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Introduction to RDF
Paul MillerUK Office for Library & Information Networking
Thomas HofmannAustralian Museums On-Line
(Based closely upon an earlier presentation by Eric Miller of OCLC)
Metadata: Definition
Traditionally: metadata has been understood as
“Data about Data”Example(s):
a library catalogue contains information (metadata) about publications (data)
a file system maintains permissions (metadata) about files (data)
Metadata: Definition (Cont.)
Metadata describes other data One application’s metadata is another
application’s data Metadata can itself be described by
metadata (but that doesn’t make it meta-metadata)
Example: Price lists (metadata) have expiration dates:
metadata about metadata. It is still just metadata.
Applications of Metadata
Cataloguing (Item and Collections)Resource DiscoveryElectronic CommerceIntelligent Software AgentsDigital SignaturesContent RatingIntellectual Property RightsPrivacy Preferences & Policies
Application: Item and Collection Cataloguing
Describing individual resources documents, pages, images, audio files,
etc.Describing the content of collections:
Web sites, databases, directories, etc.Relationships among Resources
Tables of Content, chapters, images…. Site Maps e.g. CIMI collection records
Application: Resource Discovery
Search engines can better “understand” the contents of a particular page
More accurate searchesAdditional information aids precisionMakes it possible to automate searches
because less manual “weeding” is needed to process the search results
Application: Electronic Commerce
Metadata can be used to encode information needed in all stages of electronic commerce locating seller/buyer & product
searching “yellow pages”
agreeing on terms of saleprices, terms of payment, contractual information
transactionsdelivery mechanisms, dates, terms
Application: Intelligent Agents
representation and sharing of knowledge knowledge exchange modelling
communication user-to-agent, agent-to-agent, agent-to-
serviceresource discovery
gives web-roaming agents the ability to “understand” their environment
Application: Digital Signatures
These are key to building the “Web of Trust”
Required by agents electronic commerce collaboration
RDF will become the preferred way to encode digital signatures on documents and on statements about documents
Application: Content Rating
Empowering users to select which kinds of web content they wish to see
Child Protection W3C PICS (Platform for Internet
Content Selection) working group US Communications Decency Act of 1996 simple metadata architecture precursor to RDF
Other Applications
Privacy Preferences and Policies describing a user’s willingness/reluctance
to disclose information about himself/herself
describing a site administrator’s desire to gather information about visiting users
Intellectual Property Rights contractual terms related to usage and
distribution rights to a document
Metadata Transmission
Embedded (e.g. META)
Associated With(in HTTP header)
Trusted Third Party(explicit HTTP GET)
Metadata Assertions
Metadata requirements will evolveThe Web is “machine-readable” but
not “machine-understandable”Metadata is useful
e.g A lot could be gained from structured description of pages, servers, search services, and other resources
See point 1
Introducing: RDF
Improve on PICS, HTML, and XMLMachine understandable metadataSupport structured valuesSupport metadata bureauxEncourage authenticated metadataBase for a variety of descriptions:
cataloging, privacy, accessibility, IPR, ...
Data IntegrationExample:
“The author of a document is Paul” “Paul is the author of a document” “A document is authored by Paul” “The author of a document is Paul”
Representation(s) in XML:
<author> <url> http://doc_url </url> <name> Paul </name></author>
<document> <author> <name> Paul </name> </author> <url> http://doc_url </url></document>
<document href = “http://doc_url”author = “Paul”/>
Data Integration (cont.)
