Understanding RDF: the Resource Description Framework in Context (1999)

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Institute for Learning and Research Technology

Understanding RDFThe Resource Description Framework in Context

Dan Brickley ILRT, University of Bristol

[email protected]


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Outline• RDF overview (origin, aims…)

• target applications

• components of RDF– RDF data model– RDF syntax (in XML)– RDF vocabularies (schemas)

• current status

• issues and challenges


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RDF origins

• W3C Metadata Activity 1997-1999

• PICS (Internet content selection)

• Warwick Framework / Dublin Core

• XML (XML Data, Channels etc)

• MCF (Apple, Netscape)

• PICS-NG Working Group

• URI specification for Web identifiers


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RDF - architectural aims

• W3C Metadata needs: PICS,Dsig, DC

• Defines a convention for applications that exchange metadata on the Web

• XML used for ‘serialisation syntax’

• Vocabulary semantics defined by communities of expertise, not W3C

• Fine-grained mixing of diverse metadata


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What is RDF, really? “Resource Description Framework”

• Resources: things that can be named with URIs (http, urn, doi...)• Description: statements about the properties of these resources• Framework: a common model for diversity

Builds on: URI for identifiers, XML for syntax (file format)URI: Uniform Resource IdentifiersXML: Extensible Markup Language

Self Describing! - a framework for describing resources...


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Target applications• resource discovery, indexing, cataloging

• privacy, preferences, user profiles

• sitemaps / web collections

• content rating, filtering, trust (Dsig/PICS)

• intellectual property rights...

What can these diverse applications have in common?


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Syntax: RDF uses XML(What does RDF add to XML?)

<XML> Extensible Markup Language </XML>

•A simple universal file format

•RDF adds interpretation rules: what does this data tell us?

•XML/RDF uses URIs to unambiguously name vocabularies

When we see XML using the RDF syntax, we can interpret the markup as making statements about the named properties of Web resources. Even if we’ve never heard of that vocabulary...


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RDF / Dublin Core example…

<?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/1.0/” > <rdf:Description about="http://docs” > <dc:creator> Joe Smith </dc:creator> </rdf:Description></rdf:RDF>


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RDF Model = Warwick Framework ?• No single vocabulary can describe it all...

• Dublin Core (common ‘discovery’ core)

• IMS Metadata (educational content)

• V-Card (data about people)

• Classification systems (thesauri etc.)

• Industry specific (eg. XMLNews.org)

• (your metadata application here…)


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RDF data model - example


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RDF data model: details...

R1 R2Nodes are resources connected

by named propertiesP1

R1 “foo” The degenerate case is an arc terminating in a fixed value

P1

R1 R2 R3

R6R4

An RDF description consists of a directed

graph of arbitrary complexity

R7

R5

R8

P1 P2

P3 P4 P5

P6

P7


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RDF model: one simple idea...(the same idea that underpins the Web)

“The most fundamental specification of Web architecture, while one of the simpler, is that of the Universal Resource Identifier, or URI. The principle that anything, absolutely anything, ‘on the Web’ should identified distinctly is core.” (Tim Berners-Lee)

RDF aims to build a Web of overlapping metadata vocabularies

We use URIs to define metadata vocabularies

We build ‘graphs’ using these vocabularies to say things...


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The common model: Why use “Nodes and Arcs”?• Arbitrarily extensible (just add more arcs!)

• ‘Node and arc’ model can use multiple vocabularies, distinguished by URI

• We can disagree about semantics, but share infrastructure (syntax, query, editors)

• URIs create market for data merging, aggregation, annotation and filtering services


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RDF Schema Specification

• RDF schemas are Web resources (and have URIs) and can be described using RDF

• RDF properties (size, colour, price, creator...) describe resources and their relationships

• Resources are grouped into classes, which can be organised hierarchically

• Properties ‘make sense’ in context of certain classes of Web resource, eg. ‘sizeInBytes’


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RDF Schema: example

rdfs:Resource

xyz:MotorVehicle rdfs:Class

s s

t

t

xyz:Truck

s

t

xyz:PassengerVehicle

s = rdfs:subClassOf t = rdf:type

xyz:Van s

s

xyz:MiniVan s

s

t

t

t


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RDF Schema: extensibility• properties and classes in an RDF schema have URIs• Things with URIs can be described in RDF• A foundation for mapping between vocabularies

http://mymetadata.vocab.org/author

--- rdfs:subPropertyOf --->

http://purl.org/dc/elements/1.0/Creator


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Current Status

• Model and Syntax is now W3C Recommendation

• RDF Schema is a Proposed Recommendation

• RDF Query - likely future work

• Tools are becoming available...


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Getting started…W3C Site: http://www.w3.org/RDF/

RDF-DEV developers listhttp://www.ilrt.bris.ac.uk/discovery/rdf-dev/Several RDF parsers now available (Perl, Java...)

Netscape / Mozilla RDF implementation http://www.mozilla.org/rdf/doc/

Applications are starting to appear...


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For discussion... What is the URI of a person? Of a museum

artifact? Of a Web site or page or alternate-language version of that page?

E-commerce will require unambiguity.

Progress with URIs needed

- RDF alone can’t fix the Web...

Understanding RDF: the Resource Description Framework in Context (1999)

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