se3316a_2013_11_semweb

SE3316 Web Technologies

Semantic Web

Dr. Jagath [email protected]

TEB 351

519-661-2111 x80058

Fax: 519-850-2436

SE3316 Jagath Samarabandu 27-Nov-13

Web Resources W3C RDF Primer

http://www.w3.org/TR/rdf-primer/ W3C RDF Tutorial

http://www.w3.org/Consortium/Offices/Presentations/RDFTutorial/

Introduction to RDF: talk by A. M. Kuchling http://www.amk.ca/talks/2004-12-02/


Semantic Web coined by Tim Berners-Lee (1997)

"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.

T. Berners-Lee, J. Hendler, O. Lassila,The Semantic Web, Scientific American, May 2001


Need for Semantic Web W3C project since around 1999. The existing Web of HTML documents is

designed for humans Needs only a browser to access it

Programs have a harder time with it Screen-scrapers can extract information from

HTML, but they're hard to write The Semantic Web will augment the existing

human-readable Web with structured data that's easy for software to process


Cant we just use XML?This is what a web-page in natural language looks like for a machine


XML helps

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XML allows meaningful tags to be added toparts of the text


XML machine accessible meaning

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But to your machine, the tags look like this.


Schemas take a step in the right direction

Schemas help.

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But other people use other schemas

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Someone else has one like this.


The semantics isnt there

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KR provides external referents to merge on

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SW languages add mappingsAnd structure.

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Which is what the web was meant to be!! "This is a pity, as in fact documents on the web describe

real objects and imaginary concepts, and give particular relationships between them... For example, a document might describe a person. The title document to a house describes a house and also the ownership relation with a person. ... This means that machines, as well as people operating on the web of information, can do real things. For example, a program could search for a house and negotiate transfer of ownership of the house to a new owner. The land registry guarantees that the title actually represents reality.

-Tim Berners-Lee plenary presentation at WWW Geneva, 1994


Web Semantics

Semantic Web LayerCake (Berners-Lee, 99;Swartz-Hendler, 2001)

We are stillhere


Layers: A Closer Look Web Ontology Language (OWL)

Relationships between vocabularies "Persons" in vocabulary A are the same thing as "Users" in

vocabulary B. Resource X and resource Y are referring to the same thing.

RDF Schema: Vocabulary definitions There is a class called "Person". Resource X is an instance of "Person".

Resource Description Framework (RDF) Assertions of facts Resource X is named "Drew".


Resource Description Framework RDF is a specification that defines a model for

representing the world, and a syntax for serializing and exchanging the model

A language for representing metadata about Web resources Title, author, and modification date of a web page, Copyright and licensing information about a Web

document, Availability schedule for a shared resource

Generalized to represent things that arent directly retrievable (e.g. person, house)


RDF Overview Intended when meta-information needs to be processed

by applications Provides a common framework so it can be exchanged

between applications without loss of meaning. Application designers can leverage the availability of

common RDF parsers and processing tools. The ability to exchange information between different

applications means that the information may be made available to applications other than those for which it was originally created


RDF Concepts Need to represent the fact that

John Smith created a particular web page In English, this could be

http://www.example.org/index.html has a creatorwhose value is John Smith

Important parts are The thing being described (i.e. web page) Property of the thing (i.e. creator) Value of this property (i.e. John Smith)


RDF Specifics Identify things using web identifiers (URIs) Describe resources in terms of properties and

property values Facts are 3-tuple of (subject, property, object)

Or (resource, property, value) Subject has a property of object

Resource X has a name of Drew ISBN 1234567890 has an author of resource X Resource X has a type of Person


Representing Graph Structure Graph of nodes (subject and object) connected

by arcs (property) Not necessarily a tree

This graph is represented using RDF syntax Written using XML or N3

Which is suited for tree structures Allows converting a graph structure to a

serializable format By breaking graph to several tree structures


An RDF Graph


RDF Representation

Eric Miller

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RDF Example


RDF Graph: Resources Resources are identified by URIs e.g. http://example.com/person/0042, urn:isbn:1930110111


RDF Graph: Literals A primitive string value. The interpretation of the string is up to your application.


RDF Graph: Properties An attribute or aspect of a resource. A property value can be a literal or another resource. Multiple values are allowed; no value at all is also legal.


RDF Graph: Property URIs How are properties identified?

