Inlining and Hierarchy Extensions for Atom
draft-divilly-atom-hierarchy-01

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Abstract

This specification defines mechanisms for in-lining representations of linked resources and for hierarchical navigation among Atom feeds and entries.

Editorial Note

To provide feedback on this Internet-Draft, join the atom-syntax mailing list (http://www.imc.org/atom-syntax/).

Table of Contents

1.  Introduction
    1.1.  Namespace
    1.2.  Notational Conventions
    1.3.  Terminology
2.  Hierarchy Model
    2.1.  Entry Classification
    2.2.  Entry Parent Representation
3.  Inline Representation of Resources
    3.1.  Representation of Linked Resources
    3.2.  The "ae:inline" Extension Element
4.  Child and Descendant Feeds
    4.1.  The "down" Link
    4.2.  The "down-tree" Link
    4.3.  Examples
5.  Parent Entries and Parent and Ascendant Feeds
    5.1.  The "up" Link
    5.2.  The "up-tree" Link
    5.3.  Examples
6.  Security Considerations
7.  IANA Considerations
8.  Normative References
Appendix A.  Acknowledgements
Appendix B.  Revision History
§  Authors' Addresses

1.  Introduction

Many applications, besides blogs, provide their data in the form of syndicated Web feeds using formats such as Atom [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). Some such applications organize Atom Entries in a hierarchical fashion similar to a file system.

This specification describes a means of communicating about Atom Entries that are hierarchically related to each other since resource identifiers are opaque to clients and cannot be directly manipulated for the purposes of representation exchange, i.e., navigation.

This specification proposes new XML markup to in-line representations of linked resources and new link relations for hierarchically related Atom resources.

1.1.  Namespace

The XML Namespaces URI for the XML data format described in this specification is:

    http://purl.org/atom/ext/

This specification uses the prefix "ae:" for the namespace name. The prefix "atom:" is used 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.). These namespace prefixes are not semantically significant.

1.2.  Notational Conventions

Some sections of this specification are illustrated with fragments of a non-normative RELAX NG Compact schema [RELAX-NG]. In those sections, this specification uses the atomCommonAttributes and anyElement, defined in [RFC4287].

However, the text of this specification provides the definition of conformance.

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.).

1.3.  Terminology

This specification uses Atom link relations to identify different types of links; see the Atom specification [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.) for information about their syntax, and the IANA link relation registry for more information about specific values.

2.  Hierarchy Model

A hierarchy exists when a resource indicates the likelihood of a parent and/or a child resource. The terms parent and child are indicative of the need for the former to exist before the latter can be created.

2.1.  Entry Classification

The Atom Syndication Format [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.) defines the Atom Entry construct. The extensions in this specification define two specialized kinds of Entry construct -- parent Entry and child Entry.

A parent Entry is a container for child Entries. A parent Entry could itself be a child of another parent Entry.

Every Entry construct is represented as an Atom Entry Document [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.) referred to in this specification as an "entry" and its plural. A logical Feed comprising entirely of child entries of a given Entry is called its child feed and one comprising entirely of its parent entries is called its parent feed. Both parent feed and child feed are seen from the perspective of a given Entry resource. The entries in the parent feed and child feed of an Entry SHOULD be disjoint, i.e., not share any entries.

A parent entry contains one or more "down" atom:link for its child feed. A parent entry may also contain one or more "down-tree" atom:link for a child feed of a subset of the descendants of that parent Entry.

   atom:entry
        |
        o- atom:link@rel="down"

A child entry contains one or more "up" atom:link for its parent feed or entry if the child only allows a single parent. A child entry may also contain one or more "up-tree" atom:link for a parent feed of a subset of the ascendants of that child Entry.

   atom:entry
        |
        o- atom:link@rel="up"

A child feed may contain one or more "up" atom:link for its parent Entry.

   atom:feed
        |
        o- atom:link@rel="up"

2.2.  Entry Parent Representation

Applications MAY allow more than one parent Entry to contain a given child Entry. This is similar to hard links in filesystems. On the other hand, certain applications allow only a single parent Entry.

