In a past post I mentioned that the data model I’m using is very extensible. I’d like to explore that model a little over the course of the next week to demonstrate just how it can be extended.Â

Subjects, Predicates, and Objects

The foundation of the data model is the Resource Description Framework (RDF), a language for representing information with well-defined semantics as a graph. The basic building blocks of RDF are resources, properties, and values:

• Resources are the things being represented, and are identified by Universal Resource Identifiers (URIs) such as http://www.example.com/example.htm or uri:uuid:e2f03e8d-72a1-4f7d-a9f5-1cab5f9cae80. Such a URI may or may not be the actual location of a Web page; it doesn’t really matter so long as the identifier is unique.
• Properties are the attributes of things, also identified by URIs.  An important characteristic is that they have well-defined semantic meaning. For example, http://purl.org/dc/elements/1.1/title is a property that can be used to describe the formal name of some resource, such as this Web site.
• Values of the attributes of things are the last basic building block, and can be identified by URIs or literal values. For example, the title of this Web site is the string literal “Enabling the Distributed Family Tree”.

These three building blocks are also known respectively as subjects, predicates, and objects. A subject and object connected by a predicate is called a statement, or triple.

The Semantic Meaning of Predicates

Available properties are defined in ontologies. An ontology is a sort of schema that dictates the semantic meaning of resources and predicates and how they relate to each other. The current standard for publishing RDF ontologies is with the OWL Web Ontology Language.

For example, http://purl.org/dc/elements/1.1/title is a property of the Dublin Core ontology, and is defined just so:

URI:Â	http://purl.org/dc/elements/1.1/title
Label:Â	Title
Definition:Â	A name given to the resource.
Comment:Â	Typically, a Title will be a name by which the resource is formally known.
Type of Term: Â	element
Status:Â	recommended
Date Issued:Â	1999-07-02

Properties are often given an abbreviated name.  This property, for example, is usually referred to as dc:title.

Statements About Statements

A collection of statements is a graph: subjects and objects can be thought of as nodes, while predicates serve as the connecting arcs. RDF can be extended to support multiple graphs through a concept known as Named Graphs. With this extension, each graph can be given a unique identifier which can then be used as the subject or object of other statements.

Next Time

In part two I’ll show how basic genealogical information can be recorded in RDF.

Technorati Tags: RDF, OWL, Dublin Core, Named Graphs

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