Overview

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

There are many ways you can interact with images and set up image streams, depending on where the images' registries are located, any authentication requirements around those registries, and how you want your builds and deployments to behave. The following sections cover a range of these topics.

Tagging Images

Before working with OpenShift Container Platform image streams and their tags, it will help to first understand image tags in the context of Docker generally.

Container images can have names added to them that make it more intuitive to determine what they contain, called a tag. Using a tag to specify the version of what is contained in the image is a common use case. If you have an image named ruby, you could have a tag named 2.0 for 2.0 version of Ruby, and another named latest to indicate literally the latest built image in that repository overall.

When interacting directly with images using the docker CLI, the docker tag command can add tags, which essentially adds an alias to an image that can consist of several parts. Those parts can include:

<registry_server>/<user_name>/<image_name>:<tag>

The <user_name> part in the above could also refer to a project or namespace if the image is being stored in an OpenShift Container Platform environment with an internal registry.

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.

See Red Hat Enterprise Linux 7’s Getting Started with Containers documentation for more about tagging images directly using the docker CLI.

Adding Tags to Image Streams

Keeping in mind that an image stream in OpenShift Container Platform comprises zero or more container images identified by tags, you can add tags to an image stream using the oc tag command:

$ oc tag <source> <destination>

For example, to configure the ruby image’s latest tag to always refer to the current image for the tag 2.0:

$ oc tag ruby:latest ruby:2.0

There are different types of tags available. The default behavior uses a permanent tag, which points to a specific image in time; even when the source changes, it will not reflect in the destination tag.

A tracking tag means the destination tag’s metadata will be imported during the import. To ensure the destination tag is updated whenever the source tag changes, use the --alias=true flag:

$ oc tag --alias=true <source> <destination>

You can also add the --scheduled=true flag to have the destination tag be be refreshed (i.e., re-imported) periodically. The period is configured globally at system level. See Importing Tag and Image Metadata for more details.

Avoid tagging OpenShift Container Platform-managed images (i.e., those built using an OpenShift Container Platform instance and pushed to its internal registry). There is a known issue that prevents the registry client from pulling from such a tag.

Removing Tags from Image Streams

To remove a tag completely from an image stream run:

$ oc delete istag/ruby:latest

or:

$ oc tag -d ruby:latest

Referencing Images in Image Streams

Images can be referenced in image streams using the following reference types:

  • An ImageStreamTag is used to reference or retrieve an image for a given image stream and tag. It uses the following convention for its name:

    <image_stream_name>:<tag>
  • An ImageStreamImage is used to reference or retrieve an image for a given image stream and image name. It uses the following convention for its name:

    <image_stream_name>@<id>

    The <id> is an immutable identifier for a specific image, also called a digest.

  • A DockerImage is used to reference or retrieve an image for a given external registry. It uses standard Docker pull specification for its name, e.g.:

    openshift/ruby-20-centos7:2.0

    When no tag is specified, it is assumed the latest tag will be used.

    You can also reference a third-party registry:

    registry.access.redhat.com/rhel7:latest

    Or an image with a digest:

    centos/ruby-22-centos7@sha256:3a335d7d8a452970c5b4054ad7118ff134b3a6b50a2bb6d0c07c746e8986b28e

When viewing example image stream definitions, such as the example CentOS image streams, 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 whenever 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.

You can view an image’s object definition by retrieving an ImageStreamImage definition using the image stream name and ID:

$ oc export isimage <image_stream_name>@<id>

You can find valid <id> values for a given image stream by running:

$ oc describe is <image_stream_name>

For example, from the ruby image stream asking for the ImageStreamImage with the name and ID of ruby@3a335d7:

Example 1. Definition of an Image Object Retrieved via ImageStreamImage
$ oc export isimage ruby@3a335d7

