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Re: SPARQL WG Soliciting Early Reviews of Working Drafts

From: Peter Ansell <ansell.peter@gmail.com>
Date: Fri, 2 Jul 2010 12:05:54 +1000
Message-ID: <AANLkTiniSNypsEJ9UCxUpLXEP2oGF1zh3xEN4JgS0sSD@mail.gmail.com>
To: Lee Feigenbaum <lee@thefigtrees.net>
Cc: SPARQL Working Group Comments <public-rdf-dawg-comments@w3.org>
On 2 July 2010 11:30, Lee Feigenbaum <lee@thefigtrees.net> wrote:
> On 7/1/2010 6:44 PM, Peter Ansell wrote:
>>
>> The June 1 SPARQL Federation draft [1] doesn't make it clear how
>> GRAPHS and FROM/FROM NAMED etc map to, or are omited from, Federated
>> queries. It does say "with a query Q and no default-graph-uri or
>> named-graph-uri" in section 4.1, but it doesn't make it clear in the
>> examples. Is the idea is that you can't use GRAPH/FROM/FROM NAMED when
>> you are using SERVICE.
>>
>> Personally, I would find it much more useful if Federation wasn't
>> restricted to the default graph, as any number of endpoints may not
>> have any data at all in the default graph which would make them immune
>> to federated queries. I wouldn't like to see Federation introduced at
>> the expense of graphs.
>
> Hi Peter,
>
> The SERVICE keyword is a way to effectively embed an invocation of the
> SPARQL protocol within a query. The text in 4.1 specifies that the remote
> service should be invoked without any default-graph0uri or named-graph-uri
> parameters. The effect of this is that the remote endpoint will use its
> default RDF dataset -- this default dataset consists of a default graph
> (potentially empty) and zero or more named graphs.
>
> You can indeed use a GRAPH clause within SERVICE, and the graph pattern
> within the GRAPH clause will be matched against the remote endpoint's named
> graphs.
>
> Does this explain the situation? If so, does it address your concerns?

That does explain the note about default-graph-uri etc..

Is it also allowable to put the GRAPH outside the SERVICE pattern? The
current syntax seems to put them on the same level as both are part of
[49] GraphPatternNotTriples in the syntax, so the following may be
legal?

GRAPH <http://example.com/mygraph>
{
  {
    SERVICE <http://example.com/sparql>
    {
      ?s ?p ?o .
    }
 }
UNION
{
    SERVICE <http://example.com/sparql2>
    {
      ?s ?p2 ?o2 .
    }
  }
}

Adding an example to show how GRAPH's and FROM relate to the new
SERVICE pattern would be useful. It may be useful to change the syntax
to make sure that SERVICE will always be a top level pattern, or never
be inside a GRAPH pattern, if that is the intention.

>> In the BINDINGS syntax, is it 'UNDEF' or 'UNBOUND'. Currently both are
>> used but they seem to have the same meaning.

Just out of curiosity, which keyword is currently preferred here?

>> In the section 3 example there is the variable ?human in the bindings
>> section, and ?species in the main part of the query. Is ?human
>> supposed to provide values for ?species, as it is not used apart from
>> that. Seems like a typo where ?human needs to be changed to ?species.
>
> I'll leave these two to be fixed up by the editor in due course.
>
> thanks for the review,
> Lee

Another query... How does one control the paging of results from
particular SERVICE calls? A use case would be if you know that results
from an endpoint need to be retrieved in offsets of a particular
number, but that restriction does not apply to other endpoints.

The reasoning for this is that some sparql endpoints, notably dbpedia,
may legitimately reject queries if the results are going to be too
large, so you can't rely on all results coming down in one call, or
even coming down at all if you set the default too high. The following
example roughly demonstrates the idea:

    SERVICE <http://dbpedia.org/sparql>
    {
      ?s ?p ?o .
    } ORDER BY ?s ?p ?o PAGEVALUE 500
    SERVICE <http://ontologies.localnetwork/sparql>
    {
      ?p ?p2 ?o2 .
    } ORDER BY ?p ?p2 ?o2 PAGEVALUE 10000

If it is easier, it may be useful to extend the Service Description
specification to include details about preferred OFFSET values and the
maximum results ever returned by the endpoint even if you repeatedly
page through OFFSET and LIMIT.

Thanks,

Peter
Received on Friday, 2 July 2010 02:06:28 GMT

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