API Microversions


Cyborg uses a framework we call ‘API Microversions’ for allowing changes to the API while preserving backward compatibility. The basic idea is that a user has to explicitly ask for their request to be treated with a particular version of the API. So breaking changes can be added to the API without breaking users who don’t specifically ask for it. This is done with an HTTP header OpenStack-API-Version which has as its value a string containing the name of the service, accelerator, and a monotonically increasing semantic version number starting from 2.0. The full form of the header takes the form:

OpenStack-API-Version: accelerator 2.0

If a user makes a request without specifying a version, they will get the _MIN_VERSION_STRING``(defined in ``cyborg/api/controllers/v2/versions.py) as the default version. This value is currently 2.0 and is expected to remain so for quite a long time.

There is a special value latest which can be specified, which will allow a client to always receive the most recent version (_MAX_VERSION_STRING defined in cyborg/api/controllers/v2/versions.py) of API responses from the server.


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

For full details please read the Ussuri spec for microversions and Microversion Specification.

When do I need a new Microversion?

A microversion is needed when the contract to the user is changed. The user contract covers many kinds of information such as:

  • the Request

    • the list of resource urls which exist on the accelerator

      Example: adding a new accelerator_requests/{ID}/foo which didn’t exist in a previous version of the code

    • the list of query parameters that are valid on urls

      Example: adding a new parameter is_yellow accelerator_requests/{ID}?is_yellow=True

    • the list of query parameter values for non free form fields

      Example: parameter filter_by takes a small set of constants/enums “A”, “B”, “C”. Adding support for new enum “D”.

    • new headers accepted on a request

    • the list of attributes and data structures accepted.

      Example: adding a new attribute ‘description’ to the accelerator request body

  • the Response

    • the list of attributes and data structures returned

      Example: adding a new attribute ‘description’ to the output of accelerator_requests/{ID}

    • the allowed values of non free form fields

      Example: adding a new allowed state to accelerator_requests/{ID}

    • the list of status codes allowed for a particular request

      Example: an API previously could return 200, 400, 403, 404 and the change would make the API now also be allowed to return 409.

      See [2] for the 400, 403, 404 and 415 cases.

    • new headers returned on a response.

    • changing a status code on a particular response.

      Example: changing the return code of an API from 501 to 400.


      Fixing a bug so that a 400+ code is returned rather than a 500 or 503 does not require a microversion change. It’s assumed that clients are not expected to handle a 500 or 503 response and therefore should not need to opt-in to microversion changes that fixes a 500 or 503 response from happening. According to the OpenStack API Working Group, a 500 Internal Server Error should not be returned to the user for failures due to user error that can be fixed by changing the request on the client side. See [1].

The following flow chart attempts to walk through the process of “do we need a microversion”.

digraph states {

 label="Do I need a microversion?"

 silent_fail[shape="diamond", style="", group=g1, label="Did we silently
 fail to do what is asked?"];
 ret_500[shape="diamond", style="", group=g1, label="Did we return a 500
 new_error[shape="diamond", style="", group=g1, label="Are we changing what
 status code is returned?"];
 new_attr[shape="diamond", style="", group=g1, label="Did we add or remove
 an attribute to a payload?"];
 new_param[shape="diamond", style="", group=g1, label="Did we add or remove
 an accepted query string parameter or value?"];
 new_resource[shape="diamond", style="", group=g1, label="Did we add or
 remove a resource url?"];

no[shape="box", style=rounded, label="No microversion needed"];
yes[shape="box", style=rounded, label="Yes, you need a microversion"];
no2[shape="box", style=rounded, label="No microversion needed, it's
a bug"];

silent_fail -> ret_500[label=" no"];
silent_fail -> no2[label="yes"];

 ret_500 -> no2[label="yes [1]"];
 ret_500 -> new_error[label=" no"];

 new_error -> new_attr[label=" no"];
 new_error -> yes[label="yes"];

 new_attr -> new_param[label=" no"];
 new_attr -> yes[label="yes"];

 new_param -> new_resource[label=" no"];
 new_param -> yes[label="yes"];

 new_resource -> no[label=" no"];
 new_resource -> yes[label="yes"];

{rank=same; yes new_attr}
{rank=same; no2 ret_500}
{rank=min; silent_fail}



When a microversion is not needed

A microversion is not needed in the following situation:

  • the response

    • Changing the error message without changing the response code does not require a new microversion.

