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There are a lot of pros and cons to various approaches to API versioning, but that has been covered in depth before: API Versioning Has No "Right" Way.

API evolution is making a comeback these days with GraphQL and gRPC advocates shouting about it. Whatever API paradigm or implementation you subscribe to, evolution is available to you. REST advocates have been recommending API evolution for decades, but in the past I failed to understand how exactly to handle evolution.

Luckily, as always, tooling and standards for HTTP have been improving, and these days API evolution is a lot easier to wrap your head around.

What is API Evolution

API evolution is the concept of striving to maintain the "I" in API, the request/response body, query parameters, general functionality, etc., only breaking them when you absolutely, absolutely, have to. It's the idea that API developers bending over backwards to maintain a contract, no matter how annoying that might be, is often more financially and logistically viable than dumping the workload onto a wide array of clients.

At some point change cannot be prevented, so at that time evolution suggests you provide sensible warnings to clients, letting them know if a feature they're using is going away, and not bothering them otherwise.

Examples

The property name exists, and that needs to be split into first_name and last_name.

Easy enough. However the data is handled internally (splitting on first space or last space or some other falsehood defying assumption) you now have two new properties.

The serializer can change from outputting just their name, to outputting all three properties:

class UserSerializer
  include FastJsonapi::ObjectSerializer

  attributes :name, :first_name, :last_name

  attribute :name do |object|
    "#{object.first_name} #{object.last_name}"
  end
end

When folks POST or PATCH to your API, if they send a name you can convert it, or if they send first_name and last_name it'll get picked up fine on the serializer. Job done.

The property price needs to stop being dollars/pounds/whatever as we're starting to support currencies that don't fit into "unit" and "subunit".

Switching to an integer to place your cents, pence, etc. would be just as much of a Fallacies Programmers Think About Currencies as using float dollars/pounds, etc. To support the widest array of currencies, some folks like to use "micros", a concept explained well here by Sift Science. In this case, the new property could easily be called price_micros.

If somebody grumps about that and you want a more concise name, just call it amount and point folks towards that property instead. A thesaurus is handy.

Why don't we just outright change this value from dollars to micros? Because then we'd start charging $1,000,000 for stuff that should only cost $1, and folks probably wouldn't like that.

Now clients can either send the price property, and it'll convert, or send the new price_micros property. If currency is a property in the resource (or something nearby) then it's easy enough to support price for whatever initial currencies you had (dollar/pound/euro) and throw an error if somebody tries using price for these newer currencies, pointing them instead to the micro property.

Nothing broke for existing use cases, and new functionality was added seamlessly.

We have too many old properties kicking around, we need to get rid of them.

Deprecations can be communicated in a few ways for API's. For those using OpenAPI v3, you can mark it as deprecated: true in the documentation. That's not ideal, of course, as OpenAPI is usually human-readable documentation, sat out of band on a developer portal somewhere. Rarely are OpenAPI schemas shoved into the response like JSON Schema is, so programmatically clients have no real way to access this.

JSON Schema is considering adding a deprecated keyword, and oops I think I'm in charge of making that happen. I'll get back to doing that after this blog post. The idea here would be to pair the schema with a smart SDK (client code) which detects which properties are being used. If the schema marks the foo field as deprecated, and the client code then calls $response->foo, the SDK can raise a deprecation warning. This is achieved by inspecting the schema file at runtime if you offer your schemas in the Link header, or at compile time if you're distributing schema files with the SDK.

A visualization of how property deprecation works for GraphQL.

GraphQL has the advantage when it comes to property deprecation for sure, as their type system demands clients to specify the properties they want. By knowing which clients are requesting a deprecated property, you can either reach out to that client (manually or automatically), or shove some warnings into the response somewhere to let them know they're asking for a thing which is going away. This is the sort of advantage you get when your type system, clients, etc. are all part of the same package, but HTTP in general can achieve this same functionality through standards.

All of that said, removing old properties is usually not all that much of a rush or an issue. Over time new developers will be writing new integrations, looking at your new documentation that tells them to use the new property, and your developer newsletters or changelogs just let them know to move away from it over time.

A carpooling company that has "matches" as a relationship between "drivers" and "passengers", suggesting folks who could ride together, containing properties like "passenger_id" and "driver_id". Now we need to support carpools that can have multiple drivers (i.e. Frank and Sally both take it in turns to drive), so this whole matches concept is garbage.

At a lot of startups, this sort of conceptual change is common. No number of new properties is going to help out here, as the whole "one record = one match = one driver + one passenger" concept was junk. We'd need to make it so folks could accept a carpool based on the group, and any one of those folks could drive on a given day.

Luckily, business names often change fairly regularly in the sort of companies that have fundamental changes like this. There is often a better word that folks have been itching to switch to, and evolution gives you a chance to leverage that change to your benefit.

Deprecating the whole concept of "matches", a new concept of "riders" can be created. This resource would track folks far beyond just being "matched", through the whole lifecycle of the carpool, thanks to a status property containing pending, active, inactive, blocked, etc.

By creating the /riders endpoint, this resource can have a brand new representation. As always, the same database fields can be used internally, the same internal alerting tools are used for letting folks know about matches (v1 app) or new pending riders (v2 app). The API can create and update "matches" through this new "riders" interface. Clients can then use either one, and the code just figures itself out in the background. Over time the refactoring can be done to move the internal logic more towards riders, and your integration tests / contract tests will confirm that things aren't changing on the outside.

@cebe asks: How would the matches endpoint return data where there is more than one driver? If the data does not fit the endpoint anymore, it must be broken or fail for such data?

Ex-coworker and API mastermind Nicolás Hock-Isaza says:

We only exposed the first driver match to the older apps. If the user accepted it, great. The other driver riders would be denied.

If the user rejected the first one, we would show the next one, and the next one, and the next one.

All it takes is a little ingenuity, and API evolution isn't so scary.

We have all these old endpoints hanging around, can we get rid of these slightly more intelligently than just sending some emails?

Yes! API Endpoints can be marked with a Sunset header to signal deprecation (and eventual removal) of an endpoint.

The Sunset header is an in-development HTTP response header that is aiming to standardize how URLs are marked for deprecation. tl;dr it looks a bit like this:

Sunset: Sat, 31 Dec 2018 23:59:59 GMT

The date is a HTTP date, and can be combined with a Link: <http://foo.com/something> rel="sunset" which can be anything that might help a developer know what is going on. Maybe link to your API documentation for the new resource, the OpenAPI/JSON Schema definitions, or even a blog post explaining the change.

Ruby on Rails has rails-sunset, and hopefully other frameworks will start adding this functionality. Open-source API Gateway system Tyk is adding support to an upcoming version.

Clients then add a middleware to their HTTP calls, checking for Sunset headers. We do this with faraday-sunset (Ruby), Hunter Skrasek made guzzle-sunset (PHP), and anyone can write a thing that looks for a header and logs it to whatever logging thing they're using.

We need to change some validation rules, but the clients have rules baked in. How do we let them know change is coming?

Certain validation rules are very clearly breaking. For example, lowering the maximum length of a string property would break clients who are expecting to be able to send longer names. Folks would have to shorten the property on certain devices which would be really weird, especially as the client may well be showing it as valid, only to then surface an error from the API.

Other rules may seem like they're backwards compatible, but can still break clients in all sorts of ways. For example, making a string property accept a longer value can lead to problems where an out-of-date client is expecting a length of 20, but an up-to-date client has already been used to get that property up to 40. Again they user would find that data is valid on one device, but be stuck unable to submit the form on another device.

Baking validation rules into client applications based on whatever the documentation says is brittle, so moving client-side validation logic to server-defined JSON Schema can solve these problems, and a bunch more. It also makes evolution a whole bunch easier, because this is just another category of change you are automatically communicating to client applications, without any developers needing to get involved.

Deprecating a specific type of authentication from an endpoint, it's time to say goodbye to HTTP Basic Auth.

If the client is making a request with an authorization header, they have some sort of account. If during the signup for that account you've asked them for an email, you can contact them. If you've not got any way to contact them… tough. Monitor how many folks are using HTTP basic, blog about it, shove some videos up, and eventually you're just going to have to turn it off.

The only other approach to helping out here is an SDK. If you slide some deprecation notices into the code months ahead of the cutoff date, you can throw some warnings saying the code is no longer going to work. This gives you a fighting chance for anyone that keeps a bit up to date. For those that don't, you don't have much choice.

Shoving a clear error into your HTTP response (here using the amazing RFC 7807: Problems for HTTP APIs):

HTTP/1.1 403 Forbidden
Content-Type: application/problem+json

{
  "type": "https://example.org/docs/errors#http-basic-removed",
  "title": "Basic authentication is no longer supported",
  "detail": "HTTP Basic has been deprecated since January, 1st 2018, and was removed May, 1st 2018. Applications should switch to OAuth to resume service."
}

Google Maps are using this approach to remove keyless interactions from Google Maps API. It can be handy in other situations too, like if you're dropping application/xml from your API and want people to know it won't be there forever.

More Power

The above solutions are a little ad-hoc, and can lead to branched code paths with a bunch of if statements. You can feature flag some of this stuff to help keep things a little tidy, and another approach is to write up change as libraries, something Stripe refer to as "declarative changes". This approach can be a little heavy handed, but it's something to keep in mind.

Summary

Evolution involves thinking a little differently on how you approach change. Often there are simple things you can do to keep clients ticking along, and whilst clients will have to change at some point, the whole goal here is to allow them a decent amount of time to make that switch, with the minimal change possible during that change, and no lock-step deploys required.

And yes, whilst making a new endpoint to switch /matches and /riders is essentially the same as /v1/matches and /v2/matches, you've skipped the quagmire of tradeoffs between global versioning, resource versioning, or gulp method versioning. Global versioning has its place, but so does evolution.

Think about it this way. If implementing some change takes twice as long for API developers compared to other versioning approaches, but save 6 or 7 client developer teams from having to do a whole bunch of work, testing, etc. to chase new versions, this has been worthwhile to the company in terms of engineering hours spent.

If you've only got a small number of clients (maybe an iOS and Android version) of an API that changes drastically every year or two, then global versioning is clearly the way to go.


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Phil Sturgeon

Platform Engineer @ WeWork who talks about APIs a lot. Programming Polyglot, Pragmatist, Centerist and Sarcasist. Ex-The League of Extraordinary Packages, PHP The Right Way, Ex-PHP-FIG, Ex-CodeIgniter, Ex-FuelPHP, Ex-PyroCMS.

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Phil Sturgeon

Platform Engineer @ WeWork who talks about APIs a lot. Programming Polyglot, Pragmatist, Centerist and Sarcasist. Ex-The League of Extraordinary Packages, PHP The Right Way, Ex-PHP-FIG, Ex-CodeIgniter, Ex-FuelPHP, Ex-PyroCMS.

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