API Versioning at Monite

We all love to have shiny new tools but hate the chore of constantly updating them. This applies to anything: operating systems, apps, APIs, linux packages. It is painful when our code stops working because of an update and it is double the pain when the update was not even initiated by us.

In web API development, you are constantly at risk of breaking your users' code with every new update. If your product is an API, then these updates will be terrifying every time. Monite's main products are our API and our white-label SDK. We are an API-first company so we take great care of keeping our API stable and easy to use. Hence the problem of breaking changes is near the top of our priority list.

A common solution is to issue deprecation warnings to your clients and to release breaking changes rarely. Suddenly, your releases can now take months and some features have to stay hidden or even unmerged until each next release. This slows down your development and forces your users to update their integration every few months.

If you make releases faster, your users are going to have to update their integration too often. If you lengthen the time between releases, you will move slower as a company. The more inconvenient you make it for users -- the more convenient it is going to be for you, and vice versa. This is certainly not an optimal scenario. We wanted to move at our own pace without breaking anything for existing clients which would be impossible with a regular deprecation approach. This is why we picked an alternative solution: API versioning.

It is quite a simple idea: release any breaking changes at any time but hide them under a new API version. It grants you the best of both worlds: The users will not have their integrations routinely broken and you will be able to move at any speed you like. The users will migrate whenever they want -- free of any pressure.

Considering the simplicity of the idea, it feels perfect for any company. That is what you would expect to read in a typical engineering blog. Sadly, it is not so simple.

Beware of the price

API versioning is hard, very hard. Its illusory simplicity quickly fades away once you begin implementing it. Sadly, the internet never really warns you as there are surprisingly few resources on the topic. The absolute majority of them argue about where to put the API version but only a few scarce articles attempt to answer: "How do we implement it?". The most common ones are:

• putting different versions of the same web application into separate deployments
• copying single routes that have changed between versions
• copying the entire versioned application for each version

Separate deployments can get really expensive and hard to support, copying single routes does not scale very well to large changes, and copying the entire application creates so much extra code that you will start drowning in it after just a few versions.

Even if you try to pick the cheapest one, the burden of versioning will catch up soon. At first, it will feel simple: add another schema here, another branch in business logic there, and duplicate a few routes at the end. But given enough versions your business logic will quickly become unmanageable, many of your developers will mistake application versions and API versions, and will start versioning the data within your database, and your application will become impossible to maintain.

You might hope that you will never have more than two or three API versions at the same time; that you will be able to delete old versions every few months. It is true if you only support a small number of internal consumers. But clients outside of your organization will not enjoy the experience of being forced to upgrade every few months.

API versioning can quickly become one of the most expensive parts of your infrastructure so it is critical to do diligent research beforehand. If you only support internal consumers, then you might have a simpler time with something like GraphQL but it can quickly get just as expensive as versioning.

If you are a startup, it would be wise to postpone API versioning until the later stages of your development when you have resources to do it right. Until then, deprecations and additive change strategy might suffice. Your API will not always look great but at least you will save a great deal of money by avoiding explicit versioning.

How do we implement API versioning?

After a few trials and many errors we were at the crossroads: our prior versioning approaches that we mentioned above were too expensive to maintain. As a result of our struggles, I devised the following list of requirements that would be required of a perfect versioning framework:

1. "Maintaining a large number of versions is easy" to make sure that versioning does not slow down our feature development
2. "Deleting old versions is easy" to make sure that we can clean up our code base without efforts
3. "Creating new versions is not too easy" to make sure that our developers are still incentivized to try to solve problems without versions.
4. "Maintaining a changelog between versions is easy" to make sure that both we and our clients can always be sure about the real differences between versions

Sadly, there were little to no alternatives to our existing approaches. This is when a crazy idea came to my mind: what if we try and build something sophisticated, something perfect for the job -- something like Stripe's API versioning?

As a result of countless experiments, we now have Cadwyn: an open-source API versioning framework that not only implements Stripe's approach but significantly builds on top of it. We will be talking about its Fastapi and Pydantic implementation but the core principles are language and framework agnostic.

