1 Berendt: Advanced databases, 1ste semester 2010/2011, berendt/teaching/ 1 Advanced databases – The Semantic Web (1) Bettina

1Berendt: Advanced databases, 1ste semester 2010/2011, http://www.cs.kuleuven.be/~berendt/teaching/

1

Advanced databases –

The Semantic Web (1)

Bettina Berendt

Katholieke Universiteit Leuven, Department of Computer Science

http://www.cs.kuleuven.be/~berendt/teaching/

Last update: 8 October 2010


2

Agenda

The Semantic Web: Motivation and overview

Very brief recap of XML (& why it’s not semantic)

RDF and RDFS

RDF+RDFS+OWL explained using FOAF


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The original vision

The entertainment system was belting out the Beatles' "We Can Work It Out" when the phone rang. When Pete answered, his phone turned the sound down by sending a message to all the other local devices that had a volume control. His sister, Lucy, was on the line from the doctor's office: "Mom needs to see a specialist and then has to have a series of physical therapy sessions. Biweekly or something. I'm going to have my agent set up the appointments." Pete immediately agreed to share the chauffeuring.

At the doctor's office, Lucy instructed her Semantic Web agent through her handheld Web browser. The agent promptly retrieved information about Mom's prescribed treatment from the doctor's agent, looked up several lists of providers, and checked for the ones in-plan for Mom's insurance within a 20-mile radius of her home and with a rating of excellent or very good on trusted rating services. It then began trying to find a match between available appointment times (supplied by the agents of individual providers through their Web sites) and Pete's and Lucy's busy schedules. (The emphasized keywords indicate terms whose semantics, or meaning, were defined for the agent through the Semantic Web.)

Tim Berners-Lee, James Hendler and Ora Lassila (2001). The Semantic Web. A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities. Scientific American. http://www.sciam.com/article.cfm?articleID=00048144-10D2-1C70-84A9809EC588EF21


4

Questions

1. [concrete] What (meta-)data & procedures would be needed to solve this problem?

2. [general] What do you find works poorly on the Web today when you look for information?


5Your answers [green: this problem is one of the motivations of the Semantic Web, and we will discuss its solution approaches; purple: other remarks]

#1: meta-data:

word + category ("medication") need to go beyond natural language & individual data schemas

Geographic map + certification ("a reliable map provider") trust

user authentification [ we won’t treat that here]

#2: A search query is often “not enough”:

too many irrelevant search results want autonomous query expansion that respects my context

an overly specific query may yield zero results want autonomous query generalisation that respects my context

need background knowledge, including knowledge about users’ contexts and about concepts and how to generalise them:

– Such background knowledge must exist

– Such background knowledge must be machine-processable

Information is often spread over several pages & even if one keeps clicking, the information may still not be there [ if the info really is

not on the Web, we can’t do anything about that as computer scientists; if it just hasn’t been found, we need to build better search engines more about this later]

old information is often ranked (too) high [ we’ll talk about that later]


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The Semantic Web: overview

The semantic web is an evolving extension of the World Wide Web in which web content can be expressed not only in natural language, but also in a format that can be read and used by software agents, thus permitting them to find, share and integrate information more easily.

It derives from W3C director Sir Tim Berners-Lee's vision of the Web as a universal medium for data, information, and knowledge exchange.

At its core, the semantic web comprises a philosophy, a set of design principles, collaborative working groups, and a variety of enabling technologies.

Some elements of the semantic web are expressed as prospective future possibilities that have yet to be implemented or realized.

Other elements of the semantic web are expressed in formal specifications.

Some of these include Resource Description Framework (RDF), a variety of data interchange formats (e.g. RDF/XML, N3, Turtle, N-Triples), and notations such as RDF Schema (RDFS) and the Web Ontology Language (OWL), all of which are intended to provide a formal description of concepts, terms, and relationships within a given knowledge domain.


