Network Working Group J. Snell
Internet-Draft January 25, 2006
Expires: July 29, 2006
Feed Rank
draft-snell-atompub-feed-index-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
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 . . . . . . . . . . . . . . . . . 3
3.1. Ranking Domain Scope . . . . . . . . . . . . . . . . . . . 5
3.2. Ranking Domain Identifiers . . . . . . . . . . . . . . . . 6
4. Ranking Entries . . . . . . . . . . . . . . . . . . . . . . . 6
5. Processing Rankings . . . . . . . . . . . . . . . . . . . . . 7
6. Well-known Domains and the Default Ranking Scheme . . . . . . 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 [RFC4287] and 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 set. This document specifies a
mechanism that allows feed publishers to establish numeric rankings
for entries within a feed to be used as a means of organizing and
sorting those entries. Although this document refers to Atom
normatively, the mechanism described herein can be used with similar
syndication formats, such as the various flavors of RSS.
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].
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;
"r": "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
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 conforming to a 'Ranking
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Scheme'.
'Ranking Schemes' identify the specific rules for how the numeric
ranking values associated with a given 'Ranking Domain' are to be
interpreted. Each Ranking Scheme is associated with exactly one
Ranking Domain.
Ranking Schemes are defined using the r:ranking-scheme element.
rankingScheme = element r:ranking-scheme {
atomCommonAttributes,
attribute domain { IRI }?,
attribute label { text }?,
attribute significance { 'ascending' | 'descending' }?,
attribute precision { nonNegativeInteger }?,
attribute minimum { decimal }?,
attribute maximum { decimal }?
}
o The 'domain' attribute identifies the Ranking Domain for the
Ranking Scheme.
o The 'label' attribute specifies a language-sensitive, human
readable label for the Ranking Scheme.
o The 'significance' attribute indicates how implementations are to
interpret the significance of an entries numeric ranking value. A
value of 'descending' indicates that the significance of the rank
descends as the numeric ranking value increases. A value of
'ascending' indicates that the significance of the rank increases
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 non-negative integer. If not specified the value
is considered to be 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 'minumum' attribute specifies the lowest possible numeric
ranking value (inclusive). If not specified, the minimum value is
considered to be 0.
o The 'maximum' attribute specifies the highest possible numeric
ranking value (inclusive). If not specified, no maximum value is
considered to apply.
Feeds MAY contain any number of r:ranking-scheme elements. A Feed
MUST NOT contain more than one r:ranking-scheme element with the same
Ranking Domain.
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<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:r="http://purl.org/syndication/index/1.0">
...
<r:ranking-scheme
domain="tag:example.org,2006:movie_reviews"
xml:lang="en-us"
label="Five Star Reviews"
precision="0"
significance="ascending"
minimum="0"
maximum="5" />
...
</feed>
3.1. Ranking Domain Scope
Ranking Domain's provide a logical mechanism used to associate a
numerically ranked set of resources. Ranking Domains are identified
by IRI's.
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
<r:ranking-scheme
xml:lang="en-us"
label="Five Star Reviews"
significance="ascending"
precision="0"
min-value="0"
max-value="5" />
The set of entries contained within the Feed Ranking Domain is
limited to the set of entries contained within the Feed.
Ranking Schemes and numeric rankings that specify a domain equal or
equivalent to the Feed Document's Base URI are associated with the
Document Ranking Domain. The IRI identity of the Document Ranking
Domain is the normalized form of the containing Feed Document's Base
URI.
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A Ranking-Scheme associated with the Document Ranking Domain
<r:ranking-scheme
domain=""
xml:lang="en-us"
label="Five Star Reviews"
significance="ascending"
precision="0"
min-value="0"
max-value="5" />
The set of entries contained within the Document Ranking Domain is
limited to the set entries contained within the Document.
Ranking Schemes and numeric ranking values that specify any IRI value
other than the Base URI of the containing document are associated
with a Domain Scope.
A Ranking-Scheme associated with a Domain Scope
<r:ranking-scheme
domain="tag:example.com,2006:movie_reviews"
xml:lang="en-us"
label="Five Star Reviews"
significance="ascending"
precision="0"
min-value="0"
max-value="5" />
Domain Scoped SHOULD be considered to be open sets consistings of
entries from any number of feeds.
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 r:rank elements
specifing a numeric ranking value within a given Ranking Domain. An
entry MUST NOT contain more than one r:rank element per Ranking
Domain.
rankingValue = element r:rank {
attribute domain {IRI}?,
(decimal}
}
The domain attribute identifies the Ranking Domain.
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The value of the r:rank is a 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.
<r:rank
domain="tag:example.com,2006:movie_reviews">3.5</r:rank>
5. Processing Rankings
Processing a Ranking Domain involves the following steps:
Identify the Ranking Domain
Resolve the applicable Ranking Scheme for the identified Ranking
Domain
Identify the available set of entries containing numeric ranking
values within the identified Ranking Domain. The members of this
set MAY span multiple feeds.
Remove from the set all entries whose rankings fall outside the
minimum and maximum values set by the domain's Ranking Scheme.
Sort the remaining set of ranked entries according to the
significance and precision of the numeric ranking as specified by
the domain's Ranking Scheme.
6. Well-known Domains and the Default Ranking Scheme
Feeds MAY contain ranked entries within domains that have no
corresponding r:ranking-scheme element. Software implementations MAY
attempt to match such domains to well-known Ranking Domains and
Schemes. For instance, an online search engine may choose to define
a ranking scheme that is reflective of the relevance of a given
result to a search query; rather than require that a r:ranking-scheme
be included in every feed where the Ranking Scheme may be used, the
search engine may separately publish its Ranking Scheme and
associated Ranking Domain.
If multiple Ranking Schemes are resolved for a given Ranking Domain,
implementations 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 and
the domain is not well-known), the Ranking Domain SHOULD be
associated with the Default Ranking Scheme defined below.
o label = 'Default Ranking'
o significance = 'ascending'
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o precision = unspecified
o minumum = 0
o maximum = unspecified
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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
Format", RFC 4287, December 2005.
[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
Second Edition", W3C REC REC-xmlschema-2-20041028,
October 2004.
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Author's Address
James M Snell
Phone:
Email: jasnell@gmail.com
URI: http://snellspace.com
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