    • Removing an inapplicable HTTP header, for example, suppose the Retry-After HTTP header is being returned with a 4xx code. This header should only be returned with a 503 or 3xx response, so it may be removed without bumping the microversion.

    • An obvious regression bug in an admin-only API where the bug can still be fixed upstream on active stable branches. Admin-only APIs are less of a concern for interoperability and generally a regression in behavior can be dealt with as a bug fix when the documentation clearly shows the API behavior was unexpectedly regressed.

In Code

In cyborg/api/controllers/v2/versions.py we define some constants below:

  • BASE_VERSION: value is 2 which is intended to be used as the Cyborg API version.

  • MINOR_0_INITIAL_VERSION: value is 0 to be used as the initial value of microversion.

  • MINOR_X_Y: Y is the change you want to make, X is the min version to support Y. For example, MINOR_1_PROJECT_ID means that the request project_id is supported from microversion 2.1.

  • MINOR_MAX_VERSION: the max version, which equals to latest.

  • _MIN_VERSION_STRING: the combination of BASE_VERSION and MINOR_0_INITIAL_VERSION, which means the min version of Cyborg API.

  • _MAX_VERSION_STRING with the combination of BASE_VERSION and MINOR_MAX_VERSION, which means the max version of Cyborg API.

In cyborg/api/controllers/v2/utils.py, we define the check function of microversion.

For the example of allow_project_id() function, we compare the request version and the defined MINOR_1_PROJECT_ID to check whether the request is allowed. If the user’s request with the version which is lower than MINOR_1_PROJECT_ID, we will raise “Request not acceptable.” exception to the user.

def allow_project_id():
    # v2.1 added project_id for arq patch
    return api.request.version.minor >= versions.MINOR_1_PROJECT_ID

Adding a new API method

In the controller class:

def my_api_method(self, req, id):
    if not utils.allow_project_id():
        raise exception.NotAcceptable(_(
            "Request not acceptable. The minimal required API "
            "version should be %(base)s.%(opr)s") %
            {'base': versions.BASE_VERSION,
            'opr': versions.MINOR_1_PROJECT_ID})

This method would only be available if the caller had specified an OpenStack-API-Version of >= accelerator 2.1. If they had specified a lower version (or not specified it and received the default of accelerator 2.0) the server would respond with HTTP/406.

Other necessary changes

If you are adding a patch which adds a new microversion, it is necessary to add changes to other places which describe your change:

  • Define MINOR_*{int}_** in cyborg/api/controllers/v2/versions.py

  • Update MINOR_MAX_VERSION to the defined MINOR_*{int}_** in cyborg/api/controllers/v2/versions.py

  • Add a verbose description of what changed in the new version to cyborg/api/rest_api_version_history.rst.

  • Add a release note with a features section announcing the new or changed feature and the microversion.

  • Update the expected versions in affected tests, for example in cyborg.tests.unit.api.controllers.v2.test_arqs.TestARQsController#test_apply_patch_allow_project_id.

  • Make a new commit to python-cyborgclient and update corresponding files to enable the newly added microversion API.

  • Update the API Reference documentation as appropriate. The source is located under api-ref/source/.

If applicable, add functional sample tests under cyborg_tempest_plugin/tests/api/

Allocating a microversion

If you are adding a patch which adds a new microversion, it is necessary to allocate the next microversion number. The minor number of _MAX_API_VERSION will be incremented. This will also be the new microversion number for the API change. Developers may need over time to rebase their patch calculating a new version number as above based on the updated value of _MAX_API_VERSION.

Testing Microversioned API Methods

Testing a microversioned API method is very similar to a normal controller method test, you just need to add the OpenStack-API-Version header, for example:

req = fakes.HTTPRequest.blank('/testable/url/endpoint')
req.headers = {'OpenStack-API-Version': 'accelerator 2.1'}
req.api_version_request = api_version.APIVersionRequest('2.1')

controller = controller.TestableController()

res = controller.index(req)
... assertions about the response ...

For many examples of testing, the canonical examples are in cyborg.tests.unit.api.controllers.v2.test_arqs.TestARQsController#test_apply_patch_allow_project_id.