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RDF: Resource Description Framework

RDF: Resource Description Framework. Jianguo Lu University of Windsor. Contents. Ontology Syntax of RDF and RDFS Axiomatic Semantics for RDF and RDFS Applications of RDF and RDFS Querying of RDF/RDFS Documents using RQL. Web Today. [Hendler & Miller 02]. The semantic web. Ontology.

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RDF: Resource Description Framework

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  1. RDF: Resource Description Framework Jianguo Lu University of Windsor

  2. Contents • Ontology • Syntax of RDF and RDFS • Axiomatic Semantics for RDF and RDFS • Applications of RDF and RDFS • Querying of RDF/RDFS Documents using RQL

  3. Web Today [Hendler & Miller 02]

  4. The semantic web

  5. Ontology • Long history coming from Philosophy, introduced by Aristotle • “a branch of metaphysics concerned with the nature and relations of being” --Webster • Picked up by the Artificial Intelligence • “For AI systems, what ‘exists’ is that which can be represented” • “a shared and common understanding of some domain that can be communicated between people and application systems” – Gruber

  6. What is Ontology • An ontology is a formal, explicit specification of a shared conceptualization - Gruber • Conceptualizationrefers to an abstract model of phenomena. • Explicit means that the type of concepts used, and the constraints on their use are explicitly defined. • Formal refers to the fact that the ontology should be machine readable. • Shared reflects that ontology should capture consensual knowledge accepted by the communities.

  7. Main components of an Ontology • Classes: concepts of the domain or tasks, which are usually organized in taxonomies • in a university ontology, student and professor are two classes • Relations: a type of interaction between concepts of the domain • such as: subclass-of, is-a • Axioms: model sentences that are always true • such as: if the student attends both A and B course, then he or she must be a second year student • Instances: to represent specific elements • such as: a student called Peter is the instance of Student class

  8. Contents • Ontology • Syntax of RDF and RDFS • Semantics • Applications

  9. RDF is a Web Standard • RDF Model and Syntax Specification became a W3C (World Wide Web Consortium) Recommendation in February 1999. • It has a long history, considering that XML became W3C Recommendation in 1998. • The purpose of RDF (Resource Description Framework) is to give a standard way of specifying data "about" something.

  10. XML vs. RDF <company> <treatmentOffered>Physiotherapy</treatmentOffered> <companyName>Agilitas Physiotherapy Centre </companyName> <staff> <therapist>Lisa Davenport</therapist> <therapist>Steve Matthews</therapist> <secretary>Kelly Townsend</secretary> </staff> … … </company> • How to answer the query “give me the list of staff members in this company”? • XML provides semantic information as a by-product of defining the structure of the document • XML prescribes a tree structure for documents and the different leaves of the tree have a well-defined tag and context the information can be understood with. • That is, structure and semantics of documents are interwoven • there is no intended meaning associated with the nesting of tags • It is up to each application to interpret the nesting.

  11. staff subClassOf subClassOf therapist secretary RDF and RDF Schema • RDF isA therapist Lisa • RDF Schema How to encode the rule that “a person can not be a secretary and therapist at the same time”?

  12. OWL (Web Ontology Language) isA therapist Lisa RDF isA secretary staff RDFS subClassOf therapist secretary OWL Disjoint (therapist, secretary)

  13. Basic Ideas of RDF • David Billington is a lecturer of Discrete Maths <course name="Discrete Maths"> <lecturer>David Billington</lecturer> </course> <lecturer name="David Billington"> <teaches>Discrete Maths</teaches> </lecturer> • Opposite nesting, same information! • Basic building block of rdf: subject-predicate-object triple • It is called a statement; • Sentence about Billington is such a statement; <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:mydomain="http://www.mydomain.org/my-rdf-ns#"> <rdf:Description rdf:about="http://www.cit.gu.edu.au/DiscreteMath"> <mydomain:taughtBy> David Billington </mydomain:taughtBy> </rdf:Description> </rdf:RDF>

