenShift Update Service to provide only compatible updates, a release verification pipeline drives 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 Update Service notifies you that it is available.

Two controllers run during continuous update mode. The first controller continuously updates the payload manifests, applies the manifests to the cluster, and outputs the controlled rollout status of the Operators to indicate whether they are available, upgrading, or failed. The second controller polls the OpenShift Update Service to determine if updates are available.

During the update process, the Machine Config Operator (MCO) applies the new configuration to your cluster machines. The MCO cordons the number of nodes specified by the maxUnavailable field on the machine configuration pool and marks them unavailable. By default, this value is set to 1. The MCO updates the affected nodes alphabetically by zone, based on the topology.kubernetes.io/zone label. If a zone has more than one node, the oldest nodes are updated first. For nodes that do not use zones, such as in bare metal deployments, the nodes are updated by age, with the oldest nodes updated first. The MCO updates the number of nodes as specified by the maxUnavailable field on the machine configuration pool at a time. The MCO 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 because you must update the OpenShift API on the machines first.

With the specification for the new version 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 unavailable nodes is set to ensure that normal cluster operations can continue with that number of machines out of service.

The OpenShift Update Service is composed of an Operator and one or more application instances.

The Cluster Version Operator (CVO) is the primary component that orchestrates and facilitates the OpenShift Container Platform update process. During installation and standard cluster operation, the CVO is constantly comparing the manifests of managed cluster Operators to in-cluster resources, and reconciling discrepancies to ensure that the actual state of these resources match their desired state.

A release image is the delivery mechanism for a specific OpenShift Container Platform (OCP) version. It contains the release metadata, a Cluster Version Operator (CVO) binary matching the release version, every manifest needed to deploy individual OpenShift cluster Operators, and a list of SHA digest-versioned references to all container images that make up this OpenShift version.

You can inspect the content of a specific release image by running the following command:

$ oc adm release extract <release image>

$ oc adm release extract quay.io/openshift-release-dev/ocp-release:4.12.6-x86_64
Extracted release payload from digest sha256:800d1e39d145664975a3bb7cbc6e674fbf78e3c45b5dde9ff2c5a11a8690c87b created at 2023-03-01T12:46:29Z

$ ls
0000_03_authorization-openshift_01_rolebindingrestriction.crd.yaml
0000_03_config-operator_01_proxy.crd.yaml
0000_03_marketplace-operator_01_operatorhub.crd.yaml
0000_03_marketplace-operator_02_operatorhub.cr.yaml
0000_03_quota-openshift_01_clusterresourcequota.crd.yaml 1
...
0000_90_service-ca-operator_02_prometheusrolebinding.yaml 2
0000_90_service-ca-operator_03_servicemonitor.yaml
0000_99_machine-api-operator_00_tombstones.yaml
image-references 3
release-metadata

The following steps represent a detailed workflow of the OpenShift Container Platform (OCP) update process:

The cluster reports as updated after the control plane update is finished, usually before all nodes are updated. After the update, the CVO maintains all cluster resources to match the state delivered in the release image.

Some manifests supplied in a release image must be applied in a certain order because of the dependencies between them. For example, the CustomResourceDefinition resource must be created before the matching custom resources. Additionally, there is a logical order in which the individual cluster Operators must be updated to minimize disruption in the cluster. The Cluster Version Operator (CVO) implements this logical order through the concept of Runlevels.

These dependencies are encoded in the filenames of the manifests in the release image:

0000_<runlevel>_<component>_<manifest-name>.yaml

For example:

0000_03_config-operator_01_proxy.crd.yaml

The CVO internally builds a dependency graph for the manifests, where the CVO obeys the following rules:

The CVO then applies manifests following the generated dependency graph.

The CVO waits until all cluster Operators in the Runlevel meet the following conditions before it proceeds to the next Runlevel:

Some actions can take significant time to finish. The CVO waits for the actions to complete in order to ensure the subsequent Runlevels can proceed safely. Initially reconciling the new release’s manifests is expected to take 60 to 120 minutes in total; see Understanding OpenShift Container Platform update duration for more information about factors that influence update duration.

In the previous example diagram, the CVO is waiting until all work is completed at Runlevel 20. The CVO has applied all manifests to the Operators in the Runlevel, but the kube-apiserver-operator ClusterOperator performs some actions after its new version was deployed. The kube-apiserver-operator ClusterOperator declares this progress through the Progressing=True condition and by not declaring the new version as reconciled in its status.versions. The CVO waits until the ClusterOperator reports an acceptable status, and then it will start reconciling manifests at Runlevel 25.

The Machine Config Operator (MCO) applies a new machine configuration to each control plane node and compute node. During the machine configuration update, control plane nodes and compute nodes are organized into their own machine config pools, where the pools of machines are updated in parallel. The .spec.maxUnavailable parameter, which has a default value of 1, determines how many nodes in a machine config pool can simultaneously undergo the update process.

When the machine configuration update process begins, the MCO checks the amount of currently unavailable nodes in a pool. If there are fewer unavailable nodes than the value of .spec.maxUnavailable, the MCO initiates the following sequence of actions on available nodes in the pool:

A node undergoing this process is unavailable until it is uncordoned and workloads can be scheduled to it again. The MCO begins updating nodes until the number of unavailable nodes is equal to the value of .spec.maxUnavailable.

As a node completes its update and becomes available, the number of unavailable nodes in the machine config pool is once again fewer than .spec.maxUnavailable. If there are remaining nodes that need to be updated, the MCO initiates the update process on a node until the .spec.maxUnavailable limit is once again reached. This process repeats until each control plane node and compute node has been updated.

The following example workflow describes how this process might occur in a machine config pool with 5 nodes, where .spec.maxUnavailable is 3 and all nodes are initially available:

Because the update process for each node is independent of other nodes, some nodes in the example above finish their update out of the order in which they were cordoned by the MCO.

You can check the status of the machine configuration update by running the following command:

$ oc get mcp

NAME         CONFIG                                                 UPDATED   UPDATING   DEGRADED   MACHINECOUNT   READYMACHINECOUNT   UPDATEDMACHINECOUNT   DEGRADEDMACHINECOUNT   AGE
master       rendered-master-acd1358917e9f98cbdb599aea622d78b       True      False      False      3              3                   3                     0                      22h
worker       rendered-worker-1d871ac76e1951d32b2fe92369879826       False     True       False      2              1                   1                     0                      22h