RDF Schema. Mario Arrigoni Neri

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1 RDF Schema Mario Arrigoni Neri

2 Semantic heterogeneity Standardization: commitment on common shared markup If no existing application If market-leaders can define de-facto standards Translation: create maps between different schemas Requires deep understanging of the schemas Mapping can be complex and expensive Translation mappings grow as N 2 Annotation: define correspondences between different ontologies Requires interpretation and understanding The creation of interconnections can be complex Reasoning services on the joint metamodel can support integration 2

3 Annotation Ontolgoy must provide the meaning of used terms IO Organism ISA IO Invertebrate 3 <SP_entry> <ID> </ID> <Authors> </Authors> <Organism> Dog </Organism> </SP_entry>? <EC_entry> <ID> </ID> <Authors> </Authors> <Invertebrate> Leech </Invertebrate> </EC_entry>

4 Annotation Ontolgoy must provide the meaning of used terms IO Organism ISA IO Invertebrate 4 <SP_entry> <ID> </ID> <Authors> </Authors> <Organism> Dog </Organism> </SP_entry>? <EC_entry> <ID> </ID> <Authors> </Authors> <Invertebrate> <Organism> Leech </Invertebrate> </Organism> </EC_entry>

5 Ontologies An ontology is an explicit representation of a conceptualization of the reality [Gruber, 1993] Different systems agree on an ontology (directly or in a mediated way) Models linked to nodes from the shared ontology Ontology adreement is required Information System 1 DB1 interpr. 1 Ontology conceptual. Information System 1 interpr. 2 5 DB2

6 Ontologies Technically speaking, ontology allows to project operational semantics to metadata <?xml version="1.0"?> <note> <to></to> <from></from> <message> </message> </note> Ok, what does it mean?? 6

7 Ontologies Technically speaking, ontology allows to project operational semantics to metadata <?xml version="1.0"?> <note> <to></to> <from></from> <message> </message> </note> 7 Note InformativeObject from.person ok!!!

8 A schema for RDF models RDF provides a way to describe generic assertions on resources and properties It is often necessary to define that resoruces belong to certain types rdf:type rdf:type

9 RDF(S) To describe types and relations we can use RDF itself The specific terminology is defined by RDF Vocabulary Description Language, known as RDF-Schema It does not provide a specific vocabulary for the specific application (terms:book, terms:assprof, terms:author, ecc..) It defines a mechanism to specify classes and properties and to build up the specific vocabulary (metalanguage) RDF(S) provides a (semantic) type system for RDF 9

10 Classes 1 Classes are sets of individuals. Each class represents a resource type Reference namespace is: Each class is in rdf:type relation with the resource

11 Classes 2 terms:assprof rdf:type rdfs:class <rdf:rdf xmlns:terms= xmlns:rdf= xmlns:rdfs= > <rdf:description rdf:about= > <rdf:type rdf:resource= /> </rdf:description> </rdf:rdf> We can use abbreviated RDF form of RDF and rdf:id 11 <rdf:rdf xml:base= xmlns:rdf= xmlns:rdfs= > <rdfs:class rdf:id= AssProf /> </rdf:rdf>

12 Classes hierarchies 1 Line in OO languages, RDF(S) allows to organize classes in hierarchies RDF(S) vocabulary defines the relation subclassof Transitive Ex: an AssProf is a Professor and a Professor is a Person 12 C : set of class symbols I : interpretation domain I : interpretation function A I I A, B C A rdfs:subclassof B x I : x A I x B I that is : A I B I where x A I <x rdf:type A>

13 Classes hierarchies 2 rdf:type rdfs:subclassof rdf:type 13 <rdfs:class rdf:id= Person /> <rdfs:class rdf:id= AssProf > <rdfs:subclassof rdf:resource= #Person /> </rdfs:class> <rdfs:class rdf:id= AssProf > <rdfs:subclassof> <rdfs:class rdf:id= Person /> </rdfs:subclassof> </rdfs:class>

14 RDF(S) meta(meta)model The resources we use to describe the schema are classes themselves. rdfs:class is a (meta) class to which all classes belong It is NOT the correspective of Object in java rdfs:resource is the universal class, from which all classes derive. If no rdfs:subclassof if explicitly defined, rdfs:resource is assumed. It is equivalent to the Object in java rdfs:subclassof rdf:type rdf:type 14 rdfs:subclassof

15 Instances classes metaclasses rdf:type rdfs:resource rdfs:class rdfs:subclassof rdf:type terms:person rdf:type terms:book rdfs:subclassof terms:assprof 15 Mario Arrigoni Neri rdf:type people:d02005 terms:name terms:author rdf:type bib:book0001