Complexity of querying XML documents N ways of mapping XML to logical structure Requires the normalization of all possible
representations for effective query Mean the same thing to a personMean very different things to a machineRDF much less flexible
less flexible == more interoperable! consistent way of representing statements
RDF Components
Formal data modelSyntax for interchange of dataSchema Type system (schema
model)Syntax for machine-understandable
schemasQuery and profile protocols
RDF Data Model
Imposes structural constraints on the expression of application data models for consistent encoding, exchange and
processing of metadataEnables resource description
communities to define their own semantics
Provides for structural interoperability
RDF Data Model
Directed labelled graphsModel elements
Resource Property Value Statement
RDF Model Primitives
ResourceProperty
ValueResource
Statement
Simple Example
ResourceAuthor
“Paul”
RDF Syntax
RDF Model defines a formal relationships among resources, properties and values
Syntax is required to... Store instances of the model into files Communicate files from one application to
another
W3C XML eXtensible Markup Language http://www.w3.org/XML
RDF Model Example #1
URI:R“RDF Presentation”
Title
Creatordc:
dc:
“Paul Miller”
RDF Syntax Example
URI:R“RDF Presentation”
Title
Creatordc:
dc:
“Paul Miller”
<RDF xmlns = “http://www.w3.org/TR/WD-rdf-syntax#” xmlns:dc = “http://purl.org/dc/elements/1.0/”> <Description about = “URI:R”> <dc:Title> RDF Presentation </dc:Title> <dc:Creator> Paul Miller </dc:Creator> </Description></RDF>
“Paul Miller”URI:PAUL
“[email protected]”“Paul Miller”
“UKOLN”
bib:Emailbib:Affbib:Name
RDF Model Example #2
URI:R
URI:UKOLN
“RDF Presentation”Title
Creatordc:
dc:
<RDF xmlns = “http://www.w3.org/TR/WD-rdf-syntax#” xmlns:dc = “http://purl.org/dc/elements/1.0/” xmlns:bib = “http://www.bib.org/persons#”> <Description about = “URI:R”> <dc:Title> RDF Presentation </dc:Title> <dc:Creator> <Description> <bib:Name> Paul Miller </bib:Name> <bib:Email> [email protected] </bib:Email> <bib:Aff resource = “http://www.ukoln.ac.uk” /> </Description> </dc:Creator> </Description></RDF>
RDF Syntax Example #2
“Eric Miller”
RDF Model Example #3
URI:R
URI:PAUL
“[email protected]”“Paul Miller”
“OCLC”
bib:Emailbib:Affbib:Name
URI:UKOLN
“RDF Presentation”Title
Creator admin:By
admin:On
“LOC”
“03-09-99”
admin:For“...”
dc:
dc:
Where do you stop?
Model provides enabling technologyDegree of metadata simplicity/complexity
is a matter of: Resource description communities needs,
best-practice and experience Organization/Institution’s Policy Economics Goals and requirements of implementation
RDF Schemas
Declaration of vocabularies properties defined by a particular community characteristics of properties and/or constraints
on corresponding values
Schema Type System - Basic Types Property, Class, SubClassOf, Domain, Range Minimal (but extensible) at this time minimize significant clashes with typing system
designed for XML NG DTDs (1999?)
Expressible in the RDF model and syntax
Schema Vocabularies
Enables communities to share machine readable tokens and locally define human readable labels.
dc:Creator“Nom”rdfs:label
“Author”rdfs:label
“$100 $a”
rdfs:label
Relationships among vocabularies
dc:Creator
ms:Kgrip
marc:245
bib:Author
Relationships among vocabulary elements
URI:R “John Smith”ms:Kgrip
dc:Creatorms:Kgrip
rdfs:subPropertyOf
rdfs:label“Key Grip”
dc:Creator
Bringing it together
RDF Metadata transmission Embedded (e.g. <META>), Transmitted with
resource (HTTP), Trusted 3rd Party (HTTP GET)RDF Data Model
Support consistent encoding, exchange and processing of metadata… critical when aggregating data from multiple sources
RDF Schema Declare, define, reuse vocabularies
RDF Summary
RDF is a general-purpose frameworkRDF provides structured, machine-
understandable metadata for the WebMetadata vocabularies can be
developed without central coordination
RDF Schemas describe the meaning of each property name
Signed RDF is the basis for trust
RDF Information
W3C RDF Model and Syntax Speciation Recommendation Status Feb 24, 1999
W3C RDF Schema Specifications Proposed Recommendation Status Mar 4,
1999W3C RDF Home Page
http://www.w3.org/RDF/
RDF and the Dublin Core
RDFDublin Core
Overview Data Model Examples Qualification Mechanisms
Dublin Core Data Model
Application of the RDF Data ModelSufficient richness in RDF to support
Dublin Core modelling goalsDefines implementation and
extensibility framework for Dublin Core based applications CIMI is an example of these applications MICI could be an example of these
applications
DC Data Model Qualifiers
Element Qualifier (and Terms)Value Qualifier (and Terms)Value Types (and Terms) Value ComponentsLanguage
The Dublin Core Data Model as RDF
rdf:Valuedc:Element
R
dcq:Type(element qualifier)
dcq:Scheme(value qualifier)
An example for ‘Date’
“Created”
dcq:DateType
“1998-11-10”
rdf:Valuedc:Date
Resource
“ISO 8601”
dcq:Scheme
An example for ‘Relation’
R http://parent
“IsPartOf”
dcq:RelationType
dc:Relation rdf:Value
Element Qualifier (and Terms)
Enabling mechanisms in the data model that support the qualification of the element relating the resource and the value e.g. The term “Illustrator” may be used to
qualify the “Creator” element that relates some resource and some value
Terms Resource identifying “Illustrator”
Value Qualifier (and Terms)
Identifies the encoding, parsing and/or processing rules associated with a value LNF: “Lastname,[sp]Firstname” ISO8601: 1998-10-01 DDC: 325.251 AAT: ionic column
Terms Resources defining LNF, ISO8601, DDC, AAT,
etc.
