JSON Hypertext Application Language
draft-kelly-json-hal-01

Abstract

This document proposes a media type for representing resources with hyperlinks.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 19, 2013.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Requirements
• 3. HAL Documents
• 4. Resource Objects
• 4.1. Links
• 4.2. Embedded Resources
• 5. Link Objects
• 5.1. href
• 5.2. templated
• 5.3. title
• 5.4. hreflang
• 6. Example Document
• 7. Media Type Parameters
• 7.1. profile
• 8. Recommendations
• 8.1. Link relations
• 9. Security Considerations
• 10. IANA Considerations
• 11. References
• Appendix A. Acknowledgements
• Appendix B. Frequently Asked Questions
• Appendix B.1. How should a client know the meaning/structure/semantics/type of a resource?
• Appendix B.2. Where can I find libraries for working with HAL?
• Appendix B.3. Why are the reserved properties prefixed with an underscore?
• Appendix B.4. Are all underscore-prefixed properties reserved?
• Appendix B.5. Why does HAL have no forms?
• Author's Address

1. Introduction

There is an emergence of non-HTML HTTP applications ("Web APIs") which use hyperlinks to direct clients around their resources.

The JSON Hypertext Application Language (HAL) is a standard which establishes conventions for expressing hyperlinks with JSON [RFC4627].

HAL is a generic media type with which Web APIs can be developed and exposed as series of links. Clients of these APIs can select links by their link relation type and traverse them in order to progress through the application.

HAL's conventions result in a uniform interface for serving and consuming hypertext, enabling the creation of general-purpose libraries that can be re-used on any API utilising HAL.

The primary design goals of HAL are generality and simplicity. HAL can be applied to many different domains, and imposes the minimal amount of structure necessary to cover the key hypertext requirements (links and embedded documents).

2. Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. HAL Documents

A HAL Document uses the format described in [RFC4627] and has the media type "application/hal+json".

Its root object MUST be a Resource Object.

For example:

GET /orders/523 HTTP/1.1
Host: example.org
Accept: application/hal+json

HTTP/1.1 200 OK
Content-Type: application/hal+json

{
  "_links": {
    "self": { "href": "/orders/523" },
    "warehouse": { "href": "/warehouse/56" },
    "invoice": { "href": "/invoices/873" }
  },
  "currency": "USD",
  "status": "shipped",
  "total": 10.20
}

Here, we have a HAL document representing an order resource with the URI "/orders/523". It has "warehouse" and "invoice" links, and its own state in the form of "currency", "status", and "total" properties.

4. Resource Objects

A Resource Object represents a resource.

It has two reserved properties "_links" and "_embedded", which represent Links and Embedded Resources respectively.

All other properties MUST be valid JSON, and represent the current state of the resource.

4.1. Links

The reserved "_links" property is REQUIRED.

It is an object whose property names are link relation types (as defined by [RFC5988]) and values are either a Link Object or an array of Link Objects. The subject resource of these links is the Resource Object of which the containing "_links" object is a property.

It MUST have a "self" property whose value is a Link Object who's target SHOULD be the URI of the subject resource.

4.2. Embedded Resources

The reserved "_embedded" property is OPTIONAL

It is an object whose property names are link relation types (as defined by [RFC5988]) and values are either a Resource Object or an array of Resource Objects.

5. Link Objects

A Link Object represents a hyperlink. It has the following properties:

5.1. href

The "href" property is REQUIRED.

Its value is either a URI [RFC3986] or a URI Template [RFC6570].

If the value is a URI Template then the Link Object SHOULD have a "templated" attribute whose value is true.

5.2. templated

The "templated" property is OPTIONAL.

Its value is boolean and SHOULD be true when the Link Object's "href" property is a URI Template.

Its value SHOULD be considered false if it is undefined or any other value than true.

5.3. title

The "title" property is OPTIONAL.

Its value is a string and is intended for labeling the link with a human-readable identifier (as defined by [RFC5988]).

5.4. hreflang

The "hreflang" property is OPTIONAL.

Its value is a string and is intended for indicating the language of the target resource (as defined by [RFC5988]).

