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02 Apr 08

XML.com: What Is RDF

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metadata rdf semanticweb xml

The simplicity and flexibility of the triple in combination with the use of URIs for globally unique names makes RDF unique, and very powerful.
It's a specification that fills a very particular niche for decentralized, distributed knowledge and provides a framework to enable computer applications to answer questions we wouldn't dream of asking computers today.

XML.com: What Is RDF

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metadata rdf semanticweb xml

Most of the abstract model of RDF comes down to four simple rules:
A fact is expressed as a Subject-Predicate-Object triple, also known as a statement. It's like a little English sentence.
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- Subjects, predicates, and objects are given as names for entities, also called resources (dating back to RDF's application to metadata for web resources) or nodes (from graph terminology). Entities represent something, a person, website, or something more abstract like states and relations.
- Names are URIs, which are global in scope, always referring to the same entity in any RDF document in which they appear.
- Objects can also be given as text values, called literal values, which may or may not be typed using XML Schema datatypes.
- Entities are named by Uniform Resource Identifiers (URIs), and this provides the globally unique, distributed naming system we need for distributed knowledge.
- There are also other types of URIs besides http: URIs, such as URNs and TAGs, which you'll see below.
- Since URIs can be quite long, in RDF notations they're usually abbreviated using the concept of namespaces from XML.
- In an RDF/XML document there are two types of nodes: resource nodes and property nodes.
- Notation 3 (N3), or Turtle, is another system for writing out RDF.

XML.com: What Is RDF

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metadata rdf semanticweb xml

he current web is a decentralized platform for distributed presentations, while the SemWeb is a decentralized platform for distributed knowledge.
Resource Description Framework (RDF) is the W3C standard for encoding knowledge.
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- The real content, the knowledge the files are conveying to the human, is opaque to the computer.
- What is meant by "semantic" in Semantic Web is not that computers are going to understand the meaning of anything, but that the logical pieces of meaning can be mechanically manipulated by a machine to useful human ends.
- Files on the Semantic Web need to be able to express information flexibly.
- Life can't be neatly packed into tables, as in relational databases or hierarchies, as in XML.
- Start Node
  Edge Label
  End Node
  
  vincent_donofrio
  starred_in
  law_&_order_ci
  
  law_&_order_ci
  is_a
  tv_show
  
  the_thirteenth_floor
  similar_plot_as
  the_matrix
- Files on the Semantic Web need to be able to relate to each other.
- We will use vocabularies for making assertions about things, but these vocabularies must be able to be mixed together.
- RDF was originally created in 1999 as a standard on top of XML for encoding metadata
- Since then, and perhaps especially after the updated RDF spec in 2004, the scope of RDF has really evolved into something greater. The most exciting uses of RDF aren't in encoding information about web resources, but information about and relations between things in the real world: people, places, concepts, etc.