Chapter 9

9 RDFS (RDF Schema) RDFS Part of the Ontological Primitive layer Adds features to RDF Provides standard vocabulary for describing concepts (meta-vocabulary) Chapter Overview of RDFS features Introduces classes, individuals, and properties

9.1 RDFS Overview RDFS (RDF Schema) Domain-neutral lightweight schema language Provides basic structures: classes & properties Builds on RDF Specification Namespace Prefix: rdfs Name:

9.2 RDFS Features RDFS (RDF Schema) ▫Defines additional modeling primitives ▫Extends RDF using a formal class concept ▫Permits restricting properties and classes ▫Uses RDF/XML syntax RDFS vocabulary ▫Collection of class & property descriptions ▫Can be combined ▫Describes domain using RDF resources that define characteristics of other resources ▫Managed by decentralized communities of interest

9.3 RDFS Classes RDFS Extends the RDF type concept Provides formal mechanisms for classes which represent concepts Predefines several core classes

9.3.1 RDFS Class Concept RDFS classes Sets used to describe concepts as categories of resources, typically for a particular domain Templates used to instantiate objects Have associated resources (instances of the class) Resources can be multi-classed Set of all instances called “class extension” Similar to OOP classes (but no behavior) Properties are not tied to specific classes More like entities in data models or RDBMS tables

9.3.2 Predefined RDFS Classes RDFS supports formal vocabularies by formalizing the concepts of Resource Class Property Datatype Literal XML literal using predefined resources

Defining RDFS Classes rdfs:Class type Is defined as an rdf:type Establishes class membership Superseded by OWL classes Usually uses the typed node syntax Classes are RDF resources whose rdf:type is “rdfs:Class” RDFS uses RDF to define itself

RDFS Resources rdfs:Resource class Built-in class Root class of all RDF resources and classes Similar to Java Object class All RDFS classes and RDF resources are subclasses of rdfs:Resource

RDF Properties rdf:Property class Built-in class Root class of all RDF & RDFS properties Similar to Java Property class RDFS uses the RDF definition of properties Use rdf prefix, not rdfs All RDFS properties are instances of the rdf:Property class

RDFS Classes rdfs:Class class All resources that have an rdf:type of rdfs:class are RDFS classes Defined recursively rdfs:Class resource is an rdfs:Class Instances are classes Root class of all RDFS classes Similar to Java Class class

RDFS Literals rdfs:Literal class Set of atomic plain and typed literal values Can be used to specify restriction that property’s values (range) must be string literals Instances are literal values such as strings and integers Used by OWL directly as a datatype Implicitly part of all ontologies All RDFS literals are instances of rdfs:Literal

XML Literals rdf:XMLLiteral class Subclass of rdfs:Literal Represents XML strings within RDF statements Only datatype predefined in RDF Use rdf prefix, not rdfs Can also include language identifiers Subset of XMLS datatypes

RDFS Datatype Class Datatypes define subsets of literal values that have associated semantics rdfs:Datatype class ▫Parent class for all RDFS datatypes ▫Instances of the class are also implicitly subclasses of rdfs:Literal ▫Provide mechanism for referring to XMLS datatypes Example ▫&xsd;nonNegativeInteger is instance of rdfs:Datatype and subclass of rdfs:Literal Although legal, defining new instances discouraged

RDFS Predefined Class Summary Predefined ClassDescription rdfs:ResourceRoot class of all resources rdf:PropertyClass of all properties rdfs:ClassClass of all classes rdfs:LiteralClass of all literal values rdf:XMLLiteralXML strings within RDF statements rdfs:DatatypeIdentifies datatypes

9.4 Individuals Individuals Instances of user-defined classes Associated to classes using rdf:type property Can be assigned to one or more classes Covered in more detail in Chapter 15

9.5 Properties RDFS enhances RDF Properties, allowing the specification of restrictions or constraints Limit the values a property can have Limit which properties a resource can have RDFS provides vocabulary for Core properties Clarification properties Container descriptions Documentation properties

9.5.1 Core Properties Core properties provided by RDFS rdf:type Relate individuals to classes rdfs:subClassOf Relate classes to classes rdfs:subPropertyOf Relate properties to properties rdfs:range & rdfs:domain Relate properties to classes All properties are instances of rdf:Property

RDF Type Property rdf:Type property Asserts that an individual is a member of a particular class Domain: any Resource Range: Instance of rdfs:Class

Specializing Classes Classes are organized into generalization/ specialization hierarchy (hyponymy) rdfs:subClassOf property ▫Defines a specialization relationship ▫Specified in the subclass ▫Value is the superclass ▫Domain: Instance of rdfs:Class ▫Range: Instance of rdfs:Class Similar to subset relation between classes A class can be a subclass of multiple superclasses