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The Semantic Web layer cake (T. Berners-Lee talk at XML 2000)

RDF: W3C Rec. 2004

OWL: W3C Rec. 2004OWL2: W3C Rec. 2009


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BTW: Semantic non-interoperability has real consequences ...


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Working example: People and their relations


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Approach 1: Centralised


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Approach 2: Decentralised / open


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FOAF (Friend of a Friend)

a machine-readable ontology describing persons, their activities and their relations to other people and objects.

Anyone can use FOAF to describe him or herself. FOAF is an extension to RDF and is defined using OWL. Computers may use these FOAF profiles to find, for example, all

people living in Europe, or to list all people both you and a friend of you know.

This is accomplished by defining relationships between people. Each profile has a unique identifier (such as the person's e-mail

addresses, a Jabber ID, or a URI of the homepage or weblog of the person), which is used when defining these relationships.

The FOAF project, which defines and extends the vocabulary of a FOAF profile, was started in 2000 by Libby Miller and Dan Brickley.

http://www.foaf-project.org

„possibly the single most prevalent use of Semantic Web technologies so far“ – blog software exporting FOAF + RSS (Paolillo et al., 2005)


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FOAF example (1)

<rdf:RDF

xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

xmlns:foaf="http://xmlns.com/foaf/0.1/"

xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">

<foaf:Person rdf:about="#JW">

<foaf:name>Jimmy Wales</foaf:name>

<foaf:mbox rdf:resource="mailto:[email protected]" />

<foaf:homepage rdf:resource="http://www.jimmywales.com/" />

<foaf:nick>Jimbo</foaf:nick>

<foaf:depiction rdf:resource="http://www.jimmywales.com/aus_img_small.jpg" />

<foaf:interest>

<rdf:Description rdf:about="http://www.wikimedia.org" rdfs:label="Wikipedia" />

</foaf:interest>


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FOAF example (2)

<foaf:knows>

<foaf:Person>

<foaf:name>Angela Beesley</foaf:name> 

</foaf:Person>

</foaf:knows>

</foaf:Person>

</rdf:RDF>

Social-web inferences


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FOAF extensions (1)

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"


xmlns:rel="http://www.perceive.net/schemas/relationship/">

<foaf:Person rdf:ID="spiderman">

<foaf:name>Spiderman</foaf:name>

<rel:enemyOf rdf:resource="#green-goblin"/>

</foaf:Person>

<foaf:Person rdf:ID="green-goblin">

<foaf:name>Green Goblin</foaf:name>

<rel:enemyOf rdf:resource="#spiderman"/>

</foaf:Person>


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FOAF extensions (2)

<foaf:Person rdf:ID="peter">

<foaf:name>Peter Parker</foaf:name>

<rel:friendOf rdf:resource="#harry"/>

</foaf:Person>

<foaf:Person rdf:ID="harry">

<foaf:name>Harry Osborn</foaf:name>

<rel:friendOf rdf:resource="#peter"/>

<rel:childOf rdf:resource="#norman"/>

</foaf:Person>

<foaf:Person rdf:ID="norman">

<foaf:name>Norman Osborn</foaf:name>

<rel:parentOf rdf:resource="#harry"/>

</foaf:Person>

</rdf:RDF>


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FOAF multimedia (1)

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"


xmlns:dc="http://purl.org/dc/elements/1.1/">

<foaf:Person rdf:ID="peter">

<foaf:name>Peter Parker</foaf:name>

<foaf:depicts rdf:resource="http://www.peterparker.com/peter.jpg"/>

</foaf:Person>

<foaf:Person rdf:ID="spiderman">

<foaf:name>Spiderman</foaf:name>

</foaf:Person>


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FOAF multimedia (2)

