RDF & RDF Schema Machine Understandable Metadata for the Web

1. RDF & RDF SchemaMachine Understandable Metadata for the Web Semantic Web - Fall 2005 Computer Engineering Department Sharif University of Technology

2. Outline • Metadata • RDF • RDFS • RDF(S) Tools

3. Semantic Web: Problems • Too much Web information • around 1,000,000,000 (1109) resources • Many different types of resources • text, images, graphics, • audio, video, multimedia, • databases, Web applications, …

4. Semantic Web: Problems (2) • Information not indexable • No common “scheme” for doing so • Short-lived, dynamic resources • Differing relationships between authors, publishers, info intermediaries, users • Each community uses their own approach

5. Semantic Web: Problems (3) • Information not shareable • Difficult to share information • Difficult to share information about information • no common cataloging schemes

6. Main Issues: • Metadata • Information about information • Structured data about data • Many types/forms of metadata, dependent on role:

7. Types of Metadata: Web Resource discovery (Intellectual) property rights management Document management administration Archival information / status Security & User authentication Content ratings (PICS) Process description & control Product & Services Descriptions Database / data schemas

8. Second Issue: • Language for expressing metadata must be: • universal (so all can understand) • flexible (to incorporate different types) • extensible (flexible to custom types) • simple (to encourage adoption) • modular (so that schemes can be mixed, extended)

9. RDF • RDF stands for Resource Description Framework • It is a machine understandable metadata • RDF is graphical formalism ( + XML syntax + semantics) • for representing metadata • for describing the semantics of information in a machine- accessible way

10. RDF in SW Architecture

11. Example (generated by RDFPic)

12. Predicate Statement RDF Model • A model is a collection of statements • Statement := (predicate,subject,object) • Predicate is a resource • Subject is a resource • Object is either a resource or a literal Subject Object

13. Example shown in triples view

14. RDF model and natural language • Subject. In grammar, this is the noun or noun phrase that is the doer of the action. In the sentence “The company sells batteries,” the subject is “the company.” • Predicate. In grammar, this is the part of a sentence that modifies the subject and includes the verb phrase. In our sentence, the predicate is the phrase “sells” • Object. In grammar this is a noun that is acted upon by the verb. In our sentence, the object is the noun “batteries.”

15. XML vs. RDF • RDF is not just an XML dialect. • XML: • Has a tree structure data model. • Only nodes are labeled. • RDF: • Has a graph structure data model. • Both edges (properties) and nodes (subjects/objects) are labeled.

16. CE Ganji http://ce.sharif.edu Sharif Linking Statements • The subject of one statement can be the object of another • Such collections of statements form a directed, labeled graph studentOF departmentOF hasHomePage

17. RDF Graph: ‘anonymous’ nodes Person PersonName Literal Person12345 person.name value Jonathan first last value Borden

18. Using XPointer to name Person PersonName Literal Person12345 person.name value Jonathan /1/1/1 first /1/1 last value /1/1/2 Borden

19. How can RDF be implemented • Usually RDF/XML syntax • However other notations are possible • e.g. Notation3: • Buddy Belden owns a business. • The business has a Web site accessible at http://www.c2i2.com/~budstv. • Buddy is the father of Lynne. • <#Buddy> <#owns> <#business>. • <#business> <#has-website> <http://www.c2i2.com/~budstv>. • <#Buddy> <#father-of> <#Lynne>.

20. Converting N3 to RDF • Jena toolkit can do such conversion

21. XML Syntax for RDF • RDF has an XML syntax that has a specific meaning: • Every Description element describes a resource • Every attribute or nested element inside a Description is apropertyof that Resource • We can refer to resources by using URIs <rdf:Description about="some.uri/person/ganji"> <studentOf resource="some.uri/Sharif/CE"/> </Description> <Description about="some.uri/Sharif/CE"> <hasHomePage>http://ce.sharif.edu</hasHomePage> <departmentOf resource="some.uri/~Sharif"/> </rdf:Description>

22. RDF type • RDF predifined property • Its value – a resource that represent a category or class • Its subject – Instance of that category or class prefix ex: URI: http://www.example.org/terms

