Semantic Web

Overview
 What is Semantic Web
 Semantic Web Vision
 Semantic Web Layers
 RDF, RDFS, OWL
 Tools
 GATE
 Applications

What is Semantic Web?
 Semantic means that the meaning of data
can be discovered by computers
 "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
cooperation." - Tim Berners-Lee

Definition
 The Semantic Web is a project to create a
universal medium for information exchange by
putting documents with computer-processable
meaning (semantic) on the World Wide Web
 The Semantic Web extends the Web through the
use of standards, markup languages and related
processing tools

The aims of Semantic Web
 Indexing and retrieving information
 Annotation
 The Web as a interoperable database
 Machine retrieval of data
 Web based services
 Discovery of services
 Intelligent software agents

Semantic Web Vision
 Oriented toward machine-readable
resources rather than human-readable
 Requires resources to be described so
that machines know what they mean
 Description in terms of metadata
 Use of logic interpretation for inference

Semantic Web Layers

Semantic Web Layers
 XML (Extensible Markup Language)- The
language framework that is used to define
nearly all new languages that are used to
interchange data over the Web
 XML Schema -A language used to define
the structure of specific XML language

Semantic Web Layers
 RDF (Resource Description Framework)-
a language used to describe all sort of
information and meta data
 RDF Schema-A framework that provides a
means to specify basic vocabularies for
specific RDF application language to use

Semantic Web Layers
 Ontology- defines vocabularies and
establish the usage of words and terms in
context of specific vocabulary
 Logic and Proof –is used to establish the
consistency and correctness of data sets
and to infer conclusion that aren’t explicitly
stated

Semantic Web agents
 Metadata will be used to identify and
extract information from Web sources.
 Ontologies will be used to assist in Web
searches, to interpret retrieved
information, and to communicate with
other agents.
 Logic will be used for processing retrieved
information and for drawing conclusions.

RDF

RDF
• “Resource Description Framework”
• RDF is a data model
• Originally for describing metadata for web pages
• Structured information
• Universal, machine-readable data exchange model
• Syntax uses XML for serialization
• Statements can be modeled with
• Resources: an element, a URI, a literal
• Properties: directed relation between two resources
• Statements: triples of two resources linked by property

RDF
• Generally triple can be viewed as a graph
• both “ object: and “ subject” are the graph nodes
• “properties are the edges
• XML syntax is only the tools for practical usage instead of graph
• Components
• URIs – for referencing resources
• Literals – data values
• Empty nodes (blank nodes) – talking about something which doesn’t
have a name

RDF Example
• Subject: URIs and empty nodes
• Predicate: URIs ( also called properties)
• Object: URIs and empty nodes and literals

XML syntax for RDF Example

RDF Example

RDF XML Code Example
1. <?xml version="1.0"?>
2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3. xmlns:dc="http://purl.org/dc/elements/1.1/"
4. xmlns:exterms="http://www.example.org/terms/">
5. <rdf:Description rdf:about="http://www.example.org/index.html">
6. <exterms:creation-date>August 16, 1999</exterms:creation-date>
7. <dc:language>en</dc:language>
8. <dc:creator rdf:resource="http://www.example.org/staffid/85740"/>
9. </rdf:Description>
10. </rdf:RDF>

A simple example
 “The book has the title War and Peace”
 Graphical RDF Statement
has the title War and
The book
The book Peace
 RDF in a XML document
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/element/1.1">
<rdf:Description rdf:about="http://amazon.com/books">
<dc:title> War and Peace</dc:title>
</rdf:Description>
</rdf:RDF>

Ontology
 We can express ontology as:
Ontology =<taxonomy, inference rules>
And we can express a taxonomy as:
Taxonomy <{classes}, {relations}>
 Ontology Languages (RDFS, OWL) has formal
foundations that allow us to infer additional (implicit)
statements

