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How cluster updates work

The Cluster Version Operator

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.

The ClusterVersion object

One of the resources that the Cluster Version Operator (CVO) monitors is the ClusterVersion resource.

Administrators and OpenShift components can communicate or interact with the CVO through the ClusterVersion object. The desired CVO state is declared through the ClusterVersion object and the current CVO state is reflected in the object’s status.

Do not directly modify the ClusterVersion object. Instead, use interfaces such as the oc CLI or the web console to declare your update target.

The CVO continually reconciles the cluster with the target state declared in the spec property of the ClusterVersion resource. When the desired release differs from the actual release, that reconciliation updates the cluster.

Update availability data

The ClusterVersion resource also contains information about updates that are available to the cluster. This includes updates that are available, but not recommended due to a known risk that applies to the cluster. These updates are known as conditional updates. To learn how the CVO maintains this information about available updates in the ClusterVersion resource, see the "Evaluation of update availability" section.

  • You can inspect all available updates with the following command:

    $ oc adm upgrade --include-not-recommended

    The additional --include-not-recommended parameter includes updates that are available but not recommended due to a known risk that applies to the cluster.
    Example output
    Cluster version is 4.10.22

Upstream is unset, so the cluster will use an appropriate default.
Channel: fast-4.11 (available channels: candidate-4.10, candidate-4.11, eus-4.10, fast-4.10, fast-4.11, stable-4.10)

Recommended updates:

  VERSION     IMAGE
  4.10.26     quay.io/openshift-release-dev/ocp-release@sha256:e1fa1f513068082d97d78be643c369398b0e6820afab708d26acda2262940954
  4.10.25     quay.io/openshift-release-dev/ocp-release@sha256:ed84fb3fbe026b3bbb4a2637ddd874452ac49c6ead1e15675f257e28664879cc
  4.10.24     quay.io/openshift-release-dev/ocp-release@sha256:aab51636460b5a9757b736a29bc92ada6e6e6282e46b06e6fd483063d590d62a
  4.10.23     quay.io/openshift-release-dev/ocp-release@sha256:e40e49d722cb36a95fa1c03002942b967ccbd7d68de10e003f0baa69abad457b

Supported but not recommended updates:

  Version: 4.11.0
  Image: quay.io/openshift-release-dev/ocp-release@sha256:300bce8246cf880e792e106607925de0a404484637627edf5f517375517d54a4
  Recommended: False
  Reason: RPMOSTreeTimeout
  Message: Nodes with substantial numbers of containers and CPU contention may not reconcile machine configuration https://bugzilla.redhat.com/show_bug.cgi?id=2111817#c22

    The oc adm upgrade command queries the ClusterVersion resource for information about available updates and presents it in a human-readable format.

  • One way to directly inspect the underlying availability data created by the CVO is by querying the ClusterVersion resource with the following command:

    $ oc get clusterversion version -o json | jq '.status.availableUpdates'
    Example output
    [
  {
    "channels": [
      "candidate-4.11",
      "candidate-4.12",
      "fast-4.11",
      "fast-4.12"
    ],
    "image": "quay.io/openshift-release-dev/ocp-release@sha256:400267c7f4e61c6bfa0a59571467e8bd85c9188e442cbd820cc8263809be3775",
    "url": "https://access.redhat.com/errata/RHBA-2023:3213",
    "version": "4.11.41"
  },
  ...
]

  • A similar command can be used to check conditional updates:

    $ oc get clusterversion version -o json | jq '.status.conditionalUpdates'
    Example output
    [
  {
    "conditions": [
      {
        "lastTransitionTime": "2023-05-30T16:28:59Z",
        "message": "The 4.11.36 release only resolves an installation issue https://issues.redhat.com//browse/OCPBUGS-11663 , which does not affect already running clusters. 4.11.36 does not include fixes delivered in recent 4.11.z releases and therefore upgrading from these versions would cause fixed bugs to reappear. Red Hat does not recommend upgrading clusters to 4.11.36 version for this reason. https://access.redhat.com/solutions/7007136",
        "reason": "PatchesOlderRelease",
        "status": "False",
        "type": "Recommended"
      }
    ],
    "release": {
      "channels": [...],
      "image": "quay.io/openshift-release-dev/ocp-release@sha256:8c04176b771a62abd801fcda3e952633566c8b5ff177b93592e8e8d2d1f8471d",
      "url": "https://access.redhat.com/errata/RHBA-2023:1733",
      "version": "4.11.36"
    },
    "risks": [...]
  },
  ...
]

Evaluation of update availability

The Cluster Version Operator (CVO) periodically queries the OpenShift Update Service (OSUS) for the most recent data about update possibilities. This data is based on the cluster’s subscribed channel. The CVO then saves information about update recommendations into either the availableUpdates or conditionalUpdates field of its ClusterVersion resource.

The CVO periodically checks the conditional updates for update risks. These risks are conveyed through the data served by the OSUS, which contains information for each version about known issues that might affect a cluster updated to that version. Most risks are limited to clusters with specific characteristics, such as clusters with a certain size or clusters that are deployed in a particular cloud platform.