  14. predicate Subject Object Equivalent! property Resource Value Statements • Statements assert the properties of resources • A statement is an resource-property-value triple • Sometimes also called subject, predicate, and value • Values can be resources or literals • Literals are atomic values (strings)

  15. Resource in a statement • We can think of a resource as 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. • In the following example, “http://www.cit.gu.edu.au/DiscreteMath” is a resource that we want to talk about. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:mydomain="http://www.mydomain.org/my-rdf-ns#"> <rdf:Description rdf:about="http://www.cit.gu.edu.au/DiscreteMath"> <mydomain:taughtBy> David Billington </mydomain:taughtBy> </rdf:Description> </rdf:RDF>

  16. Properties • Properties are a special kind of resources • They describe relations between resources • E.g. “taught by”, “written by”, “age”, “title”, etc.; • Properties are also identified by URIs. • Advantages of using URIs: • Α global, worldwide, unique naming scheme; • Reduces the homonym problem of distributed data representation. • <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" • xmlns:mydomain="http://www.mydomain.org/my-rdf-ns#"> • <rdf:Description rdf:about="http://www.cit.gu.edu.au/DiscreteMath"> • <mydomain:taughtBy> David Billington </mydomain:taughtBy> • </rdf:Description> • </rdf:RDF>

  17. Three Views of a Statement • A triple • A piece of a graph • A piece of XML code • Thus an RDF document can be viewed as: • A set of triples; • A graph (semantic net); • An XML document.

  18. RDF Example • The statement: http://www.example.org/index.html has a creation-date whose value is August 16, 1999 <?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:exterms="http://www.example.org/terms/"> <rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date>August 16, 1999</exterms:creation-date> </rdf:Description> </rdf:RDF>

  19. Several statements about the same resource

  20. Its XML representation • A complete representation <rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date>August 16, 1999</exterms:creation-date> </rdf:Description> <rdf:Description rdf:about="http://www.example.org/index.html"> <dc:language>en</dc:language> </rdf:Description> <rdf:Description rdf:about="http://www.example.org/index.html"> <dc:creator rdf:resource="http://www.example.org/staffid/85740"/> </rdf:Description> • An abbreviation <rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date> August 16, 1999 </exterms:creation-date> <dc:language> en </dc:language> <dc:creator rdf:resource="http://www.example.org/staffid/85740"/> </rdf:Description>

  21. Two different kinds of nodes • Nodes that are URIrefs are shown as ellipses • Nodes that are literals are shown as boxes • Using URIref to identify resource and property

  22. Structured Property Values and Blank Nodes Problem with this approach: may generate many intermediate URIRefs.

  23. Blank Node

  24. Blank node or anonymous resource

  25. Corresponding RDF/XML <?xml version="1.0"?> <rdf:RDF xmlns:rdf=“http://www.w3.org/1999/02/22-rdf-syntax-ns#” xmlns:dc=“http://purl.org/dc/elements/1.1/” xmlns:exterms="http://example.org/stuff/1.0/"> <rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar"> <dc:title>RDF/XML Syntax Specification (Revised)</dc:title> <exterms:editor rdf:nodeID="abc"/> </rdf:Description> <rdf:Description rdf:nodeID="abc"> <exterms:fullName>Dave Beckett</exterms:fullName> <exterms:homePage rdf:resource="http://purl.org/net/dajobe/"/> </rdf:Description> </rdf:RDF>

  26. Typed literals • 27 is an integer or a string?

  27. A Typed Literal for a Web Page's Creation Date <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:exterms="http://www.example.org/terms/"> <rdf:Description rdf:about="http://www.example.org/index.html"> <exterms:creation-date rdf:datatype="&xsd;date">1999-08-16 </exterms:creation-date> </rdf:Description> </rdf:RDF>

  28. An Invalid Typed Literal for John Smith's Age

  29. RDF Containers • Container is a resource to contain things • RDF defines three types of containers: • Bag: contains an unordered list of value elements • Seq: contains an ordered list of value elements • Alt: contains a list of alternative values for an element:

  30. Bag

  31. Bag (cont.) <rdf:Description rdf:about="http://example.org/courses/6.001"> <s:students> <rdf:Bag> <rdf:li rdf:resource="http://example.org/students/Amy"/> <rdf:li rdf:resource="http://example.org/students/Mohamed"/> <rdf:li rdf:resource="http://example.org/students/Johann"/> <rdf:li rdf:resource="http://example.org/students/Maria"/> <rdf:li rdf:resource="http://example.org/students/Phuong"/> </rdf:Bag> </s:students> </rdf:Description>

  32. Alt container

  33. Alt container (cont.) <rdf:Description rdf:about="http://example.org/packages/X11"> <s:DistributionSite> <rdf:Alt> <rdf:li rdf:resource="ftp://ftp.example.org"/> <rdf:li rdf:resource="ftp://ftp1.example.org"/> <rdf:li rdf:resource="ftp://ftp2.example.org"/> </rdf:Alt> </s:DistributionSite> </rdf:Description>

  34. RDF Collection • A container only says that certain resources are members • Id does not say that other members do not exist • <rdf:Description rdf:about="http://example.org/courses/6.001"> • <s:students rdf:parseType="Collection"> • <rdf:Description rdf:about="http://example.org/students/Amy"/> • <rdf:Description rdf:about="http://example.org/students/Mohamed"/> • <rdf:Description rdf:about="http://example.org/students/Johann"/> • </s:students> • </rdf:Description>

  35. RDF Reification • In RDF it is possible to make statements about statements • Grigoris believes that David Billington is the creator of http://www.cit.gu.edu.au/~db • Such statements can be used to describe belief or trust in other statements • The solution is to assign a unique identifier to each statement • It can be used to refer to the statement • Introduce an auxiliary object (e.g. belief1) • relate it to each of the 3 parts of the original statement through the properties subject, predicate and object • In the preceding example • subject of belief1 is David Billington • predicate of belief1 is creator • object of belief1 is http://www.cit.gu.edu.au/~db

  36. Reification example

  37. Reification in XML <?xml version="1.0"?> <!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc=“http://purl.org/dc/elements/1.1/” xmlns:exterms=“http://www.example.com/terms/” xml:base="http://www.example.com/2002/04/products"> <rdf:Description rdf:ID="item10245"> <exterms:weight rdf:datatype="&xsd;decimal">2.4 </exterms:weight> </rdf:Description> <rdf:Statement rdf:about="#triple12345"> <rdf:subject rdf:resource="http://www.example.com/2002/04/products#item10245"/> <rdf:predicate rdf:resource="http://www.example.com/terms/weight"/> <rdf:object rdf:datatype="&xsd;decimal">2.4</rdf:object> <dc:creator rdf:resource="http://www.example.com/staffid/85740"/> </rdf:Statement> </rdf:RDF>

  38. rdf:about vs rdf:ID • An element rdf:Descriptionhas • anrdf:about attribute indicating that the resource has been “defined” elsewhere • An rdf:ID attribute indicating that the resource is defined • Formally, there is no such thing as “defining” an object in one place and referring to it elsewhere • Sometimes it is useful (for human readability) to have a defining location, while other locations state “additional” properties.

  39. Connecting two resources <rdf:Description rdf:about="CIT1111"> <uni:courseName>Discrete Mathematics</uni:courseName> <uni:isTaughtBy rdf:resource="949318"/> </rdf:Description> <rdf:Description rdf:about="949318"> <uni:name>David Billington</uni:name> <uni:title>Associate Professor</uni:title> </rdf:Description>

  40. Nested description <rdf:Description rdf:about="CIT1111"> <uni:courseName>Discrete Maths</uni:courseName> <uni:isTaughtBy> <rdf:Description rdf:ID="949318"> <uni:name>David Billington</uni:name> <uni:title>Associate Professor</uni:title> </rdf:Description> </uni:isTaughtBy> </rdf:Description> <?xml version="1.0"?> <Resource-A> <property-A> <Resource-B> <property-B> <Resource-C> <property-C> Value-C </property-C> </Resource-C> </property-B> </Resource-B> </property-A> </Resource-A> value of property-A value of property-B