Names or serial numbers are not very scalable. Instead, properties have URIs just like resources. Pick a base URI for your RDF vocabulary:

http://amk.ca/xml/review/1.0# The base URI is assigned to a prefix, such as "rev". Properties are then referenced as 'rev:subject',

'rev:topic', etc. The RDF parser will concatenate the base URI (from the

prefix) and the name: rev:subject http://amk.ca/xml/review/1.0#subject


RDF Vocabularies DC (Dublin Core)

Namespace: http://purl.org/dc/elements/1.1/ Properties: title, creator, publisher, subject, identifier

FOAF (Friend-of-a-friend) Describes people Classes: Person Properties: name, interest, mbox, schoolHomepage,

workplaceHomepage Namespace: http://xmlns.com/foaf/0.1/

DOAP (Description of a Project) Describes open source projects Classes: Project, Repository Properties: name, homepage, mailing-list, license, maintainer Namespace: http://usefulinc.com/ns/doap#

RSS (RDF Site Summary)


Dublin Core Metadata Initiative Developed in 1995 at Metadata Workshop in Dublin,

Ohio Provides a minimal set of descriptive elements to

facilitate description and automated indexing of document-like networked objects

Intended for web-crawlers Meant to be simple Contains only 15 terms: title, creator, subject, description, publisher, contributor, date, type, format, identifier, source, language, relation, coverage, rights


Dublin Core Example A Web page described using RDF with DC term set

D-Lib Program - Research in Digital LibrariesThe D-Lib program .Corporation For National Research Initiatives1995-01-07

Research; statistical methodsEducation, research, related topicsLibrary use Studies

World Wide Web Home Pagetext/htmlen


RSS: RDF Site Summary Really Simple Syndication! An RDF vocabulary designed to handle web

content Most popular RDF application Contains three modules

Dublin core Syndication (updatePeriod, updateFrequency and

updateBase) Content (encoded, item, items, format, encoding)


RDF Advantages RDF exists and is strictly specified. Conceptually the graph model is pretty simple. RDF has a number of implementations. RDF is decentralized:

Anyone can create a vocabulary. Anyone can publish data about other resources.


RDF Disadvantages RDF/XML is rather verbose, and tedious to

read/write by hand. Programming interfaces require you to know

about triples, URIs, and all these low-level details.


Available RDF Software RDF parsers are available for most of the

languages Python (rdflib, 4RDF, pyrple, cwm) Perl (RDF::Core) C (Redland, libwww)

SWIG allows Redland to have interfaces for most of the other scripting languages.

Java (Jena, Sesame) PHP, Ruby, Prolog all have RDF parsers.


RDF Schema: Defining Vocabularies RDF allows making statements Need to define the vocabularies used to define these

statements Define classes of resources Define the names of properties used in these classes

RDF itself doesnt allow this RDF-Schema is an extension to RDF designed to handle

this It self is defined using RDF Namespace: http://www.w3.org/2000/01/rdf-schema# Typically prefixed as rdfs:


RDF Schema Example Some resources (web pages, articles, books)

are reviews of other resources (books, music, articles, web pages). Implies a Review class.

A review has one or more subjects that are the items being reviewed. Implies a subject property.


RDF Schema: Graph


RDF Schema Vs OOP Languages

RDF Schema type system is similar to that of OOP languages

But RDF schema property descriptions are global attributes while an attribute in a OOP class is local

RDF schema are descriptive (mostly) whereas OOP languages are prescriptive


OWL: Connecting vocabularies With RDF Schema, we know:

Which classes exist, What their properties are.

We don't know: When are two classes the same? Can properties have multiple values?

e.g. Can a book have more than one author? ... no authors? ... more than one publisher?

If X prop Y and Y prop Z are both true, which of the following are true?

X property Z (transitivity) Y property X (symmetry) Z property X


OWL: Web Ontology Language OWL is a W3C language for defining relationship

between vocabularies He could spell his own name WOL, and he

could spell Tuesday so that you knew it wasn't Wednesday... in Winnie the Pooh by A. A. Milne, about the wise character, Owl


What is an Ontology Defines terms used to describe and area of

knowledge Used by people, databases and applications that

need to share domain information Includes computer useable definitions of basic

concepts in the domain and the relationships among them

Encode knowledge in a domain as well as knowledge that spans a domain

Makes knowledge re-useable


OWL: Beyond RDF Schema Extends RDF Schema

To limit properties To infer properties To distinguish types of relationships (one-to-one, one-

to-many, many-to-one)

AI of the web (for the web) It's based on two existing research languages,

the American DAML (DARPA Agent Markup Language) and the European OIL (Ontology Inference Layer)


OWL: Possible relationships For classes:

equivalentClass disjointWith

For properties: equivalentProperty inverseOf TransitiveProperty SymmetricProperty minCardinality maxCardinality

For resources: sameAs differentFrom


How much RDF, RDFS and OWL How much of this is being used today? RDF

Yes, definitely! It's the base of the pyramid, and is widely implemented.

RDF Schema Useful for documentation. Will likely be important in future. But... you can get useful work done without it.

OWL Few libraries support OWL Implementations are largely research projects. For many applications theorem-proving will be irrelevant.

se3316a_2013_11_semweb

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