Frequently Asked Questions

What is the meaning of JSON API’s version?

Now that JSON API has reached a stable version 1.0, it will always be backwards compatible using a never remove, only add strategy.

A version is maintained in order to:

  • allow tracking of additive changes to the specification.
  • know what features a particular implementation may potentially support.

Why not use the HAL specification?

There are several reasons:

  • HAL embeds child documents recursively, while JSON API flattens the entire graph of objects at the top level. This means that if the same “people” are referenced from different kinds of objects (say, the author of both posts and comments), this format ensures that there is only a single representation of each person document in the payload.
  • Similarly, JSON API uses IDs for linkage, which makes it possible to cache documents from compound responses and then limit subsequent requests to only the documents that aren’t already present locally. If you’re lucky, this can even completely eliminate HTTP requests.
  • HAL is a serialization format, but says nothing about how to update documents. JSON API thinks through how to update existing records (leaning on PATCH and JSON Patch), and how those updates interact with compound documents returned from GET requests. It also describes how to create and delete documents, and what 200 and 204 responses from those updates mean.

In short, JSON API is an attempt to formalize similar ad hoc client-server interfaces that use JSON as an interchange format. It is specifically focused around using those APIs with a smart client that knows how to cache documents it has already seen and avoid asking for them again.

It is extracted from a real-world library already used by a number of projects, which has informed both the request/response aspects (absent from HAL) and the interchange format itself.

How can I discover a resource’s supported actions?

Whenever a client requests a URI, the server can include an Allow header in its response to indicate the methods the requested resource supports. Servers wishing to support method discovery should include this header. The client can then send a HEAD request (or any other type of request) to the URI in order to discover its supported methods.

For instance, a client might request HEAD /articles, and the response could contain the header Allow: GET,POST, indicating that the client can GET the collection and also POST to it to create new resources.

JSON API is still working on a way to for resources to advertise and detail non-standard actions they support. Feel free to join that discussion!

Where’s PUT?

Using PUT to partially update a resource (i.e. to change only some of its state) is not allowed by the HTTP specification. Instead, PUT is supposed to completely replace the state of a resource:

“The PUT method requests that the state of the target resource be created or replaced with the state…in the request message payload. A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being sent…”

The correct method for partial updates, therefore, is PATCH, which is what JSON API uses. And because PATCH can also be used compliantly for full resource replacement, JSON API hasn’t needed to define any behavior for PUT so far. However, it may define PUT semantics in the future.

In the past, many APIs used PUT for partial updates because PATCH wasn’t yet well-supported. However, almost all clients now support PATCH, and those that don’t can be easily worked around.

Is there a JSON Schema describing JSON API?

Yes, you can find the JSON Schema definition at http://jsonapi.org/schema. This schema is as restrictive as possible, but has flexibility to be extended within your documentation. Validation will not yield false negatives, but could yield false positives for the sake of flexibility.

You can find more information about the JSON Schema format at http://json-schema.org.

Why are resource collections returned as arrays instead of sets keyed by ID?

A JSON array is naturally ordered while sets require metadata to specify order among members. Therefore, arrays allow for more natural sorting by default or specified criteria.

Why are related resources nested in an included object in a compound document?

Primary resources should be isolated because their order and number is often significant. It’s necessary to separate primary and related resources by more than type because it’s possible that a primary resource may have related resources of the same type (e.g. the “parents” of a “person”). Nesting related resources in included prevents this possible conflict.

Does JSON API take any position on URI structure, on rules for custom endpoints, which do not fit the paradigm of GET/POST/PATCH/DELETE on the resource URI, etc.?

JSON API has no requirements about URI structure, implementations are free to use whatever form they wish.