Semantic Basics: Markup, Querying, and Reasoning

1
Semantic Basics Markup, Querying, and Reasoning

• Marlon Pierce
• Community Grids Lab
• Indiana University
• With Slides and Help from Sean Bechhofer, Carole
  Goble, Line Pouchard, and Dave De Roure

2
Preface Beyond XML
3
Reductio ad Absurdum

• Physics is the study of the harmonic
  oscillator.
• H. L. Richards
• Statistical Mechanics is the study of the Ising
  Model
• H. L. Richards
• Web Service standards are the study of ltxsdanygt
  sequences
• M. E. Pierce, soon to be anonymous

4
Which Web Service Specs?

• ltxselement name"Header" type"tnsHeader" /gt
• ltxscomplexType name"Header"gt
• ltxssequencegt
• ltxsany namespace"any" processContents"lax"
  minOccurs"0" maxOccurs"unbounded" /gt
• lt/xssequencegt
•   ltxsanyAttribute namespace"other"
  processContents"lax" /gt
•   lt/xscomplexTypegt

• ltxsdcomplexType name"SecurityHeaderType"gt
• ltxsdsequencegt
• ltxsdany
• processContents"lax"
• minOccurs"0"
• maxOccurs"unbounded"gt
• lt/xsdanygt
• lt/xsdsequencegt
•   ltxsdanyAttribute
• namespace"other"
• processContents"lax" /gt
• lt/xsdcomplexTypegt

5
Which, What, and Why?

• Which is what?
• Left is the definition of the SOAP header.
• Right is taken from Web Service Secure Messaging
  Specification.
• You will find this pattern repeated pretty often
  in web service specifications.
• Why?
• We have limited ways of linking several XML
  schema data models.
• XML maps relationships to trees.
• Graphs are a more natural way of expressing many
  inter-relationships of concepts.

6
XML for KR

• Definition of self-describing data in worldwide
  standardized, non-proprietary format.
• Structured data and knowledge exchange for
  enterprises in various industries.
• Integration of information from different sources
  to uniform documents.
• Exchange of knowledge bases between different AI
  languages, knowledge bases and databases,
  application systems, etc.
• But.

7
XML is not enough
The Creator of the Resource http//www.w3.org/Ho
me/Lassila is Ora Lassila

• XML defines grammars to verify and structure
  documents
• The grammar enforces constraints on tags
• Different grammars define the same content
• XML lacks a semantic model it only has a
  surface model which is a tree.

8
XML is not enough

• Meaning of XML documents is intuitively clear
• semantic markup tags are domain terms
• But computers do not have intuition
• Tag names per se do not provide semantics
• The semantics are encoded outside the XML
  specification
• XML makes no commitment on
• Domain specific ontological vocabulary
• Ontological modeling primitives
• ? requires pre-arranged agreement on ? ?
• Feasible for closed collaboration
• agents in a small stable community
• pages on a small stable intranet
• Semantic Web Markups often are expressed in XML
  but they carry extra meaning.

9
Enter the Semantic Web/Grid

• The Semantic Web is the representation of data
  on the World Wide Web. It is a collaborative
  effort led by W3C with participation from a large
  number of researchers and industrial partners. It
  is based on the Resource Description Framework
  (RDF), which integrates a variety of applications
  using XML for syntax and URIs for naming.

10
Resource Description Framework

• Overview of RDF basic ideas and XML encoding.

11
Building Semantic Markup Languages

• XML essentially defines syntax rules for markup
  languages.
• Human readable means humans provide meaning
• We also would like some limited ability to encode
  meaning directly within markup languages.
• The semantic markup languages attempt to do that,
  with increasing sophistication.
• Stack indicates direct dependencies DAML is
  defined in terms of RDF, RDFS.

Eric Miller, http//www.w3.org/2002/Talks/www2002-
w3ct-swintro-em/
12
Resource Description Framework (RDF)

• RDF is the simplest of the semantic languages.
• Basic Idea 1 Triples
• RDF is based on a subject-verb-object statement
  structure.
• RDF subjects are called classes
• Verbs are called properties.
• Basic Idea 2 Everything is a resource that is
  named with a URI
• RDF nouns, verbs, and objects are all labeled
  with URIs
• Recall that a URI is just a name for a resource.
• It may be a URL, but not necessarily.
• A URI can name anything that can be described
• Web pages, creators of web pages, organizations
  that the creator works for,.

