AtomPub Guidelines for Collection Discovery
draft-divilly-atompub-discovery-00

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Abstract

This document recommends best practices for discovering AtomPub Collection resources as applicable to various content representation formats.

Editorial Note

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Table of Contents

1.  Introduction
    1.1.  Notational Conventions
    1.2.  Namespace
2.  Discovering Collections from a Feed Document
3.  Discovering Service from an HTML Document
4.  Security Considerations
5.  IANA Considerations
6.  Normative References
Appendix A.  Revision History
§  Authors' Addresses

1.  Introduction

Atom Publishing Protocol [RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.) is used for several applications that consume and produce an unbounded number of Collection resources. This document introduces guidelines over existing syntactic techniques for identifying Collection resources in use from various Internet content representation formats, including feeds and HTML type resources.

Previously, AtomPub [RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.) has introduced two mechanisms for collection discovery. However, in the absence of suitable guidelines, there is no clarity about which mechanism is suited for a particular purpose. In general, where other Atom or AtomPub syntax is in use, this document recommends the use of the app:collection element. Where only a link element may be used, this document recommends the use of the service link relation.

1.1.  Notational Conventions

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

1.2.  Namespace

This specification uses the prefix "atom:" for "http://www.w3.org/2005/Atom", the namespace name of the Atom Syndication Format [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). The prefix "app:" is used for "http://www.w3.org/2007/app", the namespace name of the Atom Publishing Protocol [RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.). These namespace prefixes are not semantically significant.

2.  Discovering Collections from a Feed Document

When an Atom Processor encounters an Atom Feed Document, and the processor is capable of performing AtomPub operations, then it is valuable to determine whether that feed is modifiable. Some processors incorrectly assume that a feed may be modified at the exact same URI from which it is obtained.

AtomPub Processors need a way to determine whether and where new entries can be added to a Feed. If a syndicated feed document is modifiable using AtomPub, then the processor can indeed manipulate the feed. For this purpose, processors can benefit from Collection metadata that is present in the feed document.

The alternative is obtaining the feed's service document by following a link with the relation "service", followed by identifying the Collection resources in the AtomPub Service Document, followed by correlating these Collection resources to the feed at hand. Naturally, this alternative is more complex, resource consuming, and less likely to succeed in finding the exact Collection resource to be manipulated.

Moreover, not every feed can be modified, and servers can indicate this by omitting Collection metadata in such feeds.

Section 8.3.5 of [RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.) specifies the Collection metadata that may occur in syndicated Atom feeds. The presence of an app:collection element as a child of an atom:feed element signals server's preparedness to modify a feed. This document therefore recommnds that if a Feed is backed by a Collection, then the server SHOULD identify the backing Collection using the app:collection element as a child of the atom:feed element.

If the href attribute of this app:collection element is identical to that of the parent atom:feed element's self link relation, then the processor SHOULD treat the feed to be the Collection feed as defined in Section 4.2 of [RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.).

Example: Writable Collection backing a Feed

<atom:feed>
  <atom:link rel="self" href="http://example.org/feed"/>
  <app:collection href="http://example.org/collection">
    <atom:title type="text">Writable Feed</atom:title>
    <app:accept>application/atom+xml;type=entry</app:accept>
  </app:collection>
  ...
</atom:feed>

Example: Read-only Collection backing a Feed

<atom:feed>
  <atom:link rel="self" href="http://example.org/feed"/>
  <app:collection href="http://example.org/collection">
    <atom:title type="text">Read-only Feed</atom:title>
    <app:accept/>
  </app:collection>
  ...
</atom:feed>

Example: Writable Collection Feed

<atom:feed>
  <atom:link rel="self" href="http://example.org/collection"/>
  <app:collection href="http://example.org/collection">
    <atom:title type="text">Collection Feed</atom:title>
  </app:collection>
  ...
</atom:feed>

3.  Discovering Service from an HTML Document

When an Atom processor encounters an HTML or XHTML document, and the processor is capable of performing AtomPub operations, then it is useful to determine the available AtomPub resources with which it can interact.

The host format, though, limits the ability to specify metadata embedded in the content being processed. Therefore, the best approach for a server is to specify a link with relation "service" to provide a URI to the AtomPub Service Document that includes all the metadata corresponding to one or more Collections. The server can also specify a link with relation "feed" to provide a URI to an Atom feed with relevant content, which can in turn provide additional metadata for manipulating AtomPub resources present on that server as explained in the previous section.

4.  Security Considerations

None

5.  IANA Considerations

None

6. Normative References

Appendix A.  Revision History

00 - Initial Revision, created from draft-divilly-atompub-hierarchy-00.

Authors' Addresses