cadwyn 1.0.2

Modern Stripe-like API versioning in FastAPI

Cadwyn

Modern Stripe-like API versioning in FastAPI

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Installation

pip install cadwyn

Who is this for?

Cadwyn allows you to support a single version of your code, auto-generating the code/routes for older versions. You keep versioning encapsulated in small and independent "version change" modules while your business logic knows nothing about versioning.

Its approach will be useful if you want to:

1. Support many API versions for a long time
2. Effortlessly backport features and bugfixes to all of your versions

Otherwise, more conventional methods of API versioning may be preferable.

Tutorial

This guide provides a step-by-step tutorial for setting up automatic API versioning using Cadwyn library. I will illustrate this with an example of a User API, where we will be implementing changes to a User's address.

A dummy setup

Here is an initial API setup where the User has a single address. We will be implementing two routes - one for creating a user and another for retrieving user details. We'll be using "int" for ID for simplicity.

The first API you come up with usually doesn't require more than one address -- why bother?

So we create our file with schemas:

from pydantic import BaseModel


class UserCreateRequest(BaseModel):
    address: str


class UserResource(BaseModel):
    id: int
    address: str

And we create our file with routes:

from versions.latest.users import UserCreateRequest, UserResource
from cadwyn import VersionedAPIRouter

router = VersionedAPIRouter()


@router.post("/users", response_model=UserResource)
async def create_user(payload: UserCreateRequest):
    return {
        "id": 83,
        "address": payload.address,
    }


@router.get("/users/{user_id}", response_model=UserResource)
async def get_user(user_id: int):
    return {
        "id": user_id,
        "address": "123 Example St",
    }

Turning address into a list

During our development, we have realized that the initial API design was wrong and that addresses should have always been a list because the user wants to have multiple addresses to choose from so now we have to change the type of the "address" field to the list of strings.

from pydantic import BaseModel, Field


class UserCreateRequest(BaseModel):
    addresses: list[str] = Field(min_items=1)


class UserResource(BaseModel):
    id: int
    addresses: list[str]

@router.post("/users", response_model=UserResource)
async def create_user(payload: UserCreateRequest):
    return {
        "id": 83,
        "addresses": payload.addresses,
    }


@router.get("/users/{user_id}", response_model=UserResource)
async def get_user(user_id: int):
    return {
        "id": user_id,
        "addresses": ["123 Example St", "456 Main St"],
    }

But every user of ours will now have their API integration broken. To prevent that, we have to introduce API versioning. There aren't many methods of doing that. Most of them force you to either duplicate your schemas, your endpoints, or your entire app instance. And it makes sense, really: duplication is the only way to make sure that you will not break old versions with your new versions; the bigger the piece you duplicating -- the safer. Of course, the safest being duplicating the entire app instance and even having a separate database. But that is expensive and makes it either impossible to make breaking changes often or to support many versions. As a result, either you need infinite resources, very long development cycles, or your users will need to often migrate from version to version.

Stripe has come up with a solution: let's have one latest app version whose responses get migrated to older versions and let's describe changes between these versions using migrations. This approach allows them to keep versions for years without dropping them. Obviously, each breaking change is still bad and each version still makes our system more complex and expensive, but their approach gives us a chance to minimize that. Additionally, it allows us backport features and bugfixes to older versions. However, you will also be backporting bugs, which is a sad consequence of eliminating duplication.

Cadwyn is heavily inspired by this approach so let's continue our tutorial and now try to combine the two versions we created using versioning.

Creating the Migration

We need to create a migration to handle changes between these versions. For every endpoint whose response_model is UserResource, this migration will convert the list of addresses back to a single address when migrating to the previous version. Yes, migrating back: you might be used to database migrations where we write upgrade migration and downgrade migration but here our goal is to have an app of latest version and to describe what older versions looked like in comparison to it. That way the old versions are frozen in migrations and you can almost safely forget about them.

from pydantic import Field
from cadwyn.structure import (
    schema,
    VersionChange,
    convert_response_to_previous_version_for,
    RequestInfo,
    ResponseInfo,
)


class ChangeAddressToList(VersionChange):
    description = (
        "Change user address to a list of strings to "
        "allow the user to specify multiple addresses"
    )
    instructions_to_migrate_to_previous_version = [
        # You should use schema inheritance if you don't want to repeat yourself in such cases
        schema(UserCreateRequest).field("addresses").didnt_exist,
        schema(UserCreateRequest).field("address").existed_as(type=str, info=Field()),
        schema(UserResource).field("addresses").didnt_exist,
        schema(UserResource).field("address").existed_as(type=str, info=Field()),
    ]

