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Chapter 1. Overview of nodes

1.1. About nodes

A node is a virtual or bare-metal machine in a Kubernetes cluster. Worker nodes host your application containers, grouped as pods. The control plane nodes run services that are required to control the Kubernetes cluster. In OpenShift Container Platform, the control plane nodes contain more than just the Kubernetes services for managing the OpenShift Container Platform cluster.

Having stable and healthy nodes in a cluster is fundamental to the smooth functioning of your hosted application. In OpenShift Container Platform, you can access, manage, and monitor a node through the Node object representing the node. Using the OpenShift CLI (oc) or the web console, you can perform the following operations on a node.

Read operations

The read operations allow an administrator or a developer to get information about nodes in an OpenShift Container Platform cluster.

Management operations

As an administrator, you can easily manage a node in an OpenShift Container Platform cluster through several tasks:

  • Add or update node labels. A label is a key-value pair applied to a Node object. You can control the scheduling of pods using labels.
  • Change node configuration using a custom resource definition (CRD), or the kubeletConfig object.
  • Configure nodes to allow or disallow the scheduling of pods. Healthy worker nodes with a Ready status allow pod placement by default while the control plane nodes do not; you can change this default behavior by configuring the worker nodes to be unschedulable and the control plane nodes to be schedulable.
  • Allocate resources for nodes using the system-reserved setting. You can allow OpenShift Container Platform to automatically determine the optimal system-reserved CPU and memory resources for your nodes, or you can manually determine and set the best resources for your nodes.
  • Configure the number of pods that can run on a node based on the number of processor cores on the node, a hard limit, or both.
  • Reboot a node gracefully using pod anti-affinity.
  • Delete a node from a cluster by scaling down the cluster using a machine set. To delete a node from a bare-metal cluster, you must first drain all pods on the node and then manually delete the node.

Enhancement operations

OpenShift Container Platform allows you to do more than just access and manage nodes; as an administrator, you can perform the following tasks on nodes to make the cluster more efficient, application-friendly, and to provide a better environment for your developers.

1.2. About pods

A pod is one or more containers deployed together on a node. As a cluster administrator, you can define a pod, assign it to run on a healthy node that is ready for scheduling, and manage. A pod runs as long as the containers are running. You cannot change a pod once it is defined and is running. Some operations you can perform when working with pods are:

Read operations

As an administrator, you can get information about pods in a project through the following tasks:

Management operations

The following list of tasks provides an overview of how an administrator can manage pods in an OpenShift Container Platform cluster.

Enhancement operations

You can work with pods more easily and efficiently with the help of various tools and features available in OpenShift Container Platform. The following operations involve using those tools and features to better manage pods.

OperationUserMore information

Create and use a horizontal pod autoscaler.

Developer

You can use a horizontal pod autoscaler to specify the minimum and the maximum number of pods you want to run, as well as the CPU utilization or memory utilization your pods should target. Using a horizontal pod autoscaler, you can automatically scale pods.

Install and use a vertical pod autoscaler.

Administrator and developer

As an administrator, use a vertical pod autoscaler to better use cluster resources by monitoring the resources and the resource requirements of workloads.

As a developer, use a vertical pod autoscaler to ensure your pods stay up during periods of high demand by scheduling pods to nodes that have enough resources for each pod.

Provide access to external resources using device plug-ins.

Administrator

A device plug-in is a gRPC service running on nodes (external to the kubelet), which manages specific hardware resources. You can deploy a device plug-in to provide a consistent and portable solution to consume hardware devices across clusters.

Provide sensitive data to pods using the Secret object.

Administrator

Some applications need sensitive information, such as passwords and usernames. You can use the Secret object to provide such information to an application pod.

1.3. About containers

A container is the basic unit of an OpenShift Container Platform application, which comprises the application code packaged along with its dependencies, libraries, and binaries. Containers provide consistency across environments and multiple deployment targets: physical servers, virtual machines (VMs), and private or public cloud.

Linux container technologies are lightweight mechanisms for isolating running processes and limiting access to only designated resources. As an administrator, You can perform various tasks on a Linux container, such as:

OpenShift Container Platform provides specialized containers called Init containers. Init containers run before application containers and can contain utilities or setup scripts not present in an application image. You can use an Init container to perform tasks before the rest of a pod is deployed.

Apart from performing specific tasks on nodes, pods, and containers, you can work with the overall OpenShift Container Platform cluster to keep the cluster efficient and the application pods highly available.