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Re: FROM and FROM NAMED: To fetch or not to fetch?

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Sun, 03 Jun 2012 16:52:37 +0100
Message-ID: <4FCB8845.2030105@epimorphics.com>
To: public-rdf-dawg-comments@w3.org
David,

I'm not clear as to whether this is a discussion point or a specific 
comment on the spec.  You mention difference between SPARQL engines - 
have you discussed this with them?  If you are passing on such 
discussion, thank you very much - if not, I'm not sure how that is a 
formal comment to the SPARQL-WG. For a discussion, maybe 
public-sparql-dev@w3.org is a better place.

 > Clarification please?

This is SPARQL 1.0 and has not changed in SPARQL 1.1.

The dataset description, FROM/FROM NAMED, is a declarative description 
of the dataset to be used for the query.  It does not dictate how that 
dataset came about; that is an implementation aspect.

Data management isn't considered as part of query.  Because different 
systems are trying to do different things, it is likely that that is 
variation.  Just restricting it to read from the web (live? cached? what 
is not available?) or conversely reading from a local collection graphs, 
seems to forcing an unnecessary choice - not all systems are run with 
public web access, not all systems run off a local collection of graphs.

(Jena supports both modes in different storage systems)

	Andy




On 03/06/12 04:42, David Booth wrote:
> When a SPARQL query is issued to a server using the FROM<U>  (or FROM
> NAMED<U>) syntax, is the server supposed to fetch that graph from U or
> is it supposed to only look among its existing named graphs for U?
>
> Sec 13.2 says:
> http://www.w3.org/TR/sparql11-query/#specifyingDataset
> [[
> The FROM and FROM NAMED keywords allow a query to specify an RDF dataset
> by reference; they indicate that the dataset should include graphs that
> are obtained from representations of the resources identified by the
> given IRIs (i.e. the absolute form of the given IRI references).
> ]]
>
> And sec 13.2.2 further says:
> http://www.w3.org/TR/sparql11-query/#namedGraphs
> [[
> The FROM NAMED syntax suggests that the IRI identifies the corresponding
> graph, but the relationship between an IRI and a graph in an RDF dataset
> is indirect.
> ]]
>
> These sections certainly make it sound like the server is supposed to
> fetch from the URI (or perhaps use a cached version if it is fresh).
> But I just tried this with three different SPARQL servers, and only one
> fetches from the URI.  The others only look among their existing named
> graphs.
>
> There is a big difference between fetching and not fetching.  As a query
> writer I need to know which behavior is correct.
>
> Personally, although the fetching behavior seems convenient for some
> things, I think the non-fetching behavior seems more sensible, because:
>
> 1. If the correct behavior were to not fetch, then named graphs could be
> fetched using LOAD INTO GRAPH anyway.
>
> 2. If the correct behavior were to fetch then I don't know how one would
> specify multiple already-loaded named graphs to be used as the default
> graph in a query.
>
> 3. If the spec is intentionally ambiguous about fetching or not fetching
> then that would be a problem, because I certainly would not want a
> server trying to fetch without my control.
>
> Clarification please?
>
> Thanks
>
>
>
Received on Sunday, 3 June 2012 15:53:08 GMT

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