Re: Discovering a query endpoint associated with a given Linked Data resource

Hi Heiko,

Thanks a lot for the pointer to the paper.

In your experiment, were you able to get some insights on *why* data
publishers are not providing VoID descriptions when it is applicable to do
so (leaving out single FOAF documents etc.) ?

[[Approaches using proposed methods such as VoID and the provenance
vocabulary are scarcely in use (and sometimes not implemented according to
the specification), they lead to a valid SPARQL endpoint in less than 1% of
all cases.]]

Also did you find many occasions where actually a VoID description is
available, but it is not discoverable according to the VoID spec (such as
the case you mention about not having the description in the root level but
in another level). For instance,  http://dbpedia.org/void/Dataset exists
but is not in http://dbpedia.org/.well-known/void and the resources don't
provide a back-link.

Best Regards,
Nandana


On Wed, Aug 26, 2015 at 12:05 PM, Heiko Paulheim <
heiko@informatik.uni-mannheim.de> wrote:

> Hi all,
>
> two years ago, we conducted an empirical experiment to find out how
> promising the different approaches to discover SPARQL endpoints are. The
> results were rather disappointing, see [1].
>
> Executive summary: rather than trying to find VoID descriptions (which
> rarely exist), querying catalogues like datahub seems more promising
> (higher recall at least, precision is lower).
>
> Hth.
>
> Best,
> Heiko
>
> [1] http://www.heikopaulheim.com/docs/iswc2013_poster.pdf
>
>
>
>
> Am 26.08.2015 um 11:50 schrieb Nandana Mihindukulasooriya:
>
> Thanks all for the pointers.
>
> Yes, it seems it is quite rare in practice. I tried several hosts that
> provide Linked Data resources but couldn't find ones that provide a VoID
> description in .well-known/void.
>
> I guess there is a higher technical barrier in making that description
> available in the given location compared to providing that information in
> the response in most cases. So probably the pragmatic thing to do would be
> to include this information either in the content or as a Link relation
> header using the void properties when dereferenced.
>
> So I can use the void:inDataset back-link mechanism [1] and point to a
> VoID description that will have the necessary information about the query
> endpoints.
>
> -----
> dbpedia:Sri_Lanka void:inDataset _:DBpedia .
> _:DBpedia a void:Dataset;
>     void:sparqlEndpoint <http://dbpedia.org/sparql>;
>     void:uriLookupEndpoint <http://fragments.dbpedia.org/2014/en?subject=>
> .
> ------
> or
>
> ----
> Link: < <http://dbpedia.org/void/Dataset>http://dbpedia.org/void/Dataset>;
> rel="http://rdfs.org/ns/void#inDataset"
> ----
>
> Best Regards,
> Nandana
>
> [1] http://www.w3.org/TR/void/#discovery-links
>
> On Wed, Aug 26, 2015 at 11:05 AM, Miel Vander Sande <
> <miel.vandersande@ugent.be>miel.vandersande@ugent.be> wrote:
>
>> Hi Nandana,
>>
>> I guess VoID would be the best fit
>>
>> In case of LDF you could use
>>
>> <...> void:uriLookupEndpoint <
>> <http://fragments.dbpedia.org/2014/en?subject=>
>> http://fragments.dbpedia.org/2014/en?subject=>
>>
>> But wether these exists in practice? Probably not. I'd leave it up to the
>> dereference publisher to provide this triple in te response, rather than
>> doing the .well_known thing.
>>
>> Best,
>>
>> Miel
>>
>> On 26 Aug 2015, at 10:57, Víctor Rodríguez Doncel <
>> <vrodriguez@fi.upm.es>vrodriguez@fi.upm.es> wrote:
>>
>> >
>> > Well, you might try to look in this folder location:
>> > .well-known/void
>> > And possibly find a "void:sparqlEndpoint".
>> >
>> > But this would be too good to be true.
>> >
>> > Regards,
>> > Víctor
>> >
>> > El 26/08/2015 10:45, Nandana Mihindukulasooriya escribió:
>> >> Hi,
>> >>
>> >> Is there a standard or widely used way of discovering a query endpoint
>> (SPARQL/LDF) associated with a given Linked Data resource?
>> >>
>> >> I know that a client can use the "follow your nose" and related link
>> traversal approaches such as [1], but if I wonder if it is possible to have
>> a hybrid approach in which the dereferenceable Linked Data resources that
>> optionally advertise query endpoint(s) in a standard way so that the
>> clients can perform queries on related data.
>> >>
>> >> To clarify the use case a bit, when a client dereferences a resource
>> URI it gets a set of triples (an RDF graph) [2].  In some cases, it might
>> be possible that the returned graph could be a subgraph of a named graph /
>> default graph of an RDF dataset. The client wants to discover if a query
>> endpoint that exposes the relevant dataset, if one is available.
>> >>
>> >> For example, something like the following using the "search" link
>> relation [3].
>> >>
>> >> ------
>> >> HEAD /resource/Sri_Lanka
>> >> Host: http://dbpedia.org
>> >> ------
>> >> 200 OK
>> >> Link: <http://dbpedia.org/sparql>; rel="search"; type="sparql", <
>> <http://fragments.dbpedia.org/2014/en#dataset>
>> http://fragments.dbpedia.org/2014/en#dataset>; rel="search"; type="ldf"
>> >> ... other headers ...
>> >> ------
>> >>
>> >> Best Regards,
>> >> Nandana
>> >>
>> >> [1]
>> http://swsa.semanticweb.org/sites/g/files/g524521/f/201507/DissertationOlafHartig_0.pdf
>> >> [2]
>> http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#section-rdf-graph
>> >> [3]
>> http://www.iana.org/assignments/link-relations/link-relations.xhtml
>> >
>> >
>> > --
>> > Víctor Rodríguez-Doncel
>> > D3205 - Ontology Engineering Group (OEG)
>> > Departamento de Inteligencia Artificial
>> > Facultad de Informática
>> > Universidad Politécnica de Madrid
>> >
>> > Campus de Montegancedo s/n
>> > Boadilla del Monte-28660 Madrid, Spain
>> > Tel. (+34) 91336 3672 <%28%2B34%29%2091336%203672>
>> > Skype: vroddon3
>> >
>> >
>> > ---
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>> >
>>
>
> --
> Prof. Dr. Heiko Paulheim
> Data and Web Science Group
> University of Mannheim
> Phone: +49 621 181 2646
> B6, 26, Room C1.08
> D-68159 Mannheim
>
> Mail: heiko@informatik.uni-mannheim.de
> Web: www.heikopaulheim.com
>
>