6.4. 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: 2017-09-29T13:40:11Z
      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.

    注意

    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
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