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API Microversions
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=================
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Background
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----------
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Magnum uses a framework we call 'API Microversions' for allowing changes
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to the API while preserving backward compatibility. The basic idea is
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that a user has to explicitly ask for their request to be treated with
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a particular version of the API. So breaking changes can be added to
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the API without breaking users who don't specifically ask for it. This
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is done with an HTTP header ``OpenStack-API-Version`` which has as its
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value a string containing the name of the service, ``container-infra``,
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and a monotonically increasing semantic version number starting
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from ``1.1``.
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The full form of the header takes the form::
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OpenStack-API-Version: container-infra 1.1
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If a user makes a request without specifying a version, they will get
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the ``BASE_VER`` as defined in
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``magnum/api/controllers/versions.py``. This value is currently ``1.1`` and
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is expected to remain so for quite a long time.
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When do I need a new Microversion?
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----------------------------------
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A microversion is needed when the contract to the user is
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changed. The user contract covers many kinds of information such as:
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- the Request
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- the list of resource urls which exist on the server
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Example: adding a new clusters/{ID}/foo which didn't exist in a
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previous version of the code
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- the list of query parameters that are valid on urls
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Example: adding a new parameter ``is_yellow`` clusters/{ID}?is_yellow=True
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- the list of query parameter values for non free form fields
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Example: parameter filter_by takes a small set of constants/enums "A",
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"B", "C". Adding support for new enum "D".
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- new headers accepted on a request
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- the list of attributes and data structures accepted.
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Example: adding a new attribute 'locked': True/False to the request body
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- the Response
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- the list of attributes and data structures returned
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Example: adding a new attribute 'locked': True/False to the output
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of clusters/{ID}
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- the allowed values of non free form fields
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Example: adding a new allowed ``status`` to clusters/{ID}
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- the list of status codes allowed for a particular request
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Example: an API previously could return 200, 400, 403, 404 and the
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change would make the API now also be allowed to return 409.
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See [#f2]_ for the 400, 403, 404 and 415 cases.
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- changing a status code on a particular response
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Example: changing the return code of an API from 501 to 400.
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.. note:: Fixing a bug so that a 400+ code is returned rather than a 500 or
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503 does not require a microversion change. It's assumed that clients are
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not expected to handle a 500 or 503 response and therefore should not
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need to opt-in to microversion changes that fixes a 500 or 503 response
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from happening.
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According to the OpenStack API Working Group, a
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**500 Internal Server Error** should **not** be returned to the user for
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failures due to user error that can be fixed by changing the request on
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the client side. See [#f1]_.
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- new headers returned on a response
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The following flow chart attempts to walk through the process of "do
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we need a microversion".
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.. graphviz::
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digraph states {
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label="Do I need a microversion?"
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silent_fail[shape="diamond", style="", group=g1, label="Did we silently
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fail to do what is asked?"];
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ret_500[shape="diamond", style="", group=g1, label="Did we return a 500
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before?"];
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new_error[shape="diamond", style="", group=g1, label="Are we changing what
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status code is returned?"];
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new_attr[shape="diamond", style="", group=g1, label="Did we add or remove an
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attribute to a payload?"];
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new_param[shape="diamond", style="", group=g1, label="Did we add or remove
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an accepted query string parameter or value?"];
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new_resource[shape="diamond", style="", group=g1, label="Did we add or remove a
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resource url?"];
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no[shape="box", style=rounded, label="No microversion needed"];
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yes[shape="box", style=rounded, label="Yes, you need a microversion"];
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no2[shape="box", style=rounded, label="No microversion needed, it's
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a bug"];
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silent_fail -> ret_500[label=" no"];
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silent_fail -> no2[label="yes"];
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ret_500 -> no2[label="yes [1]"];
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ret_500 -> new_error[label=" no"];
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new_error -> new_attr[label=" no"];
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new_error -> yes[label="yes"];
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new_attr -> new_param[label=" no"];
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new_attr -> yes[label="yes"];
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new_param -> new_resource[label=" no"];
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new_param -> yes[label="yes"];
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new_resource -> no[label=" no"];
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new_resource -> yes[label="yes"];
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{rank=same; yes new_attr}
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{rank=same; no2 ret_500}
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{rank=min; silent_fail}
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}
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**Footnotes**
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.. [#f1] When fixing 500 errors that previously caused stack traces, try
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to map the new error into the existing set of errors that API call
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could previously return (400 if nothing else is appropriate). Changing
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the set of allowed status codes from a request is changing the
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contract, and should be part of a microversion (except in [#f2]_).
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The reason why we are so strict on contract is that we'd like
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application writers to be able to know, for sure, what the contract is
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at every microversion in Magnum. If they do not, they will need to write
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conditional code in their application to handle ambiguities.
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When in doubt, consider application authors. If it would work with no
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client side changes on both Magnum versions, you probably don't need a
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microversion. If, on the other hand, there is any ambiguity, a
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microversion is probably needed.
