RDF Schemas

1
RDF Schemas

• Thomas Baker, GMD
• "Combining multiple metadata
• standards in implementations"
• SCHEMAS Project Workshop
• Bath, 11 May 2000

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Web of relationships between objects

• Tim Berners-Lees original proposal for the Web
  (1989) envisioned a web of relationships among
  information objects
• Who wrote this code? Where does she work?
• Which laboratories are included in this project?
• People use circles and arrows (nodes and arcs) to
  represent entities and relations
• Today (2000) the directed labelled graph model
  of the Resource Description Framework

3
Resource Description Framework

• Provides a standard way for using XML to
  represent metadata
• Provides a language for making statements about
  properties and relationships of items on the Web
• The object of these statements can be almost
  anything that has a Web address
• Web pages, graphics, audio files, movie clips

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RDF data model (generally) and RDF Schemas

• Circles People, software modules, departments,
  projects, concepts, documents, computer
  hardware...
• Arrows "Describes", "Has the following subject",
  "Is employed by", "Is authored by", "Is designed
  by", "Is dependant on", "Is part of"
• RDF Schemas create a web of named relationships
  between metadata vocabularies

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Graphic relations expressed in XML
URIR
title
Bath presentation
creator
Tom Baker
lt?XML version1.0?gt ltrdfRDF xmlnsrdfhttp//w
ww.w3.org/TR/REC-rdf-syntax
xmlnsdchttp//purl.org/dc/elements/1.1/gt
ltrdfDescription rdfabout URIRgt
ltdctitlegt Bath presentation lt/dctitlegt
ltdccreatorgt Tom Baker lt/dccreatorgt
lt/rdfDescriptiongt lt/rdfRDFgt
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Dublin Core fits on 5 pages
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A schema declares namespaces
ltrdfRDF xmlnsrdfhttp//www.w3.org/1999/02/2
2-rdf-syntax xmlnsbibhttp//www.bib.org/voc
abgt

• Sets prefixes (abbreviations) for the URIs of
  namespaces to which the schema refers

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... schemas describe themselves...
-- "The Dublin Core Element Set Qualifier
Vocabulary" -- Created by the DC Metadata
Initiative -- Created on 13 March 2000 --
Language English.
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... and schemas declare and define metadata
vocabulary terms
-- This DC qualifier is named "Alternative" -- It
is a sub-property of the DC element Title -- It
has the following definition...
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What RDF Schemas define

• Declares Properties (Title, Author, Subject)
• Declares Classes (Book, Article...)
• Describes those Properties and Classes with
  human-readable labels and comments (definitions)
• Specify relations between entities
  subPropertyOf, subClassOf, isDefinedBy

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Annotation vocabularies

• RDF Schemas let registration authorities or third
  parties layer annotations on metadata schemas or
  elements
• Example DCMI may recommend an element or
  qualifier -- whether it is in DCMI's own
  namespace or elsewhere
• Example the SCHEMAS Project Registry may layer
  expert commentaries over short descriptions of
  metadata activities (projects, software,
  standards)

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RDF schemas in multiple languages

• Locally communities share machine-readable tokens
  and locally define human-readable labels.

dccreator
Server in Germany
DCMI Server
Server in Jakarta
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Linking multiple translations of a standard

• Multilingual Dublin Core
• Dublin Core Metadata Initiative Registry Working
  Group and Interest Group for DC in Multiple
  Languages
• Uses RDF schemas to share machine-readable tokens
  for translations of DC terms in Japanese, Arabic,
  Punjabi (26 languages to date)
• Java applets for displaying fonts (ULIS, Japan)
• Raises policy questions How can we manage the
  evolution of Dublin Core as a multilingual
  standard? How can other language communities
  help shape the global (English) standard?

14
MyRDF Toolkit

• Author Eric Miller, Online Computer Library
  Center, Dublin (Ohio) and RDF Working Group of
  W3C
• To define, search, and navigate among distributed
  collections of RDF schemas
• Works like the Web itself to add a schema, make
  it available in RDF on the Web and create
  hyperlinks to and from that schema
• Prototype registry for Dublin Core Metadata
  Initiative
• Soon to be publicly available under an open
  source license

15
RDF Schemas as a publication format for standards
and registries

• How standards are are currently defined
• in HTML pages and paper documents
• no explicit hyperlinks between related elements
  in different standards
• RDF Schema format
• URIs provide cross-references to related schemas
  and documentation
• on Web, browse related namespaces (and profiles)
• just like HTML files form a web of documents, RDF
  schemas form a web of metadata vocabularies

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RDF Schemas cross-reference terms between schemas

• Hierarchy versus point-to-point
• In 1992, Gopher helped you find the right file on
  the Internet by browsing directories
• But Mosaic (the Web) was better hyperlinks
  created point-to-point links between documents
• RDF Schemas create a web of terms with
  cross-references (hyperlinks) among them
• More useful than a fixed hierarchy
• Future relations other than exact equivalency to
  relate terms in different schemas

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Semantic Web as a huge database

• If HTML and the Web made all the online
  documents look like one huge book, RDF, schema
  and inference languages will make all the data in
  the world look like one huge database. (Tim
  Berners-Lee, 1999)
• RDF Schemas accessible over the Web can be
  treated as one huge database of metadata
  vocabularies
• MyRDF is a "metadata browser" click on
  cross-references to follow links between related
  terms in various vocabularies

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thomas.baker_at_gmd.de

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Namespaces versus Profiles

• Implementors usually need to "mix and match"
• use parts of one standard with parts of another
• coin some local terms to fill in gaps
• Application profiles (DESIRE Project)
• schemas are defined in namespaces
• namespace semantics reused in application
  profiles
• Registries should include application profiles
• expressible using RDF schema format
• will to help implementors learn from peers

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RDF Data Model Statement
Literal
Resource
Resource
Property
Statement