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Hydra: Describing API Classes, Operations & More


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We're looking at the JSON-LD documentation that describes our API. Right now, we know that we only have one API resource: DragonTreasure. But if you look down at the supportedClasses section, there are actually a bunch of supported classes. There's one called Entrypoint, another called ConstraintViolation, and another called ConstraintViolationList. Those last two will come up later when we talk about validation errors.

Entrypoint: Your API Homepage

But this Entrypoint is really interesting. It's called "The API entrypoint", and it's actually describing what the homepage for our API looks like. We don't always think about our APIs having a homepage, but they can and they should.

And, welcome to our API homepage - HTML style! If you scroll down to the bottom, you can see other formats. Click "JSON-LD" and... say "hello" to the API homepage in JSON-LD format! This returns an API resource called Entrypoint, whose whole job is to tell us where we can find info about the other API resources. It's like links on a homepage! You can discover the API by going to this Entrypoint and following the @context link... which points to this.

Hello Hydra

Anyways, the purpose of JSON-LD is to add those three extra fields to your API resources: @id, @type, and @context. Then we can leverage @context to point to other documentation to get more metadata or more context. For example, at the top of the JSON-LD documentation, it points to several other documents that add more meaning to JSON-LD.

And, there's one really important one here called hydra. Hydra is, in short an extension to JSON-LD: it describes even more fields that you can add to JSON-LD and what they mean.

Think about it: if we want to totally describe our API, we need to be able to communicate things like what classes we have, their properties, whether each is readable or writeable, and what operations each class supports. That communication is done down here... and it's actually part of Hydra. Yup, if you use JSON-LD by itself... it doesn't have a predefined way to advertise what your models look like. But then Hydra says:

What if we allow the API classes to be described with a key called hydra:supportedClasses?

Here's the big picture: API Platform allows us to fetch JSON-LD API documentation that contains extra hydra fields. The end result is a system that fully describes our API. They describe the models we have, the operations... everything.

Why Hydra and OpenAPI?

And yes, if this sounds very similar to the point of OpenAPI, you're absolutely correct. Both of them do the same thing: describe our API. In fact, if you go to /api/docs.json, this is the OpenAPI description of our API. If we replace the .json with .jsonld, this is the JSON-LD Hydra description of the same API. Why do we have both? Hydra is a bit more powerful: there are certain things it can describe that OpenAPI can't. But OpenAPI is a lot more common and has more tools built on top of it. API platform provides both... in case you need them!

Next: Let's add some serious debugging tools to our API Platform setup.