XML and “meta-tagging” Technical seminar for Pathfinder LEAs, BECTa, Coventry, 26 February 2002

XML and “meta-tagging” Technical seminar for Pathfinder LEAs,

BECTa, Coventry, 26 February 2002

Pete Johnston

UKOLN, University of Bath

Bath, BA2 7AY

UKOLN is supported by:

[email protected]://www.ukoln.ac.uk/

XML and "meta-tagging", BECTa Pathfinders, Coventry, 26 Feb 2002

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XML and “meta-tagging”

• What is metadata & what is it used for?

• Sharing metadata– semantics : introducing the Dublin Core– syntax : introducing the Extensible Markup

Language (XML)– structure : the limits of XML

• Introducing the Resource Description Framework (RDF)


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What is metadata?

• “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

• Structured data about resources that can be used to help support a wide range of operations


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What resources, objects, things?

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

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


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Who/what is metadata for?

• Used by– human agents (owner, user/researcher, 3rd party

services)– software agents (e.g. aggregators, portals, brokers)

• Different “flavours” of metadata serve different purposes

– simple, generic vs. rich, specific– published widely vs. shared within community vs.

used by resource owner/manager

• Created by– software tools (resource creation tools, indexing

robots/web crawlers)

– human agents (resource creator/owner, other parties)


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Metadata embedded in resource

Resource1

e.g. meta elements in HTML docs; summary properties in word processor docs

Can resource support embedding of metadata?

Does metadata creator have write access to resource?

Can service extract embedded metadata?

Metadata about aggregates of resources?

Metadata about people, places, concepts?

Creator = J Smith

Date = 2001-11-05

Title = Report


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Metadata linked from resourcee.g. link elements in HTML docs

Metadata record may be remote from resource

Can resource support embedding of link?

Does metadata creator have write access to resource?

Can service follow link to metadata record?

Metadata about aggregates of resources?

Metadata about people, places, concepts?

Resource1

Metadata rec 1

Metadata rec = 1

Creator = J Smith

Date = 2001-11-05

Title = Report


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Metadata points to resourcee.g. most metadata records…

Metadata record may be remote from resource

Does not require embedding of metadata or link

Does not require metadata creator to have write access to resource

Service obtains metadata record independently of resource

Metadata record can describe anything (with identifier…)

Metadata record may persist after resource deleted

Resource1

Metadata rec 1

Creator = J Smith

Date = 2001-11-05

Title = Report

Doc = 1


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Metadata managed in database

J Smith 2001-11-05 Report

Creator Date TitleDoc

1

Metadata content stored in database, exposed in form(s) appropriate for service(s)


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What operations?

Owner / manager / provider

establish control of own resources;administer/manage (through time);disclose/promote own resources widely;enable and control access/use;contextualise

Other metadata creator

disclose/promote resources (including resources owned by others); re-contextualise (re-describe, annotate)

Discovery service

disclose/promote resources from range of providers; re-contextualise (re-describe, annotate);facilitate user discovery

End user find, identify, select resources from range of providers; obtain/use; interpret


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Resources

Website

Metadata

Single resource provider Resource owner

=

Metadata creator

=

Service provider


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PortalWebsite

Metadata

Website

Metadata

Website

Multiple resource owners/Metadata

creators/Local service providers

Separate portal service provider

Multiple resource providers


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MetadataWebsite

PortalWebsite

Metadata

Website

Multiple resource owners/Metadata

creators/Local service providers

Other metadata creators

Separate portal service provider

MetadataWebsite


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PortalB

Website

MetadataWebsite

PortalA

Website

Metadata

Website

MetadataWebsite


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Metadata for resource discovery

• Metadata for resource discovery – is used beyond its creator community– is combined/compared with metadata from other

communities– is aggregated or cross-searched by services

• Challenges of “interoperability”– How does a metadata provider make metadata

records available in a commonly understood form?– (How does a service provider obtain these

metadata records from data providers?)


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How is metadata shared?

• Metadata as language; metadata records as sets of statements

• Effective transmission of information requires agreement on

– semantics– what terms mean– e.g. “cat”, “to sit”, “mat”

– structure– significance of arrangement of terms– e.g. sentence: subject -> verb -> object

(in English….)

– syntax– rules of expression– “The cat sat on the mat.”

Sharing metadata : semanticsIntroducing the Dublin Core


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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


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Introducing the Dublin Core (2)

• Title• Subject• Description• Creator• Publisher• Contributor• Date

• Type• Format• Identifier• Source• Language• Relation• Coverage• Rights


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Introducing the Dublin Core (3)

• Simplicity of semantics, ease of use• Provides basic semantic interoperability

– across domains– across language communities

• Allows for extensibility– but tension between extending DC and choosing

other, richer schema

• Interoperability requires – use of content rules/standards– clarity about resource being described

– e.g. digital surrogate v physical “original”


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Using the Dublin Core

• Not a replacement for richer descriptive standards

• A “pidgin” language for use by “tourists on the Internet commons”

– Tom Baker, “A Grammar of Dublin Core”

• Can provide 15 “windows” into richer resource descriptions

– disclose rich description in simple form– semantic cross-walks, mappings– (if you have rich descriptions, then) export

rather than create?

Sharing metadata : syntaxIntroducing XML


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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

– embedded markup delimits and describes data

• “Meta-language”– language for describing markup languages– can define unlimited number of markup languages

• Widely adopted for transferring data between programs, systems


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Introducing XML (2)

• Simple syntax• Rules of XML made public so any

programmer can write parser• Many parsers available for application

developer– reusable software components– standard programming interfaces

• Data independent of platform• Support from major software vendors

– use of XML increasingly invisible to user

• Foundation for “Web services”– distributed applications invoked over Web


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<table>

<record>

<doc>1</doc>

<creator>J Smith</text>

<date>2001-11-05</date>

<title>Report</title>

</record>

</table>

J Smith 2001-11-05 Report


1 record

title

Report

creator

J Smith

date

2001-11-05

table

record

doc

1


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<record>

...

