Intro semanticweb

Introduction to the
Semantic Web
Payam Barnaghi

2
The Semantic Web
“The Semantic Web is an extension of the
current web in which information is
given well-defined meaning, better
enabling computers and people to
work in co-operation.“
[Berners-Lee et al, 2001]

3
Today’s Web
 Currently most of the Web content is suitable
for human use.
 Typical uses of the Web today are information
seeking, publishing, and using, searching for
people and products, shopping, reviewing
catalogues, etc.
 Dynamic pages generated based on information
from databases but without original information
structure found in databases.

4
Limitations of the Web Search today
 The Web search results are high recall,
low precision.
 Results are highly sensitive to vocabulary.
 Results are single Web pages.
 Most of the publishing contents are not
structured to allow logical reasoning and
query answering.

6
What is a Web of Data?
Thinking back a bit... 1994
HTML and URIs
Markup language and means
for connecting resources
Below the file level
Stopped at the text level
[Miller 04]

7
What is a Web of Data?
(continued)
Now
XML, RDF, OWL and URIs
Markup language and means for
connecting resources
Below the file level
Below the text level
At the data level
[Miller 04]

8
The Syntactic Web
[Hendler & Miller 02]

9
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…
[Davies, 03]

10
i.e. the Syntactic Web is…
 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]

11
Web 2
 It is all about people, collaboration,
media, ...
[The mind-map pictured above constructed by Markus Angermeier, source Wikipedia]

12
Web 2.0 and Folksonomies
[http://flickr.com/photos/tags/]

13
Machine-accessible Content
 The main obstacle to provide better
support to Web users is that, at present ,
the meaning of Web content is not
machine accessible.
 Although there are tools to retrieve
texts, but when it comes to interpreting
sentence and extracting useful
information for the user, the capabilities
of current software are still very limited.

14
Distinguishing the meaning
 It is simply difficult for machines to
distinguish the meaning of:
I am a philosopher.
from
I am a philosopher, you may think.
Well,…

15
…Limitations of the Web today
The Web activities are mostly focus on Machine-to-Human,
and Machine-to-Machine activities are not particularly well
supported by software tools.
[Davies, 03]

16
How Can the Current Situation be
Improved?
 An alternative approach is to represent
Web content in a form that is more easily
machine-accessible and to use intelligent
techniques to take advantage of these
presentations.

17
Machine Accessible Meaning
CV
name
education
work
private
[Davies, 03]

18
XML
<H1>Internet and World Wide Web</H1>
<UL>
<LI>Code: G52IWW
<LI>Students: Undergraduate
</UL>
<H1>Internet and World Wide Web</H1>
<UL>
<LI>Code: G52IWW
<LI>Students: Undergraduate
</UL>
HTML:
<module>
<title>Internet and World Wide Web</title>
<code>G52IWW</code>
<students>Undergraduate</students>
</module>
<module>
<title>Internet and World Wide Web</title>
<code>G52IWW</code>
<students>Undergraduate</students>
</module>
XML:
User definable and domain specific markup

19
XML: Document = labeled tree
module
lecturertitle students
name weblink
<module date=“...”>
<title>...</title>
<lecturer>
<name>...</name>
<weblink>...</weblink>
</lecturer>
<students>...</students>
</module>
=
 DTD: describe the grammar and structure of
permissible XML trees
 node = label + contents

20
But What about this?
CV
name
education
work
private
< >
< >
< >
< >
< >
< Χς >
< ναµε >
<εδυχατιον>
<ωορκ>
<πριϖατε>
[Davies, 03]

21
XML
 Meaning of XML-Documents is intuitively clear
 due to "semantic" Mark-Up
 tags are domain-terms
 But, computers do not have intuition
 tag-names do not provide semantics for machines.
 DTDs or XML Schema specify the structure of
documents, not the meaning of the document contents
 XML lacks a semantic model
 has only a "surface model”, i.e. tree

22
XML:
limitations for semantic markup
 XML representation makes no commitment on:
 Domain specific ontological vocabulary
 Which words shall we use to describe a given set of concepts?
 Ontological modelling primitives
 How can we combine these concepts, e.g. “car is a-kind-of (subclass-
of) vehicle”
 requires pre-arranged agreement on vocabulary and
primitives
 Only feasible for closed collaboration
 agents in a small & stable community
 pages on a small & stable intranet
.. not for sharable Web-resources
[Davies, 03]

