XML RDF

What is RDF?

• RDF stands for Resource Description Framework
• RDF is a framework for describing resources on the web
• RDF is designed to be read and understood by computers
• RDF is not designed for being displayed to people
• RDF is written in XML
• RDF is a part of the W3C's Semantic Web Activity
• RDF is a W3C Recommendation from 10. February 2004

RDF - Examples of Use

• Describing properties for shopping items, such as price and availability
• Describing time schedules for web events
• Describing information about web pages (content, author, created and modified date)
• Describing content and rating for web pictures
• Describing content for search engines
• Describing electronic libraries

RDF is Designed to be Read by Computers

RDF was designed to provide a common way to describe information so it can be read and understood by computer applications.

RDF descriptions are not designed to be displayed on the web.

RDF is Written in XML

RDF documents are written in XML. The XML language used by RDF is called RDF/XML.

By using XML, RDF information can easily be exchanged between different types of computers using different types of operating systems and application languages.

RDF and "The Semantic Web"

The RDF language is a part of the W3C's Semantic Web Activity. W3C's "Semantic Web Vision" is a future where:

• Web information has exact meaning
• Web information can be understood and processed by computers
• Computers can integrate information from the web

RDF uses Web identifiers (URIs) to identify resources.

RDF describes resources with properties and property values.

RDF Resource, Property, and Property Value

RDF identifies things using Web identifiers (URIs), and describes resources with properties and property values.

Explanation of Resource, Property, and Property value:

• A Resource is anything that can have a URI, such as "http://www.w3ii.com/rdf"
• A Property is a Resource that has a name, such as "author" or "homepage"
• A Property value is the value of a Property, such as "Jan Egil Refsnes" or "http://www.w3ii.com" (note that a property value can be another resource)

The following RDF document could describe the resource "http://www.w3ii.com/rdf":

RDF Statements

The combination of a Resource, a Property, and a Property value forms a Statement (known as the subject, predicate and object of a Statement).

Let's look at some example statements to get a better understanding:

Statement: "The author of http://www.w3ii.com/rdf is Jan Egil Refsnes".

• The subject of the statement above is: http://www.w3ii.com/rdf
• The predicate is: author
• The object is: Jan Egil Refsnes

Statement: "The homepage of http://www.w3ii.com/rdf is http://www.w3ii.com".

• The subject of the statement above is: http://www.w3ii.com/rdf
• The predicate is: homepage
• The object is: http://www.w3ii.com

RDF Example

Here are two records from a CD-list:

Below is a few lines from an RDF document:

The first line of the RDF document is the XML declaration. The XML declaration is followed by the root element of RDF documents: <rdf:RDF>.

The xmlns:rdf namespace, specifies that elements with the rdf prefix are from the namespace "http://www.w3.org/1999/02/22-rdf-syntax-ns#".

The xmlns:cd namespace, specifies that elements with the cd prefix are from the namespace "http://www.recshop.fake/cd#".

The <rdf:Description> element contains the description of the resource identified by the rdf:about attribute.

The elements: <cd:artist>, <cd:country>, <cd:company>, etc. are properties of the resource.

RDF Online Validator

W3C's RDF Validation Service is useful when learning RDF. Here you can experiment with RDF files.

The online RDF Validator parses your RDF document, checks your syntax, and generates tabular and graphical views of your RDF document.

Copy and paste the example below into W3C's RDF validator:

When you parse the example above, the result will look something like this.

RDF Elements

The main elements of RDF are the root element, <RDF>, and the <Description> element, which identifies a resource.

The <rdf:RDF> Element

<rdf:RDF> is the root element of an RDF document. It defines the XML document to be an RDF document. It also contains a reference to the RDF namespace:

The <rdf:Description> Element

The <rdf:Description> element identifies a resource with the about attribute.

The <rdf:Description> element contains elements that describe the resource:

The elements, artist, country, company, price, and year, are defined in the http://www.recshop.fake/cd# namespace. This namespace is outside RDF (and not a part of RDF). RDF defines only the framework. The elements, artist, country, company, price, and year, must be defined by someone else (company, organization, person, etc).

Properties as Attributes

The property elements can also be defined as attributes (instead of elements):

Properties as Resources

The property elements can also be defined as resources:

In the example above, the property artist does not have a value, but a reference to a resource containing information about the artist.

RDF Containers

RDF containers are used to describe group of things.

The following RDF elements are used to describe groups: <Bag>, <Seq>, and <Alt>.

The <rdf:Bag> Element

The <rdf:Bag> element is used to describe a list of values that do not have to be in a specific order.

The <rdf:Bag> element may contain duplicate values.

Example

The <rdf:Seq> Element

The <rdf:Seq> element is used to describe an ordered list of values (For example, in alphabetical order).

The <rdf:Seq> element may contain duplicate values.

Example

The <rdf:Alt> Element

The <rdf:Alt> element is used to describe a list of alternative values (the user can select only one of the values).

Example

RDF Terms

In the examples above we have talked about "list of values" when describing the container elements. In RDF these "list of values" are called members.

So, we have the following:

• A container is a resource that contains things
• The contained things are called members (not list of values)

RDF Collections

RDF collections describe groups that can ONLY contain the specified members.

The rdf:parseType="Collection" Attribute

As seen in the previous chapter, a container says that the containing resources are members - it does not say that other members are not allowed.

RDF collections are used to describe groups that can ONLY contain the specified members.

A collection is described by the attribute rdf:parseType="Collection".

Example

RDF Schema and Application Classes

RDF Schema (RDFS) is an extension to RDF.

RDF describes resources with classes, properties, and values.

In addition, RDF also needs a way to define application-specific classes and properties. Application-specific classes and properties must be defined using extensions to RDF.

One such extension is RDF Schema.

RDF Schema (RDFS)

RDF Schema does not provide actual application-specific classes and properties.

Instead RDF Schema provides the framework to describe application-specific classes and properties.

Classes in RDF Schema are much like classes in object oriented programming languages. This allows resources to be defined as instances of classes, and subclasses of classes.

RDFS Example

The following example demonstrates some of the RDFS facilities:

In the example above, the resource "horse" is a subclass of the class "animal".

Example Abbreviated

Since an RDFS class is an RDF resource we can abbreviate the example above by using rdfs:Class instead of rdf:Description, and drop the rdf:type information:

That's it!

The Dublin Core

The Dublin Core Metadata Initiative (DCMI) has created some predefined properties for describing documents.

RDF is metadata (data about data). RDF is used to describe information resources.

The Dublin Core is a set of predefined properties for describing documents.

The first Dublin Core properties were defined at the Metadata Workshop in Dublin, Ohio in 1995 and is currently maintained by the Dublin Core Metadata Initiative.

A quick look at the table above indicates that RDF is ideal for representing Dublin Core information.

RDF Example

The following example demonstrates the use of some of the Dublin Core properties in an RDF document:

RDF Reference

The RDF namespace (xmlns:rdf) is: http://www.w3.org/1999/02/22-rdf-syntax-ns#

The RDFS namespace (xmlns:rdfs ) is: http://www.w3.org/2000/01/rdf-schema#

The recommended file extension for RDF files is .rdf. However, the extension .xml is often used to provide compatibility with old xml parsers.

The MIME type should be "application/rdf+xml".

RDFS / RDF Classes

RDFS / RDF Properties

RDF Attributes