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Hydra: Describing API Classes, Operations & More


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So, at least on a high level, we understand that each resource will have an @type key and that this page - via the supportedClass and supportedProperty keys - defines what that type means - what properties it has, and a lot of info about each property.

Right now, we only have one API resource, so we only have one entry under supportedClass, right? Surprise! There's another one called Entrypoint! And another one called ConstraintViolation, which defines the resource that's returned when our API has a validation error.

The Entrypoint Resource

Let's talk about this Entrypoint class: it's a pretty beautiful idea. We already know that when we go to /api, we get, sort of, the HTML version of an API "homepage". Whelp, there is also a JSON-LD version of this page! There's a link to see it at the bottom of this page - but let's get to it a different way.

Find your terminal: we can use curl to see what the "homepage" looks like for the JSON-LD format:

curl -X GET 'https://localhost:8000/api' -H "accept: application/ld+json"

In other words: please make a GET request to /api, but advertise that you would like the JSON-LD format back. I'll also pipe that to jq - a utility that makes JSON look pretty - just skip that if you don't have it installed. And... boom!

Say hello to your API's homepage! Because every URL represents a unique resource, even this page is a resource: it's... an "Entrypoint" resource. It has the same @context, @id and @type, with one real "property" called cheeseListing. That property is the IRI of the cheese listing collection resource.

Heck, this is described in our JSON-LD document! The Entrypoint class has one property: cheeseListing with the type hydra:Link - that's interesting. And, it's pretty ugly, but the rdfs:range part is apparently a way to describe that the resource this property refers to is a "collection" that will have a hydra:member property, which will be an array where each item is a CheeseListing type. Woh!

Hello Hydra

So JSON-LD is all about adding more context to your data by specifying that our resources will contain special keys like @context, @id and @type. It's still normal JSON, but if a client understands JSON-LD, it's going to be able to get a lot more information about your API, automatically.

But in API Platform, there is one other thing that you're going to see all the time, and we're already seeing it! Hydra, which is more than just a many-headed water monster from Greek mythology.

Go back to /api/docs.jsonld. In the same way that this points to the xmls external document so that we can reference things like xmls:integer, we're also pointing to an external document called hydra that defines more "types" or "vocab" we can use.

Here's the idea: JSON-LD gave us the system for saying that this piece of data is this type and this piece of data is this other type. Hydra is an extension of JSON-LD that adds new vocab. It says:

Hold on a second. JSON-LD is great and fun and an excellent dinner party guest. But to really allow a client and a server to communicate, we need more shared language! We need a way to define "classes" within my API, the properties of those classes and whether or not each is readable and writeable. Oh, and we also need to be able to communicate the operations that a resource supports: can I make a DELETE request to this resource to remove it? Can I make a PUT request to update it? What data format should I expect back from each operation? And what is the true identity of Batman?

Hydra took the JSON-LD system and added new "terminology" - called "vocab" - that makes it possible to fully define every aspect of your API.

Hydra Versus OpenAPI

At this point, you're almost definitely thinking:

But wait, this seriously sounds like the exact same thing that we got from our OpenAPI JSON doc.

And, um... yea! Change the URL to /api/docs.json. This is the OpenAPI specification. And if we change that to .jsonld, suddenly we have the JSON-LD specification with Hydra.

So why do we have both? First, yes, these two documents basically do the same thing: they describe your API in a machine-readable format. The JSON-LD and Hydra format is a bit more powerful than OpenAPI: it's able to describe a few things that OpenAPI can't. But OpenAPI is more common and has more tools built around it. So, in some cases, having an OpenAPI specification will be useful - like to use Swagger - and other times, the JSON-LD Hydra document will be useful. With API Platform, you get both.

Phew! Ok, enough theory! Let's get back to building and customizing our API.