API Microversions


Magnum 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, container-infra, and a monotonically increasing semantic version number starting from 1.1. The full form of the header takes the form:

OpenStack-API-Version: container-infra 1.1

If a user makes a request without specifying a version, they will get the BASE_VER as defined in magnum/api/controllers/versions.py. This value is currently 1.1 and is expected to remain so for quite a long time.

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 clusters/{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 clusters/{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 ‘locked’: True/False to the request body

  • the Response

    • the list of attributes and data structures returned

      Example: adding a new attribute ‘locked’: True/False to the output of clusters/{ID}

    • the allowed values of non free form fields

      Example: adding a new allowed status to clusters/{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.

In Code

In magnum/api/controllers/base.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:

def my_api_method(self, req, id):

This method would only be available if the caller had specified an OpenStack-API-Version of >= 1.2. If they had specified a lower version (or not specified it and received the default of 1.1) the server would respond with HTTP/406.

Removing an API method

In the controller class:

@base.Controller.api_version("1.2", "1.3")
def my_api_method(self, req, id):

This method would only be available if the caller had specified an OpenStack-API-Version of >= 1.2 and OpenStack-API-Version of <= 1.3. If 1.4 or later is specified the server will respond with HTTP/406.

Changing a method’s behavior

In the controller class:

@base.Controller.api_version("1.2", "1.3")
def my_api_method(self, req, id):
    .... method_1 ...

@base.Controller.api_version("1.4") #noqa
def my_api_method(self, req, id):
    .... method_2 ...

If a caller specified 1.2, 1.3 (or received the default of 1.1) they would see the result from method_1, and for 1.4 or later they would see the result from 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)

When not using decorators

When you don’t want to use the @api_version decorator on a method or you want to change behavior 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 accessed with pecan.request). Every API method has an versions object attached to the request object and that can be used to modify behavior based on its value:

def index(self):
    <common code>

    req_version = pecan.request.headers.get(Version.string)
    req1_min = versions.Version("1.1")
    req1_max = versions.Version("1.5")
    req2_min = versions.Version("1.6")
    req2_max = versions.Version("1.10")

    if req_version.matches(req1_min, req1_max):
    elif req_version.matches(req2min, req2_max):
        ....other stuff....
    elif req_version > versions.Version("1.10"):
        ....more stuff.....

    <common code>

The first argument to the matches method is the minimum acceptable version and the second is maximum acceptable version. If the specified minimum version and maximum version are null then ValueError is returned.

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 magnum/api/controllers/versions.py

  • Update CURRENT_MAX_VER in magnum/api/controllers/versions.py

  • Add a verbose description to magnum/api/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 magnum/tests/unit/api/controllers/test_base.py.

  • Make a new commit to python-magnumclient 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.

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 magnum spec for the change, the minor number of CURRENT_MAX_VER 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 CURRENT_MAX_VER.