Network Working Group J. Snell
Internet-Draft September 27, 2005
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draft-snell-atompub-feed-index-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines a mechanism for numerically ranking entries
within a syndication feed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Ranking Domains and Schemes . . . . . . . . . . . . . . . . . 4
3.1 Ranking Domain Scope . . . . . . . . . . . . . . . . . . . 5
3.2 Ranking Domain Identifiers . . . . . . . . . . . . . . . . 6
4. Ranking Entries . . . . . . . . . . . . . . . . . . . . . . . 6
5. Processing Rankings . . . . . . . . . . . . . . . . . . . . . 7
6. Resolving Ranking Schemes . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
In syndication document formats such as Atom [I-D.ietf-atompub-
format] and RSS ([http://purl.org/rss/1.0/spec][http://
blogs.law.harvard.edu/tech/rss]), the order of entries as presented
in the list is typically insignificant. This presents a challenge
when the list of entries is intended to represent an ordered or
ranked listing of items. This document specifies a mechanism that
allows feed publishers to establish numeric rankings for entries
within a feed that may be used as a means of organizing and sorting
those entries. Although the document refers to Atom normatively, the
mechanism described herein can be used with similar syndication
formats, such as the various flavors of RSS.
Section 6.3 of the Atom Format specification indicates that Atom
processors that encounter unknown extensions MUST ignore those
extensions without altering their behavior. Because of this
requirement, there can be no assumption that a particular software
implementation will support the extensions defined herein.
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 BCP 14, [RFC2119], as
scoped to those conformance targets.
In this specification, "entry" refers to an atom:entry element or
similar construct from other syndication formats (e.g., RSS) that are
contained within a feed.
In this specification, "feed" refers to an Atom Feed Document or
similar syndication format (e.g., RSS) that contains a collection of
entries.
In this specification, "head section" refers to the children of a
feed document's document-wide metadata container; e.g., the child
elements of the atom:feed element in an Atom Feed Document or the
children of the RSS 'channel' element.
This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
to uniquely identify XML element names. It uses the following
namespace prefix for the indicated namespace URI;
"i": "http://purl.org/syndication/index/1.0"
This specification uses terms from the XML Infoset [W3C.REC-xml-
infoset-20040204]. However, this specification uses a shorthand; the
phrase "Information Item" is omitted when naming Element Information
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Items. Therefore, when this specification uses the term "element,"
it is referring to an Element Information Item in Infoset terms.
3. Ranking Domains and Schemes
A 'Ranking Domain' is a uniquely identifiable logical collection of
entries containing numeric ranking values that conform to a given
'Ranking Scheme'. 'Ranking Schemes' identify the specific rules for
how the numeric ranking values associated with a given 'Ranking
Domain' should be interpreted. Every Ranking Domain is associated
with exactly one Ranking Scheme.
Ranking Schemes are defined using the r:ranking-scheme element:
rankingScheme = element r:ranking-scheme {
attribute domain { IRI }?,
attribute label { text }?,
attribute significance { 'ascending' | 'descending' }?,
attribute precision { nonNegativeInteger }?,
attribute min-value { decimal }?,
attribute max-value { decimal }?
}
o The domain attribute identifies the Ranking Domain this Ranking
Scheme is associated with. (see the section 'Ranking Domain Scope'
for more details)
o The label attribute specifies a human readable label for the
Ranking Scheme. Because the value of this attribute is intended
to be used for display purposes, it is language sensitive as
defined by Atom [I-D.ietf-atompub-format].
o The significance attribute provides an indicator of how
implementations are to interpret the significance of an entries
numeric ranking value. A value of 'descending' means that the
significance of the rank descends as the numeric ranking value
increases. A value of 'ascending' means that the significance of
the rank ascends as the numeric ranking value increases. If not
specified, the significance is considered to be 'ascending'.
o The precision attribute specifies the level of precision to be
applied to the value of the numeric ranking value. The value is
expressed as a nonNegativeInteger as defined by [W3C.REC-
xmlschema-2-20041028]. If not specified the value is
indeterminate (e.g., the number 2.0 is not distinct from the
number 2.00). Ranking schemes that are based on fractional
numeric ranking values SHOULD specify a precision.
o The min-value attribute specifies the lowest possible numeric
ranking value (inclusive). If not specified, the minimum value is
considered to be 0.
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o The max-value attribute specifies the highest possible numeric
ranking value (inclusive). If not specified, no maximum value is
considered to apply.
The head section of a feed or the atom:source element contained by an
entry MAY contain any number of r:ranking-scheme elements. A feed
MUST NOT contain more than one r:ranking-scheme element associated
with the same Ranking Domain.
...
...
3.1 Ranking Domain Scope
Ranking Domain's provide a logical mechanism that may be used to
associate the numeric ranking values contained within a collection of
entries. All numeric ranking values are associated with a domain.
Ranking Domains are identified by a unique IRI.
