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Using pods

A pod is one or more containers deployed together on one host, and the smallest compute unit that can be defined, deployed, and managed.

Understanding pods

Pods are the rough equivalent of a machine instance (physical or virtual) to a Container. Each pod is allocated its own internal IP address, therefore owning its entire port space, and containers within pods can share their local storage and networking.

Pods have a lifecycle; they are defined, then they are assigned to run on a node, then they run until their container(s) exit or they are removed for some other reason. Pods, depending on policy and exit code, might be removed after exiting, or can be retained to enable access to the logs of their containers.

OpenShift Container Platform treats pods as largely immutable; changes cannot be made to a pod definition while it is running. OpenShift Container Platform implements changes by terminating an existing pod and recreating it with modified configuration, base image(s), or both. Pods are also treated as expendable, and do not maintain state when recreated. Therefore pods should usually be managed by higher-level controllers, rather than directly by users.

For the maximum number of pods per OpenShift Container Platform node host, see the Cluster Limits.

Bare pods that are not managed by a replication controller will be not rescheduled upon node disruption.

Example pod configurations

OpenShift Container Platform leverages the Kubernetes concept of a pod, which is one or more containers deployed together on one host, and the smallest compute unit that can be defined, deployed, and managed.

The following is an example definition of a pod. It demonstrates many features of pods, most of which are discussed in other topics and thus only briefly mentioned here:

Pod object definition (YAML)
kind: Pod
apiVersion: v1
metadata:
  name: example
  labels:
    environment: production
    app: abc (1)
spec:
  restartPolicy: Always (2)
  securityContext: (3)
    runAsNonRoot: true
    seccompProfile:
      type: RuntimeDefault
  containers: (4)
    - name: abc
      args:
      - sleep
      - "1000000"
      volumeMounts: (5)
       - name: cache-volume
         mountPath: /cache (6)
      image: registry.access.redhat.com/ubi7/ubi-init:latest (7)
      securityContext:
        allowPrivilegeEscalation: false
        runAsNonRoot: true
        capabilities:
          drop: ["ALL"]
      resources:
        limits:
          memory: "100Mi"
          cpu: "1"
        requests:
          memory: "100Mi"
          cpu: "1"
  volumes: (8)
  - name: cache-volume
    emptyDir:
      sizeLimit: 500Mi
1 Pods can be "tagged" with one or more labels, which can then be used to select and manage groups of pods in a single operation. The labels are stored in key/value format in the metadata hash.
2 The pod restart policy with possible values Always, OnFailure, and Never. The default value is Always.
3 OpenShift Container Platform defines a security context for containers which specifies whether they are allowed to run as privileged containers, run as a user of their choice, and more. The default context is very restrictive but administrators can modify this as needed.
4 containers specifies an array of one or more container definitions.
5 The container specifies where external storage volumes are mounted within the container.
6 Specify the volumes to provide for the pod. Volumes mount at the specified path. Do not mount to the container root, /, or any path that is the same in the host and the container. This can corrupt your host system if the container is sufficiently privileged, such as the host /dev/pts files. It is safe to mount the host by using /host.
7 Each container in the pod is instantiated from its own container image.
8 The pod defines storage volumes that are available to its container(s) to use.

If you attach persistent volumes that have high file counts to pods, those pods can fail or can take a long time to start. For more information, see When using Persistent Volumes with high file counts in OpenShift, why do pods fail to start or take an excessive amount of time to achieve "Ready" state?.

This pod definition does not include attributes that are filled by OpenShift Container Platform automatically after the pod is created and its lifecycle begins. The Kubernetes pod documentation has details about the functionality and purpose of pods.

Additional resources