Network Working Group J. Snell
Internet-Draft April 10, 2006
Expires: October 12, 2006
Atom Ranking Extensions
draft-snell-atompub-feed-index-08.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines an extension 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. The 'r:scheme' element . . . . . . . . . . . . . . . . . . 3
3.2. The 'r:rank' element . . . . . . . . . . . . . . . . . . . 5
4. Ranking Domain Scope . . . . . . . . . . . . . . . . . . . . . 6
4.1. The Document Ranking Domain and xml:base . . . . . . . . . 7
5. Ranking Domain and Scheme Identifiers . . . . . . . . . . . . 8
6. Processing Rankings . . . . . . . . . . . . . . . . . . . . . 8
7. Default Ranking Scheme . . . . . . . . . . . . . . . . . . . . 8
8. Well-Known Ranking Schemes . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
In the Atom Syndication Format [RFC4287], the order of entries as
presented in a feed is typically considered to be insignificant.
This presents a challenge when the list of entries is intended to
represent an ordered or ranked set. This document specifies an
extension 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.
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].
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.
'Ranking Schemes' identify the specific rules for how the numeric
ranking values within a 'Ranking Domain' are to be interpreted.
3.1. The 'r:scheme' element
Ranking Schemes are defined using the r:scheme element.
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rankingScheme = element r:scheme {
atomCommonAttributes,
attribute name { IRI }?,
attribute label { text }?,
attribute significance { 'ascending' | 'descending' }?,
attribute precision { nonNegativeInteger }?,
attribute step { decimal }?,
attribute minimum { decimal }?,
attribute maximum { decimal }?
}
o The 'name' attribute provides a universally unique identifier for
the scheme in the form of an absolute IRI.
o The 'label' attribute specifies a Language-Sensitive, human
readable label for the 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 total number of decimal
digits allowed in 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 'step' attribute specifies the minimum significant increment
for numeric ranking values. For instance, if a scheme is based on
full and half values (e.g. 0, 0.5, 1.0, 1.5, etc), the step
attribute value would be 0.5. If not specified, no significant
increment is considered to apply. Numeric ranking values that do
not precisely match the pattern specified by the step attribute
MUST be rounded down to the nearest significant increment (e.g.,
in the half step example given above, the value 0.85 is equivalent
to the value 0.5).
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.
An Atom feed element MAY contain any number of r:scheme elements. A
Feed MUST NOT contain more than one r:scheme element with the same
name.
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...
...
3.2. The 'r:rank' element
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 with the same
combination of domain and scheme attribute values.
rankingValue = element r:rank {
atomCommonAttributes,
attribute domain {IRI}?,
attribute scheme {IRI}?,
attribute label { text }?,
{ decimal }
}
The 'domain' attribute identifies the Ranking Domain. See "Ranking
Domain Scope" for details.
The 'scheme' attribute identifies the Ranking Scheme. If not
specified, the scheme is assumed to be the Default Ranking Scheme.
The 'label' attribute provides a Language-Sensitive, human readable
label for the ranking value.
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.
3.5
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4. Ranking Domain Scope
Ranking Domain's provide a logical mechanism used to associate a
numerically ranked set of resources. Ranking Domains are identified
by absolute or relative 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
Numeric ranking values that do not specify a domain attribute are
associated with the Feed Ranking Domain. The IRI identity of the
Feed Ranking Domain is the same as the containing feed element's
atom:id element.
A numeric ranking associated with the Feed Ranking Domain
3.5
The set of entries contained within the Feed Ranking Domain is
limited to the set of entries contained within the feed.
Numeric rankings that specify a domain equal or equivalent to the in-
scope Base URI are associated with a Document Ranking Domain
identified by the in-scope Base URI.
A numeric ranking associated with the Document Ranking Domain
3.5
The set of entries contained within the Document Ranking Domain is
limited to the set of entries contained within the Document
identified by the Base URI.
Numeric ranking values that specify any IRI value other than the Base
URI of the containing document and the atom:id of the containing feed
are associated with a Domain Scope.
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A numeric ranking associated with the Feed Ranking Domain
3.5
Domain Scoped SHOULD be considered to be open sets consistings of
entries from any number of feeds.
4.1. The Document Ranking Domain and xml:base
When specifying or processing rankings in the Document Ranking
Domain, implementors need to be aware of the possible sides effect of
using the xml:base attribute.
For instance, in the example,
...
tag:example.org,2005:1
3.5
...
tag:example.com,2005:1
3.5
...
tag:example.com,2005:2
3.5
...
The three entries contained by the feed exist in two separate
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Document Ranking Domains, each respectively identified by the URI's
"http://example.org/feed.xml" and
"http://example.com/anotherfeed.xml".
It is posible for the set of entries within a Document Ranking Domain
to span multiple Atom Feed Documents if those documents share the
same Base URI as specified by mechanisms such as xml:base or the
Content-Location HTTP header.
5. Ranking Domain and Scheme Identifiers
The IRI's identifying Ranking Domains and Ranking Schemes are subject
to the same construction and comparison rules as the atom:id element.
Comparison's of domain and scheme identifiers MUST be performed on a
case-sensitive, character-by-character basis solely on the IRI
character strings and MUST NOT rely on dereferencing the IRI's or
URI's mapped from them.
6. Processing Rankings
Processing a Ranking Domain involves the following steps:
Select the Ranking Scheme.
Identify the Ranking Domain
Identify the available set of entries containing numeric ranking
values within the identified Ranking Domain using the selected
Ranking Scheme.
Remove from the set all entries whose rankings fall outside the
minimum and maximum values set by the selected Ranking Scheme.
Sort the remaining set of ranked entries according to the
significance and precision of the numeric ranking as specified by
the Ranking Scheme.
7. Default Ranking Scheme
Feeds MAY contain ranked entries that have no specified scheme. In
such cases the Default Ranking Scheme should be applied.
o label = 'Default Ranking'
o significance = 'ascending'
o precision = unspecified
o step = unspecified
o minimum = 0
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o maximum = unspecified
8. Well-Known Ranking Schemes
Feeds MAY contain ranked entries whose ranking scheme cannot be
resolved (i.e., no r:scheme with a 'name' attribute matching the
rankings "scheme" attribute can be found). In such cases software
implementations MAY attempt to match such rankings to well-known
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:scheme element
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. (The format of such a publication is
beyond the scope of this specification.)
A hypothetical search engine ranking using a well-known scheme
5
If a Ranking Scheme cannot be resolved this way (e.g., no r:scheme
with a matching 'name' attribute can be found and the scheme is not
well-known), the Default Ranking Scheme should be applied.
Further, it is possible that a processor may resolve multiple Ranking
Schemes for a given Ranking. In such cases, the processor may decide
which Scheme to apply.
9. Security Considerations
Because this specification defines an extension to the Atom
Syndication Format [RFC4287], it is subject to the same security
consideration as defined in section 8 of that specification.
10. IANA Considerations
There are no IANA considerations introduced by this specification.
11. 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.
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[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.
Appendix A. Acknowledgements
The author gratefully acknowledges the feedback from Andreas Sewe,
and the Atom Publishing working group during the development of this
specification.
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Author's Address
James M Snell
Phone:
Email: jasnell@gmail.com
URI: http://www.snellspace.com
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