RDF Schema
 Intended to structure RDF resources
 RDFS
 Set theory – rdfs:Class
 Relation – rdf:Property, rdfs:domain, rdfs:range
 Hierarchy – rdfs:subClassOf, rdfs:subPropertyOf
 Built-in Datatype – xsd:string, xsd:dataTime

RDF & RDFS
 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

Limitations of RDF/RDFS
 No standard for expressing primitive data types such as
integer, etc. All data types in RDF/RDFS are treated as
strings.
 No standard for expressing relations of properties
(unique, transitive, inverse etc.)
 No standard for expressing whether enumerations are
closed.
 No standard to express equivalence, disjointedness etc.
among properties

OIL and DAML
 RDFRDFS define a framework, however they have
limitations. There is a need for new semantic web
languages with following requirements
 They should be compatible with (XML, RDF/RDFS)
 They should have enough expressive power to fill in the gaps in
RDFS
 They should provide automated reasoning support
 Ontology Inference Layer (OIL) and DARPA Agent Markup
Language (DAML) are two important efforts developed to
fulfill these requirements.
 Their combined efforts formed DAML+OIL declarative
semantic language.

OIL and DAML
 DAML+OIL is built on top of RDFS.
 It uses RDFS syntax.
 It has richer ways to express primitive data types.
 DAML+OIL allows other relationships (inverse and
transitivity) to be directly expressed.
 DAML+OIL provides well defined semantics, This
provides followings:
 Meaning of DAML+OIL statements can be formally specified.
 Machine understanding and automated reasoning can be
supported.
 More expressive power can be provided.

Example
Example: T. Rex is not herbivore and not a currently living
species.
 This statement can be expressed in DAML+OIL, but not in
RDF/RDFS since RDF/RDFS cannot express disjointedness.
 DAML+OIL provides automated reasoning by providing such
expressive power.
 For instance, a software agent can find out the “list of all the carnivores
that won’t be any threat today” by processing the DAML+OIL data
representation of the example above.
 RDF/RDFS does not express “is not” relationships and exclusions.

OWL

Web Ontology Language = OWL
 OWL is an extra layer, a bit like RDFS
 own namespace, own terms
 it relies on RDF Schemas
 It is a separate recommendation
 actually… there is a 2004 version of OWL
(“OWL 1”)
 and there is an update (“OWL 2”) published in
2009

OWL- Web Ontology Language
 OWL is a vocabulary extension of the RDF and is
derived from the DAML+OIL Web Ontology Language.
 OWL
 Description Logic
 Class, Thing, Nothing
 DatatypeProperty, ObjectProperty, AnnotationProperty,…
 Class
 oneOf, disjointWith, unionOf, complementOf, intersectionOf …
 Restriction, onProperty, cardinality, hasValue…
 Property
 inverseOf , TransitiveProperty , SymmetricProperty
 FunctionalProperty, InverseFunctionalProperty
 Equality– equivalentClass , sameAs , differentFrom…
 Ontology annotation – Ontology, imports, versionInfo

Term equivalences
 For classes:
 owl:equivalentClass: two classes have the
same individuals
 owl:disjointWith: no individuals in common
 For properties:
 owl:equivalentProperty
 remember the a:author vs. f:auteur?
 owl:propertyDisjointWith

Term equivalences
 For individuals:
 owl:sameAs: two URIs refer to the same
concept (“individual”)
 owl:differentFrom: negation of owl:sameAs

Example
owl:equivalentProperty
a:author f:auteur
owl:equivalentClass
a:Novel f:Roman

Property characterization
 In OWL, one can characterize the
behavior of properties (symmetric,
transitive, functional, reflexive, inverse
functional…)
 One property can be defined as the
“inverse” of another

What this means is…
 If the following holds in our triples:
:email rdf:type owl:InverseFunctionalProperty.
<A> :email "mailto:a@b.c".
<B> :email "mailto:a@b.c".

