Update Microversion Header


Nova pioneered the microversion concept. The success of the concept has led other projects to follow suit, exposing limitations in the original implementation. One of those limitations is that the specificity of the microversion header name leads to some inflexibility on at least two dimensions:

  • The use of project name instead of service type is not aligned with cross-project goals of using service type wherever possible.

  • The placement of the name or type in the header name misrepresents that the type is actually a value of the header.

These issues have been addressed in a new microversion specification from the API working group and a related guideline on avoiding header proliferation.

Problem description

With the advent of the new microversion header specification, nova is not compliant.

Use Cases

As a user of OpenStack APIs I expect them to be consistent, coherent and sensible.

Proposed change

Nova’s microversion handling will be updated to accept the new style request headers and send the required response headers. Specifically this means that in a request the following will be accepted to indicate a microversion negotation:

OpenStack-API-Version: compute <the version requested>

In a response the following will be sent:

OpenStack-API-Version: compute <the version used>
Vary: OpenStack-API-Version

These headers will be in addition to the existing X-OpenStack-Nova-API-Version headers, to preserve compatibility in both directions. If both headers are present in a request, the newer OpenStack-API-Version style header must be preferred.

Adding these headers will, with some irony, require a microversion bump.

A microversion-parse library already exists to facilitate this change.


This specification makes no effort to deprecate and eventually remove the older microversion headers under the assumption that doing so is more trouble than it is worth. In other contexts there has been some concern that this wastes header bytes. If this is a concern shared by many we may be able to figure out a way to address it. If there is a useful way to do so, that would be an alternative.

Data model impact


REST API impact

No API methods are added, but every request will now have additional headers in each request and response. Luckily the change to do this is centralized.

The mechanics of incorrect values in the new header are the same as with the old (e.g. responding with a 406).

Security impact


Notifications impact


Other end user impact

python-novaclient will eventually need to be updated to deal with this header. If the minimum API version is ever raised above the threshold set at the change to a new microversion header then support for the old microversion header could be removed from novaclient.

Performance Impact

None, unless you think a few more bytes in the headers is a significant problem relative to the other real issues in performance.

Other deployer impact

If there are header filtering proxies somewhere in a deployer’s stack, they will need to update to allow the new header to pass.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Update nova’s WSGI framework to use microversion-parse.

  • Update the WGSI framework to send and receive both headers.

  • Update unit and functional tests.

  • Bump microversion.


None, other than the aforementioned microversion-parse, which is already accepted into global requirements.


In testing it is important to make sure that we cover situations using the old header, the new header, both headers and neither of the headers.

Documentation Impact

The api-ref will need to be updated at some point to reflect the availability of the new header. This does not need to be immediate as the old headers will continue to work indefinitely.




Release Name