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Misha and group:
my - a bit cynical - answer is: it depends on what you want to achive. This is my summary:
a) in the scope of creating valid URIs [1]: both terminating characters can be used
b) in the scope of creating URI References as used by RDF [2]: both may be used, but in the scope of practical use and implementations the # is clearly preferred
c) in the scope of publishing RDF vocabularies [3]: both can be used.
This all again more in detail:
Prerequisite: the issue is: the IPTC G2-Standards define that a scheme must have a URI for identifying it and codes representing members of the scheme shall be concatenated to this Scheme URI and the result should be again a valid URI - and preferentially a URL which resolves to a web resource.
re a) the scope which is the easiest to discuss: the current URI RFC allows a URIs with both a "/" and a "#" at the end, and appending a code is not big issue as most of the Unicode characters can be used.
re b) Things get more complex in this scope as the RDF documents are contentious about what a URI reference exactly is: an identifier - in any case - which can be resolved to a web resource ... hm, this excludes URNs per se. I guess this comes i) from different editors for the six basic RDF document and ii) that in some document an explicit or implicit assumption is made: RDF is about the Web, thus all URIs must be web URLs = http: URLs.
Go provide some references to corresponding sections in RDF document:
- in the RDF Concepts [4], under 6.4 on can read: " ...and would produce a valid URI character sequence (per RFC2396 [URI], sections 2.1) representing an absolute URI with optional fragment identifier when subjected to the encoding described below. "
- in the RDF Semantics [5], under 1.2 one can read: (2 quotes follow)
-- "This document does not take any position on the way that URI references may be composed from other expressions, e.g. from relative URIs or QNames; the semantics simply assumes that such lexical issues have been resolved in some way that is globally coherent, so that a single URI reference can be taken to have the same meaning wherever it occurs."
-- "The semantics does not assume any particular relationship between the denotation of a URI reference and a document or Web resource which can be retrieved by using that URI reference in an HTTP transfer protocol, or any entity which is considered to be the source of such documents. Such a requirement could be added as a semantic extension, but the formal semantics described here makes no assumptions about any connection between the denotations of URI references and the uses of those URI references in other protocols."
My comment: this sounds very aloof regarding the relationship between URIs as identifier and http URLs - it tells in other words: others may add more specific definitions.
- in the RDF Primer [6], on can read under 2.1, Basic Concepts:
-- "Because of this generality, RDF uses URIs as the basis of its mechanism for identifying the subjects, predicates, and objects in statements. To be more precise, RDF uses URI references [URIS]. A URI reference (or URIref) is a URI, together with an optional fragment identifier at the end. For example, the URI reference http://www.example.org/index.html#section2 ...."
My comment: This chapter is in its first section about the subject of RDF triples and it is quite obvious that this is a http URL for all Web resources - and RDF is made for them. But in this later section it extends this view to URIs for predicates and objects - and this is much less obvious.
The major problem with this http-URL focused thinking is that even standards are tweaked to abide to it. Example: the XML Schema specification provides this namespace URL: "http://www.w3.org/2001/XMLSchema". When it comes to RDF the URL is suddenly "http://www.w3.org/2001/XMLSchema#" - to be strict: that are two different namespaces.
Another major issue is: I got aware that many people working in the RDF area assume that the # termination is a MUST for RDF and many software vendors do not support a / at the end of a namespace URI - this results in URIs like this one: http://namespace.example.org/ns1/#predicate1" if a namespace is defined as http://namespace.example.org/ns1/ and a predicate of "predicate1" is applied to it. Apparently the software checks if a # is in the namespace URI and if not it is appended.
An important issue regarding http-URLs is fragment values: in case of referring to a section of an XHTML document the @id of the corresponding tag (e.g. <p>, or <div>) is used. This approach inherits the lexical space limitations for @id in XML documents: e.g. not digit is allowed as first character. And this was one of the main reasons for the IPTC not to adopt the # as recommended termination character as this would exclude any code (of a scheme) starting with a digit.
re c) This document explicitly - and completely in parallel - discribes how vocabularies with a # (hash) or a / (slash) at the end of the scheme's URI can be accessed and retrieved.
Happy considerations and drawing conclusions
Michael
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[1] RFC 3986 - http://tools.ietf.org/html/rfc3986
[2] http://www.w3.org/RDF/
[3] http://www.w3.org/TR/swbp-vocab-pub/
[4] http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Graph-URIref
[5] http://www.w3.org/TR/rdf-mt/#urisandlit
[6] http://www.w3.org/TR/rdf-primer/#basicconcepts
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On 4 Nov 2009 at 18:21 misha.wolf@... wrote:
The IPTC News Architecture group is discussing, among other matters, the pros and cons of "#" and "/" in the construction of URIs representing concepts. Your input is welcome.
Regards,
Misha
From: Wolf, Misha (M Cont Ent)
Sent: 04 November 2009 18:19
To: 'iptc-news-architecture-dev@yahoogroups.com'
Subject: RE: [IPTC-NAR-dev] Scheme URIs
The Semantic Web allows both "#" and "/". In deciding which to use, one must consider a number of things, including:
- the intended UI
- the implementation
The "#" approach lends itself to describing an entire Scheme on one static Web page.
The "/" approach implies a flat directory, containing many items.
In reality, software can be made to deliver any UI, based on either mechanism.
This W3C document discusses very detailed implementation issues for the two options:
Best Practice Recipes for Publishing RDF Vocabularies
Regards,
Misha
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