apiVersion: v1
image:
  dockerImageLayers:
  - name: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
    size: 0
  - name: sha256:ee1dd2cb6df21971f4af6de0f1d7782b81fb63156801cfde2bb47b4247c23c29
    size: 196634330
  - name: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
    size: 0
  - name: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
    size: 0
  - name: sha256:ca062656bff07f18bff46be00f40cfbb069687ec124ac0aa038fd676cfaea092
    size: 177723024
  - name: sha256:63d529c59c92843c395befd065de516ee9ed4995549f8218eac6ff088bfa6b6e
    size: 55679776
  dockerImageMetadata:
    Architecture: amd64
    Author: SoftwareCollections.org <sclorg@redhat.com>
    Config:
      Cmd:
      - /bin/sh
      - -c
      - $STI_SCRIPTS_PATH/usage
      Entrypoint:
      - container-entrypoint
      Env:
      - 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: {}
      Image: d9c3abc5456a9461954ff0de8ae25e0e016aad35700594714d42b687564b1f51
      Labels:
        build-date: 2015-12-23
        io.k8s.description: Platform for building and running Ruby 2.2 applications
        io.k8s.display-name: Ruby 2.2
        io.openshift.builder-base-version: 8d95148
        io.openshift.builder-version: 8847438ba06307f86ac877465eadc835201241df
        io.openshift.expose-services: 8080:http
        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
    ContainerConfig: {}
    Created: 2016-01-26T21:07:27Z
    DockerVersion: 1.8.2-el7
    Id: 57b08d979c86f4500dc8cad639c9518744c8dd39447c055a3517dc9c18d6fccd
    Parent: d9c3abc5456a9461954ff0de8ae25e0e016aad35700594714d42b687564b1f51
    Size: 430037130
    apiVersion: "1.0"
    kind: DockerImage
  dockerImageMetadataVersion: "1.0"
  dockerImageReference: centos/ruby-22-centos7@sha256:3a335d7d8a452970c5b4054ad7118ff134b3a6b50a2bb6d0c07c746e8986b28e
  metadata:
    creationTimestamp: 2016-01-29T13:17:45Z
    name: sha256:3a335d7d8a452970c5b4054ad7118ff134b3a6b50a2bb6d0c07c746e8986b28e
    resourceVersion: "352"
    uid: af2e7a0c-c68a-11e5-8a99-525400f25e34
kind: ImageStreamImage
metadata:
  creationTimestamp: null
  name: ruby@3a335d7
  namespace: openshift
  selflink: /oapi/v1/namespaces/openshift/imagestreamimages/ruby@3a335d7

Image Pull Policy

Each container in a pod has a container image. Once you have created an image and pushed it to a registry, you can then refer to it in the pod.

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

  • Always - always pull the image.

  • IfNotPresent - only pull the image if it does not already exist on the node.

  • Never - never pull the image.

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

  1. If the tag is latest, OpenShift Container Platform defaults imagePullPolicy to Always.

  2. Otherwise, OpenShift Container Platform defaults imagePullPolicy to IfNotPresent.

Accessing the Internal Registry

You can access OpenShift Container Platform’s internal registry directly to push or pull images. For example, this could be helpful if you wanted to create an image stream by manually pushing an image, or just to docker pull an image directly.

The internal registry authenticates using the same tokens as the OpenShift Container Platform API. To perform a docker login against the internal registry, you can choose any user name and email, but the password must be a valid OpenShift Container Platform token.

To log into the internal registry:

  1. Log in to OpenShift Container Platform:

    $ oc login
  2. Get your access token:

    $ oc whoami -t
  3. Log in to the internal registry using the token. You must have docker installed on your system:

    $ docker login -u <user_name> -e <email_address> \
        -p <token_value> <registry_server>:<port>

    Contact your cluster administrator if you do not know the registry IP or host name and port to use.

In order to pull an image, the authenticated user must have get rights on the requested imagestreams/layers. In order to push an image, the authenticated user must have update rights on the requested imagestreams/layers.

By default, all service accounts in a project have rights to pull any image in the same project, and the builder service account has rights to push any image in the same project.

Using Image Pull Secrets

Docker registries can be secured to prevent unauthorized parties from accessing certain images. If you are using OpenShift Container Platform’s internal registry and are pulling from image streams located in the same project, then your pod’s service account should already have the correct permissions and no additional action should be required.

However, for other scenarios, such as referencing images across OpenShift Container Platform projects or from secured registries, then additional configuration steps are required. The following sections detail these scenarios and their required steps.

Allowing Pods to Reference Images Across Projects

When using the internal 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:

$ oc policy add-role-to-user \
    system:image-puller system:serviceaccount:project-a:default \
    --namespace=project-b

After adding that role, the pods in project-a that reference the default service account will be able to pull images from project-b.

To allow access for any service account in project-a, use the group:

$ oc policy add-role-to-group \
    system:image-puller system:serviceaccounts:project-a \
    --namespace=project-b

Allowing Pods to Reference Images from Other Secured Registries

The .dockercfg file (or $HOME/.docker/config.json for newer Docker clients) is a Docker credentials file that stores your 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.

If you already have a .dockercfg file for the secured registry, you can create a secret from that file by running:

$ oc secrets new <pull_secret_name> .dockercfg=<path/to/.dockercfg>

Or if you have a $HOME/.docker/config.json file:

$ oc secrets new <pull_secret_name> .dockerconfigjson=<path/to/.docker/config.json>

If you do not already have a Docker credentials file for the secured registry, you can create a secret by running:

$ oc secrets new-dockercfg <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 will use; default is the default service account:

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

To use a secret for pushing and pulling build images, the secret must be mountable inside of a pod. You can do this by running:

$ oc secrets link builder <pull_secret_name>

Importing Tag and Image Metadata

An image stream can be configured to import tag and image metadata from an image repository in an external Docker image registry. You can do this using a few different methods.

