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Fwd: proposal to drop DESCRIBE from SPARQL

From: Alberto Reggiori <alberto@asemantics.com>
Date: Mon, 17 Jan 2005 15:45:12 +0100
Message-Id: <64BA5A94-6896-11D9-851A-0011242E4018@asemantics.com>
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>


This comment suggests HTTP GET HREF/SRC alike semantics for RDF/S the  
rdfs:seeAlso property, as an alternative to the DESCRIBE keyword (and  
query machinery) [1]. And replacing it by simply using a SELECT query  
on certain well-known properties (and  promoting an immediate "network  
effect" by leveraging on the protocol part).

While it is clear to me that by moving the "description of a resource"  
logic/problem out of the query language it would simplify the syntax  
and the server implementation burden -  I am not sure the rdfs:seeAlso  
proposal would be general enough to ask open questions to an arbitrary  
big RDF storage/service, perhaps containing several millions of  
triples. And when several combinations of bNodes are used to describe  
resources being queried.

I expect this needs further discussion in the WG - especially in the  
light of the upcoming WD for SPARQL query syntax.

Any thoughts?

Whom is supporting DESCRIBE?

Alberto

[1]  
http://lists.w3.org/Archives/Public/public-rdf-dawg/2004OctDec/ 
0359.html

Begin forwarded message:

> Resent-From: public-rdf-dawg-comments@w3.org
> From: Dan Brickley <danbri@w3.org>
> Date: January 17, 2005 2:30:10 PM CET
> To: public-rdf-dawg-comments@w3.org
> Cc: danbri@w3.org
> Subject: proposal to drop DESCRIBE from SPARQL
>
>
> (this is a personal review comment, like my others; it shouldn't
> be mistaken for a SWBPD or SWIG request for changes to SPARQL).
>
> I hereby propose you drop the DESCRIBE construct from SPARQL,
> and rework that part of the spec to show how queries can be
> written which ask for RDF documents in terms of their
> topics and other properties.
>
> Similar (perhaps in some ways better) functionality can be achieved by
> simply asking SPARQL questions which give as their answers
> references to RDF/XML documents.
>
> Refs:
> published WD of 2004-10-12,
> http://www.w3.org/TR/2004/WD-rdf-sparql-query-20041012/#describe
>
> live editor's copy at time of writing,
> http://www.w3.org/2001/sw/DataAccess/rq23/#describe
>
> The spec has been re-organized a bit, but the basic idea seems
> the same as published in the October WD, which is to provide a
> loose and flexible mechanism by which a server can return a
> useful bundle of information about some entities identified
> via query expressions. I should be clear here that I think this
> is a valuable facility to have available in SPARQL; my only
> objection is that it doesn't need a keyword in the language.
>
> Instead, I propose that queries which use the rdfs:seeAlso
> property could serve this purpose. If that property
> doesn't quite meet your needs, please look into designs
> which use similar properties.
>
>> From http://www.w3.org/TR/rdf-schema/#ch_seealso
>
> 	5.4.1 rdfs:seeAlso
>
> 	rdfs:seeAlso is an instance of rdf:Property that is used to indicate a
> 	resource that might provide additional information about the subject
> 	resource.
>
> 	A triple of the form:
>
> 	    S rdfs:seeAlso O
>
> 	states that the resource O may provide additional information about S.
> 	It may be possible to retrieve representations of O from the Web, but
> 	this is not required. When such representations may be retrieved, no
> 	constraints are placed on the format of those representations.
>
> 	The rdfs:domain of rdfs:seeAlso is rdfs:Resource. The rdfs:range of
> 	rdfs:seeAlso is rdfs:Resource.
>
> There are also some notes in the ESW wiki,
> http://esw.w3.org/topic/UsingSeeAlso which might be useful.
>
> Here's the first example from 10.3.2 editor's draft,
>
> 	PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
> 	DESCRIBE ?x
> 	WHERE    (?x foaf:mbox <mailto:alice@org> )
>
>
> Here's the seeAlso'd form I propose:
>
>         PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
> 	PREFIX rdfs:   <http://www.w3.org/2000/01/rdf-schema#>
>         SELECT ?doc
>         WHERE    (?x foaf:mbox <mailto:alice@org> )
> 	WHERE    (?x rdfs:seeAlso ?doc)
>
>
> Some notes on pro/cons:
>
> 1. the query is longer, but the query language is simpler
>
> 2. implementations barely change; as with DESCRIBE particular datasets
>  and services may or may not have anything to offer in response
>  to the query.
>
> 3. the query-based design is incrementally extensible - different
> types of description could be requested. We could constrain the
> query by (?doc dc:format "text/rdf+n3") or by reference to the
> rdf:type of the ?doc, eg. FOAF's "PersonalProfileDocument" or RSS1's
> notion of a channel, or by characteristics of the source of the
> describing statements (eg. we could ask for ?docs which have been
> digitally signed by people who match some SPARQL expression).
>
> 4. the main difference between the current keyword-based design and
> the property-centric alternative I'm proposing seems to be that in
> the latter, we get the actual RDF, in the former, we get a (potentially
> dangling, 404 etc) document reference. I'd expect a common setup to be
> that these are simply references back into the same SPARQL service,
> although similar queries could (depending on nature and whims of the
> dataset/service being asked) return rdfs:seeAlsos that point elsewhere
> in the Web. The property-centric approach could then mean more
> round-trips to the server --- is this a problem? But it also allows
> RDF to be used to describe the documents being referenced (eg. size in
> bytes, number of triples, etc) which could help clients be more
> efficient and selective.
>
> 5. Can we SELECT from the results of a DESCRIBE in a single SPARQL
> expression? I can see something like that making sense in terms of
> partitioning work between a client and server, eg. a dumb server  
> returns
> generic book reviews; local client selects out prices and ratings. If  
> this
> sort of thing is important, perhaps it is evidence for the existing
> keyword-based approach. But I'm not sure DESCRIBE works like that
> currently. There's also some relationship to N3's 'log:semantics'
> design, perhaps.  In N3, I believe I could query using rdfs:seeAlso
> expressions, and then dereference the RDF to populate a queriable
> context. I expect that in SPARQL, such things will get done in
> application code rather than in the QL.
>
> 6. How to deal with multiple topics?
>
> Another example (from Editor's copy)
>
> [[	More than one URI or can be given:
>
> 	PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
> 	DESCRIBE ?x ?y <http://example.org/>
> 	WHERE    (?x foaf:knows ?y)
> ]]
>
> This time we're asking for a chunk of RDF that describes
> things that are in a foaf:knows relationship and a
> document, <http://example.org/>. Triples that
> were "about" (in a loose sense, not just the rdf:subject sense)
> either ?x, or ?y, or the doc... would be relevant. Triples which
> were "about" the foaf:knows relationship connecting the two people
> might be even more relevant, etc etc. The query is pretty vague.
>
> Can this be reformulated using properties? I tried, with a
> mess of optionals. I'm sure something could be cooked up
> with seeAlso or other properties, or rdf:List or rdf:Alt. The
> sense of the query is loose enough that I doubt it worth
> trying to capture it more formally as a complicated bunch of
> OPTIONALs. Doing so is somewhat add odds to the whole point of
> DESCRIBE anyway.
>
>
> 7. seeAlso is good for Semantic Web deployment
>
> Promoting seeAlso and RDF description of RDF *documents* is
> good for the Semantic Web, since it helps people find and
> cross-reference pieces of RDF data. SPARQL's "DESCRIBE"
> mechanism is purely internal to the language, currently. It
> allows me to ask a service for RDF that describes some
> particular thing, but all I get is the actual RDF. Encouraging
> that RDF to be made available at GET-able URIs, and
> described with rdfs:seeAlso and other RDF properties, is imho
> an important part of getting the Semantic Web deployed,
> crawled, and indexed. It also directs some attention towards
> the important problem of characterising broad classes of RDF
> document, and the constraints (loose/prose or machine-readable,
> RDF-level or XML-level, etc) associated with them.
>
>
>
> That's about it. To recap: please drop the DESCRIBE keyword and
> replace the examples with ones that use rdfs:seeAlso.
>
> cheers,
>
> Dan
>
> ps. I numbered my paragraphs to make my thoughts look ordered; don't
> suppose I fooled anyone.
>
> pps. http://www.w3.org/1999/11/02-RDFServices/ was an attempt in
> similar vein; now obsoleted by Annotea and SPARQL. I think there
> almost as many usecases for 'describe' functionality as there
> are for Web pages...
>
>
>
>
>
>
-
Alberto Reggiori, Senior Partner, R&D @Semantics S.R.L.
www.asemantics.com Milan Office
Received on Monday, 17 January 2005 14:45:00 GMT

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