23
XML is a first step
 Semantic markup
 HTML  layout
 XML  content
 Metadata
 within documents, not across documents
 prescriptive, not descriptive
 No commitment on vocabulary and modelling
primitives
 RDF is the next step
[Davies, 03]

24
Resource Description
Framework (RDF)
 A standard of W3C
 Relationships between documents
 Consisting of triples or sentences:
 <subject, property, object>
 <“Mozart”, composed, “The Magic Flute” >
 RDFS extends RDF with standard “ontology
vocabulary”:
 Class, Property
 Type, subClassOf
 domain, range

25
RDF for semantic annotation
 RDF provides metadata about Web resources
 Object -> Attribute-> Value triples
 It has an XML syntax
 Chained triples form a graph
http://sepang.nottingham.edu.my/~bpayam/images/payam-barnaghi.png
has_image
#Payam payam@notting
ham
has_emailUNiM
has_teaching
http://www.nottingham.edu.my/CSIT/G53ELC
has_owner
<rdf:Description rdf:about=“#Payam”>
<has_email>payam@nottingham</has_email>
</rdf:Description>
http://sepang.nottingham.edu.my/~bpayam/#Payam

26
RDF: Basic Ideas
 Resources
 Every resource has a URI (Universal Resource
Identifier)
 A URI can be a URL (a web address) or a some other
kind of identifier;
 An identifier does not necessarily enable access to a
resources
 We can think of a resources as an object that we
want to describe it.
 Books
 Person
 Places, etc.

27
RDF: Basic Ideas
 Properties
 Properties are special kind of resources;
 Properties describe relations between
resources.
 For example: “written by”, “composed by”,
“title”, “topic”, etc.
 Properties in RDF are also identified by URIs.
 This provides a global, unique naming
scheme.

28
RDF: Basic Ideas
 Statements
 A statement is an object-attribute-value
triple.
 It consists of a resources, a property, and a
value.
http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10140
publishedBy
#MIT Press

30
RDF Example
Source: http://www.w3.org/TR/swbp-skos-core-guide/

31
RDF Schema: Basic Ideas
 RDF is a universal language that enables
users to describe their own vocabularies.
 But, RDF does not make assumption about
any particular domain.
 It is up to user to define this in RDF
schema.

32
What does RDF Schema add?
• Defines vocabulary for RDF
• Organizes this vocabulary in a typed hierarchy
• Class, subClassOf, type
• Property, subPropertyOf
• domain, range
AlanTom
Staff
Lecturer Research Assistant
subClassOf
subClassOf
type
supervisedBy
domain range
type
supervisedBy
[adapted from: Studer et al, 04]
Schema(RDFS)
Data(RDF)

33
Querying RDF data
 Query Languages such as SPARQL, RQL.
 RDF is a directed, labeled graph data format for
representing information in the Web.
 Most forms of the query languages contain a set
of triple patterns.
 Triple patterns are like RDF triples except that
each of the subject, predicate and object may
be a variable.

34
Basic Queries
 The example provided in RQL.
 Using select-from-where
 select specifies the number and order of
retrieved data.
 from is used to navigate through the data
model.
 where imposes constraints on possible
solutions

35
Basic Queries: Example
select X,Y
From {X} writtenBy {Y}
X, Y are variables, {X} writtenBy {Y}
represents a resource-property-value
triple

36
Conclusions about RDF(S)
 Next step up from plain XML:
 (small) ontological commitment to modeling
primitives
 possible to define vocabulary
 However:
 no precisely described meaning
 no inference model
[Davies, 03]

37
Ontologies
 The term ontology is originated from
philosophy. In that context it is used as
the name of a subfield of philosophy,
namely, the study of the nature of
existence.
 For the Semantic Web purpose:
 “An ontology is an explicit and formal
specification of a conceptualisation”.
(R. Studer)

38
Ontologies and Semantic Web
 In general, an ontology describes formally a
domain of discourse.
 An ontology consists of a finite list of terms and
the relationships between the terms.
 The terms denote important concepts classes of
objects) of the domain.
 For example, in a university setting, staff
members, students, courses, modules, lecture
theatres, and schools are some important
concepts.

