API Microversions


Barbican 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, key-manager, and a monotonically increasing semantic version number starting from 1.0. The full form of the header takes the form:

OpenStack-API-Version: key-manager 1.1

If a user makes a request without specifying a version, they will get the MIN_API_VERSION as calculated from the defined _MIN_MICROVERSION in barbican/api/controllers/versions.py. This value is currently 1.0 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 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 microversions are not following semver and therefore backward compatibility is not guaranteed. Clients, like python-barbicanclient, 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 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 server

      Example: adding a new servers/{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 servers/{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 ‘consumer’: ‘…’ to the request body

  • the Response

    • the list of attributes and data structures returned

      Example: adding a new attribute ‘consumers’: [] to the output of secrets/{ID}

    • the allowed values of non free form fields

      Example: adding a new allowed secret_type to secrets/{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.

    • 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].

    • new headers returned on a response

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. See [3] for an example from Nova. Intentional behavior changes to an admin-only API do require a microversion.


In Code

In barbican/api/controllers/versions.py we define the is_supported function which is intended to be used in Controller methods to check if API request version satisfies version restrictions. The function accepts min_version and max_version arguments, and returns True when the requested version meets those constrainst.


Originally Nova also implemented a decorator API, but it frequently lead to code duplication. In Barbican it was decided to limit the microversion API to just the is_supported function.

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

  • Update _MAX_MICROVERSION and bump _LAST_UPDATED in barbican/api/controllers/versions.py

  • Add a verbose description to doc/source/api/microversion_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, add new tests to test both the old and new behavior to avoid regressions.

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

  • If the microversion changes the response schema, a new schema and test for the microversion must be added to Tempest.

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

Allocating a microversion

If you are adding a patch which adds a new microversion, it is necessary to allocate the next microversion number. Except under extremely unusual circumstances and this would have been mentioned in the barbican spec for the change, the _MAX_MICROVERSION will be incremented. This will also be the new minor version number for the API change.

It is possible that multiple microversion patches would be proposed in parallel and the microversions would conflict between patches. This will cause a merge conflict. We don’t reserve a microversion for each patch in advance as we don’t know the final merge order. Developers may need over time to rebase their patch calculating a new version number as above based on the updated value of _MAX_MICROVERSION.

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 unit tests, ‘barbican.test.utils.set_version’ function can be used, for example:

def test_should_get_secret_as_json_v1(self):
    utils.set_version(self.app, '1.1')
    secret = self._test_should_get_secret_as_json()
    self.assertIn('consumers', secret)