JSON Hypertext Application Language
draft-kelly-json-hal-01
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| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
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| Author | Mike Kelly | ||
| Last updated | 2012-06-18 | ||
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draft-kelly-json-hal-01
Network Working Group M. Kelly
Internet-Draft Stateless
Intended status: Informational June 19, 2012
Expires: December 21, 2012
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 21, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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 . . . . . . . . . . . . . . . . . . . . . . . 6
7. Media Type Parameters . . . . . . . . . . . . . . . . . . . . . 7
7.1. profile . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Link relations . . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
11. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . . 9
B.1. How should a client know the
meaning/structure/semantics/type of a resource? . . . . . . 9
B.2. Where can I find libraries for working with HAL? . . . . . 9
B.3. Why are the reserved properties prefixed with an
underscore? . . . . . . . . . . . . . . . . . . . . . . . . 9
B.4. Are all underscore-prefixed properties reserved? . . . . . 9
B.5. Why does HAL have no forms? . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 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.
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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]).
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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
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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.
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9. Security Considerations
TBD
10. IANA Considerations
TBD
11. Normative References
[I-D.wilde-profile-link]
Wilde, E., "The 'profile' Link Relation Type",
draft-wilde-profile-link-02 (work in progress), June 2012.
[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.
[W3C.NOTE-curie-20101216]
McCarron, S. and M. Birbeck, "CURIE Syntax 1.0", World
Wide Web Consortium NOTE NOTE-curie-20101216,
December 2010,
<http://www.w3.org/TR/2010/NOTE-curie-20101216>.
[1] <http://stateless.co/hal_specification.html>
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.
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Appendix B. Frequently Asked Questions
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.
B.2. Where can I find libraries for working with HAL?
A list of libraries is maintained and published at the HAL Home
Page [1]
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.
B.4. Are all underscore-prefixed properties reserved?
No, HAL only reserves the names detailed in this specification.
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.
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Author's Address
Mike Kelly
Stateless
Email: mike@stateless.co
URI: http://stateless.co/
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