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Control plane architecture

The control plane, which is composed of control plane machines, manages the OKD cluster. The control plane machines manage workloads on the compute machines, which are also known as worker machines. The cluster itself manages all upgrades to the machines by the actions of the Cluster Version Operator (CVO), the Machine Config Operator, and a set of individual Operators.

Node configuration management with machine config pools

Machines that run control plane components or user workloads are divided into groups based on the types of resources they handle. These groups of machines are called machine config pools (MCP). Each MCP manages a set of nodes and its corresponding machine configs. The role of the node determines which MCP it belongs to; the MCP governs nodes based on its assigned node role label. Nodes in an MCP have the same configuration; this means nodes can be scaled up and torn down in response to increased or decreased workloads.

By default, there are two MCPs created by the cluster when it is installed: master and worker. Each default MCP has a defined configuration applied by the Machine Config Operator (MCO), which is responsible for managing MCPs and facilitating MCP upgrades. You can create additional MCPs, or custom pools, to manage nodes that have custom use cases that extend outside of the default node types.

Custom pools are pools that inherit their configurations from the worker pool. They use any machine config targeted for the worker pool, but add the ability to deploy changes only targeted at the custom pool. Since a custom pool inherits its configuration from the worker pool, any change to the worker pool is applied to the custom pool as well. Custom pools that do not inherit their configurations from the worker pool are not supported by the MCO.

A node can only be included in one MCP. If a node has multiple labels that correspond to several MCPs, like worker,infra, it is managed by the infra custom pool, not the worker pool. Custom pools take priority on selecting nodes to manage based on node labels; nodes that do not belong to a custom pool are managed by the worker pool.

It is recommended to have a custom pool for every node role you want to manage in your cluster. For example, if you create infra nodes to handle infra workloads, it is recommended to create a custom infra MCP to group those nodes together. If you apply an infra role label to a worker node so it has the worker,infra dual label, but do not have a custom infra MCP, the MCO considers it a worker node. If you remove the worker label from a node and apply the infra label without grouping it in a custom pool, the node is not recognized by the MCO and is unmanaged by the cluster.

Any node labeled with the infra role that is only running infra workloads is not counted toward the total number of subscriptions. The MCP managing an infra node is mutually exclusive from how the cluster determines subscription charges; tagging a node with the appropriate infra role and using taints to prevent user workloads from being scheduled on that node are the only requirements for avoiding subscription charges for infra workloads.

The MCO applies updates for pools independently; for example, if there is an update that affects all pools, nodes from each pool update in parallel with each other. If you add a custom pool, nodes from that pool also attempt to update concurrently with the master and worker nodes.

Machine roles in OKD

OKD assigns hosts different roles. These roles define the function of the machine within the cluster. The cluster contains definitions for the standard master and worker role types.

The cluster also contains the definition for the bootstrap role. Because the bootstrap machine is used only during cluster installation, its function is explained in the cluster installation documentation.

Control plane and node host compatibility

The OKD version must match between control plane host and node host. For example, in a 4.9 cluster, all control plane hosts must be 4.9 and all nodes must be 4.9.

Temporary mismatches during cluster upgrades are acceptable. For example, when upgrading from OKD 4.8 to 4.9, some nodes will upgrade to 4.9 before others. Prolonged skewing of control plane hosts and node hosts might expose older compute machines to bugs and missing features. Users should resolve skewed control plane hosts and node hosts as soon as possible.

The kubelet service must not be newer than kube-apiserver, and can be up to two minor versions older depending on whether your OKD version is odd or even. The table below shows the appropriate version compatibility:

OKD version Supported kubelet skew

Odd OKD minor versions [1]

Up to one version older

Even OKD minor versions [2]

Up to two versions older

  1. For example, OKD 4.5, 4.7, 4.9.

  2. For example, OKD 4.6, 4.8, 4.10.

Cluster workers

In a Kubernetes cluster, the worker nodes are where the actual workloads requested by Kubernetes users run and are managed. The worker nodes advertise their capacity and the scheduler, which is part of the master services, determines on which nodes to start containers and pods. Important services run on each worker node, including CRI-O, which is the container engine, Kubelet, which is the service that accepts and fulfills requests for running and stopping container workloads, and a service proxy, which manages communication for pods across workers.

In OKD, machine sets control the worker machines. Machines with the worker role drive compute workloads that are governed by a specific machine pool that autoscales them. Because OKD has the capacity to support multiple machine types, the worker machines are classed as compute machines. In this release, the terms worker machine and compute machine are used interchangeably because the only default type of compute machine is the worker machine. In future versions of OKD, different types of compute machines, such as infrastructure machines, might be used by default.