    @convert_request_to_next_version_for(UserCreateRequest)
    def change_address_to_multiple_items(request: RequestInfo):
        request.body["addresses"] = [request.body.pop("address")]

    @convert_response_to_previous_version_for(UserResource)
    def change_addresses_to_single_item(response: ResponseInfo) -> None:
        response.body["address"] = response.body.pop("addresses")[0]

See how we are popping the first address from the list? This is only guaranteed to be possible because we specified earlier that min_items for addresses must be 1. If we didn't, then the user would be able to create a user in a newer version that would be impossible to represent in the older version. I.e. If anyone tried to get that user from the older version, they would get a ResponseValidationError because the user wouldn't have data for a mandatory address field. You need to always keep in mind tht API versioning is only for versioning your API, your interface. Your versions must still be completely compatible in terms of data. If they are not, then you are versioning your data and you should really go with a separate app instance. Otherwise, your users will have a hard time migrating back and forth between API versions and so many unexpected errors.

See how we added a migration not only for response but also for request? This will allow our business logic to stay completely the same, no matter which version it was called from. Cadwyn will always give your business logic the request model from the latest version or from a custom schema if you want to.

Grouping Version Changes

Finally, we group the version changes in the VersionBundle class. This represents the different versions of your API and the changes between them. You can add any "version changes" to any version. For simplicity, let's use versions 2002 and 2001 which means that we had a single address in API in 2001 and added addresses as a list in 2002's version.

from cadwyn.structure import Version, VersionBundle
from datetime import date
from contextvars import ContextVar

api_version_var = ContextVar("api_version_var")

versions = VersionBundle(
    Version(date(2002, 1, 1), ChangeAddressToList),
    Version(date(2001, 1, 1)),
    api_version_var=api_version_var,
)

That's it. You're done with describing things. Now you just gotta ask cadwyn to do the rest for you. We'll need the VersionedAPIRouter we used previously, our API versions, and the module representing the latest versions of our schemas.

from versions import latest, api_version_var
from cadwyn import generate_code_for_versioned_packages, generate_versioned_routers

generate_code_for_versioned_packages(latest, versions)
router_versions = generate_versioned_routers(
    router,
    versions=versions,
    latest_schemas_module=latest,
)
api_version_var.set(date(2002, 1, 1))
uvicorn.run(router_versions[date(2002, 1, 1)])

Cadwyn has generated multiple things in this code:

• Two versions of our schemas: one for each API version
• Two versions of our API router: one for each API version

You can now just pick a router by its version and run it separately or use a parent router/app to specify the logic by which you'd like to pick a version. I recommend using a header-based router with version dates as headers. And yes, that's how Stripe does it.

Note that cadwyn migrates your response data based on the api_version_var context variable so you must set it with each request. cadwyn.get_cadwyn_dependency does that for you automatically on every request based on header value.

Obviously, this was just a simple example and cadwyn has a lot more features so if you're interested -- take a look at the reference.

Examples

Please, see tutorial examples for the fully working version of the project above.

Important warnings

1. The goal of Cadwyn is to minimize the impact of versioning on your business logic. It provides all necessary tools to prevent you from ever checking for a concrete version in your code. So please, if you are tempted to check something like api_version_var.get() >= date(2022, 11, 11) -- please, take another look into reference section. I am confident that you will find a better solution there.
2. I ask you to be very detailed in your descriptions for version changes. Spending these 5 extra minutes will potentially save you tens of hours in the future when everybody forgets when, how, and why the version change was made.
3. Cadwyn doesn't edit your imports when generating schemas so if you make any imports from versioned code to versioned code, I would suggest using relative imports to make sure that they will still work as expected after code generation.

Reference

CLI

Cadwyn has an optional CLI interface that can be installed with pip install cadwyn[cli].

Code generation

You can essentially run generate_code_for_versioned_packages using this CLI instead of creating a script file.

• cadwyn generate-code-for-versioned-packages path.to.latest.package path.to.version.bundle:my_version_bundle
• cadwyn generate-code-for-versioned-packages path.to.latest.package path.to.version.bundle:func_that_returns_version_bundle

Version checks

Run cadwyn --version to check current version of Cadwyn

Endpoints

Note that the endpoint constructor contains a second argument that describes the methods of the endpoints you would like to edit. If you have two routes for a single endpoint and you put both of their methods into the instruction -- both of them are going to be changed as you would expect.

