As noted in the "What is an API?" chapter, APIs can come in a variety of formats. XML-RPC and SOAP both use XML. REST APIs can choose whatever they want, with XML and JSON being the most common, with many custom formats of each, many with their own media types.
XML provides two very simple ways to provide relational links:
<link>tag can be used:
<link rel="foo" href="/foo" />
Alternatedly, use XLink to provide links on arbitrary elements; use
xlink:hrefto denote the URI, and
xlink:typeto denote the relation:
<?xml version="1.0" encoding="utf-8"?> <document xmlns="http://example.org/xmlns/2002/document" xmlns:xlink="http://www.w3.org/1999/xlink"> <foo xlink:type="foo" xlink:href="/foo">...</foo> </document>
Note that you need to add the relevant XML namespace to the document in order to use XLink.
JSON does not provide hypermedia linking by default, which left the Apigility project with a quandary: how should links be represented in JSON?
Several emerging standards/projects attempt to answer this question. Among the most popular solutions are:
- Hypermedia Application Language, or HAL. The specification for this comes in both XML and JSON variants. Links and embedded resources fall under reserved keys; otherwise, you return whatever you want in the payload, however you want.
- Collection+JSON. Collection+JSON is a JSON-only media type; all return values have a specific structure, with top-level keys being reserved by the format, and each item in the return value having a specific structure to indicate the canonical link, any link relations, and the data it encapsulates.
- Siren. Siren is JSON only; like Collection+JSON, all return values are in a specific structure, with top-level keys reserved by the format. In addition, you define the "entities" encapsulated in the payload, and any "actions" that may be performed. The aim is to be fully self-documenting.
Ultimately, Apigility chose to use HAL, due to its simplicity. It manages to provide the hypermedia controls in a straight-forward way, and does not complicate the payload with additional details, or nest the important bits - the data - several layers deep. That said, any of the above formats would have posed an excellent choice.
Read the HAL Primer for more details on the HAL format and how it works.