RDF & RDFS - 웹정보시스템

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1 - - RDF & RDFS Sang-goo Lee School of Computer Science & Engineering Center for E-Business Technology Seoul National University, Seoul, Korea Center for E-Business Technology Seoul National University Seoul, Korea Copyright 2008~2009 SNU Center for E-Business Technology. All rights reserved.

2 The Semantic Web A vision for the World Wide Web Ubiquitous Web: Web for everything & everyone Safe Web: Web of trust and confidence Semantic Web: Web of knowledge The Semantic Web (Web of Data) An extension of the current Web in which information is given welldefined meaning, better enabling computers and people to work in cooperation. T. Berners-Lee, 2001 Current Web: documents for people Semantic Web: data for computer programs RDF & RDFS - 2

3 Web Nodes & Links [Steve Bratt, W3C] RDF & RDFS - 3

4 Web Nodes & Links to a computer! [Steve Bratt, W3C] RDF & RDFS - 4

5 A Semantic Web convener meeting leader place PR website html [Steve Bratt, W3C] RDF & RDFS - 5

6 Ontology The study of being or existence forms the basic subject matter of metaphysics The study of the nature and structure of what is possible Ontology has one basic question: "What is there?" Some other questions in ontology What is existence? Is existence a property? What is a physical object? What are an object's properties or relations and how are they related to the object itself? Aristoteles by Francesco Hayez (1811) [Wikipedia] RDF & RDFS - 6

7 Ontology in Computer Science in Wikipedia a data model that represents a set of concepts within a domain and the relationships between those concepts. It is used to reason about the objects within that domain. Purpose enable computers to deal with semantics (meaning) An ontology language usually introduces concepts (aka classes, entities), properties of concepts (aka slots, attributes, roles), relationships between concepts (aka associations), and additional constraints. Related technology RDF, OWL, DL, RDF & RDFS - 7

8 XML Structure of XML Data XML Document Schema Querying and Transformation

9 extensible Markup Language Defined by the WWW Consortium (W3C) Originally intended as a document markup language not a database language Documents have tags giving extra information about sections of the document E.g. <title> XML </title> <slide> Introduction </slide> Derived from SGML (Standard Generalized Markup Language), but simpler to use SGML is too complex => nobody uses it! Goal was (is?) to replace HTML as the language for publishing documents on the Web RDF & RDFS - 9

10 HTML vs XML Extensible unlike HTML users can add new tags, and separately specify how the tag should be handled for display HTML is not fit for data exchange HTML tags are predefined mainly deal with how to handle the display of texts and images HTML tags do not annotate the meaning of the data Example: <H1> aaa </H1> specifies that aaa should appear as header in the display RDF & RDFS - 10

11 HTML vs XML Example HTML <H1>Invoice</H1> <B>From: PC World </B><P> To: SGLee <P> Date: 2001/11/16 <P> Amount: $100 <P> XML <Invoice> <From>PC World </From> <To> SGLee </To> <Date year= 2001 month= 11 day= 16 /> <Amount unit= dollars > 100</Amount> </Invoice> RDF & RDFS - 11

12 XML, HTML, & SGML subset SGML instance DTD HTML XML instance DTD / Schema Order Invoice Transcode XSL Display on browser RDF & RDFS - 12

13 Structure of XML Data Tag label for a section of data Element: section of data beginning with <tagname> and ending with matching </tagname> Elements must be properly nested Proper nesting <account> <balance> </balance> </account> Improper nesting <account> <balance> </account> </balance> Formally: every start tag must have a unique matching end tag, that is in the context of the same parent element. Every document must have a single top-level element RDF & RDFS - 13

14 XML Data Example <bank-1> <customer> <customer-name> Hayes </customer-name> <customer-street> Main </customer-street> <customer-city> Harrison </customer-city> <account> <account-number> A-102 </account-number> <branch-name> Perryridge </branch-name> <balance> 400 </balance> </account> <account> </account> </customer>.. </bank-1> RDF & RDFS - 14

15 Attributes Elements can have attributes <account acct-type = checking > <account-number> A-102 </account-number> <branch-name> Perryridge </branch-name> <balance> 400 </balance> </account> Attributes are specified by name=value pairs inside the starting tag of an element An element may have several attributes, but each attribute name can only occur once <account acct-type = checking monthly-fee= 5 > RDF & RDFS - 15

