Why Doesn't Stripe Automatically Upgrade API Versions?

I got an interesting question while talking to a friend last night about API versioning, “Why doesn’t Stripe automatically do API upgrades for its users?” The idea is interesting enough that it’s worth digging into a little more.

Some readers may be aware already that at Stripe we take a somewhat unconventional “safety first” approach to API versioning. A new version is introduced every time a backwards incompatible change is made, and there’s enough of them that they’re named after dates like 2017-02-14. There tends to be on the order of 10s of new versions in a year.

The first time a user account makes a request to the API, their account is automatically locked to the current version of the API. They can override it by passing a Stripe-Version header, but any requests without will implicitly be assigned the version fixed to their account.

A backwards incompatible change is one that could potentially break an existing integration. For example, removing a field from a response, or changing its JSON type. Most day-to-day changes like adding a new API endpoint or a new field to an existing response are considered forwards compatible, and a version isn’t cut for them.

You can see by perusing the API changelog that most changes are fairly minor. Upgrades can be painful and time consuming for users, so we try our hardest to get the design of the API right the first time. In cases that we don’t, the changes that are made are relatively minor. For example:

  • The response on /v1/accounts no longer returns the currencies_supported field.
  • Disputes on charge resources used to be expanded by default, but are now collapsed without an explicit request for expansion.
  • Requests with insufficient permissions now return a 403 status code instead of 401.
  • The name field under bank account responses was renamed to account_holder_name.

The Stripe API has a lot of surface area with dozens of resources and ~130 endpoints. Most people are using only a small subset of that, and any given change isn’t likely to affect them. Even if they are using an affected endpoint, it’s possible, and even likely, that they’re not using any of the fields that changed.

If most upgrades are safe for most users, then it stands to reason that we could potentially upgrade people automatically so that they wouldn’t have to do it themselves. It would also make retiring older API versions possible, which is desirable, but not currently done 1.

In at least some cases automatic upgrades would be possible. We have good information around which users call into which endpoints, and if we noticed that the endpoints changed by newly introduced upgrade and the endpoints called by a user were perfectly exclusive, we could roll them forward onto a new version. From an example above, if /v1/accounts is changing, but a user only creates charges and customers, they could be upgraded.

But in other cases it’s difficult: if disputes under charge resources start getting collapsed by default and we know that a user makes calls on charge endpoints, we can’t measure the safety of the upgrade. It’s possible that it’s still perfectly safe because although they get charge responses they never look at their disputes, but it’s also possible that they do, and we have no way of knowing.

Automatic upgrades are a great idea and they’d be a nice feature, but too many changes fall into this ambiguous area, so we don’t.

Regardless of design, all RESTful APIs will be more or less stuck in the same place because it’s so standard to respond with the entire serialized form of API resources on any request. Even hypermedia, which theoretically allows for greater flexibility through the use of HTTP content negotiation and smarter clients, doesn’t have an answer for the problem.

That’s not to say though that there aren’t other ways. GraphQL is a popular API paradigm that’s seeing some pretty good uptake. One of the things I really appreciate about it is that it requires all fields to be requested explicitly:

  human(id: "1000") {

There is no equivalent to SELECT * FROM ....

Even accounting for some error where users request fields that they don’t really need, this still gives you an ability to profile incoming requests that’s leaps and bounds better than REST, and better flexibility as a result.

1 Given the choice, most people won’t be very proactive about upgrading (and really, why should they be?), so in practice old versions tend to have at least some usage forever.

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