A child Entry MUST use a logical Feed to represent multiple parent Entries. This implies use of a feed for the URI [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) identified as the child Entry's "up" link. A child Entry MAY use an entry for the URI identified as the child Entry's "up" link if it does not allow multiple parents for that child Entry. Clients SHOULD be prepared to inspect the representation received for the "up" link of a child Entry before assuming either cardinality models.

3.  Inline Representation of Resources

A resource is linked from an Atom document using the atom:link element [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). The representation of the linked resource MAY be in-lined in the referring Atom document to reduce network usage.

3.1.  Representation of Linked Resources

The href of atom:link element identifies a resource linked from Atom documents. An Atom Processor can obtain the representation of a linked resource using its identifier. This specification defines an alternate mechanism in cases where the linked resource can be represented directly inside the referring Atom document.

An Atom document may include the in-line representation of a linked resource by using the ae:inline element inside the corresponding atom:link element. The in-lined representation is only a hint and MAY differ from the representation obtained from the URI referenced in the link. Atom Processors SHOULD use the link URI to obtain the complete representation of the linked resource.

3.2.  The "ae:inline" Extension Element

The in-line representation of a linked resource may contain XML or text content similar to atom:content [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

extensionInline =
   element ae:inline {
     atomCommonAttributes,
     inlineRepresentation
   }

inlineRepresentation =
   text | anyElement

In-line representation of linked resource is analogous to the in-line representation of an entry's content. In this sense, atom:content and atom:link behave similarly in the presence of in-line extensions. Atom processors conforming to this specification MUST, therefore, process the value of ae:inline per the processing model for atom:content as specified in Section 4.1.3.3 of [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

Additionally, even though the type attribute is optional in atom:link elements, conforming generators that in-line linked resources MUST specify a value for the type attribute in the corresponding atom:link element. The value of the ae:inline element, i.e., the inline representation MUST conform to the MIME media type specified in the type attribute of the atom:link element that holds the ae:inline element.

4.  Child and Descendant Feeds

4.1.  The "down" Link

A parent entry MUST contain an atom:link element with link relation of "down" to indicate the child Feed URI. The type attribute of this link element (if present), MUST be the Atom Feed content type, i.e., application/atom+xml;type=feed. An entry MUST NOT contain more than one atom:link element with a rel attribute value of "down" that has the same combination of type and hreflang attribute values.

4.2.  The "down-tree" Link

A parent entry MAY contain an atom:link element with link relation of "down-tree" to indicate the URI of a feed of descendant Entry resources. The type attribute of this link element (if present), MUST be the Atom Feed content type, i.e., application/atom+xml;type=feed. This specification does not prescribe any means of limiting the depth to which descendants are available. An entry MUST NOT contain more than one atom:link element with a rel attribute value of "down-tree" that has the same combination of type and hreflang attribute values.

4.3.  Examples

Example: Entry with out-of-line reference to child feed

<atom:entry>
  <atom:title type="text">My Portfolio</atom:title>
  <atom:link rel="down" type="application/atom+xml;type=feed"
    href="/finance/feeds/default/portfolios/1/positions/"/>
  <atom:link rel="down" type="text/html"
    href="/finance/feeds/default/portfolios/1/positions"/>
  ...
</atom:entry>

Example: Parent entry representation with inline child feed

<atom:entry>
  <atom:link rel="down" type="application/atom+xml;type=feed"
    href="/finance/feeds/default/portfolios/1/positions">
    <ae:inline>
      <atom:feed>
        <atom:link rel="self"
         href="/finance/feeds/default/portfolios/1/positions"/>
         ...
      </atom:feed>
    </ae:inline>
  </atom:link>
  ...
</atom:entry>

Example: Entry with out-of-line reference to descendant feed

<atom:entry>
  <atom:link rel="down-tree"
    href="/finance/feeds/default/portfolios/"/>
  ...
</atom:entry>

5.  Parent Entries and Parent and Ascendant Feeds

5.1.  The "up" Link

Child Entries identify the URIs of their parent Entry or multiple parent Entry resources in their own metadata. A child entry MUST contain an atom:link element with link relation of "up" to indicate the parent Entry URI. If the type attribute of this link is present, it MUST be an Atom content type. An entry MUST NOT contain more than one atom:link element with a rel attribute value of "up" that has the same combination of type and hreflang attribute values.