  • You can manually import tag and image information with the oc import-image command using the --from option:

    $ oc import-image <image_stream_name>[:<tag>] --from=<docker_image_repo> --confirm

    For example:

    $ oc import-image my-ruby --from=docker.io/openshift/ruby-20-centos7 --confirm
    The import completed successfully.
    
    Name:			my-ruby
    Created:		Less than a second ago
    Labels:			<none>
    Annotations:		openshift.io/image.dockerRepositoryCheck=2016-05-06T20:59:30Z
    Docker Pull Spec:	172.30.94.234:5000/demo-project/my-ruby
    
    Tag	Spec					Created			PullSpec							Image
    latest	docker.io/openshift/ruby-20-centos7	Less than a second ago	docker.io/openshift/ruby-20-centos7@sha256:772c5bf9b2d1e8...	<same>

    You can also add the --all flag to import all tags for the image instead of just latest.

  • Like most objects in OpenShift Container Platform, you can also write and save a JSON or YAML definition to a file then create the object using the CLI. Set the spec.dockerImageRepository field to the Docker pull spec for the image:

    apiVersion: "v1"
    kind: "ImageStream"
    metadata:
      name: "my-ruby"
    spec:
      dockerImageRepository: "docker.io/openshift/ruby-20-centos7"

    Then create the object:

    $ oc create -f <file>

When you create an image stream that references an image in an external Docker registry, OpenShift Container Platform communicates with the external registry within a short amount of time to get up to date information about the image.

After the tag and image metadata is synchronized, the image stream object would look similar to the following:

apiVersion: v1
kind: ImageStream
metadata:
  name: my-ruby
  namespace: demo-project
  selflink: /oapi/v1/namespaces/demo-project/imagestreams/my-ruby
  uid: 5b9bd745-13d2-11e6-9a86-0ada84b8265d
  resourceVersion: '4699413'
  generation: 2
  creationTimestamp: '2016-05-06T21:34:48Z'
  annotations:
    openshift.io/image.dockerRepositoryCheck: '2016-05-06T21:34:48Z'
spec:
  dockerImageRepository: docker.io/openshift/ruby-20-centos7
  tags:
    -
      name: latest
      annotations: null
      from:
        kind: DockerImage
        name: 'docker.io/openshift/ruby-20-centos7:latest'
      generation: 2
      importPolicy: {  }
status:
  dockerImageRepository: '172.30.94.234:5000/demo-project/my-ruby'
  tags:
    -
      tag: latest
      items:
        -
          created: '2016-05-06T21:34:48Z'
          dockerImageReference: 'docker.io/openshift/ruby-20-centos7@sha256:772c5bf9b2d1e8e80742ed75aab05820419dc4532fa6d7ad8a1efddda5493dc3'
          image: 'sha256:772c5bf9b2d1e8e80742ed75aab05820419dc4532fa6d7ad8a1efddda5493dc3'
          generation: 2

You can set a tag to query external registries at a scheduled interval to synchronize tag and image metadata by setting the --scheduled=true flag with the oc tag command as mentioned in Adding Tags to Image Streams.

Alternatively, you can set importPolicy.scheduled to true in the tag’s definition:

apiVersion: v1
kind: ImageStream
metadata:
  name: ruby
spec:
  tags:
  - from:
      kind: DockerImage
      name: openshift/ruby-20-centos7
    name: latest
    importPolicy:
      scheduled: true

Importing Images from Insecure Registries

An image stream can be configured to import tag and image metadata from insecure image registries, such as those signed with a self-signed certificate or using plain HTTP instead of HTTPS.

To configure this, add the openshift.io/image.insecureRepository annotation and set it to true. This setting bypasses certificate validation when connecting to the registry:

kind: ImageStream
apiVersion: v1
metadata:
  name: ruby
  annotations:
    openshift.io/image.insecureRepository: "true" (1)
  spec:
    dockerImageRepository: my.repo.com:5000/myimage
1 Set the openshift.io/image.insecureRepository annotation to true

The above definition only affects importing tag and image metadata. For this image to be used in the cluster (e.g., to be able to do a docker pull), each node must have Docker configured with the --insecure-registry flag. See Host Preparation for information.

Additionally, you can specify a single tag using an insecure repository. To do so, set importPolicy.insecure in the tag’s definition to true:

kind: ImageStream
apiVersion: v1
metadata:
  name: ruby
  tags:
  - from:
      kind: DockerImage
      name: my.repo.com:5000/myimage
    name: mytag
    importPolicy:
      insecure: true  (1)
1 Set tag mytag to use insecure connection to that registry.

Importing Images from Private Registries

An image stream can be configured to import tag and image metadata from private image registries, requiring authentication.

To configure this, you need to create a secret which is used to store your credentials.

