Nice. Looks like this will address the use case of categories in single-cardinality resource responses. Appendix B seems a little confused: <quote> Appendix B. Notes on use with Atom Where the cardinality is known to be one (for example, when retrieving an individual resource) it MAY be preferable to render the resource natively over HTTP without Atom structures. In this case the contents of the atom:content element SHOULD be returned as the HTTP entity-body and metadata including the type attribute and atom: category element(s) via HTTP header-field(s). This approach SHOULD NOT be used where the cardinality is guaranteed to be one (for example, search results which MAY return one result). </quote> The last sentence should say "where the cardinality is *not* guaranteed to be one", I believe. .. Shlomo On Wed, Jul 1, 2009 at 11:03 PM, Sam Johnston<samj@samj.net> wrote:
On Sun, Jun 28, 2009 at 10:44 PM, <shlomo.swidler@gmail.com> wrote:
On Sun, Jun 28, 2009 at 3:48 AM, Sam Johnston<samj@samj.net> wrote:
The categories were derived from Atom so in dropping it for individual resources we're going to have to reinvent it (think HTTP headers). <snip> I would prefer to have the categories implemented in the HTTP headers for single-resource responses.
Ok so I've written up an IETF Internet-Draft (draft-johnston-http-category-header-00) which specifies a Category: HTTP header based on atom:category (that is, with a term and optional scheme and label). This means there's one less resason to use Atom for individual resources. I've also devised (but not yet documented) a simple, intuitive mechanism for updating (or at least suggesting updates) to these metadata headers.
Although we can't reference I-Ds directly from the OCCI standard (as they're only valid for 6 months from the most recent revision), even if it doesn't proceed to a permanent status the content is also available under the usual OGF IPR rules and CC-BY-SA for good measure.
Sam
Internet Engineering Task Force S. Johnston Internet-Draft Australian Online Solutions Intended status: Experimental July 1, 2009
Expires: January 2, 2010
Web Categories draft-johnston-http-category-header-00
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Abstract
This document specifies the Category header-field for HyperText Transfer Protocol (HTTP), which enables the sending of taxonomy information in HTTP headers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Categories . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The Category Header Field . . . . . . . . . . . . . . . . . . . 4 3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Category Header Registration . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Internationalisation Considerations . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 6 Appendix A. Notes on use with HTML . . . . . . . . . . . . . . . . 7 Appendix B. Notes on use with Atom . . . . . . . . . . . . . . . . 7
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . . 8 Appendix D. Document History . . . . . . . . . . . . . . . . . . . 8 Appendix E. Outstanding Issues . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
A means of indicating categories for resources on the web has been defined by Atom [RFC4287]. This document defines a framework for
exposing category information in the same format via HTTP headers.
The atom:category element conveys information about a category associated with an entry or feed. A given atom:feed or atom:entry element MAY have zero or more categories which MUST have a "term"
attribute (a string that identifies the category to which the entry or feed belongs) and MAY also have a scheme attribute (an IRI that identifies a categorization scheme) and/or a label attribute (a human-readable label for display in end-user applications).
Similarly a web resource may be associated with zero or more categories as indicated in the Category header-field(s). These categories may be divided into separate vocabularies or "schemes"
and/or accompanied with human-friendly labels.
[[ Feedback is welcome on the ietf-http-wg@w3.org mailing list, although this is NOT a work item of the HTTPBIS WG. ]]
1.1. Requirements Language
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.
This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616], and explicitly includes the following rules from it:
quoted-string, token. Additionally, the following rules are included from [RFC3986]: URI.
2. Categories
In this specification, a category is a grouping of resources by 'term', from a vocabulary ('scheme') identified by an IRI [RFC3987].
It is comprised of:
o A "term" which is a string that identifies the category to which the resource belongs.
o A "scheme" which is an IRI that identifies a categorization scheme
(optional).
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o An "label" which is a human-readable label for display in end-user
applications (optional).
A category can be viewed as a statement of the form "resource is from the {term} category of {scheme}, to be displayed as {label}", for example "'Loewchen' is from the 'dog' category of 'animals', to be
displayed as 'Canine'".
3. The Category Header Field
The Category entity-header provides a means for serialising one or more categories in HTTP headers. It is semantically equivalent to
the atom:category element in Atom [RFC4287].
Category = "Category" ":" #category-value category-value = term *( ";" category-param ) category-param = ( ( "scheme" "=" <"> scheme <"> )
| ( "label" "=" quoted-string ) | ( "label*" "=" enc2231-string ) | ( category-extension ) ) category-extension = token [ "=" ( token | quoted-string ) ]
enc2231-string = <extended-value, see [RFC2231], Section 7> term = token scheme = URI
Each category-value conveys exactly one category but there may be multiple category-values for each header-field and/or multiple
header-fields per [RFC2616].
Note that schemes are REQUIRED to be absolute URLs in Category headers, and MUST be quoted if they contain a semicolon (";") or comma (",") as these characters are used to separate category-params
and category-values respectively.
The "label" parameter is used to label the category such that it can be used as a human-readable identifier (e.g. a menu entry). Alternately, the "label*" parameter MAY be used encode this label in
a different character set, and/or contain language information as per [RFC2231]. When using the enc2231-string syntax, producers MUST NOT use a charset value other than 'ISO-8859-1' or 'UTF-8'.
