- From: Danny Ayers <danny666@virgilio.it>
- Date: Tue, 22 Jun 2004 13:20:49 +0200
- To: www-rdf-interest@w3.org
- Cc: distobj@acm.org, ken@bitsko.slc.ut.us, Norman.Walsh@Sun.COM
I'm seeking a little clarification of the nature of identity on the web, several related issues involved. In short, here are three little puzzles: 1. Can a resource have multiple different representations of the same type? 2. If two resources have different sets of representations, can they ever be considered the same? 3. How can I ever assert: <http://mydomain.org/a> owl:sameAs <http://yourdomain.org/a> . when I can't ever be sure that you aren't going to change http://yourdomain.org/a ? Ok, why I think these might be puzzles. Any statement involving URI(ref)s presuppose the coolness assumption - the mapping between the resource and its identifier won't change. Generally this seems good enough in practice to be workable, particularly as the URI will only be used as a name. But again in practice it seems to me that there is potential for problems when we start talking in terms of two URIs identifying the same resource, as in the owl:sameAs above. Fielding says of URIs [1] : [[ The naming authority that assigned the resource identifier, making it possible to reference the resource, is responsible for maintaining the semantic validity of the mapping over time (i.e., ensuring that the membership function does not change). ]] The OWL Reference says [2]: [[ The built-in OWL property owl:sameAs links an individual to an individual. Such an owl:sameAs statement indicates that two URI references actually refer to the same thing: the individuals have the same "identity". ]] So I can say, hand on heart: <http://mydomain.org/a> owl:sameAs <http://mydomain.org/b> . but unless I have total authority over you and your minions (or at least some permanent binding agreement), how can I say: <http://mydomain.org/a> owl:sameAs <http://yourdomain.org/a> . Ok, the same problem applies to practically anything I might want to say about URIs for which I don't have authority, and a bit of handwaving and the implication that it was true when I said is enough to enable reasoning on names defined in different domains. However, I could be wrong, but it seems to me that equivalence/shared identity is a special case due to the way URIs work in practice, because of the notion of representations. If two URIs have the same identity, then surely they will have the same set of representations. Which, I would suggest, is likely to be unusual on the real web. This has come up in the context of the Atom syndication language, where a suggested approach to dealing with multiple occurences of the same entry across the web is to use a non-resolvable URI to identify items, with another URI used to provide a locator for representations of the items. i.e. a URN and a URL (A completely non-normative description of this approach can be found at [3]). It does seem to be bending the notion of URIs somewhat (as a workaround for uncoolness), it's saying that resource A has no representations except the representations of resource B. But perhaps there is some consistent way of expressing URI equivalence that can capture this. For a use case, consider a homepage at http://example.org/1994 - the original owner has long gone, this doesn't resolve into anything. But the material is still available at http://archive.org/example.org/1994. As it happens, it's now looking like the supposed gains for dual URN+URL in Atom doesn't actually bring much benefit anyhow (e.g. threading between entries will have to refer back to the URN, so a reliable URL still needs to be discovered). Cheers, Danny. [1] http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_2_1_1 [2] http://www.w3.org/TR/2004/REC-owl-ref-20040210/#sameAs-def [3] http://diveintomark.org/archives/2004/05/28/howto-atom-id -- Raw http://dannyayers.com
Received on Tuesday, 22 June 2004 07:22:55 UTC