Internet Engineering Task Force (IETF)
Request for Comments: 6573
Category: Informational
ISSN: 2070-1721

M. Amundsen
April 2012

The Item and Collection Link Relations

Abstract

RFC 5988 standardized a means of indicating the relationships between resources on the Web. This specification defines a pair of reciprocal link relation types that may be used to express the relationship between a collection and its members.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6573.

Copyright Notice

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.

1. Introduction

These link relation types can be applied to a wide range of use cases across multiple media types. For example, the 'collection' and 'item' link relation types are used in these media types:

   1.  OpenSearch 1.1: see Section 4.5.4.1 of [OpenSearch]
   
   2.  Maze+XML: see [Maze]
   
   3.  Collection+JSON: see [CollectionJSON]

2. Link Relations

The following link relations are defined.

2.1. 'item'

When included in a resource that represents a collection, the 'item' link relation identifies a target resource that represents a member of that collection.

For example, if a resource represents a catalog of products, that same representation may include one or more links to resources that represent members of that catalog.

   <html>
   ...
     <h1>Product Group X Listing</h1>
     ...
     <a href="..." rel="item">View Product X001</a>
     <a href="..." rel="item">View Product X002</a>
     ...
   </html>

or, in the case of a Link Header field

   Link: <...>; rel="item"; title="View Product X001"
   Link: <...>; rel="item"; title="View Product X002"

2.2. 'collection'

When included in a resource that represents a member of a collection, the 'collection' link relation identifies a target resource that represents a collection of which the context resource is a member.

For example, if a resource represents a single product in a catalog, that same representation may include a link to a resource that represents a product group to which this single product belongs:

<a href="..." rel="collection">Return to Product Group X</a>

or, in the case of a Link Header field

   Link: <...>; rel="collection"; title="Return to Product Group X"

Since it is possible that a resource could be a member of multiple collections, multiple 'collection' link relations may appear within the same representation:

<a href="..." rel="collection">View other widgets</a> <a href="..." rel="collection">View all discontinued items</a>

The target resource representation need not be restricted to representing a list. It may simply be a document that provides details on the collection of which the context resource is a member:

   Link: <...>; rel="collection";
         title="Shakespeare's Collected Works - A History"

It should also be noted that the same link might represent an 'item' in one collection as well as a 'collection' itself. In this case, both link relation values can be applied to the same link:

   Link: <...>; rel="collection item";
         title="A Review of Issac Asimov's Collected Works - Vol. I"

3. IANA Considerations

IANA has registered the 'collection' and 'item' link relations below as per [RFC5988].

3.1. 'item' Link Relation Registration

Relation Name:

      item

Description:

The target IRI points to a resource that is a member of the collection represented by the context IRI.

Reference:

      See Section 2

3.2. 'collection' Link Relation Registration

Relation Name:

      collection

Description:

The target IRI points to a resource that represents the collection resource for the context IRI.

Reference:

      See Section 2

4. Security Considerations

The two link relation types defined in this document do not introduce any new security issues to those which are discussed in Section 7 of RFC5988 [RFC5988].

5. Internationalisation Considerations

The 'item' and 'collection' link relation types do not have any internationalization considerations other than those which are discussed in Section 8 of RFC5988 [RFC5988].

6. References

6.1. Normative References

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

6.2. Informative References

   [CollectionJSON]  Amundsen, M., "Collection+JSON - Document Format",
                     Web Page , July 2011,
                     <http://www.iana.org/assignments/media-types>.
   
   [Maze]            Amundsen, M., "Maze+XML - Format", Web Page ,
                     December 2010,
                     <http://www.iana.org/assignments/media-types>.
   
   [OpenSearch]      Clinton, D., "Open Search 1.1", Work in Progress ,
                     March 2011, <http://www.opensearch.org/
                     Specifications/OpenSearch/1.1/>.

Appendix A. Acknowledgements

The author gratefully acknowledges the contributions of Julian Reschke and Mykyta Yevstifeyev.

Author's Address

   Mike Amundsen
   
   EMail: mca@amundsen.com
   URI:   http://amundsen.com