<foaf:Person rdf:ID="green-goblin">

<foaf:name>Green Goblin</foaf:name>

</foaf:Person>



<foaf:Image rdf:about="http://www.peterparker.com/photos/spiderman/statue.jpg">

<dc:title>Battle on the Statue Of Liberty</dc:title>

<foaf:depicts rdf:resource="#spiderman"/>

<foaf:depicts rdf:resource="#green-goblin"/>

<foaf:maker rdf:resource="#peter"/>

</foaf:Image>

</rdf:RDF>


19What inferences? Ex.: A social-network analysis of LiveJournal FOAF entries(Paolillo et al., 2005)

Interests over time remain similar

Friends over time remain similar

But: the manner in which people elect friends and interests in their LiveJournal profiles is sharply different. ... [These differences] represent fundamentally different social behaviors.

What does this mean for recommender systems?


20

Agenda



RDF and RDFS



21

You have data … How should you structure it?

medium-altitude, long-endurance unmanned aerial vehicle

14.7 meters

512 kilograms70 knots

Here's some data about an aircraft:

400 nautical miles


22The XML approach is to "wrap" each data item in start/end tags

<Aircraft> <wingspan>14.8 meters</wingspan> <weight>512 kilograms</weight> <cruise-speed>70 knots</cruise-speed> <range>400 nautical miles</range> <description> medium-altitude, long-endurance unmanned aerial vehicle </description></Aircraft>

RQ-1.xml

and define this data

schema, e.g. in a DTD


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XML Terminology

<wingspan>14.8 meters</wingspan>

Start tag End tag

Data

Element


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Why use XML?

It is a universally accepted standard way of structuring data (syntax).

It is a W3C recommendation (W3C = World Wide Web Consortium)

The marketplace supports it with a lot of free/inexpensive tools.

The alternative to using XML is to define your own proprietary data syntax, and then build your own proprietary tools to support the proprietary syntax (Not a very appealing idea).


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But: What is this XML snippet talking about, i.e., what are the semantics?

<Predator> …</Predator>

What is a Predator?


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Predator - which one?

Predator: a medium-altitude, long-endurance unmanned aerial vehicle system.

Predator : one that victimizes, plunders, or destroys, especially for one's own gain.

Predator : an organism that lives by preying on other organisms.

Predator: a company which specializes in camouflage attire.

Predator: a video game.

Predator: software for machine networking.

Predator: a chain of paintball stores.


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A little more flexibility through namespaces

<?xml version="1.0" encoding=„UTF-8"?>

<myThings

xmlns:h=http://www.mySchemas.org/TR/aircraft/ xmlns:f="http://www.yourSchemas.com/animals">

<h:Predator>

<h:name>OL231-b</hname>

<h:wingspan>14.8 metres</h:wingspan>

</h:Predator>

<f:Predator>

<f:name>Panthera</f:name>

<f:eats>antelopes</f:eats>

</f:Predator>

</myThings>


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... But this doesn‘t solve the fundamental problems

1. What does nesting mean?

2. What do syntactical variations mean?

3. What do linguistic variations mean?

4. How can we extend our knowledge?


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1. What does nesting mean?

Schema 1 allows for expressions like:

<Person>

<name>Peter Parker</name> ...

</Person>

name being an XML-element of Person means: the person HAS-A ...


<Person>

<type>Comic-book hero</type> ...

</Person>

type being an XML-element of Person means: the person IS-A ...

Problems: a) we don‘t know what nesting means, b) even if we do know, we can‘t express this in a machine-readable way (at most build it into an application that uses these XML statements, but that would bury meaning in procedures!)


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2. What do syntactical variations mean?


<Person>

<name>Peter Parker</name>

<birthday>1932-04-12</birthday> ...

</Person>


<Person name=“Peter Parker“>

<type>Comic-book hero</type> ...

</Person>

Problems: a) what does it mean for some information to be an XML-element vs. an XML-attribute? b) even if we do know that they are the same, we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)


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3. What do linguistic variations mean?


<Person>

<name>Peter Parker</name> ...

</Person>


<Person>

<naam>Peter Parker</naam> ...

