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Using image streams

An image stream and its associated tags provide an abstraction for referencing container images from within OpenShift Container Platform. The image stream and its tags allow you to see what images are available and ensure that you are using the specific image you need even if the image in the repository changes.

Image streams do not contain actual image data, but present a single virtual view of related images, similar to an image repository.

You can configure builds and deployments to watch an image stream for notifications when new images are added and react by performing a build or deployment, respectively.

For example, if a deployment is using a certain image and a new version of that image is created, a deployment could be automatically performed to pick up the new version of the image.

However, if the image stream tag used by the deployment or build is not updated, then even if the container image in the container image registry is updated, the build or deployment continues using the previous, presumably known good image.

The source images can be stored in any of the following:

  • OpenShift Container Platform’s integrated registry.

  • An external registry, for example registry.redhat.io or Quay.io.

  • Other image streams in the OpenShift Container Platform cluster.

When you define an object that references an image stream tag, such as a build or deployment configuration, you point to an image stream tag and not the repository. When you build or deploy your application, OpenShift Container Platform queries the repository using the image stream tag to locate the associated ID of the image and uses that exact image.

The image stream metadata is stored in the etcd instance along with other cluster information.

Using image streams has several significant benefits:

  • You can tag, rollback a tag, and quickly deal with images, without having to re-push using the command line.

  • You can trigger builds and deployments when a new image is pushed to the registry. Also, OpenShift Container Platform has generic triggers for other resources, such as Kubernetes objects.

  • You can mark a tag for periodic re-import. If the source image has changed, that change is picked up and reflected in the image stream, which triggers the build or deployment flow, depending upon the build or deployment configuration.

  • You can share images using fine-grained access control and quickly distribute images across your teams.

  • If the source image changes, the image stream tag still points to a known-good version of the image, ensuring that your application do not break unexpectedly.

  • You can configure security around who can view and use the images through permissions on the image stream objects.

  • Users that lack permission to read or list images on the cluster level can still retrieve the images tagged in a project using image streams.

Configuring image streams

An ImageStream object file contains the following elements.

Imagestream object definition
apiVersion: v1
kind: ImageStream
metadata:
  annotations:
    openshift.io/generated-by: OpenShiftNewApp
  creationTimestamp: 2017-09-01T13:40:00Z
  generation: 1
  labels:
    app: ruby-sample-build
    template: application-template-stibuild
  name: origin-ruby-sample (1)
  namespace: test
  resourceVersion: "633"
  selflink: /oapi/v1/namespaces/test/imagestreams/origin-ruby-sample
  uid: ee2b9405-c68c-11e5-8a99-525400f25e34
spec: {}
status:
  dockerImageRepository: 172.30.56.218:5000/test/origin-ruby-sample (2)
  tags:
  - items:
    - created: 2017-09-02T10:15:09Z
      dockerImageReference: 172.30.56.218:5000/test/origin-ruby-sample@sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d (3)
      generation: 2
      image: sha256:909de62d1f609a717ec433cc25ca5cf00941545c83a01fb31527771e1fab3fc5 (4)
    - created: 2017-09-01T13:40:11Z
      dockerImageReference: 172.30.56.218:5000/test/origin-ruby-sample@sha256:909de62d1f609a717ec433cc25ca5cf00941545c83a01fb31527771e1fab3fc5
      generation: 1
      image: sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d
    tag: latest (5)
1 The name of the image stream.
2 Docker repository path where new images can be pushed to add or update them in this image stream.
3 The SHA identifier that this image stream tag currently references. Resources that reference this image stream tag use this identifier.
4 The SHA identifier that this image stream tag previously referenced. Can be used to rollback to an older image.
5 The image stream tag name.

Image stream images

An image stream image points from within an image stream to a particular image ID.

Image stream images allow you to retrieve metadata about an image from a particular image stream where it is tagged.