How Cadwyn works

Version Changes

The problem of all other versioning approaches is that we are duplicating too much. Why would we duplicate the entire route, controller, or even application when only a tiny part of our contract got broken?

With Cadwyn, whenever API maintainers need to create a new version, they apply the breaking changes to their latest schemas, models, and business logic. Then they create a version change -- a class that encapsulates all differences between the new version and a prior version.

For example, let's say that previously our clients could create a user with an address but now we would like to allow them to specify multiple addresses instead of a single one. The version change would look like this:

class ChangeUserAddressToAList(VersionChange):
    description = (
        "Renamed `User.address` to `User.addresses` and "
        "changed its type to an array of strings"
    )
    instructions_to_migrate_to_previous_version = (
        schema(User).field("addresses").didnt_exist,
        schema(User).field("address").existed_as(type=str),
    )

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

    @convert_response_to_previous_version_for(UserResource)
    def change_addresses_to_single_item(response):
        response.body["address"] = response.body.pop("addresses")[0]

instructions_to_migrate_to_previous_version are used by Cadwyn to generate code for older API versions of schemas and the two converter functions are the trick that allows us to maintain as many versions as we would like. The process looks like the following:

1. Cadwyn converts all user requests from older API versions to the latest API version by using change_address_to_multiple_items converter, and pipes them to our business logic
2. Business logic, its API responses, and database models are always tailored to the latest API version (of course, it must still support old features even if they were dropped in new versions)
3. After business logic produces the response, Cadwyn converts it to the older API version that the client requester is currently on by using change_addresses_to_single_item converter.

After our API maintainers have created the version change, they need to add it to our VersionBundle to tell Cadwyn that this VersionChange will be included in some version:

VersionBundle(
    Version(
        date(2023, 4, 27),
        ChangeUserAddressToAList
    ),
    Version(
        date(2023, 4, 12),
        CollapseUserAvatarInfoIntoAnID,
        MakeUserSurnameRequired,
    ),
    Version(date(2023, 3, 15)),
)

That's it: we have added a breaking change but our business logic only handles a single version -- latest. Even after we add dozens of API versions, our business logic will still be free from versioning logic, constant renaming, if's and data converters.

Version Chaining

Version changes are dependent on the public interface of the API and we almost never add breaking changes into existing API versions. This means that once we have released the version – it will not be broken.

Because version changes describe breaking changes within versions and there are no breaking changes within old versions, we can be sure that our version changes are completely immutable – they will never have a reason to change. Immutable entities are much easier to maintain than if they were a part of business logic because it is always evolving. Version changes are also applied one after another -- forming a chain of transformers between versions that can migrate any request to any newer version and any response to any older version.

Side effects

API contracts are much more complex than just schemas and fields. They consist of all endpoints, status codes, errors, error messages, and even business logic behaviors. Cadwyn uses the same DSL we described above to handle endpoints and status codes but errors and business logic behaviors are a different story: they are impossible to describe using a DSL, they need to be embedded into business logic.

This makes such version changes much more expensive to maintain than all others because they affect business logic. We call this property a "side effect" and we try to avoid them at all costs because of their maintenance burden. All version changes that want to modify business logic will need to be marked as having side effects. It will serve as a way to know which version changes are "dangerous":

class RequireCompanyAttachedForPayment(VersionChangeWithSideEffects):
    description = (
        "User must now have a company_id in their account "
        "if they want to make new payments"
    )

It will also allow API maintainers to check that the client request uses an API version that includes this side effect:

if RequireCompanyToBeAttachedForPayment.is_applied:
    validate_company_id_is_attached(user)

No silver bullets

Cadwyn has many benefits: It greatly reduces the burden on our developers and can be integrated into our infrastructure to automatically generate the changelog and improve our API docs.

However, the burden of versioning still exists and even a sophisticated framework is not a silver bullet. We do our best to only use API versioning when absolutely necessary. We also try to make our API correct on the first try by having a special "API Council". All significant API changes are reviewed there by our best developers, testers, and tech writers before any implementation gets moving.

Special thanks to Brandur Leach for his API versioning article at Stripe and for the help he extended to me when I implemented Cadwyn: it would not be possible without his help.

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