RDF

Kitty Turner

Current Situation

there is hardly any metadata on the Web search engine sites do the equivalent of going through a library, reading every book, and allowing us to look things up based on the words in the text

What is RDF?

general framework to describe any Internet resources by describing and interchanging metadata provides the basis for generic tools for authoring, manipulating, and searching machine-understandable data on the Web thereby promoting the transformation of the Web into a machine-processable repository of information

What is RDF?

an XML application

- utilizes the namespace facility

- this allows RDF to scope and uniquely identify a set of properties (a schema) which can be accessed at the URI identified by the namespace- also inherits the language tag which enables support of multi-lingual metadata

Facts

originally conceived as an extension of the content rating PICS (Platform for Internet Content Selection) Recommendationnow a formal W3C Recommendationa 2nd W3C Recommendation proposes a system where descriptions related to a particular purpose would constitute a class (much like OO); theses classes could fit into a hierarchy of classes with inheritanceprovides the building blocks for supporting the Semantic Web

Uses of RDF metadata

resource discovery for search enginescataloging for describing content and content relationships available at a web siteintelligent software agents to facilitate knowledge sharingcontent ratingdescribing collections of pages that represent a single logical documentdescribing intellectual property rights of Web pages

RDF Rules

1. a Resource is anything that can have a URIweb pages, individual elements of XML docs, etc.

2. a Property is a Resource that has a name and can be used as a propertyany independent organization can invent themex: Author or Title

RDF Rules

3. a Statement is a 3-part records consisting of Resource, Property and a value ex: “The Author of http://www.textuality.com/RDF/Why.html is Tim Bray.”

4. The abstract Properties are expressed in XML as follows:

<rdf:Description about=’http://www.textuality.com/RDF/Why.html’><Author>Tim Bray</Author><Home-Page rdf:resource=’http://www.textuality.com’/>

</rdf:Description>

A Bigger Example

<? xml version="1.0" ?><RDF xmlns = "http://w3.org/TR/1999/PR-rdf-syntax-19990105#" xmlns:DC = "http://purl.org/DC#" > 

<Description about = "http://dstc.com.au/report.html" > <DC:Title> The Future of Metadata </DC:Title> <DC:Creator> Jacky Crystal </DC:Creator> <DC:Date> 1998-01-01 </DC:Date><DC:Subject> Metadata, RDF, Dublin Core</DC:Subject>

</Description></RDF>

More Basics You can have multiple schemas (sets of properties)

<? xml version="1.0" ?><RDF xmlns = "http://w3.org/TR/1999/PR-rdf-syntax-

19990105#" xmlns:DC = http://purl.org/DC#

xmlns:AGLS = "http://naa.gov.au/AGLS#" >  <Description about = "http://dstc.com.au/report.html" > <DC:Title> The Future of Metadata </DC:Title> <DC:Creator> Jacky Crystal </DC:Creator>

<DC:Date> 1998-01-01 </DC:Date><DC:Subject> Metadata, RDF, Dublin Core

</DC:Subject> <AGLS:Function> Info Man’t -

Internet</AGLS:Function> </Description>

</RDF>

More Basics

You can have complex values using properties as values

<DC:Creator parseType="Resource"> <vCard:FN> Dr Jacky J Crystal </vCard:FN> <vCard:TITLE> Director </vCard:TITLE> <vCard:EMAIL> jacky@dstc.com.au </vCard:EMAIL> <vCard:ROLE> Researcher </vCard:ROLE>

</DC:Creator>

More Basics

You can have the value be a literal which will not be interpreted

<DC:AddressLabel parseType="Literal"> <b>Dr Jacky J Crystal</b> <center>DSTC</center> <i>500 Street</i> <b>Australia</b>

</DC:AddressLabel>

More Basics

RDF can express collections of the same properties with Bag, Sequences (Seq) and Alternatives (Alt)

More Basics

a Bag is a collection of multiple values for the same property - used instead of repeating the Property type

<DC:Creator> <Bag>

<li> Maddie Azzurii </li> <li> Corky Brown </li> <li> Jacky Crystal </li>

</Bag></DC:Creator>

More Basics

a Seq is a collection of multiple values for a property where the order of the values is important

<DC:Creator> <Seq>

<li> Maddie Azzurii </li> <li> Corky Brown </li> <li> Jacky Crystal </li>

</Seq></DC:Creator>

More Basics

an Alt is used when there is a choice of values available for a property which each value being valid but dependent on some factor.