16 Attributes Vs. Subelements In the context of documents attributes are part of markup, subelement contents are part of the basic document contents In the context of data representation the difference is unclear and may be confusing Same information can be represented in two ways OR <account account-number = A-101 > </account> <account> <account-number>a-101</account-number> </account> Suggestion: Use attributes for identifiers of elements, and use subelements for contents RDF & RDFS - 16

17 XML Document Schema Database schemas constrain what information can be stored, and the data types of stored values XML documents are not required to have an associated schema However, schemas are very important for XML data exchange Otherwise, a site cannot automatically interpret data received from another site Two mechanisms for specifying XML schema Document Type Definition (DTD) Original spec. Simpler but limited XML Schema Newer, more complicated but with more features RDF & RDFS - 17

18 Document Type Definition (DTD) The type of an XML document can be specified using a DTD DTD constraints structure of XML data What elements can occur What attributes can/must an element have What subelements can/must occur inside each element, and how many times. DTD does not constrain data types All values represented as strings in XML DTD syntax <!ELEMENT element (subelements-specification) > <!ATTLIST element (attributes) > RDF & RDFS - 18

19 Bank Element <!DOCTYPE bank [ ]> <!ELEMENT bank ( ( account customer depositor)+)> <!ELEMENT account (account-number branch-name balance)> <!ELEMENT customer(customer-name customerstreet customer-city)> <!ELEMENT depositor (customer-name accountnumber)> <!ELEMENT account-number (#PCDATA)> <!ELEMENT branch-name (#PCDATA)> <!ELEMENT balance (#PCDATA)> <!ELEMENT customer-name (#PCDATA)> <!ELEMENT customer-street (#PCDATA)> <!ELEMENT customer-city (#PCDATA)> <bank> <account> <account_number> A-101 </account_number> <branch_name> Downtown </branch_name> <balance> 500 </balance> </account> <customer> <customer_name> Johnson </customer_name> <customer_street> Alma </customer_street> <customer_city> Palo Alto </customer_city> </customer> <depositor> <account_number> A101 </account_number> <customer_name> Johnson </customer_name> <depositor> </bank> RDF & RDFS - 19

20 XML Schema XML Schema is a more sophisticated schema language Addresses the drawbacks of DTDs. Typing of values E.g. integer, string, etc Also, constraints on min/max values User defined types Is itself specified in XML syntax, unlike DTDs More standard representation, but verbose Is integrated with namespaces Many more features List types, uniqueness and foreign key constraints, inheritance.. Significantly more complicated than DTDs but with more power RDF & RDFS - 20

21 XML Schema Version of Bank DTD <xsd:schema xmlns:xsd= <xsd:element name= bank type= BankType /> <xsd:element name= account > <xsd:complextype> <xsd:sequence> <xsd:element name= account-number type= xsd:string /> <xsd:element name= branch-name type= xsd:string /> <xsd:element name= balance </xsd:squence> </xsd:complextype> </xsd:element>.. definitions of customer and depositor. <xsd:complextype name= BankType > <xsd:squence> <xsd:element ref= account type= xsd:decimal /> minoccurs= 0 maxoccurs= unbounded /> <xsd:element ref= customer minoccurs= 0 maxoccurs= unbounded /> <xsd:element ref= depositor minoccurs= 0 maxoccurs= unbounded /> </xsd:sequence> </xsd:complextype> </xsd:schema> RDF & RDFS - 21

22 Namespaces XML data has to be exchanged between organizations Same tag name may have different meaning in different organizations, causing confusion on exchanged documents Solution Using (globally) unique element names avoids confusion Two part solution: unique-name:element-name Avoid using long unique names all over document by using XML namespaces (kind of an) abbreviated prefix <bank xmlns:fb= > <FB:branch> <FB:branchname>Downtown</FB:branchname> <FB:branchcity> Brooklyn </FB:branchcity> </FB:branch> </bank> RDF & RDFS - 22

23 Querying and Transforming XML Data Translation of information from one XML schema to another Querying on XML data Above two are closely related, and handled by the same tools Standard XML querying/translation languages XSLT Simple language designed for translation from XML to XML and XML to HTML XPath Simple language consisting of path expressions XQuery An XML query language with a rich set of features RDF & RDFS - 23

24 Tree Model of XML Data An XML document is modeled as a tree with nodes corresponding to elements and attributes Query and transformation languages are based on a tree model of XML data Element nodes have child nodes which can be attributes or subelements Text in an element is modeled as a text node child of the element Children of a node are ordered according to their order in the XML document Element and attribute nodes (except for the root node) have a single parent, which is an element node The root node has a single child which is the root element of the document RDF & RDFS - 24

