Semantic Web Standards

1
Semantic Web Standards
Slides based on Ian Horrocks class
2
Where we are Today the Syntactic Web
Hendler Miller 02
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The Syntactic Web is

• A hypermedia, a digital library
• A library of documents called (web pages)
  interconnected by a hypermedia of links
• A database, an application platform
• A common portal to applications accessible
  through web pages, and presenting their results
  as web pages
• A platform for multimedia
• BBC Radio 4 anywhere in the world! Terminator 3
  trailers!
• A naming scheme
• Unique identity for those documents
• A place where computers do the presentation
  (easy) and people do the linking and interpreting
  (hard).
• Why not get computers to do more of the hard
  work?

Goble 03
4
Hard Work using the Syntactic Web
Find images of Peter Patel-Schneider, Frank van
Harmelen and Alan Rector
Rev. Alan M. Gates, Associate Rector of the
Church of the Holy Spirit, Lake Forest, Illinois
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Impossible (?) using the Syntactic Web

• Complex queries involving background knowledge
• Find information about animals that use sonar
  but are not either bats or dolphins
• Locating information in data repositories
• Travel enquiries
• Prices of goods and services
• Results of human genome experiments
• Finding and using web services
• Visualise surface interactions between two
  proteins
• Delegating complex tasks to web agents
• Book me a holiday next weekend somewhere warm,
  not too far away, and where they speak French or
  English

6
What is the Problem?

• Consider a typical web page

• Markup consists of
• rendering information (e.g., font size and
  colour)
• Hyper-links to related content
• Semantic content is accessible to humans but not
  (easily) to computers

7
What information can we see

• WWW2002
• The eleventh international world wide web
  conference
• Sheraton waikiki hotel
• Honolulu, hawaii, USA
• 7-11 may 2002
• 1 location 5 days learn interact
• Registered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam, zaire
• Register now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmed
• Tim berners-lee
• Tim is the well known inventor of the Web,
• Ian Foster
• Ian is the pioneer of the Grid, the next
  generation internet

8
What information can a machine see

• WWW2002
• The eleventh international world wide web
  conference
• Sheraton waikiki hotel
• Honolulu, hawaii, USA
• 7-11 may 2002
• 1 location 5 days learn interact
• Registered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam, zaire
• Register now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmed
• Tim berners-lee
• Tim is the well known inventor of the Web,
• Ian Foster
• Ian is the pioneer of the Grid, the next
  generation internet

9
Solution XML markup with meaningful tags?

• ltnamegtWWW2002
• The eleventh international world wide
  webconlt/namegt
• ltlocationgtSheraton waikiki hotel
• Honolulu, hawaii, USAlt/locationgt
• ltdategt7-11 may 2002lt/dategt
• ltslogangt1 location 5 days learn interactlt/slogangt
• ltparticipantsgtRegistered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam,
  zairelt/participantsgt
• ltintroductiongtRegister now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmedlt/introductiongt
• ltspeakergtTim berners-leelt/speakergt
• ltbiogtTim is the well known inventor of the
  Web,lt/biogt

10
But What About

• ltconfgtWWW2002
• The eleventh international world wide
  webconlt/confgt
• ltplacegtSheraton waikiki hotel
• Honolulu, hawaii, USAlt/placegt
• ltdategt7-11 may 2002lt/dategt
• ltslogangt1 location 5 days learn interactlt/slogangt
• ltparticipantsgtRegistered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam,
  zairelt/participantsgt
• ltintroductiongtRegister now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmedlt/introductiongt
• ltspeakergtTim berners-leelt/speakergt
• ltbiogtTim is the well known inventor of the Web,

11
Machine sees

• ltnamegtWWW2002
• The eleventh international world wide webclt/namegt
• ltlocationgtSheraton waikiki hotel
• Honolulu, hawaii, USAlt/locationgt
• ltdategt7-11 may 2002lt/dategt
• ltslogangt1 location 5 days learn interactlt/slogangt
• ltparticipantsgtRegistered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam,
  zairelt/participantsgt
• ltintroductiongtRegister now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmedlt/introductiongt
• ltspeakergtTim berners-leelt/speakergt
• ltbiogtTim is the well known inventor of the
  Wlt/biogt
• ltspeakergtIan Fosterlt/speakergt
• ltbiogtIan is the pioneer of the Grid, the nelt/biogt