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

The image stream image consists of the image stream name and image ID from the repository, delimited by an @ sign:

<image-stream-name>@<image-id>

To refer to the image in the ImageStream object example, the image stream image looks like:

origin-ruby-sample@sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d

Image stream tags

An image stream tag is a named pointer to an image in an image stream. It is abbreviated as istag. An image stream tag is used to reference or retrieve an image for a given image stream and tag.

Image stream tags can reference any local or externally managed image. It contains a history of images represented as a stack of all images the tag ever pointed to. Whenever a new or existing image is tagged under particular image stream tag, it is placed at the first position in the history stack. The image previously occupying the top position is available at the second position. This allows for easy rollbacks to make tags point to historical images again.

The following image stream tag is from an ImageStream object:

Image stream tag with two images in its history
  tags:
  - items:
    - created: 2017-09-02T10:15:09Z
      dockerImageReference: 172.30.56.218:5000/test/origin-ruby-sample@sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d
      generation: 2
      image: sha256:909de62d1f609a717ec433cc25ca5cf00941545c83a01fb31527771e1fab3fc5
    - created: 2017-09-01T13:40:11Z
      dockerImageReference: 172.30.56.218:5000/test/origin-ruby-sample@sha256:909de62d1f609a717ec433cc25ca5cf00941545c83a01fb31527771e1fab3fc5
      generation: 1
      image: sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d
    tag: latest

Image stream tags can be permanent tags or tracking tags.

  • Permanent tags are version-specific tags that point to a particular version of an image, such as Python 3.5.

  • Tracking tags are reference tags that follow another image stream tag and can be updated to change which image they follow, like a symlink. These new levels are not guaranteed to be backwards-compatible.

    For example, the latest image stream tags that ship with OpenShift Container Platform are tracking tags. This means consumers of the latest image stream tag are updated to the newest level of the framework provided by the image when a new level becomes available. A latest image stream tag to v3.10 can be changed to v3.11 at any time. It is important to be aware that these latest image stream tags behave differently than the Docker latest tag. The latest image stream tag, in this case, does not point to the latest image in the Docker repository. It points to another image stream tag, which might not be the latest version of an image. For example, if the latest image stream tag points to v3.10 of an image, when the 3.11 version is released, the latest tag is not automatically updated to v3.11, and remains at v3.10 until it is manually updated to point to a v3.11 image stream tag.

    Tracking tags are limited to a single image stream and cannot reference other image streams.

You can create your own image stream tags for your own needs.

The image stream tag is composed of the name of the image stream and a tag, separated by a colon:

<imagestream name>:<tag>

For example, to refer to the sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d image in the ImageStream object example earlier, the image stream tag would be:

origin-ruby-sample:latest

Image stream change triggers

Image stream triggers allow your builds and deployments to be automatically invoked when a new version of an upstream image is available.

For example, builds and deployments can be automatically started when an image stream tag is modified. This is achieved by monitoring that particular image stream tag and notifying the build or deployment when a change is detected.

Image stream mapping

When the integrated registry receives a new image, it creates and sends an image stream mapping to OpenShift Container Platform, providing the image’s project, name, tag, and image metadata.

Configuring image stream mappings is an advanced feature.

This information is used to create a new image, if it does not already exist, and to tag the image into the image stream. OpenShift Container Platform stores complete metadata about each image, such as commands, entry point, and environment variables. Images in OpenShift Container Platform are immutable and the maximum name length is 63 characters.

The following image stream mapping example results in an image being tagged as test/origin-ruby-sample:latest:

Image stream mapping object definition
apiVersion: v1
kind: ImageStreamMapping
metadata:
  creationTimestamp: null
  name: origin-ruby-sample
  namespace: test
tag: latest
image:
  dockerImageLayers:
  - name: sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef
    size: 0
  - name: sha256:ee1dd2cb6df21971f4af6de0f1d7782b81fb63156801cfde2bb47b4247c23c29
    size: 196634330
  - name: sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef
    size: 0
  - name: sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef
    size: 0
  - name: sha256:ca062656bff07f18bff46be00f40cfbb069687ec124ac0aa038fd676cfaea092
    size: 177723024
  - name: sha256:63d529c59c92843c395befd065de516ee9ed4995549f8218eac6ff088bfa6b6e
    size: 55679776
  - name: sha256:92114219a04977b5563d7dff71ec4caa3a37a15b266ce42ee8f43dba9798c966
    size: 11939149
  dockerImageMetadata:
    Architecture: amd64
    Config:
      Cmd:
      - /usr/libexec/s2i/run
      Entrypoint:
      - container-entrypoint
      Env:
      - RACK_ENV=production
      - OPENSHIFT_BUILD_NAMESPACE=test
      - OPENSHIFT_BUILD_SOURCE=https://github.com/openshift/ruby-hello-world.git
      - EXAMPLE=sample-app
      - OPENSHIFT_BUILD_NAME=ruby-sample-build-1
      - PATH=/opt/app-root/src/bin:/opt/app-root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
      - STI_SCRIPTS_URL=image:///usr/libexec/s2i
      - STI_SCRIPTS_PATH=/usr/libexec/s2i
      - HOME=/opt/app-root/src
      - BASH_ENV=/opt/app-root/etc/scl_enable
      - ENV=/opt/app-root/etc/scl_enable
      - PROMPT_COMMAND=. /opt/app-root/etc/scl_enable
      - RUBY_VERSION=2.2
      ExposedPorts:
        8080/tcp: {}
      Labels:
        build-date: 2015-12-23
        io.k8s.description: Platform for building and running Ruby 2.2 applications
        io.k8s.display-name: 172.30.56.218:5000/test/origin-ruby-sample:latest
        io.openshift.build.commit.author: Ben Parees <bparees@users.noreply.github.com>
        io.openshift.build.commit.date: Wed Jan 20 10:14:27 2016 -0500
        io.openshift.build.commit.id: 00cadc392d39d5ef9117cbc8a31db0889eedd442
        io.openshift.build.commit.message: 'Merge pull request #51 from php-coder/fix_url_and_sti'
        io.openshift.build.commit.ref: master
        io.openshift.build.image: centos/ruby-22-centos7@sha256:3a335d7d8a452970c5b4054ad7118ff134b3a6b50a2bb6d0c07c746e8986b28e
        io.openshift.build.source-location: https://github.com/openshift/ruby-hello-world.git
        io.openshift.builder-base-version: 8d95148
        io.openshift.builder-version: 8847438ba06307f86ac877465eadc835201241df
        io.openshift.s2i.scripts-url: image:///usr/libexec/s2i
        io.openshift.tags: builder,ruby,ruby22
        io.s2i.scripts-url: image:///usr/libexec/s2i
        license: GPLv2
        name: CentOS Base Image
        vendor: CentOS
      User: "1001"
      WorkingDir: /opt/app-root/src
    Container: 86e9a4a3c760271671ab913616c51c9f3cea846ca524bf07c04a6f6c9e103a76
    ContainerConfig:
      AttachStdout: true
      Cmd:
      - /bin/sh
      - -c
      - tar -C /tmp -xf - && /usr/libexec/s2i/assemble
      Entrypoint:
      - container-entrypoint
      Env:
      - RACK_ENV=production
      - OPENSHIFT_BUILD_NAME=ruby-sample-build-1
      - OPENSHIFT_BUILD_NAMESPACE=test
      - OPENSHIFT_BUILD_SOURCE=https://github.com/openshift/ruby-hello-world.git
      - EXAMPLE=sample-app
      - PATH=/opt/app-root/src/bin:/opt/app-root/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
      - STI_SCRIPTS_URL=image:///usr/libexec/s2i
      - STI_SCRIPTS_PATH=/usr/libexec/s2i
      - HOME=/opt/app-root/src
      - BASH_ENV=/opt/app-root/etc/scl_enable
      - ENV=/opt/app-root/etc/scl_enable
      - PROMPT_COMMAND=. /opt/app-root/etc/scl_enable
      - RUBY_VERSION=2.2
      ExposedPorts:
        8080/tcp: {}
      Hostname: ruby-sample-build-1-build
      Image: centos/ruby-22-centos7@sha256:3a335d7d8a452970c5b4054ad7118ff134b3a6b50a2bb6d0c07c746e8986b28e
      OpenStdin: true
      StdinOnce: true
      User: "1001"
      WorkingDir: /opt/app-root/src
    Created: 2016-01-29T13:40:00Z
    DockerVersion: 1.8.2.fc21
    Id: 9d7fd5e2d15495802028c569d544329f4286dcd1c9c085ff5699218dbaa69b43
    Parent: 57b08d979c86f4500dc8cad639c9518744c8dd39447c055a3517dc9c18d6fccd
    Size: 441976279
    apiVersion: "1.0"
    kind: DockerImage
  dockerImageMetadataVersion: "1.0"
  dockerImageReference: 172.30.56.218:5000/test/origin-ruby-sample@sha256:47463d94eb5c049b2d23b03a9530bf944f8f967a0fe79147dd6b9135bf7dd13d