25 XPath Select parts of documents using path expressions A path expression is a sequence of steps separated by / Similar to a directory hierarchy Result of path expression E.g. set of values that match the specified path (along with their containing elements/attributes) /bank/customer/customer_name evaluated on the data shown in figure 10.1 returns <customer_name> Johnson </customer_name> <customer_name> Hayes </customer_name> /bank/customer/customer_name/text( ) returns the same names, but without the enclosing tags RDF & RDFS - 25

26 XPath (Cont.) The initial / denotes root of the document (above the top-level tag) Path expressions are evaluated left to right Each step operates on the set of instances produced by the previous step Selection predicates may follow any step in a path, in [ ] E.g. /bank/account[balance > 400] returns account elements with a balance value greater than 400 /bank/account[balance] returns account elements containing a balance subelement RDF & RDFS - 26

27 XSLT Stylesheet stores formatting options for a document usually separately from document HTML style sheet may specify font colors and sizes for headings, etc. XML Stylesheet Language (XSL) A general-purpose XML transformation language was originally designed for generating HTML from XML Can translate XML to XML, as well as XML to HTML XSLT transformations are expressed using rules called templates Templates combine selection using XPath with construction of results RDF & RDFS - 27

28 XSLT Example <xsl:template match= /bank/customer > <name> <xsl:value-of select= customer_name /> </name> </xsl:template> <xsl:template match= * /> Result: <name> Johnson </name> <name> Hayes </name> RDF & RDFS - 28

29 RDF

30 Basic Ideas of RDF Basic building block: object-attribute-value triple It is called a statement RDF has been given a syntax in XML This syntax inherits the benefits of XML Other syntactic representations of RDF possible <course name="discrete Maths"> <lecturer>david Billington</lecturer> </course> The fundamental concepts of RDF are: resources properties statements RDF & RDFS - 30

31 Resources A resource is an object, a thing we want to talk about E.g. authors, books, publishers, places, people, hotels Every resource has a URI, a Universal Resource Identifier A URI can be a URL (Web address) or some other kind of unique identifier Advantages of using URIs: Α global, worldwide, unique naming scheme Reduces the homonym problem of distributed data representation RDF & RDFS - 31

32 Properties Properties are a special kind of resources They describe relations between resources E.g. written by, age, title, etc. Properties are also identified by URIs Statements Statements assert the properties of resources A statement is an object-attribute-value triple It consists of a resource, a property, and a value Values can be resources or literals Literals are atomic values (strings) RDF & RDFS - 32

33 Three Views of a Statement A triple A pair of linked nodes An XML element Thus an RDF document can be viewed as: A set of triples A graph (semantic net) An XML document RDF & RDFS - 33

34 Statements as Triples ( David Billington, The triple (x,p,y) can be considered as a logical formula P(x,y) Binary predicate P relates object x to object y RDF offers only binary predicates (properties) RDF & RDFS - 34

35 Statements as a Piece of Graph A directed graph with labeled nodes and arcs from the resource (the subject of the statement) to the value (the object of the statement) Known in AI as a semantic net The value of a statement may be a resource Ιt may be linked to other resources RDF & RDFS - 35

36 Statements in XML Syntax Graphs are a powerful tool for human understanding but The Semantic Web vision requires machine-accessible and machine-processable representations There is a 3rd representation based on XML But XML is not a part of the RDF data model E.g. serialisation of XML is irrelevant for RDF <rdf:rdf xmlns:rdf=" xmlns:mydomain=" <rdf:description rdf:about=" <mydomain:site-owner> David Billington </mydomain:site-owner> </rdf:description> </rdf:rdf> 36 RDF & RDFS - 36

37 Statements in XML An RDF document is represented by an XML element with the tag rdf:rdf The content of this element is a number of descriptions, which use rdf:description tags. Every description makes a statement about a resource, identified in 3 ways: an about attribute, referencing an existing resource an ID attribute, creating a new resource without a name, creating an anonymous resource RDF & RDFS - 37