Defining endpoints that didn't exist in new versions

If you had an endpoint in old version but do not have it in a new one, you must still define it but mark it as deleted.

@router.only_exists_in_older_versions
@router.get("/my_old_endpoint")
async def my_old_endpoint():
    ...

and then define it as existing in one of the older versions:

from cadwyn.structure import VersionChange, endpoint


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        endpoint("/my_old_endpoint", ["GET"]).existed,
    ]

Defining endpoints that didn't exist in old versions

If you have an endpoint in your new version that must not exist in older versions, you define it as usual and then mark it as "non-existing" in old versions:

from cadwyn.structure import VersionChange, endpoint


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        endpoint("/my_new_endpoint", ["GET"]).didnt_exist,
    ]

Changing endpoint attributes

If you want to change any attribute of your endpoint in a new version, you can return the attribute's value in all older versions like so:

from cadwyn.structure import VersionChange, endpoint


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        endpoint("/my_endpoint", ["GET"]).had(description="My old description"),
    ]

Dealing with endpoint duplicates

Sometimes, when you're doing some advanced changes in between versions, you will need to rewrite your endpoint function entirely. So essentially you'd have the following structure:

from fastapi.params import Param
from fastapi.headers import Header
from typing import Annotated
from cadwyn import VersionedAPIRouter

router = VersionedAPIRouter()


@router.only_exists_in_older_versions
@router.get("/users")
def get_users_by_name_before_we_started_using_params(
    user_name: Annotated[str, Header()]
):
    """Do some logic with user_name"""


@router.get("/users")
def get_users_by_name(user_name: Annotated[str, Param()]):
    """Do some logic with user_name"""

As you see, these two functions have the same methods and paths. And when you have many versions, you can have even more functions like these two. So how do we ask cadwyn to restore only one of them and delete the other one?

from cadwyn.structure import VersionChange, endpoint


class UseParamsInsteadOfHeadersForUserNameFiltering(VersionChange):
    description = (
        "Use params instead of headers for user name filtering in GET /users "
        "because using headers is a bad API practice in such scenarios."
    )
    instructions_to_migrate_to_previous_version = [
        # We don't have to specify the name here because there's only one such deleted endpoint
        endpoint("/users", ["GET"]).existed,
        # We do have to specify the name because we now have two existing endpoints after the instruction above
        endpoint("/users", ["GET"], func_name="get_users_by_name").didnt_exist,
    ]

So by using a more concrete func_name, we are capable to distinguish between different functions that affect the same routes.

Enums

Adding enum members

Note that adding enum members can be a breaking change unlike adding optional fields to a schema. For example, if I return a list of entities, each of which has some type, and I add a new type -- then my client's code is likely to break.

So I suggest adding enum members in new versions as well.

from cadwyn.structure import VersionChange, enum
from enum import auto


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        enum(my_enum).had(foo="baz", bar=auto()),
    ]

Removing enum members

from cadwyn.structure import VersionChange, enum


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        enum(my_enum).didnt_have("foo", "bar"),
    ]

Schemas

Add a field

from pydantic import Field
from cadwyn.structure import VersionChange, schema


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        schema(MySchema)
        .field("foo")
        .existed_as(type=list[str], info=Field(description="Foo")),
    ]

You can also specify any string in place of type:

schema(MySchema).field("foo").existed_as(type="AnythingHere")

It is often the case that you want to add a type that has not been imported in your schemas yet. You can use import_from and optionally import_as to do this:

schema(MySchema).field("foo").existed_as(
    type=MyOtherSchema, import_from="..some_module", import_as="Foo"
)

Which will render as:

from ..some_module import MyOtherSchema as Foo
from pydantic import BaseModel, Field


class MySchema(BaseModel):
    foo: Foo = Field()

Remove a field

from cadwyn.structure import VersionChange, schema


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        schema(MySchema).field("foo").didnt_exist,
    ]

Change a field

from cadwyn.structure import VersionChange, schema


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        schema(MySchema).field("foo").had(description="Foo"),
    ]

Rename a schema

If you wish to rename your schema to make sure that its name is different in openapi.json:

from cadwyn.structure import VersionChange, schema


class MyChange(VersionChange):
    description = "..."
    instructions_to_migrate_to_previous_version = [
        schema(MySchema).had(name="OtherSchema"),
    ]

which will replace all references to this schema with the new name.