13
What Does This Have to Do with Grid Computing?

• RDF resources arent just web pages
• Can be computer codes, simulation and
  experimental data, hardware, research groups,
  algorithms, .
• Recall from the CMCS chemistry example that they
  needed to describe the provenance, annotation,
  and curation of chemistry data.
• Compound Xs properties were calculated by Dr. Y.
• CMCS maps all of their metadata to the Dublin
  Core.
• The Dublin Core is encoded quite nicely as RDF.

14
RDF Graph Model

• RDF is defined by a graph model.
• Resources are denoted by ovals.
• Lines (arcs) indicate properties.
• Squares indicate string literals (no URI).
• Resources and properties are labeled by a URI.

http//.../CMCS/Entries/X
http//purl.org/dc/elements/1.1/creator
http//.../CMCS/People/DrY
http//purl.org/dc/elements/1.1/title
H2O
15
Encoding RDF as Triplets

• RDF graphs may be written as triple sentences.
• A triple is just the subject, predicate, and
  object (in that order) of a graph segment.
• lthttp//.../CMCS/Entries/Xgtlthttp//purl.org/dc/ele
  ments/1.1/creatorgthttp//.../CMCS/People/DrYgt
• This structure my look trivial but is useful in
  expressing queries (more later).

16
Encoding RDF in XML

• The graph represents two statements.
• Entry X has a creator, Dr. Y.
• Entry X has a title, H2O.
• In RDF XML, we have the following tags
• ltRDFgt lt/RDFgt denote the beginning and end of the
  RDF description.
• ltDescriptiongts about attribute identifies the
  subject of the sentence.
• ltDescriptiongtlt/Descriptiongt enclose the
  properties and their values.
• We import Dublin Core conventional properties
  (creator, title) from outside RDF proper.

17
RDF XML The Gory Details

• ltrdfRDF xmlnsrdf'http//www.w3.org/1999/02/22-r
  df-syntax-ns' xmlnsdc'http//purl.org/dc/eleme
  nts/1.0/'gt
• ltrdfDescription rdfabout'http//.../Xgt
  
• ltdccreator
• rdfresource'http///people/MEP/gt
  ltdctitle
• rdfresource'H2O'/gt lt/rdfDescriptiongt
• lt/rdfRDFgt

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Creating RDF Documents

• Writing RDF XML (or DAML or OWL) by hand is not
  easy.
• Its a good way to learn to read/write, but after
  you understand it, automate it.
• Authoring tools are available
• OntoMat buggy
• Protégé preferred by CGL grad students
• IsaViz another nice tool with very good
  graphics.
• You can also generate these programmatically
  using Hewlett Packard Labs Jena toolkit for
  Java.
• This is what I did in previous example.

19
What is the Advantage?

• So far, properties are just conventional URI
  names.
• All semantic web properties are conventional
  assertions about relationships between resources.
• RDFS and DAML will offer more precise property
  capabilities.
• But there is a powerful feature we are about to
  explore
• Properties provide a powerful way of linking
  different RDF resources
• Nuggets of information.
• For example, a publication is a resource that can
  be described by RDF
• Author, publication date, URL are all metadata
  property values.
• But publications have references that are just
  other publications
• DCs hasReference can be used to point from one
  publication to another.
• Publication also have authors
• An author is more than a name
• Also an RDF resource with collections of
  properties
• Name, email, telephone number,

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vCard Representing People with RDF Properties

• The Dublin Core tags are best used to represent
  metadata about published content
• Documents, published data
• vCards are an IETF standard for representing
  people
• Typical properties include name, email,
  organization membership, mailing address, title,
  etc.
• See http//www.ietf.org/rfc/rfc2426.txt
• Like the DC, vCards are independent of (and
  predate) RDF but are map naturally into RDF.
• Each of these maps naturally to an RDF property
• See http//www.w3.org/TR/2001/NOTE-vcard-rdf-20010
  222/

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Example A vCard in RDF/XML
ltrdfRDF xmlnsrdf'http//www.w3.org/1999/02/
22-rdf-syntax-ns' xmlnsvcard'http//www.w3.
org/2001/vcard-rdf/3.0'gt ltrdfDescription
rdfabout'http//cgl.indiana.edu/people/GCF'
vcardEMAIL'gcf_at_indiana.edu'gt
ltvcardFNgtGeoffrey Foxlt/vcardFNgt
ltvcardN vcardGiven'Geoffrey'
vcardFamily'Fox'/gt
lt/rdfDescriptiongtlt/rdfRDFgt
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Linking vCard and Dublin Core Resources

