Network Working Group J. Snell Internet-Draft April 10, 2006 Expires: October 12, 2006 Atom Ranking Extensions draft-snell-atompub-feed-index-08.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 12, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document defines an extension for numerically ranking entries within a syndication feed. Snell Expires October 12, 2006 [Page 1] Internet-Draft Atom Rank April 2006 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 Snell Expires October 12, 2006 [Page 2] Internet-Draft Atom Rank April 2006 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. Snell Expires October 12, 2006 [Page 3] Internet-Draft Atom Rank April 2006 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. Snell Expires October 12, 2006 [Page 4] Internet-Draft Atom Rank April 2006 ... ... 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 Snell Expires October 12, 2006 [Page 5] Internet-Draft Atom Rank April 2006 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. Snell Expires October 12, 2006 [Page 6] Internet-Draft Atom Rank April 2006 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 Snell Expires October 12, 2006 [Page 7] Internet-Draft Atom Rank April 2006 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 Snell Expires October 12, 2006 [Page 8] Internet-Draft Atom Rank April 2006 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. Snell Expires October 12, 2006 [Page 9] Internet-Draft Atom Rank April 2006 [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. Snell Expires October 12, 2006 [Page 10] Internet-Draft Atom Rank April 2006 Author's Address James M Snell Phone: Email: jasnell@gmail.com URI: http://www.snellspace.com Snell Expires October 12, 2006 [Page 11] Internet-Draft Atom Rank April 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Snell Expires October 12, 2006 [Page 12]