16 Reasoning 1 Even the simple constructs rdf:type and rdfs:subclassof allow minimal reasoning service rdf:type rdfs:subclassof rdfs:subclassof rdfs:subclassof rdf:type rdf:type

17 Reasoning 2 people:d02005 rdf:type terms:assprof terms:assprof rdf:type rdfs:class terms:person rdf:type rdfs:class terms:assprof rdfs:subclassof terms:person term:person rdfs:subclassof rdfs:resource A rdfs:subclassof B B rdfs:subclassof C A rdfs:subclassof C A rdf:type B B rdfs:subclassof C A rdf:type C 17 Term:AssProf rdfs:subclassof rdfs:resource people:d02005 rdf:type terms:person people:d02005 rdf:type terms:resource

18 Reasoning 3 INFERENCE RULES based reasoning Inference engine combines rule antecedents with facts in the knowledge base (triples) via PATTERN MATCHING 1) 2) terms:assprof rdfs:subclassof terms:person terms:person rdfs:subclassof terms:resource A rdfs:subclassof B B rdfs:subclassof C A rdfs:subclassof C A = terms:assprof B = terms: Person C =? 18 A = terms: Person B = terms: Resource C =? A = terms:assprof B = terms: Person C = terms: Resource

19 Meta-model reasoning rdfs:resource rdfs:type rdfs:class rdfs:class rdfs:subcassof rdfs:resource A rdf:type B B rdfs:subclassof C A rdf:type C A = rdfs:resource B = rdfs:class C = rdfs:resource rdfs:resource rdf:type rdfs:recource 19

20 Properties After the description of classes we need to define specific properties Each RDF property is typed by rdf:property terms:author rdf:type rdf:property <rdf:property rdf:id= author /> 20

21 Domain and Range 1 How to link classes and properties? RDF(S) allows to restrict the types of individuals involved in a property Principal information is described by the (meta)properties rdfs:domain and rdfs:range Ex: author relation (property) connects books to persons: rdfs:domain rdf:type rdfs:range

22 Domain and Range 2 r rdfs:domain C R : set of relation names I : interpretation function R I I x I x, y I : <x,y> r I x C I r rdfs:range C x, y I : <x,y> r I y C I 22

23 Literal range 1 Optionally, the range of a property (not the domain) can be a typed literal rdfs:range

24 Literal range 2 To explicitly define the range to be a literal type we can use the class rdfs:datatype rdfs:range xsd:date rdf:type ^^xsd:date 24

25 Literal range 3 <rdf:property rdf:id= pubdate > <rdfs:domain rdf:resource= /> <rdfs:range rdf:resource= /> </rdf:property> <rdfs:datatype rdf:about= /> RDF does not define any meaning for literal types Validation is left to the application layer Literal type must be declared in the document 25 <terms:book rdf:id= book0001 > <terms:author rdf:resource= /> <terms:pubdate rdf:datatype= > </terms:pubDate> </terms:book>

26 Multiple domains and ranges <rdfs:property rdf:id= author > <rdfs:range resource= #Person /> <rdfs:range resource= #Company /> </rdfs:property> <rdfs:property rdf:id= author > <rdfs:range resource= #Person /> </rdf:property> <rdfs:property rdf:id= author > <rdfs:range resource= #Company /> </rdf:property> x, y I : <x,y> author I y Person I x, y I : <x,y> author I y Company I x, y I : <x,y> author I y Person I y Company I 26 x, y I : <x,y> author I y (Person I Company I ) = Ø

27 Subproperties Like in the class case, each property van be defined to be a subproperty of a given property rdfs:subpropertyof <rdf:property rdf:id= author > <rdfs:subpropertyof resource= #contributor /> </rdf:property> 27 R : set of property symbols I : interpretation domain I : interpretation function r I I x I r, s R r rdfs:subpropertyof s x, y I : <x, y> r I <x, y> s I

28 Subproperties, Domain and Range Domains and ranges are inherited through rdfs:subpropertyof chains r rdfs:subpropertyof s rdfs:domain D rdfs:domain x, y I : <x, y> r I <x, y> s I x, y I : <x, y> s I x D I x, y I : (<x, y> r I <x, y> s I ) (<x, y> s I x D I ) 28 x, y I : <x, y> r I x D I