Ranking Domains fall into one of three scopes:
o Feed Scope (known as the "Feed Ranking Domain")
o Document Scope (known as the "Document Ranking Domain")
o Domain Scope
Ranking Schemes and numeric ranking values that do not specify a
domain attribute are automatically associated with the Feed Ranking
Domain. The IRI identity of the Feed Ranking Domain is the same as
the feed's unique identifier (e.g., atom:id in atom:feed elements).
A Ranking-Scheme associated with the Feed Ranking Domain
Ranking Schemes and numeric ranking values that specify same document
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references are automatically associated with the Document Ranking
Domain. The IRI identity of the Document Ranking Domain is the
normalized form of the base URI of the document in which the
r:ranking-scheme and numeric ranking values are located..
A Ranking-Scheme associated with the Document Ranking Domain
Ranking Schemes and numeric ranking values that specify any IRI value
other than a same document reference are associated with a Domain
Scope.
A Ranking-Scheme associated with a Domain Scope
3.2 Ranking Domain Identifiers
The IRI's identifying Ranking Domains are subject to the same
construction and comparison rules as the atom:id element.
4. Ranking Entries
Entries within a feed MAY contain zero or more i:rank elements that
specify a numeric ranking value associated with a given Ranking
Domain.
rankingValue = element r:rank {
attribute domain {IRI}?,
(decimal}
}
The domain attribute specifies the Ranking Domain that this ranking
value is associated with. If omitted, the rank is automatically
associated with the Feed Ranking Domain. If the value is a same
document reference, the rank is associated with the Document Ranking
Domain. Any other value associates the rank with the identified
logical Ranking Domain.
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The value of the r:rank is a numeric decimal value conforming to the
XML Schema decimal data type [W3C.REC-xmlschema-2-20041028]. The
value MUST NOT contain any leading or trailing whitespace.
3.5
Feed Entries MAY contain any number of r:rank elements but MUST NOT
contain more than one r:rank element associated with the same Ranking
Domain.
5. Processing Rankings
The purpose of having a numeric ranking for a collection of feed
entries is to provide a mechanism by which those entries can be
logically organized and sorted. For instance, entries with a higher
value ranking may be presented before entries with a lower value
ranking.
Because a set of entries may be ranked in many different ways,
Ranking Domains provide a means of associating specific numeric
ranking values into logical sets. Once the ranking groups into a
domain, the Ranking Scheme may be used to interpret how to apply the
ranking values.
Processing a Ranking Domain involves the following steps:
Identify the Ranking Domain
Resolve the applicable Ranking Scheme for the identified Ranking
Domain
Identify the full set of entries containing numeric ranking values
associated with the selected Ranking Domain. The members of a
Ranking Domain's set of entries MAY span multiple feeds.
Sort the set of ranked entries according to the significance of
the numeric ranking as specified by the domain's Ranking Scheme.
6. Resolving Ranking Schemes
Resolution of an appropriate Ranking Scheme involves examining the
head section of the containing feed for an r:ranking-scheme element
associated with a given domain specified by a numeric ranking value
or by matching the domain with well-known or explicitly defined
r:ranking-schemes as described by other specifications subject to the
IESG approval.
For instance, a search engine may wish to define a ranking scheme
that is reflective of the relevance of a given entry to a search
query; rather than require that the appropriate r:ranking-scheme be
included in every feed where the Ranking Scheme may be used, the
search engine may publish its Ranking Scheme and associated Ranking
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Domain. Software implementations MAY attempt to match domains for
which no r:ranking-scheme can be found within the feed to a list of
such known Ranking Domains and Schemes. If multiple Ranking Schemes
are resolved for a given Ranking Domain, the implementation MAY
select any of the resolved schemes to apply while ignoring the
remainder.
If a Ranking Scheme for a given domain cannot be resolved (e.g., no
r:ranking-scheme with a matching domain attribute can be found), the
Ranking Domain is associated with a Default Ranking Scheme as defined
below.
o label = 'Default Ranking'
o significance = 'ascending'
o precision = {none}
o min-value = 0
o max-value = {none}
7. Security Considerations
There are no security considerations introduced by this
specification.
8. IANA Considerations
There are no IANA considerations introduced by this specification.
9. References
[I-D.ietf-atompub-format]
Sayre, R. and M. Nottingham, "The Atom Syndication
Format", draft-ietf-atompub-format-11 (work in progress),
August 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[W3C.REC-xml-infoset-20040204]
Tobin, R. and J. Cowan, "XML Information Set (Second
Edition)", W3C REC REC-xml-infoset-20040204,
February 2004.
[W3C.REC-xml-names-19990114]
Hollander, D., Bray, T., and A. Layman, "Namespaces in
XML", W3C REC REC-xml-names-19990114, January 1999.
[W3C.REC-xmlschema-2-20041028]
Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
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Second Edition", W3C REC REC-xmlschema-2-20041028,
October 2004.
Author's Address
James M Snell
Phone:
Email: jasnell@gmail.com
URI: http://snellspace.com
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