Working with image streams

The following sections describe how to use image streams and image stream tags.

Getting information about image streams

You can get general information about the image stream and detailed information about all the tags it is pointing to.

Procedure
  • Get general information about the image stream and detailed information about all the tags it is pointing to:

    $ oc describe is/<image-name>

    For example:

    $ oc describe is/python
    Example output
    Name:			python
    Namespace:		default
    Created:		About a minute ago
    Labels:			<none>
    Annotations:		openshift.io/image.dockerRepositoryCheck=2017-10-02T17:05:11Z
    Docker Pull Spec:	docker-registry.default.svc:5000/default/python
    Image Lookup:		local=false
    Unique Images:		1
    Tags:			1
    
    3.5
      tagged from centos/python-35-centos7
    
      * centos/python-35-centos7@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
          About a minute ago
  • Get all the information available about particular image stream tag:

    $ oc describe istag/<image-stream>:<tag-name>

    For example:

    $ oc describe istag/python:latest
    Example output
    Image Name:	sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
    Docker Image:	centos/python-35-centos7@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
    Name:		sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
    Created:	2 minutes ago
    Image Size:	251.2 MB (first layer 2.898 MB, last binary layer 72.26 MB)
    Image Created:	2 weeks ago
    Author:		<none>
    Arch:		amd64
    Entrypoint:	container-entrypoint
    Command:	/bin/sh -c $STI_SCRIPTS_PATH/usage
    Working Dir:	/opt/app-root/src
    User:		1001
    Exposes Ports:	8080/tcp
    Docker Labels:	build-date=20170801

More information is output than shown.

Adding tags to an image stream

You can add additional tags to image streams.

Procedure
  • Add a tag that points to one of the existing tags by using the `oc tag`command:

    $ oc tag <image-name:tag1> <image-name:tag2>

    For example:

    $ oc tag python:3.5 python:latest
    Example output
    Tag python:latest set to python@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25.
  • Confirm the image stream has two tags, one, 3.5, pointing at the external container image and another tag, latest, pointing to the same image because it was created based on the first tag.

    $ oc describe is/python
    Example output
    Name:			python
    Namespace:		default
    Created:		5 minutes ago
    Labels:			<none>
    Annotations:		openshift.io/image.dockerRepositoryCheck=2017-10-02T17:05:11Z
    Docker Pull Spec:	docker-registry.default.svc:5000/default/python
    Image Lookup:		local=false
    Unique Images:		1
    Tags:			2
    
    latest
      tagged from python@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
    
      * centos/python-35-centos7@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
          About a minute ago
    
    3.5
      tagged from centos/python-35-centos7
    
      * centos/python-35-centos7@sha256:49c18358df82f4577386404991c51a9559f243e0b1bdc366df25
          5 minutes ago

Adding tags for an external image

You can add tags for external images.

Procedure
  • Add tags pointing to internal or external images, by using the oc tag command for all tag-related operations:

    $ oc tag <repository/image> <image-name:tag>

    For example, this command maps the docker.io/python:3.6.0 image to the 3.6 tag in the python image stream.

