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Re: defn of Named Graph

From: Guus Schreiber <guus.schreiber@vu.nl>
Date: Wed, 25 Sep 2013 18:11:41 +0200
Message-ID: <52430B3D.5070501@vu.nl>
To: Sandro Hawke <sandro@w3.org>, Dan Brickley <danbri@danbri.org>, Pat Hayes <phayes@ihmc.us>, Jeremy J Carroll <jjc@syapse.com>, Gregg Reynolds <dev@mobileink.com>
CC: RDF WG <public-rdf-wg@w3.org>
Sandro, Jeremy, Pat, Dan, Gregg, (cc: RDF WG)

Thanks for having this constructive discussion.  Given the timeline of 
the RDF WG we really need to formulate a WG response to the issue [1] 
raised by Jeremy *by 2 October*.  It would be great if we can do this in 
the custom consensual way. Any chance we can achieve this over the next 
7 days?

Thanks in advance!

Best,
Guus

[1] http://www.w3.org/2011/rdf-wg/track/issues/142

For RDF WG members: see thread starting with:
http://lists.w3.org/Archives/Public/www-archive/2013Sep/0025.html:

Following that epiphany I had at the end of my last email, here's what
I'd love to see everyone agree on, more or less:

== Named Graphs

An "RDF Named Graph" is similar to an "RDF Graph", but different in one
important way.    Because RDF Graphs are defined as being mathematical
sets of RDF Triples, any two RDF Graphs which happen to contain the same
RDF Triples are, by definition, the same thing. This means that
statements made about any RDF Graph, such as metadata about provenance
and licenses, necessarily apply wherever the same set of RDF Triples
occurs.   This is not always the desired intent, and Named Graphs
provide an alternative.

Like an RDF Graph, an RDF Named Graph contains zero or more RDF
Triples.  Unlike an RDF Graph, an RDF Named Graph has an identity
distinct from those triples.  That is, two Named Graphs remain distinct
and distinguishable entities even if they happen to contain exactly the
same RDF Triples.

The term "Named Graph" has historically caused some confusion, as some
people have read the phrase to mean "an RDF Graph which happens to have
a name".   This reading is not correct, since RDF Named Graphs are not
RDF Graphs at all.   They might reasonably have been called
"Identifiable Graphs", which contrasts them to "RDF Graphs" in the same
way that a counterfeit dollar bill is not technically a dollar bill.
As in the dollar bill analogy, RDF Named Graphs and RDF Graphs have a
lot in common, but in some circumstances it is critical to distinguish
between them.    Other names that have been suggested for Named Graphs
include "surfaces" and "g-boxes", but "named graph" has been cemented by
its use in the SPARQL syntax.

Names Graphs also provide a useful semantics for RDF Datasets.  Some RDF
Datasets, hereafter NG Datasets, have this intended meaning: each
(_name_, _graph_) pair is a statement that _name_ is a Named Graph which
contains exactly the triples in _graph_.    The class rdf:NGDataset is
defined for signalling these are the intended Dataset semantics.

The class rdf:NamedGraph is defined for use in declaring the domain and
range of predicates which relate Named Graphs.   For example:

    <> a rdf:NGDataset
    GRAPH :g1 { :MtEverest :heightFeet 29002 }
    GRAPH :g2 { :MtEverest :heightFeet 29029 }
    :g1 :claimedBy :BritishIndiaSurveyOffice.
    :g2 :claimedBy :IndiaSurveyOffice.

Here, the domain of :claimedBy is rdf:NamedGraph, and it might be
defined in English as "x :claimedBy y means that all the triples in the
Named Graph x are claimed to be true by the social entity y."

The greatest differences between RDF Graphs and RDF Named Graphs appear
when one considers the possibility of them changing over time.    It is
nonsensical to consider an RDF Graph changing over time, just like it
makes no sense to talk about the value of some integer, say seven,
changing over time.   In contrast, it makes perfect sense to consider
Named Graphs changing: at one point in time the identifiable thing that
is a certain Named Graph contains some triples and at another point in
time it might contain different triples.  As of RDF 1.1, however, the
formal specifications for RDF do not provide any specific support for
handling changing data.

==

That's simple and clear enough, isn't it?     ( ... he says, clinging to
perhaps his last shred of hope. )

         -- Sandro
Received on Wednesday, 25 September 2013 16:12:11 UTC

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