A child feed MUST identify the URI of the parent Entry represented in the feed using the "up" link. This allows navigation back and forth between the parent Entry and the child feed. This is in addition to the "up" link present in individual child entries. A feed MUST NOT contain more than one atom:link element with a rel attribute value of "up" that has the same combination of type and hreflang attribute values.

5.2.  The "up-tree" Link

A child entry MAY contain an atom:link element with link relation of "up-tree" to indicate the URI of a feed of ascendant Entry resources. The type attribute of this link element (if present), MUST be the Atom Feed content type, i.e., application/atom+xml;type=feed. This specification does not prescribe any means of limiting the height to which ascendants are available. An entry MUST NOT contain more than one atom:link element with a rel attribute value of "up-tree" that has the same combination of type and hreflang attribute values.

5.3.  Examples

Example: Child Entry with inline multiple parent Entries

<atom:entry>
  <atom:title type="text">Position for NASDAQ:ORCL</atom:title>
  <atom:link rel="up" type="application/atom+xml;type=feed"
    href="/finance/feeds/default/positions/NASDAQ:ORCL/up">
    <ae:inline>
      <atom:feed>
        ...
        <atom:entry>
          <atom:link rel="self"
            href="/finance/feeds/default/portfolios/1"/>
          ...
        </atom:entry>
        <atom:entry>
          <atom:link rel="self"
            href="/finance/feeds/default/portfolios/2"/>
          ...
        </atom:entry>
      </ae:inline>
    </atom:feed>
  </atom:link>
  ...
</atom:entry>

Example: Child feed with out-of-line reference to parent Entry

<atom:feed>
  <atom:title type="text">Positions</atom:title>
  <atom:link rel="up"
    href="/finance/feeds/default/portfolios/1"/>
  ...
</atom:feed>

Example: Entry with out-of-line reference to ascendant feed

<atom:entry>
  <atom:link rel="up-tree"
    href="/finance/feeds/default/portfolios/1/positions"/>
  ...
</atom:entry>

6.  Security Considerations

Hierarchy Extensions for Atom is subject to the security considerations found in Section 8 of [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.).

The down-tree relation can overwhelm a server if reasonable limits are not placed on the depth to which hierarchy can be navigated. For this reason, applications are advised to either restrict this relation's usage to out-of-line content or apply reasonable limits on inline representation.

7.  IANA Considerations

This specification uses the following relation that has been previously registered in the Link Relations Registry

• Attribute Value: up
• Description: A URI that refers to one or more parent resources in a hierarchy of resources.
• Expected display characteristics: none
• Security considerations: See this specification

This specification defines the following new relations that have been added to the Link Relations registry:

• Attribute Value: down
• Description: A URI that refers to one or more child resources in a hierarchy of resources.
• Expected display characteristics: none
• Security considerations: See this specification

• Attribute Value: down-tree
• Description: A URI that refers to the set of descendant resources in a hierarchy of resources.
• Expected display characteristics: none
• Security considerations: See this specification

• Attribute Value: up-tree
• Description: A URI that refers to the set of ascendant resources in a hierarchy of resources.
• Expected display characteristics: none
• Security considerations: See this specification

8. Normative References

Appendix A.  Acknowledgements

Bill de hÓra and Ashish Motivala reviewed early drafts of this I-D. On the atom-syntax mailing list, Al Brown, Julian Reschke, Mark Nottingham, Pablo Castro, and Kyle Marvin provided very valuable feedback to improve the subsequent public drafts.

Appendix B.  Revision History

00 - Initial Revision.

00 - Based on feedback from Peter Keane, Julian Reschke, and members of the CMIS TC made the following changes:

Renamed the link relation "detail" to "down" and "master" to "up"

Removed Section 3, 4, 6, and 7

Changed namespace prefix from h to ah

Added new link relations "up-tree" and "down-tree"

01 - Changes include:

In-line representations of linked resources are independent of hierarchy

Removed Atom specific language in link relation definitions

Made explicit that this specification re-uses existing 'up' link relation

Changed namespace prefix from ah to ae

Removed the ah:count attribute and added the ae:inline element

Authors' Addresses