38 RDF Examples DB Table on CDs Title Artist 제조국 레이블 가격 발매일 The Sentimental Cord SG Wanna Be+ Korea Poibos 11, Come Away With Me Norah Jones USA EMI 15, <?xml version="1.0"?> <rdf:rdf xmlns:rdf=" / / 02/ 22- rdf- syntax- ns# " xmlns:cd=" / cd# "> <rdf:description rdf:about=" / cd/ SGWannaBe# 4"> <cd:artist cd:artist>sg Wanna Be+</ cd:artist> <cd cd: 제조국 >Korea</ cd: 제조국 > <cd cd: 레이블 >Poibos Poibos</ cd: 레이블 > <cd cd: 가격 >11,900</ cd: 가격 > <cd cd: 발매일 >2007</ cd: 발매일 > </ rdf:description> <rdf:description rdf:about=" / cd/ Come Away With Me"> <cd:artist cd:artist>norah Jones</ cd:artist> <cd cd: 제조국 >USA</ cd: 제조국 > <cd cd: 레이블 >EMI</ cd: 레이블 > <cd cd: 가격 >15,700</ cd: 가격 > <cd cd: 발매일 >2004</ cd: 발매일 > </ rdf:description>... </ rdf:rdf> RDF & RDFS - 38

39 Example of University Courses <rdf:rdf xmlns:rdf=" xmlns:xsd=" xmlns:uni=" <rdf:description rdf:about="949318"> <uni:name>david Billington</uni:name> <uni:title>associate Professor</uni:title> <uni:age rdf:datatype="&xsd:integer">27<uni:age> </rdf:description> <rdf:description rdf:about="cit1111"> <uni:coursename>discrete Maths</uni:courseName> <uni:istaughtby>david Billington</uni:isTaughtBy> </rdf:description> <rdf:description rdf:about="cit2112"> <uni:coursename>programming III</uni:courseName> <uni:istaughtby>michael Maher</uni:isTaughtBy> </rdf:description> </rdf:rdf> RDF & RDFS - 39

40 Introducing Structure to RDF: rdf:type <rdf:description rdf:id="cit1111"> <rdf:type rdf:resource=" <uni:coursename>discrete Maths</uni:courseName> <uni:istaughtby rdf:resource="#949318"/> </rdf:description> <rdf:description rdf:id="949318"> <rdf:type rdf:resource=" <uni:name>david Billington</uni:name> <uni:title>associate Professor</uni:title> </rdf:description> RDF & RDFS - 40

41 Abbreviated Syntax <rdf:description rdf:id="cit1111" uni:coursename="discrete Maths"> <rdf:type rdf:resource=" <uni:istaughtby rdf:resource="#949318"/> </rdf:description> <uni:course rdf:id="cit1111" uni:coursename="discrete Maths"> <uni:istaughtby rdf:resource="#949318"/> </uni:course> RDF & RDFS - 41

42 Reification Sometimes we wish to make statements about other statements We must be able to refer to a statement using an identifier RDF allows such reference through a reification mechanism which turns a statement into a resource <rdf:description rdf:about="#949352"> </rdf:description> reifies as <uni:name>grigoris Antoniou</uni:name> <rdf:statement rdf:id="statementabout949352"> </rdf:statement> <rdf:subject rdf:resource="#949352"/> <rdf:predicate rdf:resource=" uni-ns#name"/> <rdf:object>grigoris Antoniou</rdf:object> RDF & RDFS - 42

43 RDF Schema (RDFS)

44 Basic Ideas of RDF Schema RDF is a universal language that lets users describe resources in their own vocabularies RDF does not assume, nor does it define semantics of any particular application domain The user can do so in RDF Schema using: Classes and Properties Class Hierarchies and Inheritance Property Hierarchies RDF & RDFS - 44

45 Classes and their Instances We must distinguish between Concrete things (individual objects) in the domain: Discrete Maths, David Billington etc. Sets of individuals sharing properties called classes: lecturers, students, courses etc. Individual objects that belong to a class are referred to as instances of that class The relationship between instances and classes in RDF is through rdf:type RDF & RDFS - 45

46 Class Hierarchies Classes can be organised in hierarchies A is a subclass of B if every instance of A is also an instance of B Then B is a superclass of A A subclass graph need not be a tree A class may have multiple superclasses RDF & RDFS - 46

47 Inheritance in Class Hierarchies Range restriction: Courses must be taught by academic staff members only Michael Maher is a professor He inherits the ability to teach from the class of academic staff members This is done in RDF Schema by fixing the semantics of is a subclass of It is not up to an application (RDF processing software) to interpret is a subclass of RDF & RDFS - 47

