FedX throws error on ASK queries

[mhoangvslev]

Diagnosis

• While debugging #21, I enabled fedx:supportsASKQueries in FedX config:

<http://www.ratingsite2.fr/> a sd:Service ;
    fedx:store "SPARQLEndpoint";
    sd:endpoint "http://localhost:34209/sparql?default-graph-uri=http://www.ratingsite2.fr/";
    fedx:supportsASKQueries true . 

• During evaluation (res.hasNext()), FedX throws "org.eclipse.rdf4j.query.QueryEvaluationException: @ sparql_www.ratingsite6.fr_ - HTTP Error: 406 Not Acceptable on this subquery

  ASK WHERE  { <http://www.ratingsite6.fr/ProductFeature13619> <http://www.w3.org/2002/07/owl#sameAs> <http://www4.wiwiss.fu-berlin.de/bizer/bsbm/v01/vocabulary/ProductFeature12922> .  }

• Some digging on Virtuoso forums lead to this: https://github.com/openlink/virtuoso-opensource/issues/464

TL;DR

• RDF4J is tested on Virtuoso 6 and the bug appears in Virtuoso 7. Downgrade virtuoso ?
• Simple workaround:

  SPARQLRepository repo = new SPARQLRepository(host);
  Map<String, String> headers =new HashMap<>();
  headers.put("Accept", "application/sparql-results+json");
  repo.setAdditionalHttpHeaders(headers);

[mhoangvslev]

Reported to FedX here https://github.com/eclipse/rdf4j/issues/3866