• The real power of RDF is that you can link two
  independently specified resources through the use
  of properties.
• We do this using URIs as universal pointers
• Identify specific resources (nouns) and
  specifications for properties (verbs)
• The URIs may optionally be URLs that can be used
  to fetch the information.
• Linking these resource nuggets allows us to pose
  queries like
• What is the email address of the creator of this
  entry in the chemical database?
• What other entries reference directly or
  indirectly on this data entry?
• Linkages can be made at any time
• Dont have to be designed into the system

23
Graph Model Depicting vCard and DC Linking
dry_at_stateu.edu
http//.../CMCS/Entry/1
dccreator
vcardEMAIL
http//.../People/DrY
dctitle
H20
vcardN
vcardFamily
vcardGiven
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What Else Does RDF Do?

• Collections typically used as the object of an
  RDF statement
• Bag unordered collection of resources or
  literals.
• Sequence ordered collection or resources or
  literals.
• Alternative collection of resources or literals,
  from which only one value may be chosen
• And thats about it. RDF does not define
  properties, it just tells you where to put them.
• Definitions are done by specific groups for
  specific fields (Dublin Core Metadata Initiative,
  for example).
• RDF Schema provides the rules for defining
  specific resources classes and properties.

25
RDF Schema
26
Other Semantic Markup Languages

• RDF Schema (RDFS)
• Provides formal definitions of RDF
• Also provides language tools for writing more
  specialized languages.
• Well examine in more detail.
• DARPA Agent Markup Language (DAML)
• DAML-OIL is the language component of the DAML
  project.
• Defined using RDF/RDFS.
• Well examine in more detail.
• Ontology Inference Layer (OIL)
• OIL language expressed in terms of RDF/RDFS.
• The OIL project is sponsored by the European
  Union.
• Web-Ontology Language (OWL)
• Developed by the W3Cs Web-Ontology Working Group
• Based on DAML-OIL

27
RDF Schema

• RDF Schema is a rules system for building RDF
  languages.
• RDF and RDFS are defined in terms of RDFS
• DAMLOIL is defined by RDFS.
• Take the Dublin Core RDF encoding as an example
• Can we formalize this process, defining a
  consistent set of rules?
• Can we place restrictions and use inheritance to
  define resources?
• What really is the value of creator? Can I
  derive it from another class, like person?
• Can we provide restrictions and rules for
  properties?
• How can I express the fact that title should
  only appear once?
• Current DC encoding in fact is defined by RDFS.

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Some RDFS Classes
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Some RDFS Properties
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Sample RDFS Defining ltPropertygt
ltrdfsClass rdfabout"http//.../some/uri"gt
ltrdfsisDefinedBy rdfresource"http//.../some/ur
i"/gt ltrdfslabelgtPropertylt/rdfslabelgt
ltrdfscommentgtThe class of RDF properties.lt/rdfsc
ommentgt ltrdfssubClassOf
rdfresource"http//.../Resourcegt
lt/rdfsClassgt

• This is the definition of ltpropertygt, taken from
  the RDF schema.
• The about attribute labels names this nugget.
• ltpropertygt has several properties
• ltlabelgt,ltcommentgt are self explanatory.
• ltsubClassOfgt means ltpropertygt is a subclass of
  ltresourcegt
• ltisDefinedBygt points to the human-readable
  documentation.

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RDFS Takeaway

• RDFS defines a set of classes and properties that
  can be used to define new RDF-like languages.
• RDFS actually bootstraps itself.
• You can express inheritance, restriction
• If you want to learn more, see the specification
• http//www.w3.org/TR/2003/WD-rdf-schema-20030123/
• But dont trust the write up
• Concepts are best understood by looking at the
  RDF XML. English descriptions get convoluted.
• If you want to see RDFS in action, see the DC
• http//dublincore.org/2003/03/24/dces

32
Web Ontology Language(OWL)

• Eeyore W-O-L. That spells owl.
• Owl Bless my soul! So it does!

