OpenStack API Special Interest Group
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Microversion Specification

This topic document serves to provide guidance on how to work with microversions in OpenStack REST APIs.

Microversions enables the ability to introduce API changes while being able to allow clients to discover those changes. According to negotiations with servers, clients adjust their behavior to work with server correctly.


Versioning of the API should be single monotonic counter taking the form X.Y following these conventions:

  • X should only be changed if a significant backwards incompatible API change is made which affects the API as whole. That is, something that is only very rarely incremented. X should be changed in the following cases.
    • A number of endpoints get replaced by others
    • Drastic changes in API consumer workflow
  • Y should be changed when you make any change to the API. Note that this includes semantic changes which may not affect the input or output formats or even originate in the API code layer. We are not distinguishing between backwards compatible and backwards incompatible changes in the versioning system. It will however be made clear in the documentation as to what is a backwards compatible change and what is a backwards incompatible one.


Note that these versions numbers do not follow the rules of Semantic Versioning.

There are minimum and maximum versions which are used to describe what the server can understand. The minimum and maximum versions are the oldest and most recent versions supported. So clients can specify different version in the supporting range for each API call on the same server. The minimum version can be increased when supporting old clients is too great a burden. Increasing the minimum version means breaking the old clients, this happens very rarely also.

Services expose information about their minimum and maximum supported microversion range as part of version-discovery.

Each version includes all the changes since minimum version was introduced. It is not possible to request the feature introduced at microversion X.Y without accepting all the changes before X.Y in the same request. For example, you cannot request the feature which was introduced at microversion 2.100 without backwards incompatible changes which were introduced in microversion 2.99 and earlier.

Client Interaction

A client specifies the version of the API they want via the following approach, a new header:


For example, Keystone will use the header:

OpenStack-API-Version: identity 2.114

This conceptually acts like the accept header.

Clients should expect the following behavior from the server:

  • If the OpenStack-API-Version header is not provided, act as if the minimum supported version was specified.
  • If the OpenStack-API-Version header is sent, but the value does not match the current service, act as if the minimum supported version was specified.
  • If the OpenStack-API-Version header is sent, the service type matches, and the version takes the form of a version string respond with the API at the indicated version. If the version is outside the range of versions supported and is not the string latest (as described below), return 406 Not Acceptable, and a response body including the supported minimum and maximum versions.
  • The value of OpenStack-API-Version header is [SERVICE_TYPE] [VERSION_STRING]. The VERSION_STRING must match "^([1-9]d*).([1-9]d*|0)$". If the VERSION_STRING doesn't match the regex pattern, return a 400 Bad Request with an error response body that conforms to the errors guideline errors.
  • If OpenStack-API-Version header is sent, the service type matches, and the version is set to the special keyword latest behave as if maximum version 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 microversions but limit what is acceptable to the version range that it understands at the time.

Two extra headers are always returned in the response:

OpenStack-API-Version: [SERVICE_TYPE] version_number
Vary: OpenStack-API-Version

The first header specifies the version number of the API which was executed. An example:

OpenStack-API-Version: compute 2.22

The Vary header is used as a hint to caching proxies and user agents that the response is also dependent on the OpenStack-API-Version and not just the body and query parameters. See 7231#section-7.1.4 for details.


Servers must be prepared to deal with multiple OpenStack-API-Version headers. This could happen when a client designed to address multiple services always sends the headers it thinks it needs. Most Python frameworks will handle this by setting the value of the header to the values of all matching headers, joined by a ',' (comma). For example compute 2.11,identity 2.114.


A Python library called microversion-parse is available to help with server-side processing of microversion headers, both the new style described in this document and previous forms.

When the requested version is out of range for the server, the server returns status code 406 Not Acceptable along with a response body.

The error response body conforms to the errors guideline errors with two additional properties as described in the json-schema below:

   "max_version": {
    "type": "string", "pattern": "^([1-9]\d*)\.([1-9]\d*|0)$"
   "min_version": {
    "type": "string", "pattern": "^([1-9]\d*)\.([1-9]\d*|0)$"

An example HTTP Header response:

HTTP/1.1 406 Not Acceptable
Openstack-API-Version: compute 5.3
Vary: OpenStack-API-Version

An example errors body response: