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OpenAPI Specification


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Confession time: this tutorial is about a lot more than just API Platform. The world of APIs has undergone massive changes over the past few years, introducing new hypermedia formats, standards, specifications, performance tools and more! API Platform lives right in the middle of these: bringing bleeding-edge best practices right into your app. If you truly want to master API Platform, you need to understand modern API development.

I told you earlier that what we're looking at is called Swagger. Swagger is basically an API documentation interface - a sort of, interactive README. Google for Swagger and open their site. Under tools, the one we're using is called Swagger UI.


Swagger UI allows anyone to visualize and interact with your API's resources without having any of the implementation in place.

Literally, you could first describe your API - what endpoints it will have, what it will return, what fields to expect - and then use Swagger UI to visualize your future API, before writing even one line of code for it.

Let me show you what I mean: they have a live demo that looks very similar to our API docs. See that swagger.json URL on top? Copy that, open a new tab, and paste. Woh! It's a huge JSON file that describes the API! This is how Swagger UI works: it reads this JSON file and builds a visual, interactive interface for it. Heck, this API might not even exist! As long as you have this JSON description file, you can use Swagger UI.

The JSON file contains all your paths, a description of what each does, the parameters of the input, what output to expect, details related to security... it basically tries to completely describe your API.

So if you have one of these JSON configuration files, you can plug it into Swagger UI and... boom! You get a rich, descriptive interface.

Hello OpenAPI

The format of this file is called OpenAPI. So, Swagger UI is the interface and it understands this sort of, official spec format for describing APIs called OpenAPI. To make things a bit more confusing, the OpenAPI spec used to be called Swagger. Starting with OpenAPI 3.0, it's called OpenAPI and Swagger is just the interface.


Anyways, this is all really cool... but creating an API is already enough work, without needing to try to build and maintain this gigantic JSON document on the side. Which is why API Platform does it for you.

Remember: API Platform's philosophy is this: create some resources, tweak any configuration you need - we haven't done that, but will soon - and let API Platform expose those resources as an API. It does that, but to be an extra good friend, it also creates an OpenAPI specification. Check it out: go to /api/docs.json.

Hello giant OpenAPI spec document! Notice it says swagger: "2.0". OpenAPI version 3 is still pretty new, so API Platform 2 still uses the old format. Add ?spec_version=3 to the URL to see... yep! This is that same document in the latest format version.

Now, go back to our API doc homepage and view the HTML source. Ha! The OpenAPI JSON data is already being included on this page via a little swagger-data script tag! That is how this page is working!

To generate Swagger UI from OpenAPI version 3, you can add the same ?spec_version=3 to the URL. Yep, you can see the OAS3 tag. That doesn't change a lot on the frontend, but there are a few new pieces of information that Swagger can now use thanks to the new spec version.

What else Can OpenAPI Do? Code Generation!

But... other than the fact that it gives us this nice Swagger UI, why should we care that there's some giant OpenAPI JSON spec being created behind the scenes? Back on the Swagger site, one of the other tools is called Swagger CodeGen: a tool for creating an SDK for your API in almost any language! Think about it: if your API is fully-documented in a machine-understandable language, shouldn't we be able to generate a JavaScript or PHP library that's customized for talking with your API? You totally can!

The last thing I want to point out is that, in addition to the endpoints, or "paths", the OpenAPI specification also has information about "models". In the JSON spec, scroll all the way to the bottom: it describes our CheeseListing model and the fields to expect when sending and receiving this model. You can see this same info in Swagger.

And woh! It somehow already knows that the id is an integer and that it's readonly. It also knows price is an integer and createdAt is a string in a datetime format. That's awesome! API Platform reads that information directly from our code, which means that our API docs stay up-to-date without us needing to think about it. We'll learn more about how that works along the way.

But before we get there, we need to talk about one other super important thing that we're already seeing: the JSON-LD and Hydra format that's being returned by our API responses.