</Person>

Problems: a) we do not know whether elements from different data sources that differ by, e.g. natural, language, are the same or not b) even if we do know that they are the same, we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)


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4. How can we extend our knowledge?


<WebResource>

<type>Picture</type>

<hasURL>http://www.example.org/Pictures/myPic.png</hasURL>

<isAbout>Peter Parker</isAbout> ...

</WebResource>


<WebResource>

<hasURL>http://www.example.org/Pictures/myPic.png</hasURL>

<hasLicence>CreativeCommons</hasLicence> ...

</WebResource>

Problems: a) we cannot refine our schema information by that provided by another source b) even if we can be sure about principal linkability (here: via the URL), we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)


33Summary: XML not well-suited for conceptual modelling and therefore not suited for truly semantic markup

XML makes no commitment on:

Domain-specific ontological vocabulary

Ontological modeling primitives

Requires pre-arranged agreement on &

Only feasible for closed collaboration

agents in a small & stable community

pages on a small & stable intranet

Not suited for sharing Web-resources


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Solution approach of the „higher levels“ of the Semantic Web

1. Break down information into atomic statements: subject-predicate-object

2. Define (in a formal-semantics way) what each component of each statement means

a. Give it a URI (uniform resource identifier) to enable uniform meaning specification

b. Define languages to say more about (specify) the meaning (by relating it to other units of meaning – cf. a dictionary in which each word is explained by other words)

c. (exception: some components may be literals / strings – these are not defined further)

3. The languages mentioned in 2.b. each add more expressivity:

1. RDF: subject-predicate-object statements (in RDF terminology: a resource has a property with a certain value.

2. RDFS: simple ontology building blocks: class, subclass-of relation, use RDF‘s type to denote that (e.g.) an individual is a instance of a class (= make it possible to define a schema and its instances), ...

3. OWL: more advanced ontology building blocks: a class (= concept) is disjoint with another one, is the same as another one; a property is functional, symmetric, the inverse of another one; ...


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Semantic Web vs. Database

Advantages of using RDF/RDFS/OWL to define an Ontology:

Extensible: much easier to add new properties. Contrast with a database - adding a new column may break a lot of applications

Portable: much easier to move an OWL document than to move a database.

Advantages of using a Database to define an Ontology:

Mature: the database technology has been around a long time and is very mature.


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Agenda



RDF and RDFS



37

What is RDF ?

RDF is a data model

the model is domain-neutral, application-neutral

the model can be viewed as directed, labeled graphs or as an object-oriented model (object/attribute/value)

RDF data model is an abstract, conceptual layer independent of XML

consequently, XML is a transfer syntax for RDF, not a component of RDF

RDF data might never occur in XML form


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RDF model

RDF “statements” consist of

resources (= nodes)which have propertieswhich have values (= nodes,strings)

http://www.w3.org/TR/REC-rdf-syntax/

“Ora Lassila”

author

= subject= predicate= object

“http://www.w3.org/TR/REC-rdf-syntax/ has the author Ora Lassila”

resource valueproperty


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RDF Model Example


“Ora Lassila”

dc:Creator

“1999-02-22”

dc:Date

“W3C”

dc:Publisher


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Complex values

So far, values of properties have been strings

A graph node (corresponding to a resource) also can be the value of a property

arbitrarily complex tree and graph structures are possible

syntactically, values can be embedded (i.e. lexically in-line) or referenced (linked)

Example:


“Ora Lassila”

dc:Creator

“[email protected]”

p:EMail

p:Name


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Complex values (continued)

Corresponding triples

{ “http://www.w3.org/TR/PR-rdf-syntax/”, dc:Creator, x }

{ x, p:Name, “Ora Lassila” }

{ x, p:EMail, “[email protected]” }


“Ora Lassila”

dc:Creator

“[email protected]”

p:EMail

p:Name


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Containers

Containers are collections

they allow grouping of resources (or literal values)