39
Ontologies and Semantic Web
(cont’d)
 In the context of the Web, ontologies provide a
shared understanding of a domain.
 Such a shared understanding is necessary to
overcome the difference in terminology.
 Ontologies are useful for improving accuracy of
Web searches.
 Web searches can exploit
generalization/specialization information.

40
OntologyF-Logic
similar
OntologyF-Logic
similar
PhD StudentDoktoral Student
Object
Person Topic Document
Tel
PhD StudentPhD Student
Semantics
knows described_in
writes
Affiliation
described_in is_about
knowsP writes D is_about T P T
DT T D
Rules
subTopicOf
• Major Paradigms: Logic Programming, Description Logic
• Standards: RDF(S); OWL
ResearcherStudent
instance_of
is_a
is_a
is_a
Affiliation
Affiliation
Siggi
AIFB+49 721 608 6554
A Sample Ontology
[Studer et al, 04]

41
PhD StudentPhD Student AssProfAssProf
AcademicStaffAcademicStaff
rdfs:subClassOfrdfs:subClassOf
cooperate_withcooperate_with
rdfs:range
rdfs:domain
Ontology
<swrc:AssProf rdf:ID="sst">
<swrc:name>Steffen Staab
</swrc:name>
...
</swrc:AssProf>
http://www.aifb.uni-karlsruhe.de/WBS/sst
Anno-
tation
<swrc:PhD_Student rdf:ID="sha">
<swrc:name>Siegfried
Handschuh</swrc:name>
...
</swrc:PhD_Student>
Web
Page
http://www.aifb.uni-karlsruhe.de/WBS/shaURL
<swrc:cooperate_with rdf:resource =
"http://www.aifb.uni-
karlsruhe.de/WBS/sst#sst"/>
instance of
instance
of
Cooperate_with
Ontology & Annotation
Links have explicit meanings!
[Studer et al, 04]

42
Ontologies (OWL)
 RDFS is useful, but does not solve all possible
requirements
 Complex applications may want more possibilities:
 similarity and/or differences of terms (properties or classes)
 construct classes, not just name them
 can a program reason about some terms? E.g.:
 “if «Person» resources «A» and «B» have the same «foaf:email»
property, then «A» and «B» are identical”
 etc.
 This lead to the development of OWL (Web Ontology
Language)
source: Introduction to the Semantic Web, Ivan Herman, W3C

43
Ontology Languages for the Web
 RDF Schema is a vocabulary description
language for describing properties and
classes of RDF resources, with a
semantics for generalization hierarchies
of such properties and classes.
 OWL is a richer vocabulary description
language for describing properties and
classes.

44
OWL Language
 OWL is based on Description Logics knowledge representation
formalism
 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)
 Three species of OWL
 OWL full is union of OWL syntax and RDF
 OWL DL restricted to FOL fragment
 OWL Lite is “easier to implement” subset of OWL DL
 OWL DL based on SHIQ Description Logic
[Davies, 03]

45
Classes in OWL
 In RDFS, you can subclass existing
classes… that’s all.
 In OWL, you can construct classes from
existing ones:
 enumerate its content
 through intersection, union, complement
 through property restrictions

46
OWL classes can be “enumerated”
The OWL solution, where possible content is
explicitly listed:

47
Why develop an ontology?
 To make define web resources more precisely and
make them more amenable to machine processing
 To make domain assumptions explicit
 Easier to change domain assumptions
 Easier to understand and update legacy data
 To separate domain knowledge from operational
knowledge
 Re-use domain and operational knowledge separately
 A community reference for applications
 To share a consistent understanding of what information
means
[Davies, 03]

48
Ontology and Logic
 Reasoning over ontologies
 Inferencing capabilities
X is author of Y  Y is written by X
X is supplier to Y; Y is supplier to Z 
X and Z are part of the same supply chain
Cars are a kind of vehicle;
Vehicles have 2 or more wheels 
Cars have 2 or more wheels
[Davies, 03]

49
Logic and Inference
 Logic is the discipline that studies the principles
of reasoning
 Formal languages for expressing knowledge
 Well-understood formal semantics
 Declarative knowledge: we describe what holds
without caring about how it can be deduced
 Automated reasoners can deduce (infer)
conclusions from the given knowledge
source: A Semantic Web Primer, Grigoris Antoniou and Frank van Harmelen, MIT Press

