Basic Principles of Semantic WebBesnik Fetahu

What is Semantic Web?• The main vision of the inventor of the web Tim Berners-

Lee was [1]:

I have a dream for the Web [in which computers] become capable of analyzing all the data on the Web – the content, links, and transactions between people and computers. A ‘Semantic Web’, which should make this possible, has yet to emerge, but when it does, the day-to-day mechanisms of trade, bureaucracy and our daily lives will be handled by machines talking to machines. The ‘intelligent agents’ people have touted for ages will finally materialize.

Tim Berners-Lee, 1999

What is Semantic Web?

Fig.1. Semantic Web Framework

URI – Uniform Resource Identifier•URI can be as:

▫URL – Uniform Resource Locator▫URN – Uniform Resource Name

XML – Extensible Markup Language•XML was designed as an option to send

documents easier across the web, it is considered an extensible language because it allows the user to define the mark-up elements.

<sentence><person href="http://aaronsw.com">I</person> just got a new pet <animal type="dog" ref="http://aaronsw.com/myDog">dog</animal>.</sentence>

XML – Extensible Markup Language•Problems with the name of elements.•Uniquely identify element names using

XML namespaces.

<sentence     xmlns="http://example.org/xml/documents/" xmlns:c="http://animals.example.net/xmlns/"><c: person c:href="http://aaronsw.com/">I</c:person> just got a new pet <c:animal>dog</c:animal>.</sentence>

RDF – Resource Description Framework•Important layer of Semantic Web.•Created by triple element blocks

▫Object – Attribute – Value: O(A;V)▫Other notation: : [O]_A![V]▫Objects, and values can be interchanged.

RDF – Resource Description Framework

• hasName (‘http://www.famouswriters.org/twain/mark’, "Mark Twain")• hasWritten (‘http://www.famouswriters.org/twain/mark’,

‘http://www.books.org/ISBN0001047582’)• title (‘http://www.books.org/ISBN0001047582’, "The Adventures of Tom

Sawyer")

RDF – Resource Description Framework• RDF also uses XML syntax for describing relations

among these triple objects:

<rdf:Description rdf:about="http://www.famouswriters.org/twain/mark">

<s:hasName>Mark Twain</s:hasName><s:hasWritten

rdf:resource="http://www.books.org/ISBN0001047582"/></rdf:Description><rdf:Description

rdf:about="http://www.books.org/ISBN0001047582"><s:title>The Adventures of Tom Sawyer</s:title><rdf:type rdf:resource="http://www.description.org/schema#Book"/></rdf:Description>

RDFS – RDF Schema• RDF Schema is a mechanism that lets developers

specify a vocabulary for RDF data and specify objects that can be applied to a particular attribute. About RDF Schema we can think about as a simple data typing model for RDF, this lets us create classes and properties with which we can describe very clearly the triple objects.

• RDF Schema starts with: http://www.w3.org/2000/01/rdf-schema#

RDFS – RDF Schema

•:Book rdf:type rdfs:Class .• :bookTitle rdf:type rdf:Property .• :bookTitle rdfs:domain :Book .• :bookTitle rdfs:range rdfs:Literal .• :MyBook rdf:type :Book .• :MyBook :bookTitle "My Book" .

RDFS – RDF Schema

Fig.5. Defining vocabulary and class hierarchy

RDFS – RDF Schema• Now in the framework we have included three

layers:▫XML – which defines the syntactic layer▫RDF – which defines the structure layer using

triple objects▫RDFS – which defines the semantic layer, using

classes, properties and organizing all these in a hierarchical manner.

Ontology, Inferences and DAML• Ontology: used to describe a world that consists of types,

properties, relationships … etc

• Ontology components in computer science are described using ontology languages like:▫ CycL▫ Dogma (Developing Ontology-Grounded Methods and

Applications)▫ F-Logic (Frame Logic)▫ KIF (Knowledge Interchange Format)▫ KL-ONE

Ontology, Inferences and DAML•Markup language, that use markup

schema to encode knowledge:▫DAML + OIL▫Ontology Inference Layer (OIL)▫Web Ontology Language (WOL)▫Resource Description Framework▫RDF Schema▫SHOE

Ontology, Inferences and DAML•Components included:

▫Individuals▫Classes▫Attributes▫Relations▫Function terms▫Restrictions▫Rules▫Axioms▫Events

DAML + OIL

•DAML+OIL is an ontology language, it is used to describe the structure of a domain, the structure is described using classes and properties.

•DAML + OIL also uses axioms in order to present equivalencies and assertions.

DAML + OIL

•DAML + OIL triples can be represented in different syntactic forms. It assigns for each triple of RDF a specific meaning. DAML+OIL only provides a semantic interpretation for those parts of an RDF graph that instantiate the schema

DAML + OIL

•Ontology uses axioms to assert facts about classes like describing what it presents, e.g.:<daml:Class rdf:ID="Animal"/><daml:Class rdf:ID="Man"><rdfs:subClassOf rdf:resource="#Person"/><rdfs:subClassOf rdf:resource="#Male"/></daml:Class>

DAML + OIL

•DAML provides methods of creating inverse properties, unambiguous properties, lists, restrictions etc.

•An example of these inverse properties::hasName daml:inverseOf :isNameOf :Sean :hasName "Sean“"Sean" :isNameOf :Sean

Inference

•The principle of "inference" is quite a simple, being able to derive new data from data that you already know

▫:MyCar de:macht "160KW“▫de:macht daml:equivalentTo en:power ▫:MyCar de:power “160KW”

Logic

•For the Semantic Web to become expressive enough we have to create a very powerful logical language in order to make inferences.

•There is a logical language that is currently in use by the semantic web Webized version of KIF (Knowledge Interchange Format).

Logic{ { :Joe :loves :TheSimpsons } a log:Falsehood . { :Joe :is :Nuts } a log:Falsehood .} a log:Falsehood .

•This can be read as “it is not true that Joe does not love The Simpsons and is not nuts”.

Trust and Proof

•This is the layer in which very little is done.

•This has to do with information provided by different sources ▫source a) states that “x is blue”▫source b) states that “x is red”

Context

•The future applications on the semantic web will depend on context of the information to decide whether they trust or not the data. This is for example I receive information from a trustable source which he made some claims about the data, and because I know him I also trust that claim.

Proof Languages• Proof languages are just languages used to verify the

correctness of a statement. An instance of proof language generally consists of inference items that are used to derive the information into a question, and the trust of each of these items can be checked.

• For example if we have a statement to prove that “Joe loves Mary”, and we have the items like “:Joe :loves :MSJ” and another item that uses daml to state that “:MSJ dam:equivalentTo :Mary”, and for these items we’ve got the checksums in order to verify the correctness of information.

Proof Languages• Notation3:@prefix : <http://infomesh.net/2001/proofexample/#> .@prefix p: <http://www.w3.org/2001/07/imaginary-proof-ontology#> .@prefix log: <http://www.w3.org/2000/10/swap/log#> .@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .p:ProvenTruth rdfs:subClassOf log:Truth .# Proof

{ { <a.n3> p:checksum <md5:blargh>; log:resolvesTo [ log:includes { :Joe :loves :MJS } ] }

log:implies { :Step1 a p:Success } } a log:Truth . { { <b.n3> p:checksum <md5:test>;

log:resolvesTo [ log:includes { :MJS = :Mary } ] }log:implies{ :Step2 a p:Success } } a log:Truth .{ { :Step1 a p:Success . :Step2 a p:Success }log:implies { { :Joe :loves :Mary } a p:ProvenTruth } } a log:Truth