SPARQL endpoint construct scrollable cursor fails on Turtle

[jindrichmynarz]

The SPARQL endpoint construct scrollable cursor component fails if it receives Turtle (or other RDF serialization using prefixes), since it parses any response as if it was serialized in N-Triples:

Caused by: org.eclipse.rdf4j.rio.RDFParseException: Expected '<' or '_', found: @ [line 1]
    at org.eclipse.rdf4j.rio.helpers.RDFParserHelper.reportFatalError(RDFParserHelper.java:403)
    at org.eclipse.rdf4j.rio.helpers.AbstractRDFParser.reportFatalError(AbstractRDFParser.java:682)
    at org.eclipse.rdf4j.rio.ntriples.NTriplesParser.reportFatalError(NTriplesParser.java:650)
    at org.eclipse.rdf4j.rio.ntriples.NTriplesParser.parseSubject(NTriplesParser.java:347)
    at org.eclipse.rdf4j.rio.ntriples.NTriplesParser.parseTriple(NTriplesParser.java:279)
    at org.eclipse.rdf4j.rio.ntriples.NTriplesParser.parse(NTriplesParser.java:174)
    at org.eclipse.rdf4j.rio.ntriples.NTriplesParser.parse(NTriplesParser.java:116)
    at org.eclipse.rdf4j.http.client.SPARQLProtocolSession.getRDF(SPARQLProtocolSession.java:957)
    ... 7 common frames omitted

There is no way in the component to specify the requested MIME type. Moreover, the component adds Virtuoso-specific (i.e. not portable) preamble (define output:format "HTTP+TTL text/rdf+n3") to the queries it generates asking for Notation3 even though it only accept N-Triples.

The component should solve this issue as the other SPARQL endpoint components do.

Tested using the develop version a8e599812f558baf63c86995d49fc22b78ccedeb.

[jakubklimek]

This component is, however, Virtuoso specific. So it can have this hardwired and it is not intended to be used with other triplestores.

[jindrichmynarz]

If it is Virtuoso-specific, then it should be documented. Currently, link to its documentation is missing at https://etl.linkedpipes.com/components (reported in #431). Perhaps even its name should contain "Virtuoso" to make this obvious.

Nevertheless, in this case I tested it with a Virtuoso endpoint. So the error appears in Virtuoso too.

[jakubklimek]

Currently, link to its documentation is missing

Yes, we are aware

Nevertheless, in this case I tested it with a Virtuoso endpoint. So the error appears in Virtuoso too.

Then it is the same issue as https://github.com/linkedpipes/etl/issues/446

[jindrichmynarz]

It is likely caused by the same problem as in #446. However, this is a different component.

[jakubklimek]

The documentation contains sufficient number of mentions of OpenLink Virtuoso to be clear it is virtuoso-specific.