Specializing Classes cont’d Syntax Example All classes automatically subclass of rdfs:Resource Subclasses “inherit” superclass’ property restrictions Transitive property C is a subclass of B ∧ B is a subclass of A → C is a subclass of A Instance of subclass has properties of both subclass and superclass

Specializing Properties Properties can also be organized into hierarchies rdfs:subPropertyOf property ▫Defines specialization relationship b/w properties ▫Specified in the subproperty ▫Value is the superproperty ▫Domain: Instance of rdf:Property ▫Range: Instance of rdf:Property Subproperties can have multiple superproperties Transitive, like rdf:subClassOf Example ▫

Restricting Property Values rdfs:Range property ▫Restricts values of properties to classes or datatypes/data ranges ▫Applies to all uses of the property ▫Domain: Instance of rdf:Property ▫Range: Instance of rdfs:Class Example: ▫ // Only a Person can own something ▫ Joe’s Pizzaria // Valid only if Person123 is an instance of Person

Restricting Values cont’d Use global property restrictions with care ▫Might want something to be owned by a company! Recommended: make property name specific ▫“restaurantOwnerIndividual”, not “ownedBy” Range restrictions should not be separate URIs ▫Causes scalability & usability difficulties Multiple range restrictions can be specified ▫If 2 or more rdfs:range properties specified, resources used as property values must be instances of all (intersection of) classes specified in rdfs:range properties Can infer that individual is member of a class if it is the value of a property that has a range restriction

Restricting Property Subjects rdfs:domain property ▫Restricts the subject of a property to instances of a class, or to a specified intersection of classes ▫Applies to all uses of the property ▫Domain: Instance of rdf:Property ▫Range: Instance of rdfs:Class Example ▫ // Only a Restaurant can be ownedBy something ▫ Joe’s Pizzaria // Valid since JoesPizzaria instance of Restaurant

Restricting Subjects cont’d Can infer that individual belongs to class if it’s subject of a property that has a range restriction Multiple range restrictions can be specified If 2 or more rdfs:domain properties specified, resources used as property’s subject must be instances of all (intersection of) classes specified in rdfs:domain properties

RDFS Core Properties Summary Core PropertyDescription rdf:TypeIdentifies the class of an individual rdfs:subClassOfCreates a specialization of a class rdfs:subPropertyOfCreates a specialization of a property rdfs:rangeLimits the values of a property rdfs:domainLimits the individuals that can have a property

9.5.2 RDFS Clarification Properties Clarification properties Reference related resources (rdf:seeAlso) Identify a resource’s source (rdf:isDefinedBy) Non-core properties

Referencing Related Resources rdf:seeAlso property ▫Identifies a resource that provides additional information about subject ▫Domain & Range: Instance of rdfs:Resource Syntax ▫ Example ▫ Weak semantics, but occasionally useful

Identifying A Resource’s Source rdfs:isDefinedBy property Subproperty of rdfs:seeAlso Relates resource to other resource that defines it Domain & Range: Instance of rdfs:Resource Syntax Can be used to explicitly identify the defining RDF vocabulary for a resource Useful if the URI portion of a resource does not indicate the schema of the resource

rdfs:Container class Superclass of the 3 RDF containers (Bag, Seq, Alt) Encapsulates things into groups referenced as a whole Helps manage description of container types 9.5.3(.1) RDFS Container Classes

&3 Membership Properties rdfs:ContainerMembershipProperty class Used to instantiate individual subproperties for accessing members of containers Subproperties are named rdf:_1, rdf:_2, … rdf:_n Specific types of rdfs:member property rdfs:member property Superproperty of all container membership properties (rdf:_n)

9.5.4 RDFS Documentation Properties Used to associate human-readable names, labels, and descriptions of resources rdfs:label & rdfs:comment properties Can be used by user interface software to provide documentation to developers Can supply multiple values using xml:lang attributes to support internationalization Best practice: always label and comment classes and properties

rdfs:Label property ▫Provides human-readable version of resource name ▫Short textual string useful in user interface ▫Domain: Instance of rdfs:Resource ▫Range: Instance of rdfs:Literal Syntax ▫ labelString Example ▫ Key Lime Pie Best practice: label everything! Use xml:lang attributes to support labels in other languages Labeling Resources

Commenting Resources rdfs:comment property ▫Used to describe a resource with free-form text ▫Longer textual string describing the resource ▫Domain: Instance of rdfs:Resource ▫Range: Instance of rdfs:Literal Syntax ▫ commentString Example ▫ Key Lime Pie is a dessert made from milk, juice, and crackers Best practice: use instead of Comment everything, use xml:lang attribute

