API Microversions

Background

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

OpenStack-API-Version: container 1.1

If a user makes a request without specifying a version, they will get the BASE_VER as defined in zun/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 container/{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 container/{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 container/{ID}

  • the allowed values of non free form fields

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

    Note

    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”.

Footnotes

[1]

When fixing 500 errors that previously caused stack traces, try to map the new error into the existing set of errors that API call could previously return (400 if nothing else is appropriate). Changing the set of allowed status codes from a request is changing the contract, and should be part of a microversion (except in [2]).

The reason why we are so strict on contract is that we’d like application writers to be able to know, for sure, what the contract is at every microversion in Zun. If they do not, they will need to write conditional code in their application to handle ambiguities.

When in doubt, consider application authors. If it would work with no client side changes on both Zun versions, you probably don’t need a microversion. If, on the other hand, there is any ambiguity, a microversion is probably needed.

[2] (1,2)

The exception to not needing a microversion when returning a previously unspecified error code is the 400, 403, 404 and 415 cases. This is considered OK to return even if previously unspecified in the code since it’s implied given keystone authentication can fail with a 403 and API validation can fail with a 400 for invalid JSON request body. Request to url/resource that does not exist always fails with 404. Invalid content types are handled before API methods are called which results in a 415.

Note

When in doubt about whether or not a microversion is required for changing an error response code, consult the Zun Team.

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 zun/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:

@base.Controller.api_version("1.2")
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.version
    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):
        ....stuff....
    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 zun/api/controllers/versions.py

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

• Add a verbose description to zun/api/rest_api_version_history.rst. There should be enough information that it could be used by the docs team for release notes.

• Update min_microversion in .zuul.yaml.

• Update the expected versions in affected tests, for example in zun/tests/unit/api/controllers/test_root.py.

• Update CURRENT_VERSION in zun/tests/unit/api/base.py.

• Make a new commit to python-zunclient 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 zun 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.