- From: Dan Connolly <connolly@w3.org>
- Date: Thu, 14 Aug 2008 17:47:25 -0500
- To: Alan Ruttenberg <alanruttenberg@gmail.com>
- Cc: Jonathan Rees <jar@creativecommons.org>, "www-tag@w3.org WG" <www-tag@w3.org>
On Thu, 2008-08-14 at 13:36 -0400, Alan Ruttenberg wrote: > On Aug 14, 2008, at 1:32 PM, Dan Connolly wrote: > > > On Thu, 2008-08-14 at 13:09 -0400, Alan Ruttenberg wrote: > > [...] > >> Why is the note necessary? In fact it seems backwards: > >> > >>>>>>> "The FROM NAMED syntax suggests that the IRI identifies the > >>>>>>> corresponding graph, but the relationship between an IRI and a > >>>>>>> graph > >>>>>>> in an RDF dataset is indirect. The IRI identifies a resource, > >>>>>>> and the > >>>>>>> resource is represented by a graph (or, more precisely: by a > >>>>>>> document that serializes a graph). For further details see > >>>>>>> [WEBARCH]." > >> > >> > >> That is, the IRI identifies the graph, and the representation is a > >> serialization of that graph. How is it different than asking for any > >> document, and getting back either html, or rtf, or whatever? > > > > Graphs are like integers or strings; they don't have state. > > Documents, in the sense of "the front page of the new york times" > > do have state; i.e. they change over time. > > Not all documents have state. Very well, let's both be more explicit with our quantifiers. You used the term 'document' in your question; there are two relevant senses of the word: (1) in the sense of "the front page of the new york times" (2) in the sense of an XML document, i.e. a sequence of characters (or perhaps bytes) I wanted to clarify that I understood you to be using sense (1) which is pretty close to webarch:InformationResource . And yes, not every document in the 1st sense changes; documents such as RFC 822 are documents in both the 1st and 2nd sense (which leads to nearly no end of confusion; hence the distinct term information resource) > > To say that <http://example.com/graph1> identifies a graph > > leads to a contradiction when two different GET requests > > return different graphs/representations. > > How could this happen if they can't change? (as you posit above) > BTW, it's fine for two different representations to be returned - > doesn't mean the graph(= underlying resource) changed. Well, yes, it's fine for different representations to be returned, but combined with the "from URI identifies the graph", it leads to conclusions. Consider: select ?P from <http://nytimes.example/stocks> { _:someStock nyse:sym "XOM"; nyse:price ?P } Two parties run the query. When p1 sends the query to a SPARQL endpoint, which turns around a does a GET where the response is 200 Date: ... 14:00 Content-Type: text/turtle <#xom> nyse:sym "XOM"; nyse:price 10. If the SPARQL spec said that the from URI identifies the graph represented in the response, we would have <http://nytimes.example/stocks> owl:sameAs { <#xom> nyse:sym "XOM"; nyse:price 10. }. OK so far? (is it helpful to use N3 to formalize the discussion?) Then when p2 sends the query to a SPARQL endpoint which does a GET where the response is: 200 Date: ... 14:20 Content-Type: text/turtle <#xom> nyse:sym "XOM"; nyse:price 12. Again, if the SPARQL spec said that the from URI identifies the graph represented in the response, we'd have: <http://nytimes.example/stocks> owl:sameAs { <#xom> nyse:sym "XOM"; nyse:price 12. }. The graphs are distinct, right? One has a triple with 10 in the object spot and one doesn't. Formally, that is: { <#xom> nyse:sym "XOM"; nyse:price 10. } owl:differentFrom { <#xom> nyse:sym "XOM"; nyse:price 12. } Substituting equals for equals, we get: <http://nytimes.example/stocks> owl:differentFrom <http://nytimes.example/stocks> . And that's a contradiction. And that's why the SPARQL spec does *not* say that the from URI identifies the graph you happen to GET there, but rather, clarifies that the relationship is indirect. Rather than <http://nytimes.example/stocks> owl:sameAs { <#xom> nyse:sym "XOM"; nyse:price 10. }. it's more like <http://nytimes.example/stocks> webarch:representation [ turtle:parsedGraph { <#xom> nyse:sym "XOM"; nyse:price 10. } ]. and as you observed, it's fine to have more than one representation; webarch:representation is not an owl:FunctionalProperty. -- Dan Connolly, W3C http://www.w3.org/People/Connolly/ gpg D3C2 887B 0F92 6005 C541 0875 0F91 96DE 6E52 C29E
Received on Thursday, 14 August 2008 22:47:45 UTC