Federation in practice

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Anthony Arrascue

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May 28, 2018, 8:12:58 AM5/28/18
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Hi everybody,

I was trying to run a simple federated query via the RDF4J web application (v.2.3.2+5a43663).
These are the steps I followed:

1) Created a SPARQL endpoint proxy pointing to Dbpedia's SPARQL endpoint (https://dbpedia.org/sparql). I can run queries on it, so I'm sure it works.

2)   Created a second SPARQL endpoint proxy pointing to LinkedGeoData SPARQL endpoint (http://linkedgeodata.org/sparql). I can also run queries on it.

3) Created a Federation Store. I added both endpoint proxies, dbpedia_proxy and linkedgeodata_proxy.

To query I have tried the following:
1) From dbpedia_proxy my query is:
SELECT *
 WHERE { 
         # data from Dbpedia
         ?s ?p ?o .
         # data LinkedGeoData  
             ?x ?p ?y .
         }
}
I get the following error: 
Query evaluation error: Virtuoso 42000 Error SQ070:SECURITY: Must have select privileges on view DB.DBA.SPARQL_SINV_2 SPARQL query: SELECT * WHERE { # data from Dbpedia ?s ?p ?o . # data LinkedGeoData SERVICE <http://localhost:8080/rdf4j-server/repositories/linkedgeodata_proxy> { ?x ?p ?o . } }

2) From linkedgeodata proxy:
SELECT *
 WHERE { 
         # data from LinkedGeoData
         ?s ?p ?o .
         # data Dbpedia  
             ?x ?p ?y .
         }
}
I get the following error:  
Scripting is not enabled. The Open RDF Sesame Workbench application requires scripting to be enabled in order to work properly.

3) From the federation store itself.
I get the same scripting error.

Is it correct what I am doing? Or is it maybe possible that Dbpedia's or LinkedGeoData SPARQL endpoints are not supporting federated queries?

I would be very helpful for any hints in this direction. Thanks a lot in advance!

Best,

Anthony Arrascue

Jerven Bolleman

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May 28, 2018, 8:29:39 AM5/28/18
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Dear Anthony,

The DbPedia endpoint does not allow federated queries from itself to the outside nor does linkedgeodata.org.

Secondly, I am not familiar with the proxy code but I think that even if these two public stores allowed federated queries the ones you wrote would not work as the query uses localhost:8080 and not a public address.

Regards,
Jerven

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Anthony Arrascue

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May 28, 2018, 8:51:44 AM5/28/18
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Thanks for your answer.
I think it should be possible to use localhost. I found this nice explanation at stackoverflow written by Jeen Broekstra


Query:

SELECT *
 WHERE { 
         # data from Primary dataset
         ?uri a skos:Concept ;
              skos:prefLabel ?prefLabel ;
              skos:altLabel ?altLabel .

         # data from Secondary dataset  
             ?uri :customField ?customFieldValue .
         }
}  

The DbPedia endpoint does not allow federated queries from itself to the outside nor does linkedgeodata.org.

Ok, I couldn't even find the information of which public endpoints support federated queries.
Is it actually possible to query two public endpoints in any way? So far I couldn't find any concrete example but the work from Ruben Verborgh, which I believe is the only concrete thing I have seen.

What is the purpose of the the Federation Store in RDF4J? Does it work only with local repositories?

Thanks again.
Best,

Anthony Arrascue


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Jerven Bolleman
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Jerven Bolleman

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May 28, 2018, 9:05:50 AM5/28/18
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Regarding public federated stores.
Yes, wikidata.org allows outbound federation to a few stores.

Then some in the lifesciences.

sparql.nextprot.org
sparql.orthodb.org
sparql.rhea-db.org
sparql.uniprot.org
www.ebi.ac.uk/rdf/services/sparql

among others in the life sciences allow federation.

Do be warned that the uniprot one is huge, with nearly 39 billion triples.
Also please don't use these to do research on the semantic web but research new biology ;)

Regards,
Jerven

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Jerven Bolleman
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