33
Whats an Ontology?

• Ontology is an often used term in the field of
  Knowledge Representation, Information Modeling,
  etc.
• English definitions tend to be vague to
  non-specialists
• A formal, explicit specification of a shared
  conceptionalization
• Clearer definition an ontology is a taxonomy
  combined with inference rules
• T. Berners-Lee, J. Hendler, O. Lassila
• But really, if you sit down to describe a subject
  in terms of its classes and their relationships,
  you are creating an Ontology.
• You can express this in RDFS or OWL

34
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

35
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
• Of adequate expressive power
• Formally specified
• Possible to provide automated reasoning support

36
History From RDF to OWL

• Two languages developed by extending (part of)
  RDF
• OIL developed by group of (largely) European
  researchers
• 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 (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 (Feb 2004)

37
What Are Description Logics?

• A family of logic based Knowledge Representation
  formalisms
• Descendants of semantic networks and KL-ONE
• Describe domain in terms of concepts (classes),
  roles (relationships) and individuals
• Distinguished by
• Formal semantics (typically model theoretic)
• Decidable fragments of FOL
• Closely related to Propositional Modal Dynamic
  Logics
• Provision of inference services
• Sound and complete decision procedures for key
  problems
• Implemented systems (highly optimised)

38
Short History of Description Logics

• Phase 1
• Incomplete systems (Back, Classic, Loom, . . . )
• Based on structural algorithms
• Phase 2
• Development of tableau algorithms and complexity
  results
• Tableau-based systems for Pspace logics (e.g.,
  Kris, Crack)
• Investigation of optimisation techniques
• Phase 3
• Tableau algorithms for very expressive DLs
• Highly optimised tableau systems for ExpTime
  logics (e.g., FaCT, DLP, Racer)
• Relationship to modal logic and decidable
  fragments of FOL

39
Latest Developments

• Phase 4
• Mature implementations
• Mainstream applications and Tools
• Databases
• Consistency of conceptual schemata (EER, UML
  etc.)
• Schema integration
• Query subsumption (w.r.t. a conceptual schema)
• Ontologies and Semantic Web (and Grid)
• Ontology engineering (design, maintenance,
  integration)
• Reasoning with ontology-based markup (meta-data)
• Service description and discovery
• Commercial implementations
• Cerebra system from Network Inference Ltd

40
OWL Semantic Layering

• Three language layers
• OWL Full
• Union of OWL Vocabulary and RDFS
• OWL DL
• Restricted to DL/FOL fragment (?DAMLOIL)
• OWL Lite
• Subset of OWL DL
• Syntactic Layering
• Semantic Layering
• Layers should agree on semantics.

Full
DL
Lite
41
OWL Full

• No restriction on use of OWL vocabulary (as long
  as legal RDF)
• Classes as instances
• Assertions about vocabulary
• RDF style model theory
• Reasoning using FOL engines
• via axiomatisation
• Semantics should correspond with OWL DL for
  suitably restricted KBs

Full
42
OWL DL

• Use of OWL vocabulary restricted
• Cant be used to do nasty things (i.e., modify
  OWL)
• No classes as instances
• Standard DL/FOL model theory (definitive)
• Direct correspondence with (first order) logic
• Reasoning via DL engines
• Some problems with oneOf/inverse
• Reasoning for full language via FOL engines
• Would need built in datatypes for performance

DL
43
OWL Lite

• Like DL, but fewer constructs
• No explicit negation or union
• Restricted cardinality (zero or one)
• No nominals (oneOf)
• Semantics as per DL
• Reasoning via standard DL engines (datatypes)
• E.g., FaCT, RACER, Cerebra

Lite
44
An OWL Example

• An Earth Systems Grid example

45
An Example Ontology Climate Data

• The example shows how to construct a really
  simple ontology and instance.
• Two classes
• dataset
• Parameter
• One property
• hasParameter
• Several parameters cloud_medium,
  bounds_latitude, temperature
• Line Pouchard (ORNL) created this for ESG using
  Protégé and OilEd.
• Full ontology shown at the end for reference.