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.. [#f2] The exception to not needing a microversion when returning a
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previously unspecified error code is the 400, 403, 404 and 415 cases. This is
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considered OK to return even if previously unspecified in the code since
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it's implied given keystone authentication can fail with a 403 and API
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validation can fail with a 400 for invalid json request body. Request to
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url/resource that does not exist always fails with 404. Invalid content types
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are handled before API methods are called which results in a 415.
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.. note:: When in doubt about whether or not a microversion is required
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for changing an error response code, consult the `Containers Team`_.
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.. _Containers Team: https://wiki.openstack.org/wiki/Meetings/Containers
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When a microversion is not needed
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---------------------------------
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A microversion is not needed in the following situation:
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- the response
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- Changing the error message without changing the response code
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does not require a new microversion.
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- Removing an inapplicable HTTP header, for example, suppose the Retry-After
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HTTP header is being returned with a 4xx code. This header should only be
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returned with a 503 or 3xx response, so it may be removed without bumping
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the microversion.
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In Code
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-------
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In ``magnum/api/controllers/base.py`` we define an ``@api_version`` decorator
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which is intended to be used on top-level Controller methods. It is
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not appropriate for lower-level methods. Some examples:
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Adding a new API method
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~~~~~~~~~~~~~~~~~~~~~~~
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In the controller class::
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@base.Controller.api_version("1.2")
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def my_api_method(self, req, id):
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....
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This method would only be available if the caller had specified an
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``OpenStack-API-Version`` of >= ``1.2``. If they had specified a
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lower version (or not specified it and received the default of ``1.1``)
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the server would respond with ``HTTP/404``.
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Removing an API method
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~~~~~~~~~~~~~~~~~~~~~~
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In the controller class::
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@base.Controller.api_version("1.2", "1.3")
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def my_api_method(self, req, id):
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....
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This method would only be available if the caller had specified an
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``OpenStack-API-Version`` of <= ``1.3``. If ``1.4`` or later
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is specified the server will respond with ``HTTP/404``.
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Changing a method's behavior
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In the controller class::
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@base.Controller.api_version("1.2", "1.3")
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def my_api_method(self, req, id):
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.... method_1 ...
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@base.Controller.api_version("1.4") #noqa
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def my_api_method(self, req, id):
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.... method_2 ...
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If a caller specified ``1.2``, ``1.3`` (or received the default
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of ``1.1``) they would see the result from ``method_1``,
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and for ``1.4`` or later they would see the result from ``method_2``.
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It is vital that the two methods have the same name, so the second of
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them will need ``# noqa`` to avoid failing flake8's ``F811`` rule. The
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two methods may be different in any kind of semantics (schema
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validation, return values, response codes, etc)
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When not using decorators
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~~~~~~~~~~~~~~~~~~~~~~~~~
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When you don't want to use the ``@api_version`` decorator on a method
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or you want to change behavior within a method (say it leads to
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simpler or simply a lot less code) you can directly test for the
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requested version with a method as long as you have access to the api
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request object (commonly accessed with ``pecan.request``). Every API
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method has an versions object attached to the request object and that
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can be used to modify behavior based on its value::
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def index(self):
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<common code>
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req_version = pecan.request.headers.get(Version.string)
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req1_min = versions.Version("1.1")
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req1_max = versions.Version("1.5")
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req2_min = versions.Version("1.6")
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req2_max = versions.Version("1.10")
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if req_version.matches(req1_min, req1_max):
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....stuff....
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elif req_version.matches(req2min, req2_max):
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....other stuff....
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elif req_version > versions.Version("1.10"):
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....more stuff.....
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<common code>
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The first argument to the matches method is the minimum acceptable version
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and the second is maximum acceptable version. If the specified minimum
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version and maximum version are null then ``ValueError`` is returned.
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Other necessary changes
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-----------------------
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If you are adding a patch which adds a new microversion, it is
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necessary to add changes to other places which describe your change:
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* Update ``REST_API_VERSION_HISTORY`` in
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``magnum/api/controllers/versions.py``
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* Update ``CURRENT_MAX_VER`` in
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``magnum/api/controllers/versions.py``
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* Add a verbose description to
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``magnum/api/rest_api_version_history.rst``. There should
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be enough information that it could be used by the docs team for
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release notes.
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* Update the expected versions in affected tests, for example in
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``magnum/tests/unit/api/controllers/test_base.py``.
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* Make a new commit to python-magnumclient and update corresponding
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files to enable the newly added microversion API.
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* If the microversion changes the response schema, a new schema and test for
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the microversion must be added to Tempest.
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Allocating a microversion
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-------------------------
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If you are adding a patch which adds a new microversion, it is
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necessary to allocate the next microversion number. Except under
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extremely unusual circumstances and this would have been mentioned in
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the magnum spec for the change, the minor number of ``CURRENT_MAX_VER``
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will be incremented. This will also be the new microversion number for
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the API change.
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It is possible that multiple microversion patches would be proposed in
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parallel and the microversions would conflict between patches. This
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will cause a merge conflict. We don't reserve a microversion for each
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patch in advance as we don't know the final merge order. Developers
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may need over time to rebase their patch calculating a new version
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number as above based on the updated value of ``CURRENT_MAX_VER``.
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Jenkins