The CVO continuously evaluates its cluster characteristics against the conditional risk information for each conditional update. If the CVO finds that the cluster matches the criteria, the CVO stores this information in the conditionalUpdates field of its ClusterVersion resource. If the CVO finds that the cluster does not match the risks of an update, or that there are no risks associated with the update, it stores the target version in the availableUpdates field of its ClusterVersion resource.

The user interface, either the web console or the OpenShift CLI (oc), presents this information in sectioned headings to the administrator. Each supported but not recommended update recommendation contains a link to further resources about the risk so that the administrator can make an informed decision about the update.

Additional resources

Release images

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>
Example output
$ 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
1 Manifest for ClusterResourceQuota CRD, to be applied on Runlevel 03
2 Manifest for PrometheusRoleBinding resource for the service-ca-operator, to be applied on Runlevel 90
3 List of SHA digest-versioned references to all required images

Update process workflow

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

  1. The target version is stored in the spec.desiredUpdate.version field of the ClusterVersion resource, which may be managed through the web console or the CLI.

  2. The Cluster Version Operator (CVO) detects that the desiredUpdate in the ClusterVersion resource differs from the current cluster version. Using graph data from the OpenShift Update Service, the CVO resolves the desired cluster version to a pull spec for the release image.

  3. The CVO validates the integrity and authenticity of the release image. Red Hat publishes cryptographically-signed statements about published release images at predefined locations by using image SHA digests as unique and immutable release image identifiers. The CVO utilizes a list of built-in public keys to validate the presence and signatures of the statement matching the checked release image.

  4. The CVO creates a job named version-$version-$hash in the openshift-cluster-version namespace. This job uses containers that are executing the release image, so the cluster downloads the image through the container runtime. The job then extracts the manifests and metadata from the release image to a shared volume that is accessible to the CVO.

  5. The CVO validates the extracted manifests and metadata.

  6. The CVO checks some preconditions to ensure that no problematic condition is detected in the cluster. Certain conditions can prevent updates from proceeding. These conditions are either determined by the CVO itself, or reported by individual cluster Operators that detect some details about the cluster that the Operator considers problematic for the update.

  7. The CVO records the accepted release in status.desired and creates a status.history entry about the new update.

  8. The CVO begins reconciling the manifests from the release image. Cluster Operators are updated in separate stages called Runlevels, and the CVO ensures that all Operators in a Runlevel finish updating before it proceeds to the next level.

  9. Manifests for the CVO itself are applied early in the process. When the CVO deployment is applied, the current CVO pod stops, and a CVO pod that uses the new version starts. The new CVO proceeds to reconcile the remaining manifests.

  10. The update proceeds until the entire control plane is updated to the new version. Individual cluster Operators might perform update tasks on their domain of the cluster, and while they do so, they report their state through the Progressing=True condition.

  11. The Machine Config Operator (MCO) manifests are applied towards the end of the process. The updated MCO then begins updating the system configuration and operating system of every node. Each node might be drained, updated, and rebooted before it starts to accept workloads again.

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.

Understanding how manifests are applied during an update

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:

  • During an update, manifests at a lower Runlevel are applied before those at a higher Runlevel.

  • Within one Runlevel, manifests for different components can be applied in parallel.

  • Within one Runlevel, manifests for a single component are applied in lexicographic order.

The CVO then applies manifests following the generated dependency graph.

For some resource types, the CVO monitors the resource after its manifest is applied, and considers it to be successfully updated only after the resource reaches a stable state. Achieving this state can take some time. This is especially true for ClusterOperator resources, while the CVO waits for a cluster Operator to update itself and then update its ClusterOperator status.

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

  • The cluster Operators have an Available=True condition.

  • The cluster Operators have a Degraded=False condition.

  • The cluster Operators declare they have achieved the desired version in their ClusterOperator resource.

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.

A diagram displaying the sequence of Runlevels and the manifests of components within each level

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.

Additional resources

Understanding how the Machine Config Operator updates nodes

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.

The default setting for maxUnavailable is 1 for all the machine config pools in OpenShift Container Platform. It is recommended to not change this value and update one control plane node at a time. Do not change this value to 3 for the control plane pool.

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:

  1. Cordon and drain the node

    When a node is cordoned, workloads cannot be scheduled to it.

  2. Update the system configuration and operating system (OS) of the node

  3. Reboot the node

  4. Uncordon the node

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:

  1. The MCO cordons nodes 1, 2, and 3, and begins to drain them.

  2. Node 2 finishes draining, reboots, and becomes available again. The MCO cordons node 4 and begins draining it.

  3. Node 1 finishes draining, reboots, and becomes available again. The MCO cordons node 5 and begins draining it.

  4. Node 3 finishes draining, reboots, and becomes available again.

  5. Node 5 finishes draining, reboots, and becomes available again.

  6. Node 4 finishes draining, reboots, and becomes available again.

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
Example output
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
Additional resources