23. Containers • Containers are collections • they allow grouping of resources (or literal values) • It is possible to make statements about the container (as a whole) or about its members individually • It is also possible to create collections based on URI patterns • for example, all files in a particular web site

24. RDF containers • Bag: (A resource having type rdf:Bag) • Represents an unordered list of resources or literals • Duplicated values are prermitted • Sequence: (A resource having type rdf:Seq) • Represents ordered list of resources or literal • Duplicated values are permitted • Alternatives: (A resource having type rdf:Alt) • Represents group of resources or literals that are alternatives

25. http://www.w3.org/TR/REC-rdf-syntax dc:Creator rdf:Type rdf:Seq rdf:_1 rdf:_2 “Ora Lassila” “Ralph Swick” Sequence example

26. Bag example

27. RDF reification • association of a statement and a specific resource representing the statement • used to make statements about statements • Vocabulary: • type rdf:asserts • properties • rdf:subject • rdf:predicate • rdf:object

28. Reification example • In N3:

29. Reification example (cont.) • In RDF:

30. Reification example (cont.) • RDF Graph (by IsaViz):

31. RDF Schema (RDFS) • RDF gives a formalism for meta data annotation, and a way to write it down in XML, but it does not give any special meaning to vocabulary such as subClassOf or type • Interpretation is an arbitrary binary relation • RDF Schema allows you to define vocabulary terms and the relations between those terms • it gives “extra meaning” to particular RDF predicates and resources • this “extra meaning”, or semantics, specifies how a term should be interpreted

32. Core Classes & Properties rdfs:Resource rdfs:Literal rdfs:XMLLiteral rdfs:Class rdfs:Property Core Classes rdfs:Type rdfs:SubClassOf rdfs:SubPropertyOf rdfs:Domain rdfs:Range rdfs:Label rdfs:Comment Core Properties

33. RDFS Examples <Person,type,Class> <hasColleague,type,Property> <Professor,subClassOf,Person> <Carole,type,Professor> <hasColleague,range,Person> <hasColleague,domain,Person>

34. RDF/RDFS “Liberality” • No distinction between classes and instances (individuals) <Species,type,Class> <Lion,type,Species> <Leo,type,Lion> • Properties can themselves have properties <hasDaughter,subPropertyOf,hasChild> <hasDaughter,type,familyProperty> • No distinction between language constructors and ontology vocabulary, so constructors can be applied to themselves/each other <type,range,Class> <Property,type,Class> <type,subPropertyOf,subClassOf>

35. Problems with RDFS • RDFS too weak to describe resources in sufficient detail • No localised range and domain constraints • Can’t say that the range of hasChild is person when applied to persons and elephant when applied to elephants • No existence/cardinality constraints • Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents • No transitive, inverse or symmetrical properties • Can’t say that isPartOf is a transitive property, that hasPart is the inverse of isPartOf or that touches is symmetrical • … • Difficult to provide reasoning support • No “native” reasoners for non-standard semantics • May be possible to reason via FO axiomatisation

36. RDF(S) tools • Read RDF data • Parsers: Jena, Redland, SWI-Prolog • Validators: W3C RDF validation service • Editors: IsaViz, RDF Author, RDFEd, InferEd • Store RDF data (XML format, tripples or relational/oo DB) • Sesame, RSSDB, RDFLib • Use RDF data (applications, RSS news, etc.) • Manipulate RDF data (inference, query, etc.) • Jena RDQL, etc. • Example: SELECT ?person, ?knows WHERE (?x <http://xmlns.com/foap/knows> ?z), (?x <http://xmlns.com/foap/name> ?person), (?z <http://xmlns.com/foap/name> ?knows)

37. RDF Validators • RDF Validation Service • http://www.w3.org/RDF/Validator/ • In general all the RDF parsers do some kind of validation

38. References • RDF Resource Guide: • http://www.ilrt.bris.ac.uk/discovery/rdf/resources/ • http://www.w3.org/RDF • http://www.w3.org/RDF/Validator/ • Chapter 5 of the book