- From: Paskin, Norman (DOI-ELS) <n.paskin@doi.org>
- Date: Mon, 9 Apr 2001 09:05:16 +0100
- To: www-rdf-interest@w3.org
I think Lee's posting hits the nail squarely on the head here. Allow me to mention for those who may not be aware the DOI (Digital Object Identifer) work we run (www.doi.org), which allocates names, i.e. URIs (or URNs, depending on your religion...) to entities as first class objects; and adds to this a separate resolution system (Handle) which I think is excellent; and structured, extensible (not Dublin Core based) metadata. The focus is management of intellectual property entities a.k.a. "content". Rather than argue about the abstract and what ought to be, we are doing it. Comments, participation, and constructive criticism welcomed. Norman Paskin International DOI Foundation (member of W3C) -----Original Message----- From: Lee Jonas [mailto:ljonas@acm.org] Sent: 06 April 2001 21:44 To: www-rdf-interest@w3.org Subject: Re: URIs / URLs Yet the task of keeping your own links current is a real problem, particularly for sites with large numbers of them such as search engines. URLs are not without their problems. I dread to think how many man hours and CPU cycles have been spent around the globe ensuring links to other pages still work. Additionally, the web is always 'current'. There is no formal concept of version, the W3C and most other self-respecting organisations implement the necessary measures themselves (for representing Working Draft versions) in an adhoc fashion. The notion of URLs identifying representations seems a little trite to me. It indicates the nature of the true problem, without fully addressing it: a resource at the end of a location is not consistent over time. This is for at least two good reasons: resources evolve, and resources move / disappear / or worse, a second resource ousts the first at a particular location. The first issue could have been addressed more formally (and hence consistently) with a simple versioning scheme. This would have alleviated the problem of instantly breaking third party links (or invalidating metadata semantics) when you change a resource. Yes your links must change to reflect new versions of things you reference, but these changes could be a graceful migration, not an abrupt crash. The second is the main bugbear of using a resource's location to identify it. This phenomenon is well known in distributed object technology. Superior solutions leave the actual resolution of an object's location to some distributed service when a client wants to interact with it. These are compounded with the fact that the resource can be one of many formats and there is no clear way to distinguish them from the URL iself. A resource such as http://mydomain/mypic.png may safely be assumed to be a png graphic, but what about the resource at the end of http://mydomain/mydir/ ? Mime types have become pervasive for identifying a resource's type, yet URLs predate MIME by years. If you want to know its type you have to make a request to some server process. It may be that URLs have been sufficient so far because the Internet is predominently for human consumption, and identifying abstract resources that cannot be retrieved electronically has been pretty pointless to date. However, aspects of the semantic web vision (plus other technologies like WebServices, i.e. .Net?) could add a couple of new issues to the pot: 1) It may become more common to reason about abstract resources whose identifiers may not be readily representable as a location. It would be better to identify these with a URN. Hence URNs may be more widely used than at present. 2) Encouraging reuse of RDF schema resources is antagonised by the need for schema owners to update them. Currently the proposed RDFS solution is very MS COMesque - every schema update is a new resource that SHOULD be constant and available in perpituity. IMHO this approach is inferior to specifying a common versioning scheme, which when coupled with a standard change management process, allows related schemas to migrate and evolve in tandem. After all, that's exactly what happens with dependencies between software components now. 3) Data quality will be poorer if it is hard for software to detect a resource change. Transience is bad news if you are going to store facts about something that subsequently changes. What the solution to all this is I don't know. I just can't help feeling that as the semantic web progresses things are about to get a lot more complicated unless these issues are addressed. Regards Lee Dan Connolly <connolly@w3.org> wrote: 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
Received on Monday, 9 April 2001 09:09:42 UTC