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Image Metadata

Overview

Defining image metadata helps OpenShift Enterprise better consume your Docker images, allowing OpenShift Enterprise to create a better experience for developers using your image. For example, you can add metadata to provide helpful descriptions of your image, or offer suggestions on other images that may also be needed.

This topic only defines the metadata needed by the current set of use cases. Additional metadata or use cases may be added in the future.

Defining Image Metadata

You can use the LABEL instruction in a Dockerfile to define image metadata. Labels are similar to environment variables in that they are key value pairs attached to an image or a container. Labels are different from environment variable in that they are not visible to the running application and they can also be used for fast look-up of images and containers.

Docker documentation for more information on the LABEL instruction.

The label names should typically be namespaced. The namespace should be set accordingly to reflect the project that is going to pick up the labels and use them. For OpenShift Enterprise the namespace should be set to io.openshift and for Kubernetes the namespace is io.k8s.

See the Docker custom metadata documentation for details about the format.

Table 1. Supported Metadata
Variable Description

io.openshift.tags

This label contains a list of tags represented as list of comma separated string values. The tags are the way to categorize the Docker images into broad areas of functionality. Tags help UI and generation tools to suggest relevant Docker images during the application creation process.

LABEL io.openshift.tags   mongodb,mongodb24,nosql

io.openshift.wants

Specifies a list of tags that the generation tools and the UI might use to provide relevant suggestions if you don’t have the Docker images with given tags already. For example, if the Docker image wants mysql and redis and you don’t have the Docker image with redis tag, then UI might suggest you to add this image into your deployment.

LABEL io.openshift.wants   mongodb,redis

io.k8s.description

This label can be used to give the Docker image consumers more detailed information about the service or functionality this image provides. The UI can then use this description together with the Docker image name to provide more human friendly information to end users.

LABEL io.k8s.description The MySQL 5.5 Server with master-slave replication support

io.openshift.expose-services

This label contains a list of service ports that match with the EXPOSE instructions in the *Dockerfile* and provide more descriptive information about what actual service on the given port provides to consumers.

The format is PORT[/PROTO]:NAME where the [PROTO] part is optional and it defaults to tcp if it is not specified.

LABEL io.openshift.expose-services 2020/udp:ftp,8080:https

io.openshift.non-scalable

An image might use this variable to suggest that it does not support scaling. The UI will then communicate this to consumers of that image. Being not-scalable basically means that the value of replicas should initially not be set higher than 1.

LABEL io.openshift.non-scalable     true

io.openshift.min-memory and io.openshift.min-cpu

This label suggests how much resources the Docker image might need in order to work properly. The UI might warn the user that deploying this Docker image may exceed their user quota. The values must be compatible with Kubernetes quantity.

LABEL io.openshift.min-memory 8Gi
LABEL io.openshift.min-cpu     4