It is possible to make statements about the container (as a whole) or about its members individually

Different types of containers exist

bag - unordered collection

seq - ordered collection (= “sequence”)

alt - represents alternatives

It is also possible to create collections based on URI patterns

for example, all files in a particular web site

Duplicate values are permitted

there is no mechanism to enforce unique value constraints


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Containers (continued)

http://www.w3.org/TR/REC-rdf-syntax

“Ora Lassila”

rdf:_1

rdf:Seq

dc:Creator

rdf:Type

“Ralph Swick”

rdf:_2


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Higher-order statements

One can make RDF statements about other RDF statements

example: “Ralph believes that the web contains one billion documents”

Higher-order statements

allow us to express beliefs (and other modalities)

are important for trust models, digital signatures,etc.

also: metadata about metadata

are represented by modeling RDF in RDF itself


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Reification

RDF is not really second-order

But it does provide a built-in predicate vocabulary for reification

http://www.w3.org/TR/REC-rdf-syntax “Ora Lassila”dc:Creator

“Library of Congress”

dc:Creator

• The dotted box corresponds to the following statements

• { x, rdf:predicate, “dc:creator” }• { x, rdf:subject, “http://www.w3.org/TR/RED-rdf-syntax }• { x, rdf:object, “Ora Lassila” }• { x, rdf:type, “rdf:statement” }


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Reification

pers05 ISBN...Author-of

NYT claims

<rdf:Description rdf:about=“#NYT”> <claims> <rdf:Description rdf:about=“#pers05”> <authorOf>ISBN...</authorOf> </rdf:Description> </claims></rdf:Description>

Any statement can be an objectgraphs can be nested - reification


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RDF Schema

• Defines small vocabulary for RDF: • Class, subClassOf, type• Property, subPropertyOf• domain, range

• Vocabulary can be used to define other vocabularies for your application domain

Person

Student Researcher

subClassOfsubClassOf

Jeentype

hasSuperVisordomain range

Frank

type

hasSuperVisor


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<rdf:Description ID="MotorVehicle"> <rdf:type resource="http://www.w3.org/...#Class"/> <rdfs:subClassOf rdf:resource="http://www.w3.org/...#Resource"/></rdf:Description>

<rdf:Description ID="Truck"> <rdf:type resource="http://www.w3.org/...#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdf:Description>

<rdf:Description ID="registeredTo"> <rdf:type resource="http://www.w3.org/...#Property"/> <rdfs:domain rdf:resource="#MotorVehicle"/> <rdfs:range rdf:resource="#Person"/></rdf:Description>

<rdf:Description ID=”ownedBy"> <rdf:type resource="http://www.w3.org/...#Property"/> <rdfs:subPropertyOf rdf:resource="#registeredTo"/></rdf:Description>

RDF Schema syntax in XML


49

Ontologies and concepts

An ontology is a conceptual model.

An Ontology is the collection of semantic definitions for a domain.

Example: an Aircraft Ontology is the set of semantic definitions for the Aircraft domain, e.g.,

Predator is a subClassOf Aircraft.

sensorID is a FunctionalProperty.

Platform is an equivalentClass to Aircraft.

Predator, Aircraft etc. are concepts.


50Basic idea of conceptual modelling (not only in SW): The semiotic triangle


51What is an ontology?(A commonly accepted informal definition and one formal definition)

An ontology is „an explicit specification of a shared conceptualisation.“ (Gruber, 1993)


52

Ontologies, decentralization, and bottom-up engineering

Communities of users (application builders, ...) can

Re-use existing ontologies

Established domain-specific ontologies (e.g., real-estate, medicine, bioinformatics)

All kinds: see the Semantic Web search engine http://swoogle.umbc.edu/

„The big one“: Cyc, see www.cyc.com

Link to existing ontologies ( Ontology matching / alignment)