9.6 RDFS Summary RDFS ▫Provides many of the features required in a Semantic Web language by extending RDF with additional semantic features ▫Supports specification of limited ontologies through standardization of  Classes  Properties  Individuals  Generalizations  Property restrictions

9.6 RDFS Summary cont’d Complete example showing most RDFS features Key Lime Pie This class represents Key lime pies condensed milk

9.6 RDFS Summary cont’d CategoryConstructDomainRange Core Propertiesrdf:Type&rdfs;Resource&rdfs;Class rdfs:subClassOf&rdfs;Class rdfs:subPropertyOf&rdfs;Property Constraint Properties rdfs:range&rdfs;Property&rdfs;Class rdfs:domain&rdfs;Property&rdfs;Class Clarification Properties rdfs:seeAlso&rdfs;Resource rdfs:isDefinedBy&rdfs;Resource Documentation Properties rdfs:label&rdfs;Resource&rdfs;Literal rdfs:comment&rdfs;Resource&rdfs;Literal

9.6 Why RDFS is not Enough RDFS Adds critical ontological primitives Provides several features for specifying ontologies Does not support sufficient property restrictions No restrictions on cardinality Provides few descriptors to support inferencing Can’t conclude that an object belongs to a class based on its property values Can’t do class unions, complements OWL provides the missing pieces

OWL Chapter 10

10(.1) OWL (Web Ontology Language) OWL ▫Logical layer ▫Builds on & extends RDF & RDFS ▫Adds language features for describing ontologies ▫Developed to satisfy requirements of Semantic Web Chapter ▫Overview of OWL features ▫Introduces the 3 species  OWL Lite  OWL Full  OWL DL

OWL Definition OWL W3C’s recommended ontology language Used to define an ontology for a particular domain OWL ontology Set of axioms describing classes, properties, and relationships between them RDF/XML used to define conforming instance data

OWL History OWL evolved from ▫DARPA Agent Markup Language (DAML) ▫Ontology Inference Layer (OIL) ▫Both combined into DAML+OIL DARPA Agent Markup Language (DAML) ▫Developed by DARPA & MIT  Defense Advanced Research Projects Agency  Research branch of US Dept of Defense ▫Initiated Aug 2000  Led by Dr. James Hendler

OWL History cont’d Ontology Inference Layer (OIL) European Union developed in parallel with DAML DAML+OIL Merged in March 2001 Designed to enable agent communication “Thin layer” on top of RDFS Starting point for Web Ontology Working Group

OWL History cont’d W3C WebOnt Working Group Led by Hendler and Guus Schreiber Mission Define a formal semantics to precisely define the meaning of expressions in a language and valid inferences that can be made from those expressions Use XML syntax and datatypes Retain compatibility with XML & RDF if possible OWL adopted by W3C as official recommendation February 10, 2004

OWL Specification OWL Overview (introduction) OWL Guide (good examples) OWL Reference (reference guide) OWL Semantics and Abstract Syntax (abstruse!)

OWL Specification cont’d OWL Test Cases (used to certify applications) OWL Use Cases & Requirements (scenarios) Namespace Prefix: owl Name: Mime type: application/rdf+xml

OWL Features OWL Provides features that extend RDFS Supports better inferencing Enables the creation of domain-agile tools and reasoners to provide new functionality Transitions effort from coding complex software to representing information

10.2 OWL Species Three dialects (or species) of OWL OWL Full OWL DL OWL Lite Each supports a different group of users that need a different level of expressional capability

OWL Full Complete set of all language constructs Superset of RDF All RDF documents are OWL Full documents No restrictions RDF statements can be mixed in as desired No guarantee of decidability! NP-complete problem

OWL DL Sublanguage of OWL DL Description Language (DL) dialect Restricts use of some OWL Full constructs Primary goal: Keep OWL decidable Guarantee that computations will finish in a finite amount of time Compromise between expressivity and decidable reasoning Extension of restricted view of RDF

OWL Lite Sublanguage of OWL DL Minimal set of features for users that want to benefit from ontologies without a significant investment in encoding complex relationships Provides easier transition path for upgrading existing database, XML, or RDF(S) data Eases burden on tool developers Extension of restricted view of RDF

OWL Species Summary SpeciesSummarySubset of Constructs Restrictions on Use OWL FullSuperset of RDF OWL DLSame constructs as OWL Full, but restrictions on use √ OWL LiteSubset of OWL Full constructs, same restrictions as OWL DL √√

10.3 OWL Language Summary OWL (Web Ontology Language) ▫New language supporting Semantic Web ▫Defined by the W3C ▫Three species  OWL Full: complete language  OWL DL: decidable sublanguage  OWL Lite: simplified sublanguage Section 3 covers OWL Lite Section 4 covers OWL DL, OWL Full, and OWL applications