46
Ontology header With Dublin Core Parameters.
Class Definitions
hasParameter Definition
47
Parameter Cloud_medium
Parameter Bounds_latitude
Parameter temperature
48
OWL Enriched RDF Metadata about
PCM.B06.10.dataset1
49
OWL Equivalence and Inheritance

• ltowlClass rdfIDusergt
• ltowlequivalentClass rdfresourcepersongt
• ltowlClassgt

• ltowlClass rdfaboutmagneticSpectrometergt
• ltrdfssubClassOfgt
• ltowlRestrictiongt
• ltowlonProperty rdfresourcehasMagnetsgt
• ltowlallValuesFrom rdfresourceSpectrometergt
  
• lt/owlRestrictiongt
• lt/rdfs subClassOfgt
• lt/owlClassgt

• Other logical relationships
• that can be asserted
• inverseOf,
• TransitveProperty,
• SymmetricProperty,
• FunctionalProperty,
• InverseFunctionalProperty

50
Illustration of Inverse Properties
51
Querying Semantic Data

• The Data Access Working Group (DAWG)

52
What Is Semantic Querying?

• Dont confuse querying with inference.
• Querying just means retrieving data from Semantic
  data models.
• Post a query to the world of distributed RDF data
  nuggets.
• For RDF-like structures, this amounts to querying
  triples

• Examples
• Finding an Email address from a persons vCard.
• Searching across subgraphs get me the email of
  the author of this document (Dublin Core
  vCard).
• Persistent/scheduled queries on updates to
  several multimedia databases.

53
The DAWG Working Group

• Unfortunately, there are no standards for
  querying RDF, etc.
• There are solutions, like RDQL/SquishQL
• These are just not official
• The W3C Data Access Working Group DAWG is filling
  the query gap.
• Formed Feb 2004.
• This is a work in progress
• Use Cases and Requirements http//www.w3.org/TR/r
  df-dawg-uc/
• BRQL Query Language http//www.w3.org/2001/sw/Dat
  aAccess/rq23/

54
A Simple Query

• Consider the following RDF triple
• lthttp//example.org/book/book1gt
  lthttp//purl.org/dc/elements/1.1/titlegt "BRQL
  Tutorial
• Recall this is equivalent to the sentence book1
  has title BRQL Tutorial
• We may have a large set of such triples in our
  data store.
• We want to make a query on this data like this
  What is the title of book1?

55
The Query and the Results

• We can construct queries on any of the parts of
  the triple, such as
• SELECT ?title
• WHERE lthttp//example.org/book/book1gt
  lthttp//purl.org/dc/elements/1.1/titlegt ?title .
  
• Thus just means what is the title of book1?
• ?title "BRQL Tutorial

56
So What?

• This was a trivial example in which we posed a
  query on the triples object, which was a string.
• But the object of the triple may be a URI (an RDF
  resource), not just a literal.
• Or we may construct queries against subjects or
  verbs of triples.
• For complicated graphs, this means that the query
  returns a pointer to another section of the
  graph.
• This means that we can make linked queries that
  allow us to navigate graphs.

57
Linked Queries Across Graph Sections
dry_at_stateu.edu
http//.../CMCS/Entry/1
dccreator
vcardEMAIL
http//.../People/DrY
dctitle
H20
vcardN
What is the given name Of the creator of Entry 1?
vcardFamily
vcardGiven
58
What If You Cant Wait?

• BRQL is still a work in progress.
• If you need something now, there is Jenas RDQL.
• RDQL allows you to pose triplet queries similar
  BRQL
• Jena has a programming interface that allows you
  to construct and execute these queries against
  RDF.

59
A Simple Jena RDQL Example

• Model modelnew ModelMem()
• Model.read(new FileReader(a.rdf))
• String queryString "SELECT ?x, ?fname WHERE
  (?x,lthttp//www.w3.org/2001/vcard-rdf/3.0EMAILgt,
  ?fname)"
• Query querynewQuery(queryString)
• query.setSource(model)
• QueryExecution qenew QueryEngine(query)
• QueryResults resultsqe.exec()

60
Advanced OWL Tutorial

• The Details

61
OWL Syntaxes

• Abstract Syntax
• Used in the definition of the language and the
  DL/Lite semantics
• OWL as RDF triples (and thus as, e.g. RDF/XML or
  N3)
• the official concrete syntax
• mapping rules describe how to translate from
  abstract syntax to triples.
• XML Presentation Syntax
• XML Schema definition

62
OWL Ontologies

• An OWL ontology consists of a number of Classes,
  Properties and Individuals
• All identified via URIs.
• Classes
• Have definitions providing their
  characteristics
• Properties
• Characteristics such as transitivity or
  functionality
• Domains and Ranges
• Individuals
• Class membership
• Relationships to other individuals
• Concrete values.