</record>

<record>

...

</record>

Serialisation

Transmission

De-serialisation

Remote application


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XML : document types & vocabularies

• “XML lets me make up names for element types! Great!”

• But….– XML says nothing about what your names mean– will a human recipient of your document recognise

your <level> element? – will a software agent process your <level> element

correctly?

• Communication requires consensus on– structural model of class of document/data– labelling of components– semantics of components

• Shared use of common XML “vocabularies”


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XML : DTDs, XML Schemas

• Means to codify syntax rules of vocabulary– what markup is allowed– structural constraints on use of markup– N.B. say nothing about what markup means

• Document Type Definition– part of XML Recommendation

• W3C XML Schema– recent W3C recommendation– data-typing i.e. tighter control on element content– support for combining vocabularies– uses XML syntax

• Parser/authoring tool can validate markup of instance against rules in DTD or Schema


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XML : namespaces

• Applications wish to use element from multiple vocabularies (DTDs/Schemas)

– particularly true of metadata applications– problems of “name collisions”

• XML Namespaces– recommendation of W3C– provides universal naming mechanism

• Namespace – a collection of names– given a name, which has the form of a URI

• Element type names, attribute names qualified by a namespace name (a URI)

– through use of prefix

Sharing metadata : structure The limits of XML


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The problem with XML

• Statement– this resource (song, document, picture... etc!) has

dc:creator “Don Van Vliet”

• Multiple expressions in XML<song id=“123”><title>Frownland</title><creator>Don Van Vliet</creator></song>

<lyric id=“456” title=“Frownland”><creator name=“Don Van Vliet”/></lyric>

<music id=“789” creator=“Don Van Vliet”><title text=“Frownland”/></music>


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The problem with XML (2)

• Different communities make different design choices for DTDs/XML Schemas

– all “good” (and valid)– human reader of document can interpret (maybe)– program needs prior “knowledge” of structural

conventions in each XML schema

• Within resource description community, meaning(s) of structure(s) may be limited

• Across communities, potentially unlimited – not scalable in an “open” environment– how to manage ever increasing set of conventions– always encountering unknown schemas


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The problem with XML (3)

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

– Berners-Lee, 2001

• Consensus on syntax– use of XML

• Consensus on semantics of terms– meaning of (uniquely named through XML

namespace) elements/attributes

• No consensus on meaning of structure– e.g. parent-child element relations


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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


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Introducing RDF (2)

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

syntax

• 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


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The RDF model

http://js.org/doc/1author

John

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

– The resource identified by the URI http://js.org/doc/1 has a property “author” whose value is “John”

– Or, “John” is the “author” of the resource identified by http://js.org/doc/1


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The RDF model (2)

… or (ii) another resource...

http://js.org/doc/1author

John [email protected]

name email

– The value of property “author” is another resource which has a property “name” with value “John” and a property “email” with value “[email protected]”


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The RDF model (3)

… which may itself have a URI

http://js.org/doc/1

author

John

http://js.org/person/john

[email protected]

name email


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The power of the RDF model

• Extensible model– supports any vocabularies

• Supports arbitrary complexity of description• URIs as unique fixed points to identify

– resources– properties

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

– i.e. supports distributed metadata


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First source

http://js.org/doc/1

author

John


[email protected]

name email


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Second source

http://js.org/doc/1subject

XML


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Third source


organisation

JS Foundation


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http://js.org/doc/1

author

John


[email protected]

name email


organisation

JS Foundation

http://js.org/doc/1

subject

XML

Three descriptions merged


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The RDF XML syntax

• XML representation of model– to store/exchange descriptions

• All property names made unique through use of XML namespaces

• Conventions for the meaning of structures in XML document

• Service can “know in advance” the meaning of structures

– even if unanticipated vocabularies used

– “partial understanding”

– can read multiple descriptions into store and “merge” on URIs

• Generated by tools…. more later!


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RDF Schema

• Resource Description Framework Schema– Candidate Recommendation of W3C, 2000

• Provides mechanisms to describe– terms used in RDF statements– semantic relationships between terms

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

• Defines type system– resources grouped into classes– classes related hierarchically (subClassOf)– properties related hierarchically (subPropertyOf)– use of properties constrained (domain, range)


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RDF Schema (2)

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

• Other “ontology languages” building on RDF/RDFS

– e.g. DAML-OIL– describe more complex relations between entities

• Berners-Lee’s vision of “Semantic Web”– software agents navigating web of machine-

processable descriptions and “ontologies” – making inferences about data collected– communicating via “partial understanding”


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Summary

• Resource discovery metadata is shared across boundaries of domain, sector etc

• Effective sharing requires consensus on– semantics : shared vocabularies of uniquely

named terms– syntax : XML– structure : common XML DTD/schema or RDF?

• Simple RDF model as basis of “machine-processable” statements about resources


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Acknowledgements

UKOLN is funded by Resource: the Council for Museums, Archives and Libraries, the Joint Information Systems Committee (JISC) of the UK higher and further education funding councils, as well as by project funding from the JISC and the European Union. UKOLN also receives support from the University of Bath where it is based.

http://www.ukoln.ac.uk/

Documents

XML and “meta-tagging” Technical seminar for Pathfinder LEAs, BECTa, Coventry, 26 February 2002