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There are 6 key documents in the RDF suite, and just the Primer [1] now runs
to 89 pages of A4. To counter allegations that RDF is altogether too
heavy/complicated/difficult/wordy, I thought I try Andrew's 500 word
challenge on it.
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In RDF, a resource is something that can be identified on the web, and a
description is something said about a resource.
Resources have a universal identifier, their URI, which in the case of web
pages will be the same as their address (URL). Pretty much anything else
(people, places, concepts) can be identified in this way by assigning URIs.
Descriptions are made in RDF using statements. A statement has three parts:
the thing being described, the characteristic of interest and the value of
that characteristic. For example, the thing being described might be a book,
say "A Christmas Carol" the characteristic of interest (property) the
author, and the value would be the name of the author, "Charles Dickens". In
RDF jargon these three parts are the subject, predicate and object, and
together they form a triple. The subject is a resource, the predicate is a
special kind of resource and the object can either be another resource or
literal text.
As a resources, the predicates are also unambiguously identified using URIs,
but the same predicate can be reused - when we ask who the author of a book
is, we are asking the same question whichever book we are talking about or
whoever happens to be the author. If we want to say more about a particular
book, we can use its identifier in another statement with a different
predicate (property) and object (value). The basic nature of resources and
predicates are defined with the help of a small set of terms in the RDF
specifications. This set of terms allows us to give more information in our
descriptions, so we could define a classification 'paperback' and say that
this is a kind of book. The class 'book' would in turn be described as a
kind of resource.
In this example we have identified the author by the text of their name, but
usually it is more useful to use a URI as that will be unambiguous, and
allow us to say things about the author as well. So we could have another
statement that says that this author's favourite colour is blue. Our
knowledge can be expressed as these two statements, but as the author is a
common feature in this we can visualise the knowledge as the three resources
linked by the connection from the book to the author and from the author to
the colour blue. This structure is an example of an RDF graph. There may be
other resources that we can link in as well, like books by the same author
or the book's publisher.
It isn't entirely always necessary or even possible to identify resources.
Let's say we have identified the book and the colour blue. We can still make
two statements, "A Christmas Carol" was written by X, and the X's
favourite colour is blue. This can still be visualised as a graph with three
items and two connections, and in the jargon X is known as a blank node.
seeAlso: http://w3.org/RDF
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Cheers,
Danny.
[1] http://www.w3.org/TR/2002/WD-rdf-primer-20021111/
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Danny Ayers
Semantic Web Log :
http://www.citnames.com/blog
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