RDF. Mario Arrigoni Neri

Transcription

1 RDF Mario Arrigoni Neri

2 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 level.. VB-Script, J-Script, DHTML Dynamic contents (Servlet, JSP, ASP, Applet, ) Application interacts with the user Internet phase 3: semantic web Documents readable by artificial agents Services usable by artifical agents 2

3 The semantics of documents Documents are not simply for human reading, but can be processed by artificial agents as well In documents information is hidden in text. Human beings can extract it We need a semantic sovrastructure to capture the meaning of resources in a machine processable formalism In Semantic Web we can substitute (or add) to human-readable content a set of assertions to represent the subset to be processed by machine 3

4 The Semantic Web metadata, that are data about data, describe information about resources to be processed automatically. Represents knowledge, that is information used by the system while elaboration and action Internal metadata Example: tags inserted in a text document to describe specific text properties Procedural use: print this in bold Informative use: extract the titles of all the chapters External metadata Use a descriptor external to the document Distinguish content and content description Typically I gain expressive power 4 I can standardize the language independently from the domain

5 Semantic Web Stack Before representing knowledge we have to address two problems: How to refer to onbjects in our formal model? Even if metadata are internal, the description can refer to external elements How to guarantee syntactic interoperability? Solutions from XML stack: URI (Uniform Resource Identifiers) and URIref XML (extensible Markup Language) and Unicode RDF syntax XML XSD / DTD Namespaces 5 infrastructure URI Unicode

6 URL or URI? The domain is wider than network accessible documents Example: a document has an author; this is not an internet resource by its own URI: are unique strings for: Network resources: URL (Uniform Resource Locator): ftp://ftp.server.it/file.txt Physical resources: Abstract concepts: 6

7 URI reference A more general case of URI It's an URI with an optional fragment identifier example: Used in HTML to refer to specific document fragments In RDF it is used: URI to identify RDF specification file Fragment identifier to refer to a specific resource 7

8 RDF metamodel 1 To express an assertion we must identify: The object we are describing The property of the object (or relation between objects) on which we are predicating The value of the property for the described object or the object it is in relation with has an author which value is Mario Arrigoni Neri The three components of the assertion are named: Subject Predicate Object 8

9 RDF metamodel 2 Resource: whatever is represented by an URIref. Literal: text string Property: an attribute I want to assign to a resource. Corresponds to the couples: Attribute - value Relation - filler Predicate - object Assertions: are the fundamental elements of any RDF description: Triple subject predicate - object Couple resource property Literals can be only objects in a triple 9 Subject Predicate Object author Mario Arrigoni Neri

10 RDF metamodel 2 10 Resource: whatever is represented by an URIref. Literal: text string Property: an attribute I want to assign to a resource. Corresponds to the couples: Attribute - value Relation - filler Predicate - object Assertions: are the fundamental elements of any RDF description: Triple subject predicate - object Couple resource property Literals can be only objects in a triple Subject Predicate Object R : resources L : literals R L = Ø Tr R x R x (R L) author Mario Arrigoni Neri

11 Representation To disambiguate terms of disclosure we can use URIrefs for relations too (predicates) Subject Predicate Object Mario Arrigoni Neri RDF defines a logical schema, NOT a language. Different languages to represent models (serializations) N3: native format, assertions described via triples subject predicate- object Graphic: resources al ellipses, triples as arcs 11 RDF/XML: XML serialization Different languages for different purposes

12 N3 1 URIs between < and > Literals between apices < < Mario Arrigoni Neri The use of an URI to represent predicate allows the application to understand what we mean by author and the introduction of a common terminology 12

13 N3 2 Triple notation requires complete URIrefs to be written For simplicity we use qualified names (QName) like in XML prefix : unqualifiedname (fragment) Prefix to namespace association Complete URIref is obtained by the composition of the namespace and the fragment identifier rdf terms bib bib:book0001 terms:author Mario Arrigoni Neri 13 <

14 Graphical representation Is targeted to human beings Resources represented by ellipses Literals represented by rectangles Predicates represented by arrows (oriented links) connecting resources to resources and resources to literals Mario Arrigoni Neri 14

15 Multiple relations Each resource can be subject and(or object of different triples In N3 resource is repeated In graph the node is represented only once bib:book0001 terms:author Mario Arrigoni Neri bib:book0001 terms:author Marco Colombetti Marco Colombetti Mario Arrigoni Neri

16 Using URIrefs 1 Problems in describing the author as its name (literal). It can be a non-sense saying that the author is a string What if we have different authors with the same name? What if I ant do predicat on the string itself? I define an URIref to identify the resource By using an URIref the object becomes a resource. I can further describe the author The use of URIrefs facilitates the definiton of rich shared vocabularies 16

