williams-01, proposal to close (20030411)

With reference to:

I propose that this comment is addressed by revised text at:
(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].  The text has been revised from my original proposal in response to 
discussion on the RDFcore list.  The change in terminology (property -> 
predicate) means that the GIF image used section 3.1 is revised.

[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.


3.1 Graph data model

The underlying structure of any expression in RDF is a collection of 
triples, each consisting of a subject, a predicate and an object. A set of 
such triples is called an RDF graph (defined more formally in section 6). 
This can be illustrated by a node and directed-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, predicate, object)]

Each triple represents a statement of a relationship between the things 
denoted by the nodes that it links. Each triple has three parts:

    1. a subject,
    2. an object, and
    3. a predicate (also called a property) that denotes a relationship.

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

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

The assertion of an RDF triple says that some relationship, indicated by 
the predicate, holds between the things denoted by 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 the statements corresponding to all the triples 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 predicate identifies the 
relationship between the nodes it connects.  A predicate 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 reference the same unidentified resource 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 triples containing blank nodes is entirely dependent on the particular 
concrete syntax used.

Graham Klyne
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E

Received on Friday, 11 April 2003 09:33:20 UTC