What this means is…
 If the following holds in our triples:
:email rdf:type owl:InverseFunctionalProperty.
<A> :email "mailto:a@b.c".
<B> :email "mailto:a@b.c".
then, processed through OWL, the following
holds, too:
<A> owl:sameAs <B>.

Keys
“if two persons have the same emails and the same
homepages then they are identical”
 Identification is based on the identical
values of two properties
 The rule applies to persons only

Previous rule in OWL
:Person rdf:type owl:Class;
owl:hasKey (:email :homepage) .

What it means is…
If:
<A> rdf:type :Person ;
:email "mailto:a@b.c";
:homepage "http://www.ex.org".
<B> rdf:type :Person ;
:email "mailto:a@b.c";
:homepage "http://www.ex.org".
then, processed through OWL, the following holds,
too:
<A> owl:sameAs <B>.

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
 etc

Enumerate class content
:Currency
rdf:type owl:Class;
owl:oneOf (:€ :£ :$).
 I.e., the class consists of exactly of those
individuals and nothing else

Union of classes
:Novel rdf:type owl:Class.
:Short_Story rdf:type owl:Class.
:Poetry rdf:type owl:Class.
:Literature rdf:type owl:Class;
owl:unionOf (:Novel :Short_Story :Poetry).
 Other possibilities: owl:complementOf,
owl:intersectionOf, …

For example…
If:
:Novel rdf:type owl:Class.
:Short_Story rdf:type owl:Class.
:Poetry rdf:type owl:Class.
:Literature rdf:type owl:Class;
owl:unionOf (:Novel :Short_Story :Poetry).
<myWork> rdf:type :Novel .
then the following holds, too:
<myWork> rdf:type :Literature .

What we have so far…
 The OWL features listed so far are already
fairly powerful
 E.g., various databases can be linked via
owl:sameAs, functional or inverse
functional properties, etc.
 Many inferred relationship can be found
using a traditional rule engine

The most used Semantic Web
Tools
 RDF Gateway- it runs both a Web
application server and database design to
handle RDF content
 Jena -Java API for RDF
 Smore: Semantic Markup, Ontology and
RDF Editor
 Drive - a C# API. It parses and validate
RDF documents.

General Architecture for Text
Engineering (GATE)

What is GATE?
An architecture
A macro-level organisational picture for LE software systems.
A framework
For programmers, GATE is an object-oriented class library that
implements the architecture.
A development environment
For language engineers, computational linguists et al, GATE is a
graphical development environment bundled with a set of tools for doing
e.g. Information Extraction.
Some free components... ...and wrappers for other
people's components
Tools for: evaluation; visualise/edit; persistence; IR; IE; dialogue;
ontologies; etc.
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Where did GATE come from?
A number of researchers realised in the early- mid-1990s (e.g. in
TIPSTER):
• Increasing trend towards multi-site collaborative projects
• Role of engineering in scalable, reusable, and portable HLT solutions
• Support for large data, in multiple media, languages, formats, and
locations
• Lower the cost of creation of new language processing components
• Promote quantitative evaluation metrics via tools and a level playing field
History:
• 1996 – 2002: GATE version 1, proof of concept
• March 2002: version 2, rewritten in Java, component based, more users
• Fall 2003: new development cycle
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Applications
 Swoogle
 DBpedia
 Flickr
 PhotoStuff

Swoogle
• Swoogle is a crawler based indexing and retrieval
system for Semantic Web
• Swoogle crawls and discovers documents written in
RDF,OWL
• Swoogle classifies a Semantic Web
Document(SWD) as –
• Semantic Web Ontology (SWO) – Defines new
terms
• Semantic Web Databases (SWDB) – Makes
assertions about individuals

Reference & Resources
 http://www.w3.org/TR/rdf-primer/
 http://www.w3.org/DesignIssues/Semantic.html
 http://www.w3.org/TR/rdf-syntax-grammar/
 http://www.authorstream.com/Presentation/sneha.ch
 http://www.w3.org/TR/owl-semantics/