12
Need to Add Semantics

• External agreement on meaning of annotations
• E.g., Dublin Core
• Agree on the meaning of a set of annotation tags
• Problems with this approach
• Inflexible
• Limited number of things can be expressed
• Use Ontologies to specify meaning of annotations
• Ontologies provide a vocabulary of terms
• New terms can be formed by combining existing
  ones
• Meaning (semantics) of such terms is formally
  specified
• Can also specify relationships between terms in
  multiple ontologies

13
History of the Semantic Web

• Web was invented by Tim Berners-Lee (amongst
  others), a physicist working at CERN
• TBLs original vision of the Web was much more
  ambitious than the reality of the existing
  (syntactic) Web
• TBL (and others) have since been working towards
  realising this vision, which has become known as
  the Semantic Web
• E.g., article in May 2001 issue of Scientific
  American

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Scientific American, May 2001
Beware of the Hype
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Beware of the Hype

• Hype seems to suggest that Semantic Web means
  semantics web AI
• A new form of Web content that is meaningful to
  computers will unleash a revolution of new
  abilities
• More realistic to think of it as meaning
  semantics web AI more useful web
• Realising the complete vision is too hard for
  now (probably)
• But we can make a start by adding semantic
  annotation to web resources

Images from Christine Thompson and David Booth
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Web Schema Languages

• Existing Web languages extended to facilitate
  content description
• XML ? XML Schema (XMLS)
• RDF ? RDF Schema (RDFS)
• XMLS not an ontology language
• Changes format of DTDs (document schemas) to be
  XML
• Adds an extensible type hierarchy
• Integers, Strings, etc.
• Can define sub-types, e.g., positive integers
• RDFS is recognisable as an ontology language
• Classes and properties
• Sub/super-classes (and properties)
• Range and domain (of properties)

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RDF and RDFS

• RDF stands for Resource Description Framework
• It is a W3C candidate recommendation
  (http//www.w3.org/RDF)
• RDF is graphical formalism ( XML syntax
  semantics)
• for representing metadata
• for describing the semantics of information in a
  machine- accessible way
• RDFS extends RDF with schema vocabulary, e.g.
• Class, Property
• type, subClassOf, subPropertyOf
• range, domain

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The RDF Data Model

• Statements are ltsubject, predicate, objectgt
  triples

• Can be represented using XML serialisation, e.g.
• ltIan,hasColleague,Uligt
• Statements describe properties of resources
• A resource is a URI representing a (class of)
  object(s)
• a document, a picture, a paragraph on the Web
• http//www.cs.man.ac.uk/index.html
• a book in the library, a real person (?)
• isbn//5031-4444-3333
• Properties themselves are also resources (URIs)

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URIs

• URI Uniform Resource Identifier
• "The generic set of all names/addresses that are
  short strings that refer to resources
• URIs may or may not be dereferencable
• URLs (Uniform Resource Locators) are a particular
  type of URI, used for resources that can be
  accessed on the WWW (e.g., web pages)
• In RDF, URIs typically look like normal URLs,
  often with fragment identifiers to point at
  specific parts of a document
• http//www.somedomain.com/some/path/to/filefragme
  ntID

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Linking Statements

• The subject of one statement can be the object of
  another
• Such collections of statements form a directed,
  labeled graph
• Note that the object of a triple can also be a
  literal (a string)
• Note also that RDF triples dont by themselves
  give meaning
• You know that (1) Ian and Carol are most likely
  colleagues (barring multiple jobs for Uli (2)
  (Uli hasCollegue Ian) holds (colleagueness
  unlike love is symmetric). But DOES YOUR
  PROGRAM KNOW THIS?

21
RDF Syntax

• RDF has an XML syntax that has a specific
  meaning
• Every Description element describes a resource
• Every attribute or nested element inside a
  Description is a property of that Resource with
  an associated object resource
• Resources are referred to using URIs
• ltDescription about"some.uri/person/ian_horrocks"
  gt
• lthasColleague resource"some.uri/person/uli_sa
  ttler"/gt
• lt/Descriptiongt
• ltDescription about"some.uri/person/uli_sattler"gt
  
• lthasHomePagegthttp//www.cs.mam.ac.uk/sattlerlt
  /hasHomePagegt
• lt/Descriptiongt
• ltDescription about"some.uri/person/carole_goble"
  gt
• lthasColleague resource"some.uri/person/uli_sa
  ttler"/gt
• lt/Descriptiongt

