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API Version Parameter

Cadwyn adds another routing layer to FastAPI by default: by version parameter. This means that before FastAPI tries to route us to the correct route, Cadwyn will first decide on which version of the route to use based on a version parameter. Feel free to look at the example app with URL path version prefixes and arbitrary strings as versions here.

API Version location

The version parameter can be passed in two different ways:

  • As a custom header
  • As a URL path parameter

Cadwyn will use the following defaults:

from cadwyn import Cadwyn, Version, VersionBundle

app = Cadwyn(
    api_version_location="custom_header",
    api_version_parameter_name="X-API-VERSION",
    versions=VersionBundle(Version("2022-01-02")),
)

but you can change the header name if you want to:

from cadwyn import Cadwyn, Version, VersionBundle

app = Cadwyn(
    api_version_location="custom_header",
    api_version_parameter_name="MY-GREAT-HEADER",
    versions=VersionBundle(Version("2022-01-02")),
)

or you can use a path parameter:

from cadwyn import Cadwyn, Version, VersionBundle

app = Cadwyn(
    api_version_location="path",
    api_version_parameter_name="api_version",
    versions=VersionBundle(Version("2022-01-02")),
)

API Version format

The version parameter can be formatted in two different ways:

  • As an ISO date
  • As an arbitrary string

Cadwyn uses the following default:

from cadwyn import Cadwyn, Version, VersionBundle

app = Cadwyn(
    api_version_format="date",
    versions=VersionBundle(Version("2022-01-02")),
)

In the example above only dates will be accepted as valid versions.

You can also use an arbitrary string:

from cadwyn import Cadwyn, Version, VersionBundle

app = Cadwyn(
    api_version_format="string",
    versions=VersionBundle(
        Version("v2"),
        Version("anything_can_be_a_version"),
        Version("v1"),
    ),
)

In the example above any string will be accepted as a valid version. Arbitrary strings can be used, Cadwyn will not sort them. Cadwyn will assume their actual order matches the order of the versions in the VersionBundle.

API Version waterfalling

For historical reasons, date-based routing also supports waterfalling the requests to the closest earlier version of the API if the request date parameter doesn't match any of the versions exactly.

If the app has two versions: 2022-01-02 and 2022-01-05, and the request date parameter is 2022-01-03, then the request will be routed to the 2022-01-02 version, as it is the closest version, but lower than the request date parameter.

An exact match is always preferred to a partial match and a request will never be matched to the higher-versioned route.

We implement routing like this because Cadwyn was born in a microservice architecture and it is extremely convenient to have waterfalling there. For example, imagine that you have two Cadwyn services: Payables and Receivables, each defining its own API versions. Payables service might contain 10 versions while Receivables service might contain only 2 versions because it didn't need as many breaking changes. If a client requests a version that does not exist in Receivables, we will just waterfall to some earlier version, making Receivables behavior consistent even if API keeps getting new versions.

API Version Parameter Title and Description

You can pass a title and/or a description to the Cadwyn constructor. It is equivalent to passing title and description to fastapi.Path or fastapi.Header constructors.

app = Cadwyn(
    ...,
    api_version_title="My Great API version parameter",
    api_version_description="Description of my great API version parameter",
)

API Version Context Variables

Cadwyn automatically converts your data to a correct version and has "version checks" when dealing with side effects as described in the section above. It can only do so using a special context variable that stores the current API version.

You can also pass a different compatible contextvar to your cadwyn.VersionBundle constructor.