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architecture-machine-roles.adoc

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Machine roles in {product-title}

{product-title} 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.

Note

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 {product-title} version must match between control plane host and node host. For example, in a 4.11 cluster, all control plane hosts must be 4.11 and all nodes must be 4.11.

Temporary mismatches during cluster upgrades are acceptable. For example, when upgrading from {product-title} 4.10 to 4.11, some nodes will upgrade to 4.11 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 {product-title} version is odd or even. The table below shows the appropriate version compatibility:

{product-title} version Supported kubelet skew

Odd {product-title} minor versions [1]

Up to one version older

Even {product-title} minor versions [2]

Up to two versions older

  1. For example, {product-title} 4.5, 4.7, 4.9, 4.11.

  2. For example, {product-title} 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 {product-title}, 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 {product-title} 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 {product-title}, different types of compute machines, such as infrastructure machines, might be used by default.

Note

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 {product-title}, the control plane machines are the control plane. They contain more than just the Kubernetes services for managing the {product-title} 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.

Note

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 {product-title}, 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. {product-title} {product-version} 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