3.1. Examples
NOTE: Non-ASCII characters used in prose for examples are encoded using the format "Backslash-U with Delimiters", defined in Section 5.1 of [RFC5137].
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For example: Category: dog
indicates that the resource is in the "dog" category.
Category: dog; label="Canine"; scheme="http://purl.org/net/animals"
indicates that the resource is in the "dog" category, from the
"http://purl.org/net/animals" scheme, and should be displayed as
"Canine".
The example below shows an instance of the Category header encoding multiple categories, and also the use of [RFC2231] encoding to represent both non-ASCII characters and language information.
Category: dog; label="Canine"; scheme="http://purl.org/net/animals", lowchen; label*=UTF-8'de'L%c3%b6wchen";
scheme="http://purl.org/net/animals/dogs"
Here, the second category has a label encoded in UTF-8, uses the German language ("de"), and contains the Unicode code point \u'00F6' ("LATIN SMALL LETTER O WITH DIAERESIS").
4. IANA Considerations
4.1. Category Header Registration
This specification adds an entry for "Category" in HTTP to the Message Header Registry [RFC3864] referring to this document:
Header Field Name: Category Protocol: http Status: standard Author/change controller: IETF (iesg@ietf.org) Internet Engineering Task Force
Specification document(s):
[ this document ]
5. Security Considerations
The content of the Category header-field is not secure, private or integrity-guaranteed, and due caution should be exercised when using it.
6. Internationalisation Considerations
Category header-fields may be localised depending on the Accept-
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Language header-field, as defined in section 14.4 of [RFC2616].
Scheme IRIs in atom:category elements may need to be converted to URIs in order to express them in serialisations that do not support
IRIs, as defined in section 3.1 of [RFC3987]. This includes the Category header-field.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, December 2005.
[RFC5137] Klensin, J., "ASCII Escaping of Unicode Characters", RFC 5137, February 2008.
7.2. Informative References
[OCCI] Open Grid Forum (OGF), Edmonds, A., Metsch, T., Johnston, S., and A. Richardson, "Open Cloud Computing Interface (OCCI)", <http://purl.org/occi>.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
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RFC 2068, January 1997.
[W3C.REC-html401-19991224] Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
Specification", <http://www.w3.org/TR/1999/REC-html401-19991224>.
[W3C.WD-html5-20090423]
Hyatt, D. and I. Hickson, "HTML 5", April 2009,
<http://www.w3.org/TR/2009/WD-html5-20090423>.
[draft-nottingham-http-link-header] Nottingham, M., "Web Linking",
draft-nottingham-http-link-header-05 (work in progress), April 2009.
[rel-tag-microformat] Celik, T., Marks, K., and D. Powazek, "rel="tag" Microformat", <http://microformats.org/wiki/rel-tag>.
Appendix A. Notes on use with HTML
In the absence of a dedicated category element in HTML 4 [W3C.REC-html401-19991224] and HTML 5 [W3C.WD-html5-20090423], category information (including user supllied folksonomy
classifications) MAY be exposed using HTML A and/or LINK elements by concatenating the scheme and term: category-link = scheme term scheme = URI term = token
These category-links MAY form a resolveable "tag space" in which case
they SHOULD use the "tag" relation-type per [rel-tag-microformat].
Alternatively META elements MAY be used:
o where the "name" attribute is "keywords" and the "content"
attribute is a comma-separated list of term(s)
o where the "http-equiv" attribute is "Category" and the "content" attribute is a comma-separated list of category-value(s)
Appendix B. Notes on use with Atom
Where the cardinality is known to be one (for example, when retrieving an individual resource) it MAY be preferable to render the
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resource natively over HTTP without Atom structures. In this case the contents of the atom:content element SHOULD be returned as the
HTTP entity-body and metadata including the type attribute and atom: category element(s) via HTTP header-field(s).
This approach SHOULD NOT be used where the cardinality is guaranteed to be one (for example, search results which MAY return one result).
Appendix C. Acknowledgements
The author would like to thank Mark Nottingham for his work on Web Linking [draft-nottingham-http-link-header] (on which this document was based) and to the authors of [RFC2068] for specification of the
Link: header-field on which this is based.
The author would like to thank members of the OGF's Open Cloud Computing Interface [OCCI] working group for their contributions and others who commented upon, encouraged and gave feedback to this
draft.
Appendix D. Document History
[[ to be removed by the RFC editor should document proceed to publication as an RFC. ]]
-00
* Initial draft based on draft-nottingham-http-link-header-05
Appendix E. Outstanding Issues
[[ to be removed by the RFC editor should document proceed to publication as an RFC. ]]
The following issues are oustanding and should be addressed:
1. Is extensibility of Category headers necessary as is the case for Link: headers? If so, what are the use cases?
2. Is supporting multi-lingual representations of the same category(s) necessary? If so, what are the risks of doing so?
3. Is a mechanism for maintaining Category header-fields required? If so, should it use the headers themselves or some other mechanism?
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4. Does this proposal conflict with others in the same space? If so, is it an improvement on what exists?
Author's Address
Sam Johnston Australian Online Solutions GPO Box 296 Sydney, NSW 2001
Email: samj@samj.net URI: http://samj.net/
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