Machine sets are groupings of machine resources under the machine-api namespace. Machine sets are configurations that are designed to start new machines on a specific cloud provider. Conversely, machine config pools (MCPs) are part of the Machine Config Operator (MCO) namespace. An MCP is used to group machines together so the MCO can manage their configurations and facilitate their upgrades.

Cluster masters

In a Kubernetes cluster, the control plane nodes run services that are required to control the Kubernetes cluster. In OKD, the control plane machines are the control plane. They contain more than just the Kubernetes services for managing the OKD cluster. Because all of the machines with the control plane role are control plane machines, the terms master and control plane are used interchangeably to describe them. Instead of being grouped into a machine set, control plane machines are defined by a series of standalone machine API resources. Extra controls apply to control plane machines to prevent you from deleting all control plane machines and breaking your cluster.

Exactly three control plane nodes must be used for all production deployments.

Services that fall under the Kubernetes category on the master include the Kubernetes API server, etcd, the Kubernetes controller manager, and the Kubernetes scheduler.

Table 1. Kubernetes services that run on the control plane
Component Description

Kubernetes API server

The Kubernetes API server validates and configures the data for pods, services, and replication controllers. It also provides a focal point for the shared state of the cluster.

etcd

etcd stores the persistent master state while other components watch etcd for changes to bring themselves into the specified state.

Kubernetes controller manager

The Kubernetes controller manager watches etcd for changes to objects such as replication, namespace, and service account controller objects, and then uses the API to enforce the specified state. Several such processes create a cluster with one active leader at a time.

Kubernetes scheduler

The Kubernetes scheduler watches for newly created pods without an assigned node and selects the best node to host the pod.

There are also OpenShift services that run on the control plane, which include the OpenShift API server, OpenShift controller manager, OpenShift OAuth API server, and OpenShift OAuth server.

Table 2. OpenShift services that run on the control plane
Component Description

OpenShift API server

The OpenShift API server validates and configures the data for OpenShift resources, such as projects, routes, and templates.

The OpenShift API server is managed by the OpenShift API Server Operator.

OpenShift controller manager

The OpenShift controller manager watches etcd for changes to OpenShift objects, such as project, route, and template controller objects, and then uses the API to enforce the specified state.

The OpenShift controller manager is managed by the OpenShift Controller Manager Operator.

OpenShift OAuth API server

The OpenShift OAuth API server validates and configures the data to authenticate to OKD, such as users, groups, and OAuth tokens.

The OpenShift OAuth API server is managed by the Cluster Authentication Operator.

OpenShift OAuth server

Users request tokens from the OpenShift OAuth server to authenticate themselves to the API.

The OpenShift OAuth server is managed by the Cluster Authentication Operator.

Some of these services on the control plane machines run as systemd services, while others run as static pods.

Systemd services are appropriate for services that you need to always come up on that particular system shortly after it starts. For control plane machines, those include sshd, which allows remote login. It also includes services such as:

  • The CRI-O container engine (crio), which runs and manages the containers. OKD 4.9 uses CRI-O instead of the Docker Container Engine.

  • Kubelet (kubelet), which accepts requests for managing containers on the machine from master services.

CRI-O and Kubelet must run directly on the host as systemd services because they need to be running before you can run other containers.

The installer-* and revision-pruner-* control plane pods must run with root permissions because they write to the /etc/kubernetes directory, which is owned by the root user. These pods are in the following namespaces:

  • openshift-etcd

  • openshift-kube-apiserver

  • openshift-kube-controller-manager

  • openshift-kube-scheduler

Operators in OKD

Operators are among the most important components of OKD. Operators are the preferred method of packaging, deploying, and managing services on the control plane. They can also provide advantages to applications that users run.

Operators integrate with Kubernetes APIs and CLI tools such as kubectl and oc commands. They provide the means of monitoring applications, performing health checks, managing over-the-air (OTA) updates, and ensuring that applications remain in your specified state.

Operators also offer a more granular configuration experience. You configure each component by modifying the API that the Operator exposes instead of modifying a global configuration file.

Because CRI-O and the Kubelet run on every node, almost every other cluster function can be managed on the control plane by using Operators. Components that are added to the control plane by using Operators include critical networking and credential services.

While both follow similar Operator concepts and goals, Operators in OKD are managed by two different systems, depending on their purpose:

  • Cluster Operators, which are managed by the Cluster Version Operator (CVO), are installed by default to perform cluster functions.

  • Optional add-on Operators, which are managed by Operator Lifecycle Manager (OLM), can be made accessible for users to run in their applications.

