_chapter3_rdf

8/6/2019 _Chapter3_RDF

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RDF: Resource Description

Framework


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RDF is...

? A standard syntax to represent (edgelabeled) directed graphs in XML

?

It is a standard by World Wide WebConsortium


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An Example

http://www.srdc.metu.edu.tr/~asuman Asuman DogacCreator

* Asuman Dogac is the value of property Creator for resource http://

www.srdc.metu.edu.tr/~asuman

* Resource http:// www.srdc.metu.edu.tr/~asuman has a property Creatorwith a value Asuman Dogac

*Creator ofhttp:// www.srdc.metu.edu.tr/~asuman isAsuman Dogac


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RDF Schemas

? Describe rules for using RDF properties

? Are expressed in RDF

? Are not to be confused with XML Schemas


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Schema URIs

? Ordinary XML namespace URIs are just toguarantee uniqueness: there is no assumption thatthe URI refers to anything useful (or even refers at

all)? URIs for namespaces used in RDF, though, should

refer to an RDF schema document

? RDF namespace uses rdf prefix by convention

? RDF Schema namespace uses rdfs prefix by

convention? Properties are declared in other namespaces, so

they are Web-unique


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Resources, Properties, Statements

? All things being described by RDF expressions are called resources

? Resources are always specified by URIs

? A property is a specific aspect, characteristic, attribute, or relationused to describe a resource

? A specific resource together with a named property plus the value ofthat property for that resource is an RDF statement

? Statement: subject (resource), predicate (named property), object(property value)

? Object can be another resource or it can be literal


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Basic RDF Serialization Syntax

? The Description element names, in an about attribute,the resource to which each of the statements apply

? If the resource does not yet exist (i.e. does not have aresource identifier) than a Description element can create

the identifier for the resource using an ID attribute? Property names must always be associated with a schema

? This can be done by qualifying the element names with anamespace prefix to connect the property definition withthe corresponding RDF schema


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Core Classes

? rdfs:Resource - All things being described by RDFexpressions are resources and are considered to beinstances of the class rdfs:Resource

? rdfs:Class - represents the generic concept of a type orcategory and can be defined to represent almosteverything, e.g. Web pages, people, document types

? rdf:Property - represents the subset of RDF resourcesthat are properties


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Example Classes and Subclass property

BusinessDocs

PurchaseOrder


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What is the rdf:type property?

? It specifies a class (there may be more than one) towhich the resource belongs

? Its value is always a Web resource representing the

class? It can be expressed as a type attribute on a

Description element

? It can also be implied by using a special element

instead of a Description element


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Example Classes and Subclass property

The same definition with different syntax

BusinessDocs

PurchaseOrder


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Example Property Definition

? An Example Property Definition:


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An Example Class Instance


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Core Properties

? rdfs:subClassOf - This property specifies asubset/superset relation between classes

? The rdfs:subClassOf property is transitive

? If class A is a subclass of some broader class B,and B is a subclass of C, then A is also implicitly asubclass of C

? Consequently, resources that are instances of classA will also be instances of C, since A is a sub-set ofboth B and C

? rdfs:subPropertyOf - is an instance of rdf:Propertythat is used to specify that one property is aspecialization of another


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Core Properties

? rdfs: range - is used to define that the valuesof a property are instances of one or morestated classes

? rdfs: domain - is used to state that anyresource that has a given property is aninstance of one or more classes


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Property-centric classes

? In typical OO classes, each class specifiescompletely what properties it has and whattheir types are

? In RDF classes, each property specifies whatclasses of subjects and objects it relates

? Therefore, new properties can be added to a

class without modifying the class


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Summary of useful properties

? rdf:type relates any resource to its class

? rdfs:subClassOf relates a subclass to its

superclass (multiple inheritance is OK)? rdfs:subPropertyOf relates a subproperty to

its superproperty


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Summary of useful properties

? rdfs:domain specifies the domain of aproperty (the classes of its subjects); ifunknown, anything can be a subject

? rdfs:range specifies the range of a property(the single class of its objects); if unknown,anything can be an object


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Containers

? Frequently it is necessary to refer to a collection of resources;RDF containers are used to hold such list of resources or literals

? Container model

? Bag

? Sequence

? Alternative


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Containers

? Bag: a simple collection with no ordering

? Seq: a collection with implicit ordering

? Alt: a set of alternatives (first one preferred)

? Elements are properties named _1, _2, _3, etc.


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The Bag

Ceng 720Rdf:Bag

/Sudents/Ali

/Students/Ayse

students

Rdf:type

Rdf._1

Rdf._2


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RDF Description of the Example


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An Example on the use of Collections

Yayinlar

Papers


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Example


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An Example to Container object Alternative


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Reified statements

? We reify statements so that we can talk about them rather thanasserting them

? Ali believes that creator of http://srdc.metu.edu.tr is AsumanDogac

? In order to model statements about statements RDF models the

original statement with 4 properties? subject: The resource about which the original statement is

made? predicate: Identifies the original property in the modeled

statement? object: Identifies the property value in the modelled

statement? type: describes the type of the new resource. All reified

statements are instances of RDF:Statement; that is, theyhave a typeproperty whose object is RDF:Statement


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An Example


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Dublin Core

? A set of fifteen basic properties for describinggeneralized Web resources

? The obvious mapping of Dublin Coreproperties into RDF properties has not yetbeen approved by the Dublin Core initiative,but is generally a good example


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Dublin Core

? Title: the name given to the resource

? Creator: the person or organization primarilyresponsible for the resource

? Subject: what the resource is about

? Description: a description of the content


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Dublin Core

? Publisher: the person or organizationresponsible for making the resource available

? Contributor: someone who has providedcontent to the resource other than the creator

? Date: date of creation or publication


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Dublin Core

? Type: type of resource, such as home page,technical report, novel, photograph

? Format: data format of the resource

? Identifier: URL, ISBN number,

? Source: another resource that this resourceis derived from


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Dublin Core

? Language: the language of the content

? Relation: another resource and itsrelationship to this one

? Coverage: the portion of time or spacedescribed by this resource (atlases, histories,etc.)


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Dublin Core

? Rights: the intellectual property rightsadhering to this resource, or a pointer to them

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