<SOFT:Location> <Alt>

<li> ftp://soft-sales.com.us/abc.exe </li> <li> ftp://soft-sales.com.au/abc.exe </li> <li> ftp://soft-sales.com.de/abc.exe </li> <li> ftp://soft-sales.com.uk/abc.exe </li>

</Alt></SOFT:Location>

More Basics

RDF allows statements to be made about other statements- useful when expressing properties of metadata

Example

<Description about = "http://www.com/report.html" bagID = "ID001" >

<DC:Title> The Metadata Report </DC:Title> <DC:Creator> Jacky Crystal </DC:Creator> <ECOMM:Price> $AUD20.00 </ECOMM:Price>

</Description> 

<Description aboutEach = "#ID001" > <ADMIN:ValidFrom> 1998-01-01 </ADMIN:ValidFrom> <ADMIN:ValidTo> 1999-12-31 </ADMIN:ValidTo>

</Description>

More Basics

RDF has 2 types of syntax:1. serialization: full expression

seen in the examples above

2.  abbreviated: values of the properties are expressed within XML tags

- useful when including RDF in HTML documents so that no value is rendered in the Web browser- does have some limitations: the Properties are non-repeatable and the Property values can only be strings

Example of Abbreviated RDF

<? xml version="1.0" ?><RDF xmlns = "http://w3.org/TR/1999/PR-rdf-syntax-

19990105#" xmlns:DC = "http://purl.org/DC#" xmlns:AGLS = "http://naa.gov.au/AGLS#" > 

<Description about = "http://dstc.com.au/report.html" DC:Title = "The Future of Metadata"

DC:Creator = "Jacky Crystal" DC:Date = "1998-01-01" DC:Subject = "Metadata, RDF, Dublin Core"

AGLS:Function = "Info Management - Internet" /></RDF>

More Basicsa Resource can be a the value of a Propertyex: a list of research papers that make up a collection for a conference

<Description about = "http://ieee.org/xml-conf98.html" > <IEEE:Title> Proceedings of the XML'98 Conf</IEEE:Title> <IEEE:Editor> Jacky Crystal </IEEE:Editor> <IEEE:Papers>

<Bag ID="XML98-PAPERS"> <li resource = "http://ieee.org/papers/xml98-1.xml"

/> <li resource = "http://ieee.org/papers/xml98-2.xml" /> <li resource = "http://ieee.org/papers/xml98-3.xml" /> <li resource = "http://ieee.org/papers/xml98-4.xml" /> </Bag> </IEEE:Papers>

</Description>

More Basics

RDF can describe individual properties within a collection of properties

ex: to say that 5 of the papers list previously are all copyrighted by IEEE

<Description aboutEach = "#XML98-PAPERS" > <IEEE:Copyright> Copyright IEEE 1998 </IEEE:Copyright>

</Description>

More Basics

another alternative to the previous example

<Description aboutEachPrefix = "http://ieee.org/papers" > <IEEE:Copyright> Copyright IEEE 1998 </IEEE:Copyright>

</Description>

More Basics

RDF includes preliminary work on the definition of metadata schema which will define the meaning, characteristic and relationships of a set of properties  the schema is based on the same model as the RDF syntax specification

Example of RDF schema

<? xml version="1.0" ?><RDF xmlns = "http://w3.org/TR/PR-rdf-syntax#"

xmlns:RDFS = "http://w3.org/TR/WD-rdf-schema#" >  <Description ID = "Title" >

<type resource = "http://w3.org/TR/PR-rdf-syntax#Property" />

<RDFS:label> Title </RDFS:label> <RDFS:comment> The name given to the resource,

usually by the Creator or Publisher </RDFS:comment> </Description>  <Description ID = "Creator" >

<type resource = "http://w3.org/TR/PR-rdf-syntax#Property" /> <RDFS:label> Author or Creator </RDFS:label>

<RDFS:comment> The person or organisation primarily responsible for the intellectual

content of the resource </RDFS:comment> </Description>

</RDF>

More Basics

RDF provides a model for metadata and a syntax so that independent parties can exchange and use it. BUT it doesn’t provide any Properties of its own

The idea is that Properties will come in packages called Vocabularies. ex: basic bibliographic Properties will include Author, Title, Date, Publisher, etc.

RDF and XML RDF: because statements are simple, 3-part records, they are easy to handle and to use for looking up things even in large numbers- with the continuing growth of the Web, scalability is very important

XML: isn’t scalable because- the order in which elements appear is significant which is anathema in the metadata world and very difficult to maintain with millions of data items

- certain XML constructions are hard to handle