Control plane architecture

The OKD version must match between control plane host and node host. For example, in a 4 cluster, all control plane hosts must be 4 and all nodes must be 4.

Temporary mismatches during cluster upgrades are acceptable. For example, when upgrading from the previous OKD version to 4, some nodes will upgrade to 4 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:

In a Kubernetes cluster, worker nodes run and manage the actual workloads requested by Kubernetes users. The worker nodes advertise their capacity and the scheduler, which is a control plane service, determines on which nodes to start pods and containers. The following important services run on each worker node:

In OKD, compute machine sets control the compute machines, which are assigned the worker machine role. 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 machines with the worker role 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.

In a Kubernetes cluster, the master nodes run services that are required to control the Kubernetes cluster. In OKD, the control plane is comprised of control plane machines that have a master machine role. They contain more than just the Kubernetes services for managing the OKD cluster.

For most OKD clusters, control plane machines are defined by a series of standalone machine API resources. For supported cloud provider and OKD version combinations, control planes can be managed with control plane machine sets. Extra controls apply to control plane machines to prevent you from deleting all of the control plane machines and breaking your cluster.

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

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.

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:

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:

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.

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.

By using etcd, you can benefit in several ways:

To ensure a reliable approach to cluster configuration and management, etcd uses the etcd Operator. The Operator simplifies the use of etcd on a Kubernetes container platform like OKD. With the etcd Operator, you can create or delete etcd members, resize clusters, perform backups, and upgrade etcd.

The etcd Operator observes, analyzes, and acts:

etcd holds the cluster state, which is constantly updated. This state is continuously persisted, which leads to a high number of small changes at high frequency. As a result, it is critical to back the etcd cluster member with fast, low-latency I/O. For more information about best practices for etcd, see "Recommended etcd practices".