Metadata Definitions Catalog

A common API hosted by the Glance service for vendors, admins, services, and users to meaningfully define available key / value pair and tag metadata. The intent is to enable better metadata collaboration across artifacts, services, and projects for OpenStack users.

This is about the definition of the available metadata that can be used on different types of resources (images, artifacts, volumes, flavors, aggregates, etc). A definition includes the properties type, its key, it's description, and it's constraints. This catalog will not store the values for specific instance properties.

For example, a definition of a virtual CPU topology property for number of cores will include the key to use, a description, and value constraints like requiring it to be an integer. So, a user, potentially through Horizon, would be able to search this catalog to list the available properties they can add to a flavor or image. They will see the virtual CPU topology property in the list and know that it must be an integer. In the Horizon example, when the user adds the property, its key and value will be stored in the service that owns that resource (Nova for flavors and in Glance for images).

Diagram: https://wiki.openstack.org/w/images/b/bb/Glance-Metadata-API.png