Logics for Data and Knowledge Representation

Resource Description Framework (RDF)

Feroz Farazi

RDF

A language for representing Web resources and information

about them in the form of metadata [RDF Primer]

A language to represent all kinds of things that can be

identified on the Web [RDF Primer]

A domain independent data model for representing

information on the Web [G. Antoniou and F.v. Harmelen,

2004]

A language with an underlying model designed to publish

data on the Semantic Web [F. Giunchiglia et al., 2010]

Definitions

RDF

Resource

A thing or a class or an entity we want to talk about

For example, web pages, articles, authors, etc.

Property

metadata of the resources to be described

For example, creator, date of creation, publisher, etc.

Statement

A piece of information about a resource represented using a property and a value

For example, Tim Berners-Lee authored Weaving the Web. In other words,

Weaving the Web has an author (or creator) whose value is Tim Berners-Lee.

A subject (Weaving the Web)–predicate (creator)–object (Tim Berners-Lee) triple

Keys

Statements

In RDF, statements become machine comprehensible as

Triple elements are represented in a form of machine processable identifiers

They are encoded in a machine processable language

Identifiers

URL: Uniform Resource Locator

URI: Uniform Resource Identifier

URI includes

Things that can be located on the web (using URL), e.g., a home page of a person

Things that are not web accessible, e.g., a real world concept or entity

Language

RDF defines a specific XML language called RDF/XML

RDF Data Model A graph data model with directed edges

URIs to identify nodes (subject and object) and edges (predicate)

Objects can be literals: plain and typed

GeoNames has coverage of all countries

GeoNames was modified on April 25, 2009

RDF Representations The graph is a powerful tool for the understanding of the people

For the Semantic Web we need machines to understand and process

Simple statements are represented as triples in RDF/XML

RDF statements have similarity to The entries in the data processing systems The rows in the relational database tables The assertions in logic languages

An RDF document

Contains an element enclosed in XML tag rdf:RDF

A set of descriptions can be defined within this element with the tag rdf:Description

Each description makes a set of statements about a resource, where

The resource can be defined newly with rdf:ID

It can refer to an existing resource with rdf:about

RDF Representations Structured attributes (e.g., address) can be represented

Blank nodes or anonymous resources are used

Group of things (e.g., a conference participants) can be represented

RDF Containers can be used

Constructs are rdf:Bag, rdf:Seq, rdf:Alt

Container content are called members, which are listed using rdf:li

RDF Collections can be used

Can represent a close list overcoming the limitation of Containers

Constructs are rdf:List, rdf:first, rdf:rest, rdf:nil

Provenance of the statements (e.g., who, when) can be represented

RDF Reification is the means to represent statements about statements

Constructs are rdf:Stament, rdf:subject, rdf:predicate and rdf:object

XML Syntax for RDF RDF/XML:

<?xml version=“1.0”?>

<rdf:RDF

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

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

xmlns:dc="http://purl.org/dc/terms#">

<rdf:Description rdf:about="http://www.geonames.org">

<rdfs:label>GeoNames</rdfs:label>

<dc:coverage rdf:resource="http://www.geonames.org/countries"/>

<dc:modified>April 25, 2009</dc:modified>

</rdf:Description>

</rdf:RDF>

RDF Schema (RDFS)

A language for depicting the vocabulary of RDF developed to

describe resources and relations between them [RDF Schema]

A language defined to provide mechanisms to add semantics to RDF

resources [RDF Schema]

From the type system (classes and properties) perspective it has

similarity to the object-oriented programming (OOP) paradigm

Differs from this paradigm in terms of defining classes and properties

In OOP, usually, classes are defined with respect to the properties an instance

may have

In RDF Schema properties are defined with respect to the classes of the resources

they can be attached to

Query Engine

Database

Application

Query Engine

RDF Store(merge)

Application

RDF Files

Unstructured formats (Web

pages),Structured

formats (CSV, Excel and

Databases)

Parser

Serializ

er

Converters

and scrapers

Application: RDF vs Database

Both applications have

A query engine

A storage

RDF applications also have parser, serializer, converter and scraper RDF merge functionality Some example applications are calendar integration, map integration and annotation

[D. Allemang and J. Hendler, 2008]

RDF Inferencing

In RDFS, semantics can be expressed through inferences

It allows type (rdf:type) propagation through rdfs:subClassOf:Fausto_Giunchiglia rdf:type :Prfoessor

:Professor rdfs:subClassOf :Faculty

:Fausto_Giunchiglia rdf:type :Faculty (inferred)

It allows relationship propagation thgorugh rdfs:subPropertOf:prfoessorshipAt rdfs:subProperytOf :affiliationWith

:Fausto Giunchiglia :professorshipAt :UniTN

:Fausto_Giunchiglia :affiliationWith :UniTN (inferred)

It allows type identification through rdfs:domain:professorshipAt rdfs:domain :Person

:Fausto_Giunchiglia :professrshipAt :UniTn

:Fausto_Giunchiglia rdf:type :Person

RDF Inferencing It allows type identification through rdfs:range

:professorshipAt rdfs:range:Educational_Institution

:Fausto_Giunchiglia :professrshipAt :UniTn

:UniTn rdf:type :Educational_Institution (inferred)

Inferencing through rdfs:domain and rdfs:subClassOf:Researcher rdfs:subClassOf :Scientist

:hIndex rdfs:domain :Researcher

:Fausto_Giunchiglia :hIndex 44

:Fausto_Giunchiglia rdf:type :Researcher (inferred)

:Fausto_Giunchiglia rdf:type :Scientist (inferred)

Inferencing through rdfs:range and rdfs:subClassOf:Educational_Institution rdfs:subClassOf :Organization

:professorshipAt rdfs:range: Educational_Institution

:Fausto_Giunchiglia :professorshipAt :UniTn

:UniTn rdf:type :Educational_Institution (inferred)

:UniTn rdf:type :Organization (inferred)

RDF Modeling Modeling (logical) Set Intersection in RDF

Can we represent that “If an entity e is in X, it is also in both Y and Z”?

No explicit modeling construct is provided to do that but Can be modeled as:X rdfs:subClassOf Y

X rdfs:subClassOf Z

e rdf:type X

e rdf:type Y (inferred)

e rdf:type Z (inferred)

Modeling (logical) Set Union in RDF

Can we represent “Any entity e that belongs either to Y or Z also belongs to X”?

No explicit modeling construct is provided to do that but Can be modeled as:Y rdfs:subClassOf X

Z rdfs:subClassOf X

e rdf:type Y or

e rdf:type Z

e rdf:type X (inferred)

References RDF Primer (2004). W3C Recommendation. RDF Schema (2004). W3C Recommendation. G. Antoniou & F. van Harmelen (2004). A Semantic Web Primer

(Cooperative Information Systems). MIT Press, Cambridge MA, USA. F. Giunchiglia, F. Farazi, L. Tanca, and R. D. Virgilio. The semantic

web languages. In Semantic Web Information management, a model based perspective. Roberto de Virgilio, Fausto Giunchiglia, Letizia Tanca (Eds.), Springer, 2009.

D. Allemang and J. Hendler. Semantic web for the working ontologist: modeling in RDF, RDFS and OWL. Morgan Kaufmann Elsevier, Amsterdam, NL, 2008.