6. Example Document

The following is an example document representing a list of orders

GET /orders HTTP/1.1
Host: example.org
Accept: application/hal+json

HTTP/1.1 200 OK
Content-Type: application/hal+json

{
  "_links": {
    "self": { "href": "/orders" },
    "next": { "href": "/orders?page=2" },
    "find": { "href": "/orders{?id}", "templated": true }
  },
  "_embedded": {
    "orders": [{
        "_links": {
          "self": { "href": "/orders/123" },
          "basket": { "href": "/baskets/98712" },
          "customer": { "href": "/customers/7809" }
        },
        "total": 30.00,
        "currency": "USD",
        "status": "shipped",
      },{
        "_links": {
          "self": { "href": "/orders/124" },
          "basket": { "href": "/baskets/97213" },
          "customer": { "href": "/customers/12369" }
        },
        "total": 20.00,
        "currency": "USD",
        "status": "processing"
    }]
  },
  "currentlyProcessing": 14,
  "shippedToday": 20
}

Here, the order list document provides a "next" link directing to the next page, and a "find" link containing a URI Template which can be expanded with an 'id' variable to go directly to a specific order.

It also has two embedded resources, "orders". Each of these has its own links to the associated "basket" and "customer" resources, and properties showing their "total", "currency" and "status".

Additionally, the order list resource has its own properties "currentlyProcessing" and "shippedToday".

7. Media Type Parameters

7.1. profile

The media type identifier application/hal+json MAY also include an additional "profile" parameter (as defined by [I-D.wilde-profile-link])

8. Recommendations

8.1. Link relations

Custom link relation types (Extension Relation Types in [RFC5988]) SHOULD be URIs that when dereferenced in a web browser provide relevant documentation, in the form of an HTML page, about the meaning and/or behaviour of the target Resource. This will improve the disocverability of your API.

The CURIE Syntax [W3C.NOTE-curie-20101216] MAY be used for brevity for these URIs. A CURIE is established within a HAL document via a "curie" link on the root Resource Object. This link contains a URI Template with the token 'rel', and is named via the "name" property of the Link Object.

{
  "_links": {
    "self": { "href": "/orders" },
    "curie": {
      "name": "acme",
      "href": "http://docs.acme.com/relations/{rel}",
      "templated": true
    },
    "acme:widgets": { "href": "/widgets" }
  }
}

The above demonstrates the relation "http://docs.acme.com/relations/widgets" being abbreviated to "acme:widgets" via CURIE syntax.

9. Security Considerations

TBD

10. IANA Considerations

TBD

11. References

Appendix A. Acknowledgements

Thanks to Darrel Miller, Mike Amundsen, and everyone in hal-discuss for their suggestions and feedback.

The author takes all responsibility for errors and omissions.

Appendix B. Frequently Asked Questions

Appendix B.1. How should a client know the meaning/structure/semantics/type of a resource?

There are two main approaches to solving this problem. Both involve exposing additional documentation describing the resource which may be human and/or machine readable (i.e. an HTML page and/or a JSON Schema document). The difference between the two approaches is in where that URI is shared with the client, which is either:

(1)

The URI that was the preceding link relation type.

(2)

A 'profile' link from the resource itself.

Appendix B.2. Where can I find libraries for working with HAL?

A list of libraries is maintained and published at the HAL Home Page

Appendix B.3. Why are the reserved properties prefixed with an underscore?

We elected for a prefix character to minimize risk of collisions with properties that represent the resource's state, and underscore was the character picked.

Another reason for prefixing the reserved properties is to make it visually apparent that the reserved properties are distinct from standard properties belonging to the resource.

Appendix B.4. Are all underscore-prefixed properties reserved?

No, HAL only reserves the names detailed in this specification.

Appendix B.5. Why does HAL have no forms?

Omitting forms from HAL was an intentional design descision that was made to keep it focused on linking for APIs. HAL is therefore a good candidate for use as a base media type on which to build more complex capabilities. An additional media type is planned for the future which will add form-like controls on top of HAL.

Author's Address