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XML RDF


RDF Document Example

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:si="http://www.w3schools.com/rdf/">

<rdf:Description rdf:about="http://www.w3schools.com">
  <si:title>W3Schools</si:title>
  <si:author>Jan Egil Refsnes</si:author>
</rdf:Description>

</rdf: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.w3schools.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.w3schools.com" (note that a property value can be another resource)

The following RDF document could describe the resource "http://www.w3schools.com/rdf":

<?xml version="1.0"?>

<RDF>
  <Description about="http://www.w3schools.com/rdf">
    <author>Jan Egil Refsnes</author>
    <homepage>http://www.w3schools.com</homepage>
  </Description>
</RDF>
Note The example above is simplified. Namespaces are omitted.

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.w3schools.com/rdf is Jan Egil Refsnes".

  • The subject of the statement above is: http://www.w3schools.com/rdf
  • The predicate is: author
  • The object is: Jan Egil Refsnes

Statement: "The homepage of http://www.w3schools.com/rdf is http://www.w3schools.com".

  • The subject of the statement above is: http://www.w3schools.com/rdf
  • The predicate is: homepage
  • The object is: http://www.w3schools.com

RDF Example

Here are two records from a CD-list:

Title Artist Country Company Price Year
Empire Burlesque Bob Dylan USA Columbia 10.90 1985
Hide your heart Bonnie Tyler UK CBS Records 9.90 1988

Below is a few lines from an RDF document:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire Burlesque">
  <cd:artist>Bob Dylan</cd:artist>
  <cd:country>USA</cd:country>
  <cd:company>Columbia</cd:company>
  <cd:price>10.90</cd:price>
  <cd:year>1985</cd:year>
</rdf:Description>

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Hide your heart">
  <cd:artist>Bonnie Tyler</cd:artist>
  <cd:country>UK</cd:country>
  <cd:company>CBS Records</cd:company>
  <cd:price>9.90</cd:price>
  <cd:year>1988</cd:year>
</rdf:Description>
.
.
.
</rdf:RDF>

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:si="http://www.w3schools.com/rdf/">
<rdf:Description rdf:about="http://www.w3schools.com">
  <si:title>W3Schools.com</si:title>
  <si:author>Jan Egil Refsnes</si:author>
</rdf:Description>
</rdf:RDF>

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  ...Description goes here...
</rdf:RDF>

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire Burlesque">
  <cd:artist>Bob Dylan</cd:artist>
  <cd:country>USA</cd:country>
  <cd:company>Columbia</cd:company>
  <cd:price>10.90</cd:price>
  <cd:year>1985</cd:year>
</rdf:Description>

</rdf:RDF>

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):

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire Burlesque"
cd:artist="Bob Dylan" cd:country="USA"
cd:company="Columbia" cd:price="10.90"
cd:year="1985" />

</rdf:RDF>

Properties as Resources

The property elements can also be defined as resources:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire Burlesque">
  <cd:artist rdf:resource="http://www.recshop.fake/cd/dylan" />
  ...
  ...
</rdf:Description>

</rdf:RDF>

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

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Beatles">
  <cd:artist>
    <rdf:Bag>
      <rdf:li>John</rdf:li>
      <rdf:li>Paul</rdf:li>
      <rdf:li>George</rdf:li>
      <rdf:li>Ringo</rdf:li>
    </rdf:Bag>
  </cd:artist>
</rdf:Description>

</rdf:RDF>

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

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Beatles">
  <cd:artist>
    <rdf:Seq>
      <rdf:li>George</rdf:li>
      <rdf:li>John</rdf:li>
      <rdf:li>Paul</rdf:li>
      <rdf:li>Ringo</rdf:li>
    </rdf:Seq>
  </cd:artist>
</rdf:Description>

</rdf:RDF>

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

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description
rdf:about="http://www.recshop.fake/cd/Beatles">
  <cd:format>
    <rdf:Alt>
      <rdf:li>CD</rdf:li>
      <rdf:li>Record</rdf:li>
      <rdf:li>Tape</rdf:li>
    </rdf:Alt>
  </cd:format>
</rdf:Description>

