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williams-01, proposal to close (revised)

From: Graham Klyne <GK@NineByNine.org>
Date: Fri, 04 Apr 2003 15:47:51 +0100
Message-Id: <5.1.0.14.2.20030404154248.02d85088@127.0.0.1>
To: w3c-rdfcore-wg@w3.org

I have revised the proposed text to close in response to comments 
received.  I  have tried to address the issues raised, but not always in 
exactly the way suggested.

With reference to:
   http://www.w3.org/2001/sw/RDFCore/20030123-issues/#williams-01

I propose that this comment is addressed by revised text at:
   http://www.ninebynine.org/wip/RDF-concepts/20030401/Overview.html#section-data-model
   http://www.ninebynine.org/wip/RDF-concepts/20030401/Overview.html#section-URI-Vocabulary
(copied below)

This revision is to bring the introduction of the RDF graph concept, and 
its use of URIs, into line with the agreed model [1], as articulated by Pat 
[2].

[1] [[[ref?]]]
[2] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2003Feb/0152.html

I also note that the description of abstract graph syntax in section 6 is 
already consistent with this model.

#g
--


[[
3.1 Graph data model

The underlying structure of any expression in RDF is a collection of 
triples, each consisting of a subject, a property and an object. A set of 
such triples is called an RDF graph (defined more formally in section 6). 
The structure can be illustrated by a directed node-arc diagram in which 
each triple is represented as a node-arc-node link (hence the term "graph").

   [image of the RDF triple comprising (subject, property, object)]

Each property arc represents a statement of a relationship between the 
things denoted by the nodes that it links, having three parts:

    1. a property that describes some relationship (also called a predicate),
    2. a value that is the subject of the statement, and
    3. a value that is the object of the statement.

The direction of an arc is significant: it always points toward the object 
of a statement.

The nodes of an RDF graph are its subjects and objects.

The assertion of an RDF triple says that some relationship, indicated by 
the property, holds between the subject and object of the triple. The 
assertion of an RDF graph amounts to asserting all the triples in it, so 
the meaning of an RDF graph is the conjunction (logical AND) of all the 
statements it contains. A formal account of the meaning of RDF graphs is 
given in [RDF-SEMANTICS].


3.2 URI-based vocabulary

A node may be a URI with optional fragment identifier (URI reference, or 
URIref), a literal, or blank (having no separate form of identification). 
Properties are URI references. (See [URI], section 4, for a description of 
URI reference forms, noting that relative URIs are not used in an RDF 
graph. See also section 6.4.)

A URI reference or literal used as a node identifies what that node 
represents. A URI reference used as a property identifies the relationship 
between the nodes connected by that property. A property URI reference may 
also be a node in the graph.

A blank node is a node that is not a URI reference or a literal. In the RDF 
abstract syntax, a blank node is just a unique node that can be used in one 
or more RDF statements, and has no globally distinguishing identity.

A convention used by some linear representations of an RDF graph, to allow 
several statements to contain the same blank node, is to use a blank node 
identifier, which is a local identifier that can be distinguished from all 
URIs and literals. When graphs are merged, their blank nodes must be kept 
distinct if meaning is to be preserved; this may call for re-allocation of 
blank node identifiers. Note that such blank node identifiers are not part 
of the RDF abstract syntax, and the representation of statements containing 
blank nodes is entirely dependent on the particular concrete syntax used.
]]


-------------------
Graham Klyne
<GK@NineByNine.org>
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E
Received on Friday, 4 April 2003 09:55:27 EST

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