How should I handle HATEOAS links and references in JSON?

Question

I'm in the process of designing a REST api and to be as RESTful as it gets. I want to incorporate HATEOAS into the json responses.

Adding URLs to related resources is easy enough, but there was some discussion over the structure to use for those links.

A LOT of articles I found use a structure borrowed from ATOM feeds:

"links": [ 
    {"rel": "self", "href":"http://example.org/entity/1"},
    {"rel": "friends", "href":"http://example.org/entity/1/friends"}, ... 
]

This raised some questions:

• Why use an array as a container? According to a javascript developer I know, access to the links would be easier with the links as properties of an object. For example:

  "self":    { "href":"http://example.org/entity/1" }, /* (facebook uses this) */  
  "friends": { "href":"http://example.org/entity/1/friends", "type": "..."}
  

• Is there a common json structure (beside adapting atom again) to describe references in resource properties? (for example the sender of a message).

  The reference should probably be resolved as a url again, but would it be bad to include the simple id as well? kind of like:

  "sender": { 
      "id": 12345,
      "href": "resource-uri"
  }
  

My way of thought is that while HATEOAS makes it so a client doesn't need a lot of knowledge to use an API, I'm kind of reluctant to remove the possibility to USE that knowledge (like accessing the profile picture by building the link client-side without looking up the user first).

Answer 1

I restarted this topic on the API-Craft google group and got some great responses.

The main advantages of the Array design are:

• multiple links for the same relationship
• multiple relationships for the same link without writing the link aggain
• the ability to order the links

The map of cause has better accessibility.

As far as structure goes there are a lot of possibilities:

• JSON-HAL: http://blog.stateless.co/post/13296666138/json-linking-with-hal or also http://stateless.co/hal_specification.html
• JSON-LD: http://json-ld.org/ (optionally using Hydra vocabulary)
• JSON-Schema: http://json-schema.org/ (theHyper Meta-Schema on the bottom of the page)
• Collection+JSON: http://amundsen.com/media-types/collection/

I guess i will go with HAL as it is the cleanest solution, the rest all look kind of... strange for json.