22
RDF Schema (RDFS)

• RDF gives a formalism for meta data annotation,
  and a way to write it down in XML, but it does
  not give any special meaning to vocabulary such
  as subClassOf or type
• Interpretation is an arbitrary binary relation
• I.e., ltPerson,subClassOf,Animalgt has no special
  meaning
• RDF Schema defines schema vocabulary that
  supports definition of ontologies
• gives extra meaning to particular RDF
  predicates and resources (such as subClasOf)
• this extra meaning, or semantics, specifies how
  a term should be interpreted

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Background Theory
RDF Schema is really RDF background knowledge!
Instances
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RDF/RDFS vs. General Knowledge Rep Reasoning

• We noted that RDF can be seen as base level
  facts and RDFS can be seen as background
  theory/facts/rules
• At this level, inference with RDF/RDFS seems to
  be just a special case of Knowledge
  Representation Reasoning
• This is good (CSE471 Ahoy!) and bad (reasoning
  over most non-trivial logics is NP-hard or much
  much worse).
• RDF/RDFS can be seen as an attempt to limit the
  complexity of reasoning by limiting the
  expressiveness of what can be expressed
• RDF/RDFS together can be seen as capturing a
  certain tractable subset of First Order Logic
• ..already there is trouble in paradise with
  people complaining that the expressiveness is not
  enough
• Enter OWL, which attempts to provide
  expressiveness equivalent to description logics
  (a sort of inheritance reasoning in First-order
  logic)

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Problems with RDFS

• RDFS too weak to describe resources in sufficient
  detail
• No localised range and domain constraints
• Cant say that the range of hasChild is person
  when applied to persons and elephant when applied
  to elephants
• No existence/cardinality constraints
• Cant say that all instances of person have a
  mother that is also a person, or that persons
  have exactly 2 parents
• No transitive, inverse or symmetrical properties
• Cant say that isPartOf is a transitive property,
  that hasPart is the inverse of isPartOf or that
  touches is symmetrical
• Difficult to provide reasoning support
• No native reasoners for non-standard semantics
• May be possible to reason via FO axiomatisation

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RDFS Examples

• RDF Schema terms (just a few examples)
• Class
• Property
• type
• subClassOf
• range
• domain
• These terms are the RDF Schema building blocks
  (constructors) used to create vocabularies
• ltPerson,type,Classgt
• lthasColleague,type,Propertygt
• ltProfessor,subClassOf,Persongt
• ltCarole,type,Professorgt
• lthasColleague,range,Persongt
• lthasColleague,domain,Persongt

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RDF/RDFS Liberality

• No distinction between classes and instances
  (individuals)
• ltSpecies,type,Classgt
• ltLion,type,Speciesgt
• ltLeo,type,Liongt
• Properties can themselves have properties
• lthasDaughter,subPropertyOf,hasChildgt
• lthasDaughter,type,familyPropertygt
• No distinction between language constructors and
  ontology vocabulary, so constructors can be
  applied to themselves/each other
• lttype,range,Classgt
• ltProperty,type,Classgt
• lttype,subPropertyOf,subClassOfgt

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RDF Schema is now being superseded by OWL
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Web Ontology Language Requirements

• Desirable features identified for Web Ontology
  Language
• Extends existing Web standards
• Such as XML, RDF, RDFS
• Easy to understand and use
• Should be based on familiar KR idioms
• Formally specified
• Of adequate expressive power
• Possible to provide automated reasoning support

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From RDF to OWL

• Two languages developed to satisfy above
  requirements
• OIL developed by group of (largely) European
  researchers (several from EU OntoKnowledge
  project)
• DAML-ONT developed by group of (largely) US
  researchers (in DARPA DAML programme)
• Efforts merged to produce DAMLOIL
• Development was carried out by Joint EU/US
  Committee on Agent Markup Languages
• Extends (DL subset of) RDF
• DAMLOIL submitted to W3C as basis for
  standardisation
• Web-Ontology (WebOnt) Working Group formed
• WebOnt group developed OWL language based on
  DAMLOIL
• OWL language now a W3C Recommendation (i.e., a
  standard like HTML and XML)

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OWL Language

• Three species of OWL
• OWL full is union of OWL syntax and RDF
• OWL DL restricted to FOL fragment (¼ DAMLOIL)
• OWL Lite is easier to implement subset of OWL
  DL
• Semantic layering
• OWL DL ¼ OWL full within DL fragment
• DL semantics officially definitive
• OWL DL based on SHIQ Description Logic
• In fact it is equivalent to SHOIN(Dn) DL
• OWL DL Benefits from many years of DL research
• Well defined semantics
• Formal properties well understood (complexity,
  decidability)
• Known reasoning algorithms
• Implemented systems (highly optimised)