48 Property Hierarchies Hierarchical relationships for properties E.g., is taught by is a subproperty of involves If a course C is taught by an academic staff member A, then C also involves Α The converse is not necessarily true E.g., A may be the teacher of the course C, or a tutor who marks student homework but does not teach C P is a subproperty of Q, if Q(x,y) is true whenever P(x,y) is true RDF & RDFS - 48

49 RDF Layer vs RDF Schema Layer (2) RDF & RDFS - 49

50 RDF Schema in RDF The modeling primitives of RDF Schema are defined using resources and properties (RDF itself is used!) To declare that lecturer is a subclass of academic staff member Define resources lecturer, academicstaffmember, and subclassof define property subclassof Write triple (subclassof,lecturer,academicstaffmember) We use the XML-based syntax of RDF RDF & RDFS - 50

51 Core Classes rdfs:resource, the class of all resources rdfs:class, the class of all classes rdfs:literal, the class of all literals (strings) rdf:property, the class of all properties. rdf:statement, the class of all reified statements <rdfs:class rdf:about="#lecturer"> <rdfs:subclassof rdf:resource="#staffmember"/> </rdfs:class> RDF & RDFS - 51

52 Core Properties rdf:type, which relates a resource to its class The resource is declared to be an instance of that class rdfs:subclassof, which relates a class to one of its superclasses All instances of a class are instances of its superclass rdfs:subpropertyof, relates a property to one of its superproperties rdfs:domain, which specifies the domain of a property P The class of those resources that may appear as subjects in a triple with predicate P If the domain is not specified, then any resource can be the subject rdfs:range, which specifies the range of a property P The class of those resources that may appear as values in a triple with predicate P RDF & RDFS - 52

53 Example: A University <rdfs:class rdf:id="lecturer"> <rdfs:subclassof rdf:resource="#academicstaffmember"/> </rdfs:class> <rdfs:class rdf:id="course"> <rdfs:comment>the class of courses</rdfs:comment> </rdfs:class> <rdf:property rdf:id= involves"> <rdfs:domain rdf:resource="#course"/> <rdfs:range rdf:resource= #lecturer"/> </rdf:property> <rdf:property rdf:id="istaughtby"> <rdfs:comment> Inherits its domain ("course") and range ("lecturer") from its superproperty "involves" </rdfs:comment> <rdfs:subpropertyof rdf:resource="#involves"/> </rdf:property> RDF & RDFS - 53

54 Subclass Hierarchy of Primitives of RDFS RDF & RDFS - 54

55 Instance Relationships RDF & RDFS - 55

56 Instance Relationships - Core Properties of RDF and RDFS RDF & RDFS - 56

57 The Namespace of RDF <rdfs:class rdf:id="statement" rdfs:comment="the class of triples consisting of a predicate, a subject and an object (that is, a reified statement)"/> <rdfs:class rdf:id="property" rdfs:comment="the class of properties"/> <rdfs:class rdf:id="bag" rdfs:comment="the class of unordered collections"/> <rdf:property rdf:id="predicate" </rdf:property> rdfs:comment="identifies the property of a statementin reified form"/> <rdfs:domain rdf:resource="#statement"/> <rdfs:range rdf:resource="#property"/> RDF & RDFS - 57

58 The Namespace of RDF Schema <rdfs:class rdf:id="resource" rdfs:comment="the most general class"/> <rdfs:class rdf:id="class" rdfs:comment="the concept of classes. All classes are resources"/> <rdfs:subclassof rdf:resource="#resource"/> </rdfs:class> <rdf:property rdf:id="subpropertyof"> <rdfs:domain rdf:resource= " <rdfs:range rdf:resource= " </rdf:property> <rdf:property rdf:id="subclassof"> <rdfs:domain rdf:resource="#class"/> <rdfs:range rdf:resource="#class"/> </rdf:property> RDF & RDFS - 58

59 Namespace versus Semantics Consider rdfs:subclassof The namespace specifies only that it applies to classes and has a class as a value The meaning of being a subclass not expressed The meaning cannot be expressed in RDF If it could RDF Schema would be unnecessary External definition of semantics required Respected by RDF/RDFS processing software RDF & RDFS - 59

60 Axiomatic Semantics We formalize the meaning of the modeling primitives of RDF and RDF Schema By translating into first-order logic We make the semantics unambiguous and machine accessible We provide a basis for reasoning support by automated reasoners manipulating logical formulas RDF & RDFS - 60

