1. RDF & RDF Schema Machine Understandable Metadata for the Web
Semantic Web - Spring 2007
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 Many types/forms of metadata, dependent on role:
Information about information
Structured data about data
Many types/forms of metadata, dependent on role:

7. Types of Metadata: Web Resource discovery Document management
administration
(Intellectual)
property rights
management
Archival
information /
status
Security &
User
authentication
Content
ratings (PICS)
Product &
Services
Descriptions
Process
description &
control
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 (cont.)
The XML tags can often add meaning to data, however, actually understanding the tags is meaningful only to humans.
For example:
<book>
<title>G¨odel, Escher, Bach: An Eternal Golden Braid<title>
</book>
A human might infer that: “The book has the title G¨odel, Escher, Bach: An Eternal Golden Braid.”
A machine, however, could not make this inference based upon the XML alone.
For machines to do more automatically, it is necessary to go beyond the notion of the HTML display model, or XML data model, toward a “meaning”

11. RDF (cont.)
RDF is an assertional language intended to be used to express propositions using precise formal vocabularies.
It is intended to provide a basic foundation for more advanced assertional languages with a similar purpose
The overall design goals emphasise generality and precision in expressing propositions about any topic, rather than conformity to any particular processing model
An RDF document can delineate precise relationships between vocabulary items by constructing a grammatical representation.
Assertions in different RDF documents can be combined to provide far more information together than they could separately.

12. RDF in SW Architecture

13. 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
Predicate
Statement

14. Example
“The book [subject] has the title [predicate] G¨odel, Escher, Bach: An Eternal Golden Braid [object].”
This can be represented as the triple:
(The book, has the title, G¨odel, Escher, Bach: An Eternal Golden Braid).

15. Example (generated by RDFPic)

16. Example shown in triples view

17. 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.”

18. XML vs. RDF RDF is not just an XML dialect. XML: RDF:
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.

19. Linking Statements
The subject of one statement can be the object of another
Such collections of statements form a directed, labeled graph
studentOF CE
Ganji
departmentOF
hasHomePage
Sharif

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

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

22. 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
Buddy is the father of Lynne.
<#Buddy> <#owns> <#business>.
<#business> <#has-website> <
<#Buddy> <#father-of> <#Lynne>.

23. Converting N3 to RDF
Jena toolkit can do such conversion

24. 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 a property of 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>
<departmentOf resource="some.uri/~Sharif"/>
</rdf:Description>

25. RDF document parts

26. 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:

27. 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
Simple introduction to containers. Can be skipped if pressed with time.

28. 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

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

30. Bag example

31. 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

32. Reification example
In N3:

33. Reification example (cont.)
In RDF:

34. Reification example (cont.)
RDF Graph (by IsaViz):

35. 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

36. 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

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

38. 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>

39. RDF and RDFS Layers

40. Friend of a Friend (FOAF) application
Many communities, such as companies, professional organizations, and social groupings, have proliferated on the Internet.
The Friend of a Friend (FOAF) RDF vocabulary, originated by Dan Brickley and Libby Miller, gives a basic expression for community membership.
The FOAF project describes people and their basic properties, such as name, address, and so on.
Friend of a Friend allows the expression of personal information and relationships.
As a result, search engines can find people with similar interests through FOAF.
Friend of a Friend is simply an RDF vocabulary. You can create FOAF files on your Web server and share the URLs so that software can use the information.
The creation of FOAF data is decentralized since it allows many to contribute independently.

41. FOAF example

42. Rich Site Summary (RSS)
A form of publishing content such that machines can process them.
One dialect is based on RDF
Very popular for news sites
Tools like RSS reader can syndicate news and bring them to the desktop

43. 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

44. 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 < ?z),
(?x < ?person),
(?z < ?knows)

45. RDF Validators RDF Validation Service
In general all the RDF parsers do some kind of validation

46. References RDF Resource Guide: http://www.w3.org/RDF/Validator/
Chapter 5 of the book