17 Using URIrefs Mario Arrigoni Neri arrigoni@elet.polimi.it people 17 bib:book0001 terms:author people:d02005 people:d02005 terms:name Mario Arrigoni Neri people:d02005 terms: arrigoni@elet.polimi.it

18 Structured properties More comlex than binary relations Example: the name of a person can be decomposed in first name and last name We can materialize the name in an aggregated resource arrigoni@elet.polimi.it Mario 18 Arrigoni Neri

19 Anonymous resources 1 names:d02005 resource is used only as an aggregation of name and surname properties In a complex model we should use many different intermediate nodes We can use empty nodes arrigoni@elet.polimi.it Mario 19 Arrigoni Neri

20 Anonymous resources 2 Empy nodes for anonymous resources in graphical syntax Never referenced outside the document bib:book0001 terms:author people:d02005 people:d02005 terms: arrigoni@elet.polimi.it people:d02005 terms:name?????? names:name Mario??? names:surname Arrigoni Neri It can be necessay to define local identifiers for different anonymous resources bib:book0001 terms:author people:d02005 people:d02005 terms: arrigoni@elet.polimi.it people:d02005 terms:name _:D02005Name _:D02005Name names:name Mario _:D02005Name names:surname Arrigoni Neri 20

21 Typed literals 1 In RDF it is possible to define a type for each literal ^^xsd:date bib:book0001 terms:pubdate ^^xsd:date 21

22 Typed literals 2 RDF does not define built-in types RDF simply allows for the binding of a literal to its type Type interpretation is for application level The most common type library is XSD No special meaning for XSD types 22

23 Typed resources Resources can be defined as instances of a specific category Programming languages: class, Logic: concept As an example, if an agent wants to retrieve a book it must know that is of type book In RDF the association between an instance and its type (represented by a resource as well) is represented by a special predicate: rdf:type rdf:type rdf:type

24 RDF / XML XML is anatural aption for RDF models serialization Root element is rdf:rdf Assertions are represented by the element Description <?xml version= 1.0?> <rdf:rdf xmlns:rdf= xmlns:terms = xmlns:bib = xmlns:people= > 24 <rdf:description rdf:about= > <terms:author>mario Arrigoni Neri</terms:author> </rdf:rdf>

25 Assertions groups An RDF/XML document contains a set of Description elements <rdf:description rdf:about= > <terms:author>mario Arrigoni Neri</terms:author> <rdf:description rdf:about= > <terms:author>marco Colombetti</terms:author> Assertions about the same subject can be grouped together <rdf:description rdf:about= > <terms:author>mario Arrigoni Neri</terms:author> <terms:author>marco Colombetti</terms:author> 25

26 References to resources <rdf:description rdf:about= > <terms:author rdf:resource= /> <rdf:description rdf:about= > <terms:name>mario Arrigoni Neri</terms:name> <rdf:description rdf:about= > <terms:author> <rdf:description rdf:about= > <terms:name>mario Arrigoni Neri</terms:name> </terms:author> 26

27 Anonymous resources Anonymous resources are represented by Description elements using rdf:id instead of rdf:about <rdf:description rdf:about= > <terms:name rdf:resource= #D02005Name /> <rdf:description rdf:id= D02005Name > <names:name>mario</names:name> <names:surname>arrigoni Neri</names:surname> 27 Some technicalities (we will see later), but a common choice. True anonymous resources described inline <rdf:description rdf:about= > <terms:name><rdf:description> <names:name>mario</names:name> <names:surname>arrigoni Neri</names:surname> </terms:name>

28 Typization literals <rdf:description rdf:about= > <terms:name rdf:id= D02005Name /> <terms:age rdf:datatype= > 28</terms:age> resources 28 <rdf:description rdf:about= > <terms:name rdf:id= D02005Name /> <terms:age rdf:datatype= > 28</terms:age> <rdf:type rdf:resource= />

29 Abbreviations 1 Literal filled triples can be defined by attributes <rdf:description rdf:id= D02005Name names:name= Mario > <names:surname>arrigoni Neri</names:surname> Internal descriptions can be removed by enriching predicate element (for anonymous resources only) 29 <rdf:description rdf:about= > <terms:author rdf:parsetype= resource > <terms:name rdf:resource= /> <terms:age rdf:datatype=.. >28</terms:age> </terms:author>

30 Abbreviations 2 An rdf:type relation can be promoted by substituting description element with the URIref of the resource denoting type <rdf:description rdf:about= > <rdf:type rdf:resource= /> <terms:book rdf:about= > </terms.book> Advantage: I can use prefixes 30

31 QNames and attributes 1 Pay attenction: Qnames can be used in elements and attributes names only. RDF could not resolve qualifications in the text, like in attribute values <rdf:description rdf:about= bib:book0001 > <rdf:type rdf:resource= terms:book /> 31