Data migration

Response data migration

As described in the tutorial, cadwyn can convert your response data into older versions. It does so by running your "migration" functions whenever it encounters a version change:

from cadwyn.structure import VersionChange, convert_response_to_previous_version_for
from typing import Any


class ChangeAddressToList(VersionChange):
    description = "..."

    @convert_response_to_previous_version_for(MyEndpointResponseModel)
    def change_addresses_to_single_item(response: ResponseInfo) -> None:
        response.body["address"] = response.body.pop("addresses")[0]

It is done by applying a versioning decorator to each endpoint with the given response_model which automatically detects the API version by getting it from the contextvar and applying all version changes until the selected version in reverse. Note that if the version is not set, then no changes will be applied.

Request data migration

from cadwyn.structure import VersionChange, convert_request_to_next_version_for
from typing import Any
from my_schemas.latest import UserCreateRequest


class ChangeAddressToList(VersionChange):
    description = "..."

    @convert_request_to_next_version_for(UserCreateRequest)
    def change_addresses_to_single_item(request: RequestInfo) -> None:
        request.body["addresses"] = [request.body.pop("address")]

Internal request schemas

Let's say our CreateUserRequest had a field email which was str | None in version 1 but became a required str in version 2. How do we migrate our request from version 1 to version 2? The schema from version 2 will simply raise an error if you try to put a None into the email field.

That's because the understanding that you migrate your requests to the latest schema is incomplete. In reality, your goal is to migrate them to some schema that represents the union of all schemas. Latest schema is the best candidate because our business logic is closest to it and API changes are usually additive in nature. But as you see from the aforementioned situation, that's not always the case, which is why sometimes we need another schema: an internal representation of the request which not confined by our API and can have any structure we want. Now let's solve our email problem using an internal schema.

from .versioned_schemas.latest import CreateUserRequest
from cadwyn import internal_representation_of


@internal_representation_of(CreateUserRequest)
class InternalCreateUserRequest(CreateUserRequest):
    email: str | None

Now cadwyn will always use InternalCreateUserRequest when pushing body field into your business logic instead of CreateUserRequest. Note that users will not be able to use any fields from the internal schema and their requests will still be validated by your regular schemas. So even if you added a field foo in an internal schema, and your user has passed this field in the body of the request, this field will not get to the internal schema because it will be removed at the moment of validation (or even an error will occur if you use extra="ignore").

I would, however, advise you put it in an unversioned directory and inherit it from your latest schema to minimize the chance of human errors.

Version changes with side effects

Sometimes you will use API versioning to handle a breaking change in your business logic, not in the schemas themselves. In such cases, it is tempting to add a version check and just follow the new business logic such as:

if api_version_var.get() >= date(2022, 11, 11):
    # do new logic here
    ...

In cadwyn, this approach is highly discouraged. It is recommended that you avoid side effects like this at any cost because each one makes your core logic harder to understand. But if you cannot, then I urge you to at least abstract away versions and versioning from your business logic which will make your code much easier to read.

To simplify this, cadwyn has a special VersionChangeWithSideEffects class. It makes finding dangerous versions that have side effects much easier and provides a nice abstraction for checking whether we are on a version where these side effects have been applied.

As an example, let's use the tutorial section's case with the user and their address. Let's say that we use an external service to check whether user's address is listed in it and return 400 response if it is not. Let's also say that we only added this check in the newest version.

from cadwyn.structure import VersionChangeWithSideEffects


class UserAddressIsCheckedInExternalService(VersionChangeWithSideEffects):
    description = (
        "User's address is now checked for existense in an external service. "
        "If it doesn't exist there, a 400 code is returned."
    )

Then we will have the following check in our business logic:

from src.versions import versions, UserAddressIsCheckedInExternalService


async def create_user(payload):
    if UserAddressIsCheckedInExternalService.is_applied:
        check_user_address_exists_in_an_external_service(payload.address)
    ...

So this change can be contained in any version -- your business logic doesn't know which version it has and shouldn't.

API Version header and 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.

Use cadwyn.get_cadwyn_dependency to get a fastapi.Depends that automatically sets this contextvar based on a header name that you pick.

You can also set the variable yourself or even pass a different compatible contextvar to your cadwyn.VersionBundle constructor.

Similar projects

The following projects are trying to accomplish similar results with a lot more simplistic functionality.

• https://github.com/sjkaliski/pinned
• https://github.com/phillbaker/gates
• https://github.com/lukepolo/laravel-api-migrations
• https://github.com/tomschlick/request-migrations
• https://github.com/keygen-sh/request_migrations