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Layer 4½ Mapping Between Ontologies

• Taxonomy Crisis
• How can your agent know that my title is your
  name?!
• How can my agent know that some of your address
  objects are post-boxes, not physical addresses?!
• How can my agent know that many Asian first names
  correspond to Western surnames?
• Semantic Web Solution Services for
  translating/mapping between related ontologies.
• Suppose Amazon.com uses Dublin Core (title),
  while Fred Hanna uses its own document ontology
  (name). So far my agent is forced to choose
  a ontology, or must be carefully crafted to
  understand both lanuages
• A better solution A niche now exists for a
  independent entity (UniversalBookInfo.com) that
  maps title ? name etc

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without UniversalBookInfo.com
Nick wants tobuy War Peace
Nicksvery complicatedagent
Programmersbank account

Amazonontology
FredHannaontology
Amazon
Fred Hanna
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with UniversalBookInfo.com
Nick wants tobuy War Peace
Nicks agent
Joes agent


Janes Agent

UniversalBookInfo.com
Amazon
Fred Hanna

Bank Account
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(In)famous Layer Cake
? Semanticsreasoning
?
? Relational Data
?
? Data Exchange

• Relationship between layers is not clear
• OWL DL extends DL subset of RDF

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Who will annotate the data?

• Semantic web works if the users annotate their
  pages using some existing ontology (or their own
  ontology, but with mapping to other ontologies)
• But users typically do not conform to standards..
• and are not patient enough for delayed
  gratification
• Two Solutions
• 1. Intercede in the way pages are created (act as
  if you are helping them write web-pages)
• What if we change the MS Frontpage/Claris
  Homepage so that they (slyly) add annotations?
• E.g. The Mangrove project at U. Wash.
• Help user in tagging their data (allow graphical
  editing)
• Provide instant gratification by running services
  that use the tags.
• 2. Collaborative tagging!
• Folksonomies (look at Wikipedia article)
• FLICKR, Technorati, deli.cio.us etc
• 3. Automated information extraction (next topic)

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FolksonomiesThe good

• Bottom-up approach to taxonomies/ontologies
• In systems like Furl, Flickr and Del.icio.us...
  people classify their pictures/bookmarks/web
  pages with tags (e.g. wedding), and then the most
  popular tags float to the top (e.g. Flickr's tags
  or Del.icio.us on the right)....
• Folksonomies can work well for certain kinds of
  information because they offer a small reward for
  using one of the popular categories (such as your
  photo appearing on a popular page). People who
  enjoy the social aspects of the system will
  gravitate to popular categories while still
  having the freedom to keep their own lists of
  tags.

Classic case of research playing catch-up with
practice -)
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Works best when Many people Tag the same Info
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Folksonomies the bad

• On the other hand, not hard to see a few reasons
  why a folksonomy would be less than ideal in a
  lot of cases
• None of the current implementations have synonym
  control (e.g. "selfportrait" and "me" are
  distinct Flickr tags, as are "mac" and
  "macintosh" on Del.icio.us).
• Also, there's a certain lack of precision
  involved in using simple one-word tags--like
  which Lance are we talking about? (Though this is
  great for discovery, e.g. hot or Edmonton)
• And, of course, there's no heirarchy and the
  content types (bookmarks, photos) are fairly
  simple.
• For indexing and library people, folksonomies are
  about as appealing as Wikipedia is to
  encyclopedia editors.
• But.. there's some interesting stuff happening
  around them.

Computizing Eyeballs
(brain) cycle stealing
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Collaborative Computing AKA Brain Cycle
StealingAKA Computizing Eyeballs

• A lot of exciting research related to web
  currently involves co-opting the masses to help
  with large-scale tasks
• It is like cycle stealingexcept we are
  stealing human brain cycles (the most idle of
  the computers if there is ever one -)
• Remember the mice in the Hitch Hikers Guide to
  the Galaxy? (..who were running a mass-scale
  experiment on the humans to figure out the
  question..)
• Collaborative knowledge compilation (wikipedia!)
• Collaborative Curation
• Collaborative tagging
• Many big open issues
• How do you pose the problem such that it can be
  solved using collaborative computing?
• How do you incentivize people into letting you
  steal their brain cycles?