50
An Inference Example
prof(X) → faculty(X)
faculty(X) → staff(X)
prof(michael)
We can deduce the following conclusions:
faculty(michael)
staff(michael)
prof(X) → staff(X)
source: A Semantic Web Primer, Grigoris Antoniou and Frank van Harmelen, MIT Press

51
Semantic Web Vision
Machine-processable, global
Web standards:
 Assigning unambiguous
names (URI)
 Expressing data, including
metadata (RDF)
 Capturing ontologies (OWL)
 Query, rules,
transformations,
deployment, application
spaces, logic, proofs, trust
(in progress)
[Source: Emerging Web Technologies to
Watch, Steve Bratt, W3C]

52
Semantic Web and AI?
 No human-level intelligence claims
 As with today’s WWW
 large, inconsistent, distributed
 Requirements
 scalable, robust, decentralised
 tolerant, mediated
 Semantic Web will make extensive use of current AI,
 any advancement in AI will lead to a better Semantic Web
 Current AI is already sufficient to go towards realizing the
semantic web vision
 As with WWW, Semantic Web will (need to) adapt fast
[Davies, 03]

53
Semantic Web & Knowledge
Management
 Organising knowledge in conceptual
spaces according to its meaning.
 Enabling automated tools to check for
inconsistencies and extracting new
knowledge.
 Replacing query-based search with query
answering.
 Defining who may view certain parts of
information

55
Web Services
 Web Services provide data and services to other
applications.
 Thee applications access Web Services via
standard Web Formats (HTTP, HTML, XML, and
SOAP), with no need to know how the Web
Service itself is implemented.
 You can imagine a web service like a remote
procedure call (RPC) which it returns a
message in an XML format.

56
Web Services
 loosely coupled, reusable components
 encapsulate discrete functionality
 distributed
 programmatically accessible over
standard internet protocols
 add new level of functionality on top of
the current web
[Stollberg et al., 05]

57
The Promise of Web Services

58
Deficiencies of WS Technology
 Current technologies allow usage of Web Services
 but:
 only syntactical information descriptions
 syntactic support for discovery, composition and execution
=> Web Service usability, usage, and integration needs to be
inspected manually
 no semantically marked up content / services
 no support for the Semantic Web
=> current Web Service Technology Stack failed to
realize the promise of Web Services

59
Semantic Web Technology
+
Web Service Technology
Semantic Web Services
=> Semantic Web Services as integrated solution for
realizing the vision of the next generation of the Web
• allow machine supported data interpretation
• ontologies as data model
automated discovery, selection, composition,
and web-based execution of services

60
Semantic Web Services
 define exhaustive description frameworks for
describing Web Services and related aspects
(Web Service Description Ontologies)
 support ontologies as underlying data model to
allow machine supported data interpretation
(Semantic Web aspect)
 define semantically driven technologies for
automation of the Web Service usage process
(Web Service aspect)

61
Acknowledgements
 Some of the slides are adapted from the following resources:
 Semantic Web, John Davies, Next Generation Web Research, BT.
 A Short Semantic Web Tutorial, Andreas Hotho & York Sure,
Knowledge Management Group, Institute AIFB, University of Karlsruhe.
 Semantic Web and Ontology Management, Rudi Studer, York Sure,
Christoph Tempich, Peter Haase,Institute AIFB, University of
Karlsruhe.
 A Semantic Web Primer, Grigoris Antoniou and Frank van Harmelen,
ISBN 0-262-01210-3, 2004, the MIT press.
 The Semantic Web: A Web of Machine Processible Data, Eric Miller,
W3C Semantic Web Activity Lead, 2004.
 Stollberg et al, Semantic Web Services Tutorial, 5th International
Conference on Web Engineering (ICWE 2005), Sydney, Australia.
 Introduction to the Semantic Web, Ivan Herman, W3C, 2007.

62
Suggested Readings
 A Semantic Web Primer, Grigoris Antoniou and Frank
van Harmelen, ISBN 0-262-01210-3, 2004, the MIT press.
 W3C Semantic Web
http://www.w3.org/2001/sw/
 The Semantic Web Community Portal,
http://www.semanticweb.org

Intro semanticweb

More Related Content

What's hot

Similar to Intro semanticweb

Recently uploaded

Intro semanticweb

Editor's Notes