API v1.0 supports microversions: small, documented changes to the API. A user can use microversions to discover the latest API microversion supported in their cloud. A cloud that is upgraded to support newer microversions will still support all older microversions to maintain the backward compatibility for those users, who depend on older microversions. Users can also discover new features easily with microversions, so that they can benefit from all the advantages and improvements of the current cloud.

There are multiple cases which you can resolve with microversions:

  • Older clients with new cloud

Before using an old client to talk to a newer cloud, the old client can check the minimum version of microversions to verify whether the cloud is compatible with the old API. This prevents the old client from breaking with backwards incompatible API changes.

Currently the minimum version of microversions is 1.0, which is a microversion compatible with the legacy v1 API. That means the legacy v1 API user doesn’t need to worry that their older client software will be broken when their cloud is upgraded with new versions. The cloud operator doesn’t need to worry that upgrading their cloud to newer versions will break any user with older clients that don’t expect these changes.

  • User discovery of available features between clouds

The new features can be discovered by microversions. The user client should first check the microversions supported by the server. New features are only enabled when clouds support it. In this way, the user client can work with clouds that have deployed different microversions simultaneously.

Version Discovery

The Version API will return the minimum and maximum microversions. These values are used by the client to discover the API’s supported microversion(s).

Requests to ‘/’ will get version info for all endpoints. A response would look as follows:

  "versions": [
          "id": "v1.0",
          "links": [
                  "href": "http://openstack.example.com/v1/",
                  "rel": "self"
          "max_version": "1.1",
          "min_version": "1.0",
          "updated": "2021-02-10T00:00:00Z"

“max_version” is the maximum microversion, “min_version” is the minimum microversion. The client should specify a microversion between (and including) the minimum and maximum microversion to access the endpoint.

Client Interaction

A client specifies the microversion of the API they want by using the following HTTP header:

OpenStack-API-Version: key-manager 1.1


For more detail on the syntax see the Microversion Specification.

This acts conceptually like the “Accept” header. Semantically this means:

  • If OpenStack-API-Version (specifying key-manager) is not provided, act as if the minimum supported microversion was specified.

  • If OpenStack-API-Version is provided, respond with the API at that microversion. If that’s outside of the range of microversions supported, return 406 Not Acceptable.

  • OpenStack-API-Version has a value of latest (special keyword), act as if maximum was specified.


The latest value is mostly meant for integration testing and would be dangerous to rely on in client code since microversions are not following semver and therefore backward compatibility is not guaranteed. Clients should always require a specific microversion but limit what is acceptable to the microversion range that it understands at the time.

This means that out of the box, an old client without any knowledge of microversions can work with an OpenStack installation with microversions support.

From microversion 1.1 two additional headers are added to the response:

OpenStack-API-Version: key-manager microversion_number
Vary: OpenStack-API-Version