Re: [namedgraphs] [Semantic Web Client Library] T-Box and different sources

[Cristina S. <csa...@vi...>]

Dear Olaf,

Thanks a lot for your prompt response. I understand now why this happens.

I have enabled the SemanticWebClientConfig.ENABLE_SINDICE parameter, and
executed several queries but I still get results from only the foaf
implementation. Could you please give me an example where the Sindice
configuration provides more information?

Kind regards,

Cristina



2010/9/20 Olaf Hartig <ha...@in...>

> Hello Cristina,
>
> On Monday 20 September 2010 07:52:55 Cristina Sarasua wrote:
> > Hi,
> >
> > I would like to ask something about the Semantic Web Client Library.
> >
> > I've been executing some SPARQL queries using the code specified in [1].
> >
> > With the following query:
> >
> > String queryString =
> >             "PREFIX foaf: <http://xmlns.com/foaf/0.1/> " +
> >             "SELECT ?i ?p WHERE {" +
> >             "?i ?p <http://www.w3.org/People/Berners-Lee/card#i> . " +
> >             "}";
> >
> > I get resources from different sources.
> >
> > But, with this query:
> >
> > String queryString =
> >             "PREFIX foaf: <http://xmlns.com/foaf/0.1/> " +
> >             "SELECT ?i ?p WHERE {" +
> >             "?i ?p foaf:Person . " +
> >             "}";
> >
> > all the results belong to the same source - the document where foaf is
> > implemented.
>
> That's a result of the way the SWClLib executes queries. It uses a new
> approach for query execution which we call link traversal based query
> execution. The SWClLib Web site outlines the idea of this approach. Did
> you read how this approach works?
> You find a more detailed description of this approach in my paper at
> ISWC 2009.
>
> So, what the query engine does for your example query is the following:
> Given
> we start with an empty queried dataset (a.k.a. cache). SWClLib looks up the
> URI for foaf:Person, retrieves the RDF graph that defines the FOAF
> vocabulary,
> and adds this graph to the queried dataset. Now it tries to find RDF
> triples in
> the queried dataset that match the triple pattern in your query. That's
> only
> triples from the FOAF vocab graph (because that's the only one we have).
> The
> FOAF vocab graph does not contain RDF links that SWClLib can follow (where
> "following" here means looking up URIs to retrieve further RD graphs).
> Hence,
> SWClLib cannot discover more data. That's always a problem with this query
> approach when the query contains only URIs for vocabulary terms (or t-Box
> concepts as you call them) but no URIs for individual things (i.e.
> instances
> of classes defined in vocabularies).
>
> Notice, however, that I added Sindice support to the SWClLib. With Sindice
> support enabled the SWClLib does not only look up URI (using the HTTP
> protocol) but it also queries the Sindice search engine for additional RDF
> graphs that contain RDF triples with the URI. This practice may give you
> more
> results for your second query. But, it's very expensive in terms of query
> execution time. Therefore, Sindice support is disabled by default. The Web
> site documents how to enable it.
>
> > I wonder whether the Semantic Web Client Library does not retrieve
> > resources from different sources when querying about T-Box, or whether
> > this is just a coincidence.
>
> It's not about t-Box or a-Box. To the SWClLib everything are just RDF
> graphs,
> SWClLib does not do any kind of reasoning.
>
> Greetings,
> Olaf
>
> > Thanks in advance.
> >
> > Kind regards,
> >
> > Cristina
> >
> > [1] http://www4.wiwiss.fu-berlin.de/bizer/ng4j/semwebclient/#using
>
>
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