Create the secret first, before importing the image from the private repository:

$ oc secrets new-dockercfg <secret_name> \
    --docker-server=<docker_registry_server> \
    --docker-username=<docker_user> \
    --docker-password=<docker_password> \
    --docker-email=<docker_email>

For more options, see:

$ oc secrets new-dockercfg --help

After the secret is configured, proceed with creating the new image stream or using the oc import-image command. During the import process, OpenShift Container Platform will pick up the secrets and provide them to the remote party.

Importing Images Across Projects

An image stream can be configured to import tag and image metadata from the internal registry, but from a different project. The recommended method for this is to use the oc tag command as shown in Adding Tags to Image Streams:

$ oc tag <source_project>/<image_stream>:<tag> <new_image_stream>:<new_tag>

Another method is to import the image from the other project manually using the pull spec:

The following method is strongly discouraged and should be used only if the former using oc tag is insufficient.

  1. First, add the necessary policy to access the other project:

    $ oc policy add-role-to-group \
        system:image-puller \
        system:serviceaccounts:<destination_project> \
        -n <source_project>

    This allows <destination_project> to pull images from <source_project>.

  2. With the policy in place, you can import the image manually:

    $ oc import-image <new_image_stream> --confirm \
        --from=<docker_registry>/<source_project>/<image_stream>

Creating an Image Stream by Manually Pushing an Image

An image stream can also be automatically created by manually pushing an image to the internal registry. This is only possible when using an OpenShift Container Platform internal registry.

Before performing this procedure, the following must be satisfied:

  • The destination project you push to must already exist.

  • The user must be authorized to {get, update} "imagestream/layers" in that project. The system:image-pusher role can be added to a user to provide these permissions. If you are a project administrator, then you would also have these permissions.

To create an image stream by manually pushing an image:

  1. First, log in to the internal registry.

  2. Then, tag your image using the appropriate internal registry location. For example, if you had already pulled the docker.io/centos:centos7 image locally:

    $ docker tag docker.io/centos:centos7 172.30.48.125:5000/test/my-image
  3. Finally, push the image to your internal registry. For example:

    $ docker push 172.30.48.125:5000/test/my-image
    The push refers to a repository [172.30.48.125:5000/test/my-image] (len: 1)
    c8a648134623: Pushed
    2bf4902415e3: Pushed
    latest: digest: sha256:be8bc4068b2f60cf274fc216e4caba6aa845fff5fa29139e6e7497bb57e48d67 size: 6273
  4. Verify that the image stream was created:

    $ oc get is
    NAME       DOCKER REPO                        TAGS      UPDATED
    my-image   172.30.48.125:5000/test/my-image   latest    3 seconds ago

    == Writing Image Streams for S2I Builders

Image streams for S2I builders that are displayed in the management console’s catalog page require additional metadata to provide the best experience for end users.

Example 2. Definition of an Image Stream Object with Catalog Metadata
apiVersion: v1
kind: ImageStream
metadata:
  name: ruby
  annotations:
    openshift.io/display-name: Ruby (1)
spec:
  tags:
    - name: '2.0' (2)
      annotations:
        openshift.io/display-name: Ruby 2.0 (3)
        description: >- (4)
          Build and run Ruby 2.0 applications on CentOS 7. For more information
          about using this builder image, including OpenShift considerations,
          see
          https://github.com/sclorg/s2i-ruby-container/tree/master/2.0/README.md.
        iconClass: icon-ruby (5)
        sampleRepo: 'https://github.com/openshift/ruby-ex.git' (6)
        tags: 'builder,ruby' (7)
        supports: 'ruby' (8)
        version: '2.0' (9)
      from:
        kind: DockerImage
        name: 'openshift/ruby-20-centos7:latest'
1 A brief, user-friendly name for the whole image stream.
2 The tag is referred to as the version. Tags appear in a drop-down menu.
3 A user-friendly name for this tag within the image stream. This should be brief and include version information when appropriate.
4 A description of the tag, which includes enough detail for users to understand what the image is providing. It can include links to additional instructions. Limit the description to a few sentences.
5 The icon to show for this tag. Pick from our existing logo icons when possible. Icons from FontAwesome and Patternfly can also be used. Alternatively, provide icons through CSS customizations that can be added to an OpenShift Container Platform cluster that uses your image stream. You must specify an icon class that exists, or it will prevent falling back to the generic icon.
6 A URL to a source repository that works with this builder image tag and results in a sample running application.
7 Categories that the image stream tag is associated with. The builder tag is required for it to show up in the catalog. Add tags that will associate it with one of the provided catalog categories. Refer to the id and categoryAliases in CATALOG_CATEGORIES in the console’s constants file. The categories can also be customized for the whole cluster.
8 Languages this image supports. This value will be used during oc new-app invocations to try to match potential builder images to the provided source repository.
9 Version information for this tag.