Quantum Jim's Journal: A Brief Introduction to RDF

A Brief Introduction to the Resource Description Framework
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Note: The following is a draft.  Will revise and webify.  ASCII art is formatted for 80 character lines.

Definitions
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RDF is a language framework for describing directed graphs.  First, some definitions:

* A _directed graph_ is a bunch of nodes connected by arrows.
* The node at the beginning of an arrow is called the _subject_.
* The node at the end of an arrow is called the _object_.
* The arrow is called the _predicate_.
* The whole thing is called a _statement_.

Example Graphs in Pictures
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Here's a graph composed of only one statement:

-----------   predicate    ----------
| subject |--------------->| object |
-----------                ----------

A subject node for one statement can be the object node of another statment:

-----------   predicate    -----------   predicate2    -----------
| subject |--------------->| object1 |---------------->| object2 |
-----------                -----------                 -----------

Node and Arrow String Values
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Now each node can have a string value:

* It could have no string value; these nodes are called _blank nodes_.

* It can also have a URI value; a URL is an example of an URI.  In fact, every "node" with the same URI value is actually the same node (i.e. they are not seperate nodes).

* Objects can also be any arbitrary string - including the empty string ("") - but subjects can only be URI strings.  These types of objects are called _literals_.

Note that every predicate MUST have a URI value; they can't be blank or arbitrary strings.  It is also a VERY BAD IDEA to let predicates end with anything other than the characters a-z, A-Z, or 0-9 (that is, the end of the uri SHOULD be a valid xml name).

URI Examples
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Here are some examples of URIs that could be a subjects, predicates, or objects.  There is no way of knowing in isolation which is which.  Each URI is DIFFERENT (even if they point to the same thing on the web).

1. http://example.com/subject
2. http://example.com/subject/index.html
3. http://example.com#predicate
4. file:///C:/Files/old/Video%20Game/mario.jpg
5. http://www.google.com/search?q=mario

If the URIs are all predicates, then 1, 3, and 5 are formatted best.  Here are some real-world examples of precicates:

* http://purl.org/dc/elements/1.1/creator
* http://www.w3.org/2000/01/rdf-schema#isDefinedBy

Here are some example strings that could be literals:

* http://purl.org/dc/elements/1.1/creator
* Mary had a little lamb
* <strong><acronym>HTML</acronym> is cool</strong>
* 47

Datatypes
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Literals can also have a property called its _datatype_.  For example, the literal string "47" is also an integer, so it could be described by a datatype that means integer.  This isn't required, of course, and many literals have no defined datatype.

The datatype is like a predicate and can only be a URI.  Here are some real-life examples of datatypes:

* http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral
* http://www.w3.org/2001/XMLSchema#int

Review of Values of Nodes and Arrows
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In review:

* A subject can either have:
  * No string value.
  * URI string value.

* A predicate MUST have ONLY a:
  * URI string value.

* An object can either have:
  * No string value.
  * URI string value.
  * Literal string value.

* A literal object can optional have:
  * A datatype

* A datatype MUST have ONLY a:
  * URI string value.

Graphs as Databases
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Graphs can be thought of as collections of statements, and statements can be thought of as subject/predicate/object tuples.  I tend to think of statements as English sentences and graphs as paragraphs or stories.  The analogy holds up pretty well.

Graphs can also be looked at as a tables in a database.  Here's an example (First set is the heading):

Subject
  Prediate
    Object
      (Datatype) .

http://example.com#subject1
  http://purl.org/dc/elements/1.1/creator
    "Jimmy"
      http://www.w3.org/2001/XMLSchema#string .

http://example.com#subject1
  http://purl.org/dc/elements/1.1/creator
    "John" .

http://example.com#subject1
  http://purl.org/dc/elements/1.1/creator
    http://example.com/subject2 .

http://example.com/subject2
  http://purl.org/dc/elements/1.1/creator
    "Jimmy"
      http://www.w3.org/2001/XMLSchema#string .

Conclusion
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There's a lot more, but these are the basics.