63
XML Datatypes in OWL

• OWL supports XML Schema primitive datatypes
• Clean separation between object classes and
  datatypes
• Philosophical reasons
• Datatypes structured by built-in predicates
• Not appropriate to form new datatypes using
  ontology language
• Practical reasons
• Ontology language remains simple and compact
• Implementability not compromised can use hybrid
  reasoner

64
OWL Class constructors

• OWL has a number of operators for constructing
  class expressions.
• Boolean operators
• and, or, not
• Restrictions
• slot fillers with explicit quantification
• Enumerated Classes.
• explicit enumerations of the class members

65
OWL Class Constructors
66
OWL Class constructors

• The operators have an associated semantics
• Given in terms of a domain
• D
• and an interpretation function I
• Iconcepts ! Ã(D)
• Iproperties ! Ã(D D)
• Iindividuals ! D
• I is then extended to concept expressions.

67
OWL Constructor Semantics
68
OWL Constructor Semantics
69
OWL Axioms

• Axioms allow us to add further statements about
  arbitrary concept expressions and properties
• Disjointness, equivalence, transitivity of
  properties etc.
• An interpretation is then a model of the axioms
  iff it satisfies every axiom in the ontology.

70
Basic Inference Tasks

• Inference can now be defined w.r.t.
  interpretations/models.
• C subsumes D w.r.t. K iff for every model I of K,
  I(D) µ I(C)
• C is equivalent to D w.r.t. K iff for every model
  I of K, I (C) I (D)
• C is satisfiable w.r.t. K iff there exists some
  model I of K s.t. I (C) ?
• Querying knowledge
• x is an instance of C w.r.t. K iff for every
  model I of K, I(x) 2 I(C)
• hx,yi is an instance of R w.r.t. K iff for, every
  model I of K, (I(x),I(y)) 2 I(R)

71
Why Reasoning?

• Why do we want it?
• Semantic Web aims at machine understanding
• Understanding closely related to reasoning
• Given key role of ontologies in the Semantic Web,
  it will be essential to provide tools and
  services to help users
• Design and maintain high quality ontologies,
  e.g.
• Meaningful all named classes can have instances
• Correct captured intuitions of domain experts
• Minimally redundant no unintended synonyms
• Richly axiomatised (sufficiently) detailed
  descriptions
• Answer queries over ontology classes and
  instances, e.g.
• Find more general/specific classes
• Retrieve annotations/pages matching a given
  description
• Integrate and align multiple ontologies

72
Why Decidable Reasoning?

• OWL DL constructors/axioms restricted so
  reasoning is decidable
• Consistent with Semantic Web's layered
  architecture
• XML provides syntax transport layer
• RDF(S) provides basic relational language and
  simple ontological primitives
• OWL DL provides powerful but still decidable
  ontology language
• Further layers may (will) extend OWL
• Will almost certainly be undecidable
• Facilitates provision of reasoning services
• Known practical algorithms
• Several implemented systems
• Evidence of empirical tractability
• Understanding dependent on reliable consistent
  reasoning

73
Other Links
74
Tools for Playing with Things

• Jena Toolkit Java packages from HPLabs for
  building Semantic Web applications.
• http//www.hpl.hp.com/semweb/
• Both IsaViz and Protégé use this.
• IsaViz A nice authoring/graphing tool
• http//www.w3.org/2001/11/IsaViz/
• Protégé Another ontology authoring tool
• http//protege.stanford.edu/
• SiRPAC
• Allows you to parse RDF, convert RDF/XML into
  graphs and triplets.
• http//www.w3.org/RDF/Validator/

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Other Tutorials

• Original Semantic Grid GGF tutorial material is
  here
• http//www.semanticgrid.org/presentations/ontologi
  es-tutorial/
• Beginner and Advanced OWL tutorials are here
• http//www.co-ode.org/resources/
• Lectures cover working examples (pizza ontology)
  built with Protégé.
• http//www.semanticgrid.org/presentations/ontologi
  es-tutorial/

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XML Primer

• General characteristics of XML

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Basic XML

• XML consists of human readable tags
• Schemas define rules for a particular dialect.
• XML Schema is the root, defines the rules for
  making other XML schemas.
• Tree structure tags must be closed in reverse
  order that they are opened.
• Tags can be modified by attributes
• name, minOccurs
• Tags enclose either strings or structured XML

• ltcomplexType name"FaultType"gt
• ltsequencegt
•   ltelement name"FaultName"
• type"xsdstring" /gt
•   ltelement name"MapView/gt
•   ltelement name"CartView/gt
•   ltelement name"MaterialProps"
  minOccurs"0" /gt
• ltchoicegt
•   ltelement name"Slip" /gt 
• ltelement name"Rate" /gt
•   lt/choicegt
•   lt/sequencegt
•  lt/complexTypegt

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Namespaces and URIs

• XML documents can be composed of several
  different schemas.
• Namespaces are used to identify the source schema
  for a particular tag.
• Resolves name conflictsfull path
• Values of namespaces are URIs.
• URI are just structured names.
• May point to something not electronically
  retrievable
• URLs are special cases.