61 Basic Predicates PropVal(P,R,V) A predicate with 3 arguments, which is used to represent an RDF statement with resource R, property P and value V An RDF statement (triple) (P,R,V) is represented as PropVal(P,R,V). Type(R,T) Short for PropVal(type,R,T) Specifies that the resource R has the type T Type(?r,?t) PropVal(type,?r,?t) RDF & RDFS - 61

62 RDF Classes Constants: Class, Resource, Property, Literal All classes are instances of Class Type(Class,Class) Type(Resource,Class) Type(Property,Class) Type(Literal,Class) Resource is the most general class: every class and every property is a resource Type(?p,Property) Type(?p,Resource) Type(?c,Class) Type(?c,Resource) The predicate in an RDF statement must be a property PropVal(?p,?r,?v) Type(?p,Property) RDF & RDFS - 62

63 The type Property type is a property PropVal(type,type,Property) type can be applied to resources (domain) and has a class as its value (range) Type(?r,?c) (Type(?r,Resource) Type(?c,Class)) RDF & RDFS - 63

64 The Auxiliary FuncProp Property P is a functional property if, and only if, it is a property, and there are no x, y1 and y2 with P(x,y1), P(x,y2 ) and y1 y2 Type(?p, FuncProp) (Type(?p, Property)?r?v1?v2 (PropVal(?p,?r,?v1) PropVal(?p,?r,?v2)?v1 =?v2)) RDF & RDFS - 64

65 Subclass subclassof is a property: Type(subClassOf,Property) If a class C is a subclass of a class C', then all instances of C are also instances of C': PropVal(subClassOf,?c,?c') (Type(?c,Class) Type(?c',Class)?x (Type(?x,?c) Type(?x,?c'))) RDF & RDFS - 65

66 Subproperty P is a subproperty of P', if P'(x,y) is true whenever P(x,y) is true: Type(subPropertyOf,Property) PropVal(subPropertyOf,?p,?p') (Type(?p,Property) Type(?p',Property)?r?v (PropVal(?p,?r,?v) PropVal(?p',?r,?v))) RDF & RDFS - 66

67 Domain and Range If the domain of P is D, then for every P(x,y), x D PropVal(domain,?p,?d)?x?y (PropVal(?p,?x,?y) Type(?x,?d)) If the range of P is R, then for every P(x,y), y R PropVal(range,?p,?r)?x?y (PropVal(?p,?x,?y) Type(?y,?r)) RDF & RDFS - 67

68 Semantics based on Inference Rules Semantics in terms of RDF triples instead of restating RDF in terms of first-order logic and sound and complete inference systems This inference system consists of inference rules of the form: IF E contains certain triples THEN add to E certain additional triples where E is an arbitrary set of RDF triples RDF & RDFS - 68

69 Examples of Inference Rules IF E contains the triple (?x,?p,?y) THEN E also contains (?p,rdf:type,rdf:property) IF E contains the triples (?u,rdfs:subclassof,?v) and (?v,rdfs:subclassof,?w) THEN E also contains the triple (?u,rdfs:subclassof,?w) IF E contains the triples (?x,rdf:type,?u) and (?u,rdfs:subclassof,?v) THEN E also contains the triple (?x,rdf:type,?v) RDF & RDFS - 69

70 Examples of Inference Rules (2) Any resource?y which appears as the value of a property?p can be inferred to be a member of the range of?p This shows that range definitions in RDF Schema are not used to restrict the range of a property, but rather to infer the membership of the range IF E contains (?x,?p,?y) and (?p,rdfs:range,?u) THEN E also contains the triple (?y,rdf:type,?u) RDF & RDFS - 70

71 Summary RDF provides a foundation for representing and processing metadata RDF has a graph-based data model RDF has an XML-based syntax to support syntactic interoperability. XML and RDF complement each other because RDF supports semantic interoperability RDF has a decentralized philosophy and allows incremental building of knowledge, and its sharing and reuse RDF & RDFS - 71

72 Summary (2) RDF is domain-independent RDF Schema provides a mechanism for describing specific domains RDF Schema is a primitive ontology language It offers certain modelling primitives with fixed meaning Key concepts of RDF Schema are class, subclass relations, property, subproperty relations, and domain and range restrictions There exist query languages for RDF and RDFS RDF & RDFS - 72

73 RDF, RDFS Center for E-Business Technology Seoul National University Seoul, Korea Copyright 2008~2009 SNU Center for E-Business Technology. All rights reserved.