Metadata :an overview

XML and Educational Metadata, SBU, London, 10 July 2001

Pete Johnston

UKOLN, University of Bath

Bath, BA2 7AY

UKOLN is supported by:

Emailp.johnston@ukoln.ac.ukURLhttp://www.ukoln.ac.uk/

XML and Educational Metadata, SBU, London, 10 July 2001

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Metadata: an overview

• What is metadata?• An introduction to the Dublin Core• An introduction to XML for metadata• An introduction to RDF• RDF, XML and interoperability

XML and Educational Metadata, SBU, London, 10 July 2001

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What is metadata?

• “Data about data”• “Data associated with objects which

relieves their potential users of having to have full advance knowledge of their existence or characteristics. A user might be a program or a person.”

– Dempsey and Heery, 1998

• “Machine understandable information about web resources or other things.”

– Berners-Lee, 1997

XML and Educational Metadata, SBU, London, 10 July 2001

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Resources, objects, things?

• HTML documents• digital images• databases• books• museum objects• archival records• metadata records

• collections• services• physical places• people• abstract “works”• concepts• events

XML and Educational Metadata, SBU, London, 10 July 2001

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What operations?

• User wants to– find – identify– select– obtain / use

– (based on IFLA Functional Requirements for Bibliographical Record)

XML and Educational Metadata, SBU, London, 10 July 2001

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What operations? (2)

• Owner / manager / provider wants to– describe

– classify– link, relate

– enable and control access and use– commerce– property rights– content rating– authenticity– privacy

– manage – administer – preserve

XML and Educational Metadata, SBU, London, 10 July 2001

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Metadata in practice

• Where is metadata created?– embedded in resource– separate entity linked to/from resource– remote database entry

• Where is metadata used?– harvested/aggregated to

– central database?– multiple distributed databases?

– queried by user– used by software agents in service of user

XML and Educational Metadata, SBU, London, 10 July 2001

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Metadata for a purpose

• Different “flavours” of metadata serve different purposes

• Simple, generic vs. rich, specific• Automatic generation vs. human

creation• Standards and specifications

available…• ...but need to choose appropriate

standard for context

XML and Educational Metadata, SBU, London, 10 July 2001

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Standards for metadata

• Benefit of others’ experience, expertise• Provide basis for good practice• Reflect consensus, so facilitate

exchange, access, interoperability• May have support in software tools• Standards for

– semantics– syntax– structure

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing the Dublin Core

• Initiative to improve resource discovery on Web

– not for complex resource description– simple “document-like objects”– extended to other classes of resource

• Interdisciplinary consensus on simple element set

– 15 elements– all optional– all repeatable

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing the Dublin Core

• Provides basic semantic interoperability

– across domains– across language communities– may disclose rich description in simple,

commonly understood form

• Allows for extensibility– but tension between extending DC and

choosing other, richer schema

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing the Dublin Core

• Simplicity of semantics, ease of use• Requires clarity about what resource is

being described– e.g. work, expression, manifestation, item

• Real resources more complex than (stable) “document-like object”?

– characteristics of resources change through time

– agents perform actions which produce changes

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing XML

• Extensible Markup Language• Recommendation of W3C, 1998, 2000• Defines means of describing tree-

structured data in text-based format• Subset of SGML

– embedded markup delimits and describes data

• Platform-independent syntax• Support for validation against structural

model (DTD, XML Schema)

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing XML (2)

• Initially addressing HTML’s limitations for describing document structure

• Now widely adopted syntax for transferring data between programs, systems

• Standard programming interfaces– reusable software components

• Support from major software vendors• Foundation for “Web services”

– distributed applications invoked over Web

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing XML (3)

“XML allows users to add arbitrary structure to their documents but says nothing about what the structures mean.”

– Berners-Lee, 2001

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing RDF

• Resource Description Framework Model & Syntax

• Recommendation of W3C, 1999• Generic “architecture” for metadata

– set of conventions for applications exchanging metadata

– allow semantics to be defined by different resource description communities

– accommodate mixing of metadata from diverse sources

XML and Educational Metadata, SBU, London, 10 July 2001

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Introducing RDF (2)

• Defines – model for making statements about resources– conventions for encoding statements using

XML syntax

• Object types– Resource : any object identified by URI

– not necessarily accessible via Web

– Property : attribute to describe resource– properties also uniquely identified by URI

– Statement : triple of specific resource, named property, and value

XML and Educational Metadata, SBU, London, 10 July 2001

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The RDF model

http:/my.domain/doc/author

Pete

A resource has some property whose value is either (i) a simple string value (literal)….

