JSON Hypertext Application Lanuage
draft-kelly-json-hal-00

Network Working Group                                           M. Kelly
Internet-Draft                                                 Stateless
Intended status: Informational                              June 7, 2012
Expires: December 9, 2012

                   JSON Hypertext Application Lanuage
                        draft-kelly-json-hal-00

Abstract

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

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 9, 2012.

Copyright Notice

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

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  HAL Documents . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Resource Objects  . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Links . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.2.  Embedded Resources  . . . . . . . . . . . . . . . . . . . . 5
   5.  Link Objects  . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  href  . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.2.  templated . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.3.  title . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.4.  hreflang  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Example Document  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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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:

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   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 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.

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4.2.  Embedded Resources

   The reserved "_embedded" property is OPTIONAL

   It is an object whose 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.

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

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   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".

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7.  Security Considerations

   TBD

8.  IANA Considerations

   TBD

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.

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.

Author's Address

   Mike Kelly
   Stateless

   Email: mike@stateless.co
   URI:   http://stateless.co/

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