Network Working Group                                           J. Snell
Internet-Draft                                          December 6, 2005
Expires: June 9, 2006


                               Feed Rank
                 draft-snell-atompub-feed-index-04.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 . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11




































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1.  Introduction

   In syndication document formats such as Atom [RFC4287] 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 this 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;
   "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



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   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 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 [RFC4287].
   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.
     <feed xmlns="http://www.w3.org/2005/Atom"
           xmlns:r="http://purl.org/syndication/index/1.0">
       ...
       <r:ranking-scheme
         domain="urn:example.com:/my_five_star_movie_reviews"
         xml:lang="en-us"
         label="Five Star Reviews"
         precision="0"
         significance="ascending"
         min-value="0"
         max-value="5" />
       ...
     </feed>

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
     <r:ranking-scheme
       xml:lang="en-us"
       label="Five Star Reviews"
       significance="ascending"
       precision="0"
       min-value="0"
       max-value="5" />




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   Ranking Schemes and numeric ranking values that specify same document
   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
     <r:ranking-scheme
       domain=""
       xml:lang="en-us"
       label="Five Star Reviews"
       significance="ascending"
       precision="0"
       min-value="0"
       max-value="5" />

   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
     <r:ranking-scheme
       domain="urn:example.com:/my_five_star_movie_reviews"
       xml:lang="en-us"
       label="Five Star Reviews"
       significance="ascending"
       precision="0"
       min-value="0"
       max-value="5" />

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



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   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.

   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.
     <r:rank
       domain="urn:example.com:/my_five_star_movie_reviews">3.5</r:rank>

   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



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   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
   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

   [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.



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   [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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