Cluster Operators

In OKD, all cluster functions are divided into a series of default cluster Operators. Cluster Operators manage a particular area of cluster functionality, such as cluster-wide application logging, management of the Kubernetes control plane, or the machine provisioning system.

Cluster Operators are represented by a ClusterOperator object, which cluster administrators can view in the OKD web console from the Administration → Cluster Settings page. Each cluster Operator provides a simple API for determining cluster functionality. The Operator hides the details of managing the lifecycle of that component. Operators can manage a single component or tens of components, but the end goal is always to reduce operational burden by automating common actions.

Additional resources

Add-on Operators

Operator Lifecycle Manager (OLM) and OperatorHub are default components in OKD that help manage Kubernetes-native applications as Operators. Together they provide the system for discovering, installing, and managing the optional add-on Operators available on the cluster.

Using OperatorHub in the OKD web console, cluster administrators and authorized users can select Operators to install from catalogs of Operators. After installing an Operator from OperatorHub, it can be made available globally or in specific namespaces to run in user applications.

Default catalog sources are available that include Red Hat Operators, certified Operators, and community Operators. Cluster administrators can also add their own custom catalog sources, which can contain a custom set of Operators.

Developers can use the Operator SDK to help author custom Operators that take advantage of OLM features, as well. Their Operator can then be bundled and added to a custom catalog source, which can be added to a cluster and made available to users.

OLM does not manage the cluster Operators that comprise the OKD architecture.
Additional resources

About the Machine Config Operator

OKD 4.9 integrates both operating system and cluster management. Because the cluster manages its own updates, including updates to Fedora CoreOS (FCOS) on cluster nodes, OKD provides an opinionated lifecycle management experience that simplifies the orchestration of node upgrades.

OKD employs three daemon sets and controllers to simplify node management. These daemon sets orchestrate operating system updates and configuration changes to the hosts by using standard Kubernetes-style constructs. They include:

  • The machine-config-controller, which coordinates machine upgrades from the control plane. It monitors all of the cluster nodes and orchestrates their configuration updates.

  • The machine-config-daemon daemon set, which runs on each node in the cluster and updates a machine to configuration as defined by machine config and as instructed by the MachineConfigController. When the node detects a change, it drains off its pods, applies the update, and reboots. These changes come in the form of Ignition configuration files that apply the specified machine configuration and control kubelet configuration. The update itself is delivered in a container. This process is key to the success of managing OKD and FCOS updates together.

  • The machine-config-server daemon set, which provides the Ignition config files to control plane nodes as they join the cluster.

The machine configuration is a subset of the Ignition configuration. The machine-config-daemon reads the machine configuration to see if it needs to do an OSTree update or if it must apply a series of systemd kubelet file changes, configuration changes, or other changes to the operating system or OKD configuration.

When you perform node management operations, you create or modify a KubeletConfig custom resource (CR).

When changes are made to a machine configuration, the Machine Config Operator (MCO) automatically reboots all corresponding nodes in order for the changes to take effect.

To prevent the nodes from automatically rebooting after machine configuration changes, before making the changes, you must pause the autoreboot process by setting the spec.paused field to true in the corresponding machine config pool. When paused, machine configuration changes are not applied until you set the spec.paused field to false and the nodes have rebooted into the new configuration.

Make sure the pools are unpaused when the CA certificate rotation happens. If the MCPs are paused, the MCO cannot push the newly rotated certificates to those nodes. This causes the cluster to become degraded and causes failure in multiple oc commands, including oc debug, oc logs, oc exec, and oc attach. You receive alerts in the Alerting UI of the OKD web console if an MCP is paused when the certificates are rotated.

The following modifications do not trigger a node reboot:

  • When the MCO detects any of the following changes, it applies the update without draining or rebooting the node:

    • Changes to the SSH key in the spec.config.passwd.users.sshAuthorizedKeys parameter of a machine config.

    • Changes to the global pull secret or pull secret in the openshift-config namespace.

    • Automatic rotation of the /etc/kubernetes/kubelet-ca.crt certificate authority (CA) by the Kubernetes API Server Operator.

  • When the MCO detects changes to the /etc/containers/registries.conf file, such as adding or editing an ImageDigestMirrorSet or ImageTagMirrorSet object, it drains the corresponding nodes, applies the changes, and uncordons the nodes.The node drain does not happen for the following changes:

    • The addition of a registry with the pull-from-mirror = "digest-only" parameter set for each mirror.

    • The addition of a mirror with the pull-from-mirror = "digest-only" parameter set in a registry.

    • The addition of items to the unqualified-search-registries list.
Additional information

For information on preventing the control plane machines from rebooting after the Machine Config Operator makes changes to the machine config, see Disabling Machine Config Operator from automatically rebooting.