– The resource identified by the URI http://my.domain/doc/ has a property “author” whose value is “Pete”

– Or, “Pete” is the “author” of the resource identified by http://my.domain/doc/

XML and Educational Metadata, SBU, London, 10 July 2001

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The RDF model (2)

… or (ii) another resource

http://my.domain/doc/author

Pete pete@my.domain

name email

– The value of property “author” is another resource which has a property “name” with value “Pete” and a property “email” with value “pete@my.domain”

XML and Educational Metadata, SBU, London, 10 July 2001

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The RDF XML syntax

• XML representation of model– store/exchange descriptions

• Property names made unique through use of XML namespaces.

• Variant syntaxes

<rdf:RDF xmlns:uc=“http://www.ukoln.ac.uk/core/”> <rdf:Description about=”http://my.domain/doc/”> <uc:author>Pete</uc:author> </rdf:Description></rdf:RDF>

XML and Educational Metadata, SBU, London, 10 July 2001

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The power of RDF

• Extensible model• Supports arbitrary complexity of

description• URIs as unique fixed points to identify

– resources– properties

• Descriptions created independently can be “merged” using URIs as “anchors”

XML and Educational Metadata, SBU, London, 10 July 2001

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RDF Schema

• Resource Description Framework Schema

• Candidate Recommendation of W3C, 2000

• Provides mechanisms to define vocabularies used in RDF statements

– e.g. Dublin Core metadata element set defined using RDF(S)

XML and Educational Metadata, SBU, London, 10 July 2001

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RDF Schema (2)

• Defines type system– resources grouped into classes– classes related hierarchically (subClassOf)– properties related hierarchically

(subPropertyOf)– use of properties constrained (domain,

range)

• RDF Schema employs RDF model– expressible using RDF/XML syntax

XML and Educational Metadata, SBU, London, 10 July 2001

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RDF, XML & interoperability

• Why isn’t XML enough?– simple statement could be expressed in

XML in many different ways– human reader makes interpretation/guess– application program requires prior

knowledge of schema/DTD design– RDF imposes extra syntactic constraints on

how statement expressed– with RDF/XML, both human and program

can interpret description consistently

• Less flexibility, greater interoperability

XML and Educational Metadata, SBU, London, 10 July 2001

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RDF, XML & interoperability

• Use XML for exchange when– applications both “know” semantics

conveyed by structure of (meta)data

• Use RDF/XML for exchange when– (meta)data potentially used by

applications without prior “knowledge” of specific schema

– (meta)data incorporates overlapping structures from different domains

XML and Educational Metadata, SBU, London, 10 July 2001

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The Semantic Web

• Project of W3C– Present: info on Web for human reader,

navigated by simple link– Future: data processed by programs

designed independently of data

• Requires machine-readable statements about resources and their relationships

– using common model– using vocabulary terms tied to unique

definitions– definitions available to programs

XML and Educational Metadata, SBU, London, 10 July 2001

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The Semantic Web (2)

• Vision– software agents navigating web of

descriptions and “ontologies” (including unknown vocabularies)

– making inferences about data collected– communicating via partial understanding

• But…– A vision (only?)– Mistrust of the “hype”?– XML (Schema) vs. RDF (Schema)?– Doubts about RDF from KR community?

XML and Educational Metadata, SBU, London, 10 July 2001

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Conclusions

• Meaningful discussion of interoperability requires scope, context

• Syntactic interoperability - XML• Structural interoperability - RDF• Semantic interoperability

– adoption of standard schema– terminological control– access to RDFS representation of schema

XML and Educational Metadata, SBU, London, 10 July 2001

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Acknowledgements / further reading

UKOLN metadata pages:http://www.ukoln.ac.uk/metadata/

Dublin Core Metadata Initiative:http://dublincore.org/

IFLA, Functional Requirements for Bibliographic Recordhttp://www.ifla.org/VII/s13/frbr/frbr.htm

W3C RDF : http://www.w3.org/RDF/

W3C Semantic Web :http://www.w3.org/2001/sw/