One document matched: draft-divilly-atom-hierarchy-00.txt
Network Working Group C. Divilly
Internet-Draft N. Mehta
Intended status: Experimental Oracle Corp.
Expires: November 21, 2009 May 20, 2009
Hierarchy Extensions for Atom
draft-divilly-atom-hierarchy-00
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Abstract
This specification defines mechanisms for hierarchical navigation
among Atom feeds and entries.
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Editorial Note
To provide feedback on this Internet-Draft, join the atom-syntax
mailing list (http://www.imc.org/atom-syntax/) [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Namespace . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Hierarchy Model . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Entry Classification . . . . . . . . . . . . . . . . . . . 4
2.2. Entry parent representation . . . . . . . . . . . . . . . . 4
3. Inline Representation of Hierarchical Resources . . . . . . . . 5
3.1. Representation of Linked Resources . . . . . . . . . . . . 5
3.1.1. The "ah:count" Extension Attribute . . . . . . . . . . 5
3.2. Child and descendant feeds . . . . . . . . . . . . . . . . 5
3.2.1. The "down" Link . . . . . . . . . . . . . . . . . . . . 5
3.2.2. The "down-tree" Link . . . . . . . . . . . . . . . . . 5
3.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Parent Entries and Parent and Ascendant Feeds . . . . . . . 6
3.3.1. The "up" Link . . . . . . . . . . . . . . . . . . . . . 6
3.3.2. The "up-tree" Link . . . . . . . . . . . . . . . . . . 7
3.3.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
6. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
Appendix B. Revision History . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Many applications, besides blogs, provide their data in the form of
syndicated Web feeds using formats such as Atom [RFC4287]. 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 extend the Atom
Syndication Format and new link relations to obtain representations
of 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/hierarchy/
This specification uses the prefix "ah:" 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]. These
namespace prefixes are not semantically significant.
1.2. 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].
1.3. Terminology
This specification uses Atom link relations to identify different
types of links; see the Atom specification [RFC4287] 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.
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2.1. Entry Classification
The Atom Syndication Format [RFC4287] 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] 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 a "down" atom:link for its child feed. A
parent entry may also contain a "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" (1..1)
o- atom:link@rel="down-tree" (0..1)
A child entry contains an "up" atom:link for its parent feed or entry
if the child only allows a single parent. A child entry may also
contain an "up-tree" atom:link for a parent feed of a subset of the
ascendants of that child Entry.
atom:feed
|
o- atom:link@rel="up" (1..1)
o- atom:link@rel="up-tree" (0..1)
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 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
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inspect the representation received for the "up" link of a child
Entry before assuming either cardinality models.
3. Inline Representation of Hierarchical Resources
A parent or child feed or a parent entry MAY be inlined in an entry.
Clients SHOULD NOT assume that the inline representations are
identical to the one available from the linked URI. Clients SHOULD
use URI identified by the relevant link relation to obtain its
complete representation.
3.1. Representation of Linked Resources
An entry references parent and child Entries via various links. The
representation of a hierarchically linked resource can be provided in
the following ways:
1. Out-of-line reference: The Atom Processor can retrieve the
representation by following the URI specified in the link
element.
2. Inline content with out-of-line reference: The Atom Processor can
use the content specified inside the link element as an
approximation of the server representation as detailed below.
The embedded representation is only a hint and MAY differ from
the representation obtained from the URI referenced in the link.
3.1.1. The "ah:count" Extension Attribute
On the atom:link element, the value of the "ah:count" attribute MAY
be a non-negative integral value identifying an approximate count of
the number of entries in the inlined feed document. If the inlined
representation or the type identified in the atom:link is the Atom
Entry content type, i.e., application/atom+xml;type=entry, then this
attribute MUST NOT be used.
3.2. Child and descendant feeds
3.2.1. 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.
3.2.2. 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
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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.
3.2.3. 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" ah:count="0"/>
...
</atom:entry>
Example: Parent entry representation with inline child feed
<atom:entry>
<atom:link rel="down"
href="/finance/feeds/default/portfolios/1/positions">
<atom:feed>
<atom:link rel="self"
href="/finance/feeds/default/portfolios/1/positions"/>
...
</atom:feed>
</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>
3.3. Parent Entries and Parent and Ascendant Feeds
3.3.1. 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.
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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.
3.3.2. 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.
3.3.3. 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"
href="/finance/feeds/default/positions/NASDAQ:ORCL/up">
<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>
</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" ah:count="2"/>
...
</atom:feed>
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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>
4. Security Considerations
Hierarchy Extensions for Atom is subject to the security
considerations found in Section 8 of [RFC4287].
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.
5. IANA Considerations
This specification defines the following new relations that have been
added to the Link Relations registry:
o Attribute Value: down
o Description: A URI that refers to a feed of child entries in a
hierarchy of Atom Entry resources.
o Expected display characteristics: none
o Security considerations: See this draft
o Attribute Value: down-tree
o Description: A URI that refers to the feed of descendant entries
in a hierarchy of Atom Entry resources.
o Expected display characteristics: none
o Security considerations: See this draft
o Attribute Value: up
o Description: A URI that refers to one or more parent entry
resources in a hierarchy of Atom Entry resources.
o Expected display characteristics: none
o Security considerations: See this draft
o Attribute Value: up-tree
o Description: A URI that refers to the feed of ascendant entries in
a hierarchy of Atom Entry resources.
o Expected display characteristics: none
o Security considerations: See this draft
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6. 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.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
[W3C.REC-xmlbase-20010627]
Marsh, J., "XML Base", World Wide Web Consortium
FirstEdition REC-xmlbase-20010627, June 2001,
<http://www.w3.org/TR/2001/REC-xmlbase-20010627>.
[1] <http://www.imc.org/atom-syntax/>
Appendix A. Acknowledgements
Bill de hOra and Ashish Motivala reviewed early drafts of this I-D.
Appendix B. Revision History
00 - Initial Revision.
01 - 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"
Authors' Addresses
Colm Divilly
Oracle Corp.
Email: colm.divilly@oracle.com
URI: http://cdivilly.wordpress.com
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Nikunj R. Mehta
Oracle Corp.
Email: nikunj.mehta@oracle.com
URI: http://o-micron.blogspot.com/
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