nova/doc/source/api_microversion_dev.rst

API Microversions

Background

Nova 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 X-OpenStack-Nova-API-Version which is a monotonically increasing semantic version number starting from 2.1.

If a user makes a request without specifying a version, they will get the DEFAULT_API_VERSION as defined in nova/api/openstack/wsgi.py. This value is currently 2.1 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.

For full details please read the Kilo spec for microversions

In Code

In nova/api/openstack/wsgi.py we define an @api_version decorator which is intended to be used on top-level Controller methods. It is not appropriate for lower-level methods. Some examples:

Adding a new API method

In the controller class:

@wsgi.Controller.api_version("2.4")
def my_api_method(self, req, id):
    ....

This method would only be available if the caller had specified an X-OpenStack-Nova-API-Version of >= 2.4. If they had specified a lower version (or not specified it and received the default of 2.1) the server would respond with HTTP/404.

Removing an API method

In the controller class:

@wsgi.Controller.api_version("2.1", "2.4")
def my_api_method(self, req, id):
    ....

This method would only be available if the caller had specified an X-OpenStack-Nova-API-Version of <= 2.4. If 2.5 or later is specified the server will respond with HTTP/404.

Changing a method's behaviour

In the controller class:

@wsgi.Controller.api_version("2.1", "2.3")
def my_api_method(self, req, id):
    .... method_1 ...

@wsgi.Controller.api_version("2.4")  # noqa
def my_api_method(self, req, id):
    .... method_2 ...

If a caller specified 2.1, 2.2 or 2.3 (or received the default of 2.1) they would see the result from method_1, 2.4 or later method_2.

It is vital that the two methods have the same name, so the second of them will need # noqa to avoid failing flake8's F811 rule. The two methods may be different in any kind of semantics (schema validation, return values, response codes, etc)

A method with only small changes between versions

A method may have only small changes between microversions, in which case you can decorate a private method:

@api_version("2.1", "2.4")
def _version_specific_func(self, req, arg1):
    pass

@api_version(min_version="2.5")  # noqa
def _version_specific_func(self, req, arg1):
    pass

def show(self, req, id):
    .... common stuff ....
    self._version_specific_func(req, "foo")
    .... common stuff ....

A change in schema only

If there is no change to the method, only to the schema that is used for validation, you can add a version range to the validation.schema decorator:

@wsgi.Controller.api_version("2.1")
@validation.schema(dummy_schema.dummy, "2.3", "2.8")
@validation.schema(dummy_schema.dummy2, "2.9")
def update(self, req, id, body):
    ....

This method will be available from version 2.1, validated according to dummy_schema.dummy from 2.3 to 2.8, and validated according to dummy_schema.dummy2 from 2.9 onward.

When not using decorators

When you don't want to use the @api_version decorator on a method or you want to change behaviour within a method (say it leads to simpler or simply a lot less code) you can directly test for the requested version with a method as long as you have access to the api request object (commonly called req). Every API method has an api_version_request object attached to the req object and that can be used to modify behaviour based on its value:

def index(self, req):
    <common code>

    req_version = req.api_version_request
    if req_version.matches("2.1", "2.5"):
        ....stuff....
    elif req_version.matches("2.6", "2.10"):
        ....other stuff....
    elif req_version > api_version_request.APIVersionRequest("2.10"):
        ....more stuff.....

    <common code>

The first argument to the matches method is the minimum acceptable version and the second is maximum acceptable version. A specified version can be null:

null_version = APIVersionRequest()

If the minimum version specified is null then there is no restriction on the minimum version, and likewise if the maximum version is null there is no restriction the maximum version. Alternatively a one sided comparison can be used as in the example above.

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:

• Update REST_API_VERSION_HISTORY in nova/api/openstack/api_version_request.py
• Update _MAX_API_VERSION in nova/api/openstack/api_version_request.py
• Add a verbose description to nova/api/openstack/rest_api_version_history.rst. There should be enough information that it could be used by the docs team for release notes.
• Update the expected versions in affected tests, for example in nova/tests/unit/api/openstack/compute/test_versions.py.

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 nova spec for the change, the minor number of _MAX_API_VERSION will be incremented. This will also be the new microversion 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_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 X-OpenStack-Nova-API-Version header, for example:

req = fakes.HTTPRequest.blank('/testable/url/endpoint')
req.headers = {'X-OpenStack-Nova-API-Version': '2.2'}
req.api_version_request = api_version.APIVersionRequest('2.6')

controller = controller.TestableController()

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

For many examples of testing, the canonical examples are in nova/tests/unit/api/openstack/compute/test_microversions.py.