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Re: Dereferencing a URI vs querying a SPARQL endpoint

From: Kingsley Idehen <kidehen@openlinksw.com>
Date: Thu, 21 May 2009 07:28:31 -0400
Message-ID: <4A153ADF.7010107@openlinksw.com>
To: Daniel Schwabe <dschwabe@inf.puc-rio.br>
CC: public-lod@w3.org, semantic-web@w3.org
Daniel Schwabe wrote:
> Kingsley Idehen wrote:
>> Pierre-Antoine Champin wrote:
>>> I would expect that a DESCRIBE query to the SPARQL endpoint return what
>>> I get when dereferencing the URI.
>>>   pa
>> Daniel,
>> Is this your problem:
>> Linked Data Servers publish URIs. The mechanism that delivers these 
>> URIs tends to vary since they are the product of URL-rewrite rules 
>> that may or may not be associated with SPARQL queries, and when 
>> SPARQL Query based you may be dealing with a CONSTRUCT or a DESCRIBE.
> If a URI actually refers to an RDF document, I would imagine there is 
> no URL rewriting involved; it resolves to the document itself. 
On the surface yes, but not necessarily under the covers. For instance, 
in Virtuoso we have virtual WebDAV resources and most of the time this 
is what you are interacting with when you request an RDF information 
resource (we call these DETs - Dynamic Extensions Types for our WebDAV 
> And for SPARQL based, who knows what I may be dealing with? Peter 
> already exemplified that you may get something that is neither a 
>> Ideally, you would like to be able to discern via SPARQL, what SPARQL 
>> query patterns sits behind the re-write rule for a given 
>> de-referencable URI.
> Basically yes, although I'm not even requiring being able to do it 
> directly via SPARQL (that would be actually nice)...
If you can describe a SPARQL Query in RDF, it can be done. SPIN from 
TopQuadarant [1] does have a specification for doing just that.
> Also, I'd be curious to know what is more efficient - dereferencing or 
> issuing the query through the endpoint.
Depends, in the case of Virtuoso you're talking different paths to the 
same mechanics i.e. a SPARQL Query and information resource caching schemes.


1. http://spinrdf.org/sp.html

> Cheers
> D



Kingsley Idehen	      Weblog: http://www.openlinksw.com/blog/~kidehen
President & CEO 
OpenLink Software     Web: http://www.openlinksw.com
Received on Thursday, 21 May 2009 11:29:25 UTC

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