Exerts from
http://www.xml.com/pub/a/2001/01/24/rdf.html?page=2
Resource Description Framework:
Resource Description Framework, as its name implies, is a framework for describing and interchanging metadata. It is built on the following rules.
- A Resource is anything that can have a URI; this includes all the Web's pages, as well as individual elements of an XML document. An example of a resource is a draft of the document you are now reading and its URL is http://www.textuality.com/RDF/Why.html
- A Property is a Resource that has a name and can be used as a property, for example Author or Title. In many cases, all we really care about is the name; but a Property needs to be a resource so that it can have its own properties.
- A Statement consists of the combination of a Resource, a Property, and a value. These parts are known as the 'subject', 'predicate' and 'object' of a Statement. An example Statement is "The Author of http://www.textuality.com/RDF/Why.html is Tim Bray." The value can just be a string, for example "Tim Bray" in the previous example, or it can be another resource, for example "The Home-Page of http://www.textuality.com/RDF/Why.html is http://www.textuality.com."
- There is a straightforward method for expressing these abstract Properties in XML, for example:
<rdf:Description about='http://www.textuality.com/RDF/Why-RDF.html'>
<Author>Tim Bray</Author>
<Home-Page rdf:resource='http://www.textuality.com' />
</rdf:Description>
RDF is carefully designed to have the following characteristics.
Independence
Since a Property is a resource, any independent organization (or even person) can invent them. I can invent one called Author, and you can invent one called Director (which would only apply to resources that are associated with movies), and someone else can invent one called Restaurant-Category. This is necessary since we don't have a GOD to take care of it for us.
Interchange
Since RDF Statements can be converted into XML, they are easy for us to interchange. This would probably be necessary even if we did have a GOD.
Scalability
RDF statements are simple, three-part records (Resource, Property, value), so they are easy to handle and look things up by, even in large numbers. The Web is already big and getting bigger, and we are probably going to have (literally) billions of these floating around (millions even for a big Intranet). Scalability is important.
Properties are Resources
Properties can have their own properties and can be found and manipulated like any other Resource. This is important because there are going to be lots of them; too many to look at one by one. For example, I might want to know if anyone out there has defined a Property that describes the genre of a movie, with values like Comedy, Horror, Romance, and Thriller. I'll need metadata to help with that.
Values Can Be Resources
For example, most web pages will have a property named Home-Page which points at the home page of their site. So the values of properties, which obviously have to include things like title and author's name, also have to include Resources.
Statements Can Be Resources
Statements can also have properties. Since there's no GOD to provide useful assertions for all the resources, and since the Web is way too big for us to provide our own, we're going to need to do lookups based on other people's metadata (as we do today with Yahoo!). This means that we'll want, given any Statement such as "The Subject of this Page is Donkeys", to be able to ask "Who said so? And When?" One useful way to do this would be with metadata; so Statements will need to have Properties.
