Chapter 5. Managing images

An image stream comprises any number of container images identified by tags. It presents a single virtual view of related images, similar to a container image repository.

By watching an image stream, builds and deployments can receive notifications when new images are added or modified and react by performing a build or deployment, respectively.

An image tag is a label applied to a container image in a repository that distinguishes a specific image from other images in an image stream. Typically, the tag represents a version number of some sort. For example, here :v3.11.59-2 is the tag:

registry.access.redhat.com/openshift3/jenkins-2-rhel7:v3.11.59-2

You can add additional tags to an image. For example, an image might be assigned the tags :v3.11.59-2 and :latest.

OpenShift Container Platform provides the oc tag command, which is similar to the docker tag command, but operates on image streams instead of directly on images.

Images evolve over time and their tags reflect this. Generally, an image tag always points to the latest image built.

If there is too much information embedded in a tag name, like v2.0.1-may-2019, the tag points to just one revision of an image and is never updated. Using default image pruning options, such an image is never removed. In very large clusters, the schema of creating new tags for every revised image could eventually fill up the etcd datastore with excess tag metadata for images that are long outdated.

If the tag is named v2.0, image revisions are more likely. This results in longer tag history and, therefore, the image pruner is more likely to remove old and unused images.

Although tag naming convention is up to you, here are a few examples in the format <image_name>:<image_tag>:

If you require dates in tag names, periodically inspect old and unsupported images and istags and remove them. Otherwise, you can experience increasing resource usage caused by retaining old images.

An image stream in OpenShift Container Platform comprises zero or more container images identified by tags.

There are different types of tags available. The default behavior uses a permanent tag, which points to a specific image in time. If the permanent tag is in use and the source changes, the tag does not change for the destination.

A tracking tag means the destination tag’s metadata is updated during the import of the source tag.

You can remove tags from an image stream.

You can use tags to reference images in image streams using the following reference types.

When viewing example image stream definitions you may notice they contain definitions of ImageStreamTag and references to DockerImage, but nothing related to ImageStreamImage.

This is because the ImageStreamImage objects are automatically created in OpenShift Container Platform when you import or tag an image into the image stream. You should never have to explicitly define an ImageStreamImage object in any image stream definition that you use to create image streams.

When OpenShift Container Platform creates containers, it uses the container imagePullPolicy to determine if the image should be pulled prior to starting the container. There are three possible values for imagePullPolicy:

If a container imagePullPolicy parameter is not specified, OpenShift Container Platform sets it based on the image tag:

When using the OpenShift image registry, to allow pods in project-a to reference images in project-b, a service account in project-a must be bound to the system:image-puller role in project-b.

To pull a secured container from other private or secured registries, you must create a pull secret from your container client credentials, such as Docker or Podman, and add it to your service account.

Both Docker and Podman use a configuration file to store authentication details to log in to secured or insecure registry:

These files store your authentication information if you have previously logged in to a secured or insecure registry.

{
   "auths":{
      "cloud.openshift.com":{
         "auth":"b3Blb=",
         "email":"you@example.com"
      },
      "quay.io":{
         "auth":"b3Blb=",
         "email":"you@example.com"
      },
      "quay.io/repository-main":{
         "auth":"b3Blb=",
         "email":"you@example.com"
      }
   }
}

apiVersion: v1
data:
  .dockerconfigjson: ewogICAiYXV0aHMiOnsKICAgICAgIm0iOnsKICAgICAgIsKICAgICAgICAgImF1dGgiOiJiM0JsYj0iLAogICAgICAgICAiZW1haWwiOiJ5b3VAZXhhbXBsZS5jb20iCiAgICAgIH0KICAgfQp9Cg==
kind: Secret
metadata:
  creationTimestamp: "2021-09-09T19:10:11Z"
  name: pull-secret
  namespace: default
  resourceVersion: "37676"
  uid: e2851531-01bc-48ba-878c-de96cfe31020
type: Opaque

You can update the global pull secret for your cluster by either replacing the current pull secret or appending a new pull secret.