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Re: Dereferencing a URI vs querying a SPARQL endpoint

From: Peter Ansell <ansell.peter@gmail.com>
Date: Thu, 21 May 2009 08:16:45 +1000
Message-ID: <a1be7e0e0905201516jc467f8ev814b4dd4fdd0444d@mail.gmail.com>
To: Pierre-Antoine Champin <swlists-040405@champin.net>
Cc: semantic-web@w3.org, public-lod@w3.org
Hi,

If you have a dataset that is very large and highly interlinked on
particular URI's, the DESCRIBE response may be too large to reasonably
transmit to a user over the internet (and to expect a sparql endpoint
to give out in one chunk). This is assuming the typical DESCRIBE
behaviour that sparql vendors implement which picks out r ?p1 ?o
(forward) and ?s ?p2 r (reverse) .

If you know that you want both forward and reverse behaviour then to
be you should probably utilise a SPARQL endpoint and page through the
possible results with OFFSET and LIMIT until you don't get anymore
results.

In relation to the Bio2RDF results, the URI that you dereference with
the federated queries is a mixture of what you could get at a
particular set of endpoints, with some forward and some reverse
relations, configured so that the system won't go down just from the
weight of someone trying to effectively do DESCRIBE
<http://bio2rdf.org/taxon:9606>. That would be linked to in a few
hundred thousand places, but still only has a few forward construct
triples that come out of the taxonomy database. In this case, the
direction of the relationship is important in real world terms because
it the size of the relationship.

Insisting that whenever someone wants to get information about a
taxonomy identifier (or some other classification method) that they
have to also get everything else possibly related to it would cause a
mountain of information. This is why [1] [2] [3] etc. are available
for people wanting to get more related links. (although there may be
slow endpoints that make each of those quite long operations)

Admittedly, the results for resolving Bio2RDF URI's come from multiple
endpoints, so if you just focused on a single Bio2RDF SPARQL endpoint
you would get reasonable results from DESCRIBE most of the time.

Cheers,

Peter

[1] http://qut.bio2rdf.org/pageoffset1/links/taxon:9606
[2] http://qut.bio2rdf.org/pageoffset2/links/taxon:9606
[3] http://qut.bio2rdf.org/pageoffset3/links/taxon:9606

2009/5/21 Pierre-Antoine Champin <swlists-040405@champin.net>:
> I would expect that a DESCRIBE query to the SPARQL endpoint return what
> I get when dereferencing the URI.
>
>  pa
Received on Wednesday, 20 May 2009 22:17:25 GMT

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