Transcript Semantic Web and its Logical Foundations

Semantic Web and its Logical
Foundations
Serguei Krivov,
Ecoinformatics Collaboratory
Gund Institute for Ecological Economics, UVM
Overview
• Semantic Web
• Semantic Web Languages and Tools
• Description Logics (DL) – Formalism of
Web Ontology Languages
• OWL –latest Web Ontology Language
Why Semantic Web?
• Most information on the Web is designed for
human consumption, and even if it was derived
from a database with well defined meanings for
its columns, that the structure of the data is not
evident to a robot browsing the web.
• Leaving aside the artificial intelligence problem
of training machines to behave like people, the
Semantic Web approach instead develops
languages for expressing information in a
machine processable form.
The Semantic Web Vision
+ Web made possible through established standards
TCP/IP for transporting bits down a wire
HTTP & HTML for transporting and rendering hyperlinked text
+ Applications able to exploit this common infrastructure
Result is the WWW as we know it
+ 1st generation web mostly handwritten HTML pages
+ 2nd generation (current) web often machine generated/active
+ Both intended for direct human processing/interaction
+ In next generation web, resources should be more accessible to
automated processes
To be achieved via semantic markup
Metadata annotations that describe content/function
+ Coincides with Tim Berners-Lee’s vision of a Semantic Web
Ian Horroks, Logical Foundations for the Semantic Web
What is Semantic Web?
•
Semantic Web is an initiative of World Wide Web consortium
(http://www.w3.org/2001/sw/)
•
Semantic Web is a set of Languages and Tools for machine processing of
information stored in WWW
•
You can think of it as being an efficient way of representing data on the
World Wide Web, or as a globally linked Knowledge Base
.
•
•
Semantic Web is about efficient Knowledge Representation mechanism for
WWW. (AI)
Semantic Web is about efficient Reasoning Systems requered for
integration of distributed data .
Ontologies
+ Semantic markup must be meaningful to automated processes
+ Ontologies will play a key role
Source of precisely defined terms (vocabulary) Can be shared
across applications (and humans)
+ Ontology typically consists of:
Hierarchical description of important concepts in domain
Descriptions of properties of instances of each concept
+ Degree of formality can be quite variable
+ Increased formality and regularity facilitates machine understanding
+ Ontologies can be used, e.g.:
To facilitate agent-agent communication in e-commerce
In semantic based search
To provide richer service descriptions that can be more flexibly
interpreted by intelligent agents
Ian Horroks, Logical Foundations for the Semantic Web
RDF –Ressource Definition Framework
[ressource] [property] [ressource]
[Hamlet] [Author] [W. Shakespeare ]
<?xml version="1.0"?>
<Class rdf:ID="Resource"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns="uri">
<property>value</property>
<property>value</property>
...
</Class>
RDF Usage
– Add ”annotations” for web-pages ?
– Add ”annotations” for web-services ?
– Instead of building a menu for a web-site,
declare the content in RDF, and let the useragent build the menu dynamically ?
– Describe all images on your website ?
– Market the web.service your web.site
provides ?
RDF is used as a syntax for more
expressive languages: DAML+OIL,
OWL
”Semantic Web In A Box”
• A Semantic Web in A Box Architecture
RDF enhanced Website
Internet
Search
From Jens Jacob Anderson’s Semantic Web Tutorial
View
Update Manage
RDF Repository
*.jpg
*.html
*.gif *.asp *.jsp
Agents
Companies ,Tools, and Resources
Ontoprise :
OntoEdit, OntoBrocker
see: http://www.ontoprise.de/products/index_html_en
Unicorn
http://www.unicorn.com/
KAON :
http://kaon.semanticweb.org/
Protégé -Free Ontology Editor
http://protege.stanford.edu/
Semantic Web Tools Tutorial
http://www.daml.org/2003/05/swmu-tools-tutorial/Overview.html
First Semantic Web Website:
http://owl.mindswap.org/
Web Languages
+ Web languages already extended to facilitate content description
XML Schema (XMLS)
RDF and RDF Schema (RDFS)
+ RDFS recognisable as an ontology language
Classes and properties
Range and domain of properties
Sub/super-classes (and properties)
+ But RDFS not a suitable foundation for Semantic Web
Too weak to describe resources in sufficient detail
+ Requirements for web ontology language:
Compatible with existing Web standards (XML, RDF, RDFS)
Easy to understand and use (based on familiar KR idioms)
Formally specified and of “adequate” expressive power
possible to provide automated reasoning support
Ian Horroks, Logical Foundations for the Semantic Web
Formalism for Ontology Languages: Description Logics
extracts from Franconi’s course
DL-examples
DL definition of “Happy Father”
(Example from Ian Horrocks, U Manchester, UK)
A Few Fundamental Definitions
Example:
KB={
A&B,
(C or (not D)),
D
}
The following is true:
KB |= A
KB |= C
• Decidability: A formal system is decidable if it has
inference procedure i that always terminates with
definite answer (either KB|-A or not KB|-A)
Why DL are used as formalism of Semantic Web ?
• FOL –sound, complete, but not decidable
• Proposition Logic- sound, complete, decidable,
but has low expressive power
• DL- has sound, complete, decidable inference
procedure; has reasonable expressive power
• Alternatives to DL- decidable fragments of FOLHorn logic (prolog), F-Logic
OWL-Web Ontology Language
Extracts from: Ian Horroks, Logical Foundations for the Semantic Web
+ Three species of OWL
OWL full is union of OWL syntax and RDF
OWL DL restricted to FOL fragment ( DAML+OIL)
Lite is
“easier to implement” subset of OWL DL
OWL
+ Semantic layering
OWL DL OWL full within DL fragment
DL semantics officially definitive
+ OWL DL based on SHIQ Description Logic
+ Benefits from many years of DL research
Well defined semantics
Formal properties well understood (complexity, decidability)
Known reasoning algorithms
Implemented systems (highly optimised)