Language
Defines the Language of the value lang=fr “chat” vs. lang=en “chat”
XML provides a way of handling of language (xml:lang) RDF adopts this, DC adopts this
What do you get for this?
The separation of these constructs is important for extensibility by other resource description communities
Dublin Core Metadata Initiative will not define an all inclusive set Terms
It will define a canonical set and the enabling mechanisms for other communities to extend these
RDF Data Model
Designed to impose structural constraint on syntax to support consistent encoding, exchange and processing of metadata
Enables resource description communities to define their own semantics
Provides for structural interoperability
ResourceProperty
ValueResource2
Dublin Core Data Model
http://purl.orgdc:Creator
“Paul Miller”
Dublin Core Data Model
http://purl.org SN_001dc:Creator rdf:Value
“Paul Miller”
Dublin Core Data Model
http://purl.org SN_001
dct:Illustrator
dc:Creator
dcq:CreatorType
rdf:Value“Paul Miller”
Dublin Core Data Model
http://purl.org SN_001
dct:Illustrator dct:LNF
dc:Creator
dcq:AgentType
dcq:AgentScheme
rdf:Value“Miller, Paul”
Dublin Core Data Model
http://purl.org SN_001
dct:Persondct:LNF
dc:Creator
dcq:AgentType
dcq:AgentScheme
rdf:Value“Miller, Paul”
dct:Illustrator
rdf:Type
Dublin Core Data Model
http://look.org SN_001
Author
dc:Creator
dcq:Type
rdf:Value http://411.org/pmiller
Dublin Core Data Model
http://look.org SN_001
Author
dc:Creator
dcq:Type
rdf:Value http://411.org/pmiller
Dublin Core Data Model
dct:Illustrator
dcq:CreatorType
http://look.orgdc:Creator http://
rdf.411.org/pmiller
Dublin Core Data Model
http://look.org
dct:Illustrator
dc:Creator
dcq:CreatorType
http://rdf.411.org/
pmiller
vc:fn
vc:mn
vc:ln
“A.”
“Paul”
“Miller”
Dublin Core Data Model
dcq:CreatorType
http://look.orgdc:Creator http://
rdf.411.org/pmiller
Dublin Core Data Model
dcq:CreatorType
Bag
cimi:sculptor
rdf:Type rdf:_1
rdf:_2
dct:Author
http://look.orgdc:Creator http://
rdf.411.org/pmiller
Key Data Model Decisions
Application of RDF Data ModelDC namespace defines the core elements
http://purl.org/dc/elements/1.0/DCQ namespace defines the Dublin Core
qualifier and extensibility mechanisms http://purl.org/dc/qualifiers/1.0/
DCT namespace defines the Dublin Core terms and extensibility mechanisms http://purl.org/dc/terms/1.0/
Key Data Model Open Issues
There still a fewDublin Core Data Model Working
Draft out in April DC Proposed Recommendation shortly
afterDublin Core Data Model Home Page
http://purl.org/dc/groups/datamodel.htm
Additional Information
RDF Home Page http://www.w3.org/RDF
Dublin Core Metadata Initiative http://purl.org/dc/