    $ oc tag docker.io/python:3.6.0 python:3.6
    Example output
    Tag python:3.6 set to docker.io/python:3.6.0.

    If the external image is secured, you must create a secret with credentials for accessing that registry.

Updating image stream tags

You can update a tag to reflect another tag in an image stream.

Procedure
  • Update a tag:

    $ oc tag <image-name:tag> <image-name:latest>

    For example, the following updates the latest tag to reflect the 3.6 tag in an image stream:

    $ oc tag python:3.6 python:latest
    Example output
    Tag python:latest set to python@sha256:438208801c4806548460b27bd1fbcb7bb188273d13871ab43f.

Removing image stream tags

You can remove old tags from an image stream.

Procedure
  • Remove old tags from an image stream:

    $ oc tag -d <image-name:tag>

    For example:

    $ oc tag -d python:3.5
    Example output
    Deleted tag default/python:3.5.

See Removing deprecated image stream tags from the Cluster Samples Operator for more information on how the Cluster Samples Operator handles deprecated image stream tags.

Configuring periodic importing of image stream tags

When working with an external container image registry, to periodically re-import an image, for example to get latest security updates, you can use the --scheduled flag.

Procedure
  1. Schedule importing images:

    $ oc tag <repository/image> <image-name:tag> --scheduled

    For example:

    $ oc tag docker.io/python:3.6.0 python:3.6 --scheduled
    Example output
    Tag python:3.6 set to import docker.io/python:3.6.0 periodically.

    This command causes OpenShift Container Platform to periodically update this particular image stream tag. This period is a cluster-wide setting set to 15 minutes by default.

  2. Remove the periodic check, re-run above command but omit the --scheduled flag. This will reset its behavior to default.

    $ oc tag <repositiory/image> <image-name:tag>

Importing images and image streams from private registries

An image stream can be configured to import tag and image metadata from private image registries requiring authentication. This procedures applies if you change the registry that the Cluster Samples Operator uses to pull content from to something other than registry.redhat.io.

When importing from insecure or secure registries, the registry URL defined in the secret must include the :80 port suffix or the secret is not used when attempting to import from the registry.

Procedure
  1. You must create a secret object that is used to store your credentials by entering the following command:

    $ oc create secret generic <secret_name> --from-file=.dockerconfigjson=<file_absolute_path> --type=kubernetes.io/dockerconfigjson
  2. After the secret is configured, create the new image stream or enter the oc import-image command:

    $ oc import-image <imagestreamtag> --from=<image> --confirm

    During the import process, OpenShift Container Platform picks up the secrets and provides them to the remote party.

Allowing pods to reference images from other secured registries

The .dockercfg $HOME/.docker/config.json file for Docker clients is a Docker credentials file that stores your authentication information if you have previously logged into a secured or insecure registry.

To pull a secured container image that is not from OpenShift Container Platform’s internal registry, you must create a pull secret from your Docker credentials and add it to your service account.

The Docker credentials file and the associated pull secret can contain multiple references to the same registry, each with its own set of credentials.

Example config.json file
{
   "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"
      }
   }
}
Example pull secret
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
Procedure
  • If you already have a .dockercfg file for the secured registry, you can create a secret from that file by running:

    $ oc create secret generic <pull_secret_name> \
        --from-file=.dockercfg=<path/to/.dockercfg> \
        --type=kubernetes.io/dockercfg
  • Or if you have a $HOME/.docker/config.json file:

    $ oc create secret generic <pull_secret_name> \
        --from-file=.dockerconfigjson=<path/to/.docker/config.json> \
        --type=kubernetes.io/dockerconfigjson
  • If you do not already have a Docker credentials file for the secured registry, you can create a secret by running:

    $ oc create secret docker-registry <pull_secret_name> \
        --docker-server=<registry_server> \
        --docker-username=<user_name> \
        --docker-password=<password> \
        --docker-email=<email>
  • To use a secret for pulling images for pods, you must add the secret to your service account. The name of the service account in this example should match the name of the service account the pod uses. The default service account is default:

    $ oc secrets link default <pull_secret_name> --for=pull