</rdf:RDF>

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

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://recshop.fake/cd#">

<rdf:Description
rdf:about="http://recshop.fake/cd/Beatles">
  <cd:artist rdf:parseType="Collection">
    <rdf:Description rdf:about="http://recshop.fake/cd/Beatles/George"/>
    <rdf:Description rdf:about="http://recshop.fake/cd/Beatles/John"/>
    <rdf:Description rdf:about="http://recshop.fake/cd/Beatles/Paul"/>
    <rdf:Description rdf:about="http://recshop.fake/cd/Beatles/Ringo"/>
  </cd:artist>
</rdf:Description>

</rdf:RDF>

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://www.animals.fake/animals#">

<rdf:Description rdf:ID="animal">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
</rdf:Description>

<rdf:Description rdf:ID="horse">
  <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
  <rdfs:subClassOf rdf:resource="#animal"/>
</rdf:Description>

</rdf:RDF>

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://www.animals.fake/animals#">

<rdfs:Class rdf:ID="animal" />

<rdfs:Class rdf:ID="horse">
  <rdfs:subClassOf rdf:resource="#animal"/>
</rdfs:Class>

</rdf:RDF>

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.

Property Definition
Contributor An entity responsible for making contributions to the content of the resource
Coverage The extent or scope of the content of the resource
Creator An entity primarily responsible for making the content of the resource
Format The physical or digital manifestation of the resource
Date A date of an event in the lifecycle of the resource
Description An account of the content of the resource
Identifier An unambiguous reference to the resource within a given context
Language A language of the intellectual content of the resource
Publisher An entity responsible for making the resource available
Relation A reference to a related resource
Rights Information about rights held in and over the resource
Source A Reference to a resource from which the present resource is derived
Subject A topic of the content of the resource
Title A name given to the resource
Type The nature or genre of the content of the resource

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:

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc= "http://purl.org/dc/elements/1.1/">

<rdf:Description rdf:about="http://www.w3schools.com">
  <dc:description>W3Schools - Free tutorials</dc:description>
  <dc:publisher>Refsnes Data as</dc:publisher>
  <dc:date>2008-09-01</dc:date>
  <dc:type>Web Development</dc:type>
  <dc:format>text/html</dc:format>
  <dc:language>en</dc:language>
</rdf:Description>

</rdf:RDF>

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

Element Class of Subclass of
rdfs:Class All classes  
rdfs:Datatype Data types Class
rdfs:Resource All resources Class
rdfs:Container Containers Resource
rdfs:Literal Literal values (text and numbers) Resource
rdf:List Lists Resource
rdf:Property Properties Resource
rdf:Statement Statements Resource
rdf:Alt Containers of alternatives Container
rdf:Bag Unordered containers Container
rdf:Seq Ordered containers Container
rdfs:ContainerMembershipProperty Container membership properties Property
rdf:XMLLiteral XML literal values Literal

RDFS / RDF Properties

Element Domain Range Description
rdfs:domain Property Class The domain of the resource
rdfs:range Property Class The range of the resource
rdfs:subPropertyOf Property Property The property is a sub property of a property
rdfs:subClassOf Class Class The resource is a subclass of a class
rdfs:comment Resource Literal The human readable description of the resource
rdfs:label Resource Literal The human readable label (name)  of the resource
rdfs:isDefinedBy Resource Resource The definition of the resource
rdfs:seeAlso Resource Resource The additional information about the resource
rdfs:member Resource Resource The member of the resource
rdf:first List Resource  
rdf:rest List List  
rdf:subject Statement Resource The subject of the resource in an RDF Statement
rdf:predicate Statement Resource The predicate of the resource in an RDF Statement
rdf:object Statement Resource The object of the resource in an RDF Statement
rdf:value Resource Resource The property used for values
rdf:type Resource Class The resource is an instance of a class

RDF Attributes

Attribute Description
rdf:about Defines the resource being described
rdf:Description Container for the description of a resource
rdf:resource Defines a resource to identify a property
rdf:datatype Defines the data type of an element
rdf:ID Defines the ID of an element
rdf:li Defines a list
rdf:_n Defines a node
rdf:nodeID Defines the ID of an element node
rdf:parseType Defines how an element should be parsed
rdf:RDF The root of an RDF document
xml:base Defines the XML base
xml:lang Defines the language of the element content