  41. Introducing some Structure to RDF Documents using the rdf:type Element • What does the following RDF describe? Not very straightforward. <rdf:Description rdf:ID="CIT1111"> <uni:courseName>Discrete Maths</uni:courseName> <uni:isTaughtBy rdf:resource="#949318"/> </rdf:Description> • Add type declaration <rdf:Description rdf:ID="CIT1111"> <rdf:type rdf:resource="http://www.mydomain.org/uni-ns#course"/> <uni:courseName>Discrete Maths</uni:courseName> <uni:isTaughtBy rdf:resource="#949318"/> </rdf:Description> • An equivalent, more concise notation <uni:course rdf:ID="CIT1111"> <uni:courseName>Discrete Maths</uni:courseName> <uni:isTaughtBy rdf:resource="#949318"/> </rdf:Description>

  42. Equivalent Representations <River rdf:ID="Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#" xml:base="http://www.china.org/geography/rivers"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> <River rdf:about="http://www.china.org/geography/rivers#Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> <rdf:Description rdf:about="http://www.china.org/geography/rivers#Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#"> <rdf:type rdf:resource="http://www.geodesy.org/river#River"/> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau </startingLocation> <endingLocation>East China Sea</endingLocation> </rdf:Description> Note: In the RDF literature the examples are typically shown in this form. From Costello

  43. Resource URI = concatenation(xml:base, '#', rdf:ID) = concatenation(http://www.china.org/geography/rivers, '#', "Yangtze") = http://www.china.org/geography/rivers#Yangtze

  44. Describe a property that has more than one value <?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river" xmlns:uom="http://www.measurements.org/units-of-measure#"> <length uom:units="kilometers">6300</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> RDF does not allow attributes on the properties (except for special RDF attributes such as rdf:resource). So we need to make the uom:units attribute a child element. Your first instinct might be to modify length to have two child elements: <?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river" xmlns:uom="http://www.measurements.org/units-of-measure#"> <length> <value>6300</value> <uom:units>kilometers</uom:units> </length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> However, now the length property has as its value two values. RDF allows only binary relations i.e., a single value for a property. From Costello

  45. Rdf:value • length has two values - 6300 and kilometers. • RDF provides a special property, rdf:value, to be used for specifying the "primary" value. • 6300 is the primary value • kilometers is a value which provides additional information about the primary value. 6300 length kilometers From Costello

  46. Anonymous resource <?xml version="1.0"?> <River rdf:ID="Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#" xmlns:uom="http://www.measurements.org/units-of-measure#"> <length> <rdf:Description> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </rdf:Description> </length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> An anonymous resource From Costello

  47. rdf:parseType="Resource" If the value of a property is comprised of several values then one option is to create an anonymous resource, as we saw. RDF provides a shorthand, so that you don't need to create an rdf:Description element, by using rdf:parseType="Resource", as shown here: <?xml version="1.0"?> <River rdf:ID="Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#" xmlns:uom="http://www.measurements.org/units-of-measure#"> <length rdf:parseType="Resource"> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River> From Costello

  48. Equivalent <length> <rdf:Description> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </rdf:Description> </length> <length rdf:parseType="Resource"> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </length>

  49. Exercise Modify the following XML document so that it is also a valid RDF document: <?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river" xmlns:uom="http://www.measurements.org/units-of-measure#"> <length uom:units="kilometers">6300</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> <Dam id="ThreeGorges" xmlns="http://www.geodesy.org/dam"> <name>The Three Gorges Dam</name> <width>1.5 miles</width> <height>610 feet</height> <cost>$30 billion</cost> </Dam> </River>

  50. Answer <?xml version="1.0"?> <River rdf:ID="Yangtze" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/river#" xmlns:uom="http://www.measurements.org/units-of-measure#" xml:base="http://www.china.org/geography/rivers"> <length rdf:parseType="Resource"> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> <obstacle> <Dam rdf:ID="ThreeGorges" xmlns="http://www.geodesy.org/dam#"> <name>The Three Gorges Dam</name> <width>1.5 miles</width> <height>610 feet</height> <cost>$30 billion</cost> </Dam> </obstacle> </River>

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