29 OWA Open World Assumption RDF/RDF(S) semantics is designed to allow inference even if assuming to not have complete knowledge on the topic I can even use classes and properties not defined in the schema Inference remains valid (monotonic logic) as the knowledge increaes. This is violated by disjunctive interpretation of multiple domains and ranges 29 <rdf:property rdf:id= author > <rdfs:range resource= #Person /> </rdf:property> <terms:book rdf:about= bib:book0001 > <terms:author rdf:resource= #D02005 /> </terms:book> <rdf:property rdf:id= author > <rdfs:range resource= #Company /> </rdf:property> <terms:person rdf:about= #D02005 /> <terms:people rdf:about= #D02005 > <rdf:type resource=..company /> </terms:people>

30 DL semantics RDF DL <A rdfs:subclassof B> A B <R rdfs:range B> T= R.B <R rdfs:domain A> T= R -.A // R A <R rdfs:subpropertyof S> R S <x rdf:type A> x:a // A(x) <x R y> R(x,y) 30 RDFS misses many operators to be in AL logics family

31 Documentation rdfs:label rdf:type rdf:property rdfs:domain rdfs:resource rdfs:range rdfs:literal Human readable rdfs:comment rdf:type rdf:property rdfs:domain rdfs:resource rdfs:range rdfs:literal Human readable 31

32 Annotations rdfs:seealso rdf:type rdf:property rdfs:domain rdfs:resource rdfs:range rdfs:resource rdfs:isdefinedby rdf:type rdf:property rdfs:domain rdfs:resource rdfs:range rdfs:resource rdfs:subpropertyof rdfs:seealso 32

33 Semantic schema? XML-Schema and DTD describe the syntax of documents RDF Schema defines the intended semantics of resources It does not contrain document structure, but provides information usable to interpret intended model 33 <rdf:description rdf:id= D02005Name > <names:name>mario</names:name> <names:surname>arrigoni Neri</names:surname> </rdf:description> <rdf:description rdf:id= D02005Name names:name= Mario > <names:surname>arrigoni Neri</names:surname> </rdf:description> Different syntax variations for the same model

34 RDF(S) and OOP languages RDF(S) has some similarities with OOP languages, but some distinctions too Different descriptions organization: Java public class Person { } public class Book {.. public Person author; } RDF(S) <rdfs:class rdf:id= Book /> <rdfs:class rdf:id= Person /> <rdf:property rdf:id= author > <rdfs:domain rdf:resource= #Book /> <rdfs:range rdf:resource= #Person /> </rdf:property> 34

35 Properties scope 35 This is not only a syntax caveats In a programming language, properties are semantically contextual to reference classes. If i did the same in RDF(S) I should conclude that each author is both a person and a company at the same time We can define weight property without domain nor range to facilitare property reuse public class Book {.. Public Person author; } public class Specification { Two different omonimous properties.. Public Company author; }

36 Schema uses In programming languages (and classic databases) chema is always used in a PRESCRICTIVE way. If the probram does not respects the constraints it is simply incorrect In RDF(S) schema provides additional informations for different uses Scenario 1 prescrittive Scenario 2 deductive use on the model: we can use schema knowledge to enrich model with new knowledge. Ex: I have a book with a known author, I can deduce that such a resource is a person Scenario 3 deductive use on the schema: example: we have a book with a Company as author. If inconsistency is excluded we can derive that <Company subclassof Person> 36

37 RDF Schema limits Limited expressive power (subclassof, subpropertyof, range, domain) P2 rdfs:domain C2 rdfs:subpropertyof rdfs:subclassof P1 rdfs:domain C1 Ontology languages expressive syntax, formal semantics, expressive power Compatible (with RDF) 37

38 Dublin Core 1 Standard for metadata by a consortium of librarians Dublin 1995 Borned before than the Semantic Web, but easy to be integrated into RDF framework Dublin Core 1: 15 categories of meta-informations Dublin Core 2: subclassing system of categories (qualificators) 38

39 Dublin Core 2 Content Copyright Instance Title Subject Description Type Source Relation Coverage Creator Publisher Contributor Rights Date Format Identifier language Modifiers can specialize the contents of the properties Date : created, valid, ecc.. 39 Can define specific codes for the values: Subject : LCSH (Library of Congress Subject Headings), MeSH, ecc..

40 Dublin Core 3 rdf:statement rdf:type bib:assertion01 rdf:subject rdf:predicate bib:book0001 terms:author dc:creator rdf:object people:d02005 people: terms:name names:name names:surname Marco Colombetti 40

RDF. Mario Arrigoni Neri

RDF. Mario Arrigoni Neri RDF Mario Arrigoni Neri WEB Generations Internet phase 1: static contents HTML pages FTP resources User knows what he needs and where to retrieve it Internet phase 2: web applications Custom presentation