Extend existing ontologies


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Ontologies as conceptual models / schemas; or:Database (knowledge base) = Ontology + Instances

My Life and Times

Illusions

First and Last Freedom

Paul McCartney

Richard Bach

J. Krishnamurti

June, 1998

1972

1974

title author date

BookCatalogue

<owl:Class rdf:ID="BookCatalogue"/>

<owl:DatatypeProperty rdf:ID="title"> <rdfs:domain rdf:resource="#BookCatalogue"/> <rdfs:range rdf:resource="&xsd;#string"/></owl:DatatypeProperty>

<owl:DatatypeProperty rdf:ID="author"> <rdfs:domain rdf:resource="#BookCatalogue"/> <rdfs:range rdf:resource="&xsd;#string"/></owl:DatatypeProperty>

<owl:DatatypeProperty rdf:ID="date"> <rdfs:domain rdf:resource="#BookCatalogue"/> <rdfs:range rdf:resource="&xsd;#date"/></owl:DatatypeProperty>

<?xml version=“1.0”?><BookCatalogue> <title>My Life and Times</title> <author>Paul McCartney</author> <date>June, 1998</date></BookCatalogue>


54

Agenda



RDF and RDFS



55

The FOAF specification

Textual form: http://xmlns.com/foaf/spec/

RDF: http://xmlns.com/foaf/spec/index.rdf

An older version that integrated the WordNet ontology:

Tabular: http://www.schemaweb.info/schema/SchemaInfo.aspx?id=29

as RDF: http://www.schemaweb.info/webservices/rest/GetRDFByID.aspx?

id=29


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OWL: more details

Not that new, but excellent tutorial:

Roger L. Costello & David B. Jacobs (2003). OWL Web Ontology Language.

http://www.racai.ro/EUROLAN-2003/html/presentations/JamesHendler/owl/OWL.ppt

(please note: the other tutorials referenced on slide 3 of that slide set are not available)

(please spot the French-language typos ;-) )


57

Agenda



RDF and RDFS


The Semantic Web (2): Linked Data


58

Used sources

(From or based on):

p. 6: http://en.wikipedia.org/wiki/Semantic_Web

pp. 21-26, 49: Costello, R.L. & Jacobs, D.B. (2003). A Two Minute Intro to XML. www.daml.org/meetings/2003/05/SWMU/briefings/07_1045_Essential_Building_Blocks.ppt

p. 33, pp. 37-48: Unnamed (no date). RDF and XML tutorial. http://lsdis.cs.uga.edu/SemWebCourse/RDF.ppt

p. 35, 53: Costello, R.L. & Jacobs, D.B. (2003). OWL Web Ontology Language.

http://www.racai.ro/EUROLAN-2003/html/presentations/JamesHendler/owl/OWL.ppt

pp. 12-14: http://en.wikipedia.org/wiki/FOAF_(software)

pp. 15-18: Dodds, L. (2004). An Introduction to FOAF. http://www.xml.com/pub/a/2004/02/04/foaf.html

Picture credits: see PPT „comments“ field


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Further references, background reading; acknowledgements

J. C. Paolillo, S. Mercure, and E. Wright. (2005). The social semantics of Livejournal FOAF: Structure and change from 2004 to 2005. In G. Stumme, B. Hoser, C. Schmitz, and H. Alani, editors, Proceedings of the 1st Workshop on Semantic Network Analysis at the ISWC 2005 Conference, pages 69 – 80. http://www.blogninja.com/paolillo-mercure-wright.final.pdf

Specifications:

RDF: http://www.w3.org/RDF/ , http://www.w3.org/TR/rdf-primer

OWL: http://www.w3.org/TR/owl-features

OWL2: http://www.w3.org/TR/owl2-overview/

FOAF: http://xmlns.com/foaf/spec

Documents

1 Berendt: Advanced databases, 1ste semester 2010/2011, berendt/teaching/ 1 Advanced databases – The Semantic Web (1) Bettina