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 thelatest
image stream tag are updated to the newest level of the framework provided by the image when a new level becomes available. Alatest
image stream tag tov3.10
can be changed tov3.11
at any time. It is important to be aware that theselatest
image stream tags behave differently than the Dockerlatest
tag. Thelatest
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 thelatest
image stream tag points tov3.10
of an image, when the3.11
version is released, thelatest
tag is not automatically updated tov3.11
, and remains atv3.10
until it is manually updated to point to av3.11
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