- From: Dan Connolly <connolly@w3.org>
- Date: Fri, 06 Apr 2001 08:54:05 -0500
- To: Pierre-Antoine CHAMPIN <champin@bat710.univ-lyon1.fr>
- CC: www-rdf-interest@w3.org, www-rdf-logic@w3.org
Pierre-Antoine CHAMPIN wrote: [...] > HTML and PDF version available at > http://www710.univ-lyon1.fr/~champin/urls/ This document promulgates a number of myths about Web Architecture... "However, other W3C recommendations use URLs to identify namespaces [6,8,11]. Those URLs do not locate the corresponding namespaces (which are abstract things and hence not locatable with the HTTP protocol), " I don't see any justification for the claim that namespaces are disjoint from HTTP resources. One of the primary motivations for the XML namespaces recommendation is to make the Web self-describing: each document carries the identifiers of the vocabularies/namespaces it's written in; the system works best when you can use such an identifier to GET a specification/description of the vocabulary. "For example, the URI of the type property of RDF is http://www.w3.org/1999/02/22-rdf-syntax-ns#type. As a matter of fact, the property itself is not located by that URL: its description is. " Again, I see no justification for the claim that this identifier doesn't identify a property. "URLs are transient That means they may become invalid after a certain period of time [9]. " That's a fact of life in a distributed system. URNs may become invalid after a period of time too. It's true of all URIs. URIs mean what we all agree that they mean. Agreement is facilitated by a lookup service like HTTP. In practice, URIs are quite reliable: 6% linkrot according to http://www.useit.com/alertbox/980614.html and I think the numbers get better when you measure per-request rather than per-link, since popular pages are maintained more activly than average. Unless urn: URIs provide value that http: URIs do not, the won't be deployed. I think the fact that (a) urn:'s have been standardized (IETF Proposed Standard, that is) since 1995 (b) support for them is available in popular browsers and has been for several generations and yet (c) still their use is negligible speaks for itself. They don't provide any value. Naming is a social contract, and the http: contract works and the urn: contract doesn't. "In the immediate interpretation, a URL identifies the resource retrieved through it." to be precise: it identifies the resource accessed thru it. In the general case, you can't retrieve a resource, but only a representation of one. Other text that makes this error includes: "... the retrieved resource ..." Another falsehood: "Contrarily to URLs, URNs (Uniform Resource Names) are designed to persistently identify a given resource." URIs in general are designed to persistently identifiy a given resource. Especially HTTP URIs. I recommend a series of articles by the designer of URIs, HTTP, and HTML to clarify a number of these myths: World Wide Web Design Issues http://www.w3.org/DesignIssues/ esp The Web Model: Information hiding and URI syntax (Jan 98) http://www.w3.org/DesignIssues/Model The Myth of Names and Addresses http://www.w3.org/DesignIssues/NameMyth Persistent Domains- an idea for persistence of URIs(2000/10) http://www.w3.org/DesignIssues/PersistentDomains (Hmm... this one is an interesting idea, but I think freenet: might be easier to deploy.) and regarding the intent of the design of namespaces, see: cf Web Architecture: Extensible Languages W3C Note 10 Feb 1998 http://www.w3.org/TR/NOTE-webarch-extlang -- Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Friday, 6 April 2001 09:54:09 UTC