32 QNames and attributes 2 However, we can use XML entities to obtain the same functionality <!DOCTYPE uridef [ <!ENTITY rdf " <!ENTITY terms " <!ENTITY bib " ]> <rdf:rdf xmlns:rdf= &rdf; xmlns:terms= &terms; xmlns:bib= &bib, > <rdf:description rdf:about= &bib;book0001 > <rdf:type rdf:resource= &terms;book /> </rdf:rdf> 32

33 Unique identifiers 1 rdf:id attribute is similar to ID in XML: is unique across the file Identifier is built up by combining base URI di base (phisical address of the document), # symbol and the value of rdf:id parameter, that becomes the fragment identifier When using rdf:resource= #foo fragment starting from local file address is calculated If the document comes from: 33 <rdf:description rdf:about= > <terms:name rdf:resource= #D02005Name /> <rdf:description rdf:id= D02005Name > <names:name>mario</names:name> <names:surname>arrigoni Neri</names:surname>

34 Unique identifiers 2 Referring to the node from another file <rdf:description rdf:about= > <terms:name rdf:resource= /> Problem: what if I use a mirror to access the file? #D02005Name resource could not correspond to the original one <rdf:rdf xml:base= > <rdf:description rdf:about= > <terms:name rdf:resource= /> </rdf:rdf> 34

35 Containers It is often useful to explicitly represent the collection of some resources instead of connecting each element to the subject of assertions. Example: I want to represent the set of all the authors of a book rdf:type rdf:_1 rdf:bag rdf:_2 Marco Colombetti 35 rdf:bag container represents a collection of individuals

36 Containers in RDF / XML <rdf:description rdf:about= > <terms:author> <rdf:description> <rdf:type resource= /> <rdf:_1 rdf:resource= /> <rdf:_2>marco Colombetti</rdf:_2> </terms:author> 36 <rdf:description rdf:about= > <terms:author> <rdf:bag> <rdf:li rdf:resource= /> <rdf:li>marco Colombetti</rdf:li> </rdf:bag> </terms:author>

37 Alt and Seq rdf:alt: alternatives <rdf:description rdf:about= > <terms:mirror> <rdf:alt> <rdf:li rdf:resource= /> <rdf:li rdf:resource= /> <rdf:li rdf:resource= /> </rdf:bag> </terms:mirror> I can use rdf:_n to express preference order 37 rdf:seq for ordered sets

38 Collections 1 RDF works under open world assumption, Whoever can add assertions on resources which was defined by others. The final joint model is the union of all the assertion sets By using containers we have no way to limit the number of potential components To close the components a recurrent structure is used Terms for collections are: The rdf:list type: is the type for all the nodes involved in lists The predicate rdf:first: connects each list node to the head (first element) The predicate rdf:rest: connects each list node to the queue 38 The resource rdf:nil (of type rdf:list): the empty list

39 Collections rdf:type rdf:first rdf:list rdf:rest rdf:type rdf:first Marco Colombetti rdf:type rdf:rest 39 rdf:nil

40 WEB of trust RDF works under OWA. I can add a new Description element with the attribute rdf:about to bind to a resource described elsewhere What if third party information is not correct? System will act incorrectly What if information is contradictory? In a contradictory theory all w.f.f. Are theorems 40 System is able to derive any consequency A, * The need for information certification criteria This requiers to make meta-assertions, that are assertions about other assertions (like in modal logics)

41 Reification 1 Example: Marco Colombetti says that Mario Arrigoni Neri is the author of the book A way is to define an assertion with say as predicate, Marco Colombetti ad subject and having as object the assertiong Mario Arrigoni Neri is the author of the book. It is necessary to associate to the assertion a new resource, which is available for further assertions This process is called reification (render as an object) It is quite similar to the process of translating ER many-to-many relationships to logical schemas 41

42 Reification 2 An assertion is a ternary relation between subject, predicate and object To reify it we need to build a resource and by translating ternary relation to three new object relations The lexicon is composed of: rdf:statement type for reified assertions rdf:subject, rdf:predicate and rdf:object predicates rdf:statement rdf:type terms:says people: terms:name bib:assertion01 rdf:subject rdf:predicate rdf:object names:name names:surname 42 bib:book0001 terms:author people:d02005 Marco Colombetti

43 Compact syntax for reification rdf:bagid attribute identifies assertion as a resource <rdf:description rdf:about= bagid= assertion0001 > <terms:author rdf:resource= /> <rdf:description rdf:about= > <terms:says rdf:resource= #assertion0001 /> <rdf:description> <rdf:description rdf:about= #assertion0001 > <terms:by rdf:resource= /> RDF does NOT identify the reified assertion to its representing resource 43