• ltxsdschema xmlnsxsd"http//www.w3.org/2001/XML
  Schema" xmlnsgem"http//commgrids.indiana.edu/GC
  WS/Schema/GEMCodes/Faultsgt
• ltxsdannotationgt
•   lt/xsdannotationgt
• ltgemfaultgt
• lt/gemfaultgt
• lt/xsdschemagt

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Metadata and the Dublin Core

• Define metadata and describe its use in physical
  and computer science.

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What is Metadata?

• Common definition data about data
• Traditional Examples
• Prescriptions of database structure and contents.
• File names and permissions in a file system.
• HDF5 metadata describes scientific/numerical
  data set characteristics such as array sizes,
  data formats, etc.
• Metadata may be queried to learn the
  characteristics of the data it describes.
• Traditional metadata systems are functionally
  tightly coupled to the data they describe.
• Prescriptive, needed to interact directly with
  data.

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Descriptive Metadata and the Web

• Traditional metadata concepts must be extended as
  systems become more distributed, information
  becomes broader
• Tight functional integration not as important
• Metadata used for information, becomes
  descriptive.
• Metadata may need to describe resources, not just
  data.
• Everything is a resource
• People, computers, software, conference
  presentations, conferences, activities, projects.
• Well next look at several examples that use
  metadata, featuring
• Dublin Core digital libraries
• CMCS chemistry

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The Dublin Core Metadata for Digital Libraries

• The Dublin Core is a set of simple name/value
  properties that can describe online resources.
• Usually Web content but generally usable (CMCS)
• Intended to help classify and search online
  resources.
• DC elements may be either embedded in the data or
  in a separate repository.
• Initial set defined by 1995 Dublin, Ohio meeting.

83
Thought Experiment Construct Your Own Metadata
Set

• Describe yourself your occupation, your
  interests, your place of residence, your parents,
  spouse, children,.
• Take each sentence
• The verbs become properties
• The verbs objects are property values.
• Metadata is just a collection of these name/value
  pairs.
• For particular fields (like publishing), we can
  define a conventional set of property names.

84
The Dublin Core Metadata for Digital Libraries

• The Dublin Core is a set of simple name/value
  properties that can describe online resources.
• Usually Web content but generally usable (CMCS)
• Intended to help classify and search online
  library resources.
• Digital library card catalog.
• DC elements may be either embedded in the data or
  in a separate repository.
• Initial set defined by 1995 Dublin, Ohio meeting.

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Dublin Core Elements

• Content elements
• Subject, title, description, type, relation,
  source, coverage.
• Intellectual property elements
• Contributor, creator, publisher, rights
• Instantiation elements
• Date, format, identifier, language
• In RDF, these are called properties.

86
Encoding the Dublin Core

• DC elements are independent of the encoding
  syntax.
• Rules exist to map the DC into
• HTML
• RDF/XML
• We provide more detailed info on RDF/XML encoding
  in this seminar.

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Sample RDF/HTML

• ltheadgt
• lttitlegtExpressing Dublin Core in HTML/XHTML meta
  and link elementslt/titlegt
• ltmeta name"DC.title" content"Expressing Dublin
  Core in HTML/XHTML meta and link elements" /gt
• ltmeta name"DC.creator" content"Andy Powell,
  UKOLN, University of Bath" /gt
• ltmeta name"DC.type" content"Text" /gt
• lt/headgt

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Where Do I Put the Dublin Core Metadata?

• Dublin core elements may be placed directly in
  HTML pages.
• Still need DC-aware crawlers or applications to
  find and use them.
• Or you may have a large database on DC entries
  that are used by DC-aware applications.
• Well examine a WebDAV-based scheme for chemistry
  in a second.

89
Dublin Core Element Refinements

• Many of these, and extensible
• See http//dublincore.org/documents/dcmi-terms/
  for the comprehensive list of elements and
  refinements
• Examples
• isVersionOf, hasVersion, isReplacedBy,
  references, isReferencedBy.