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Chapter 7: Resource Description Framework (RDF)

This chapter provides a comprehensive overview of the Resource Description Framework (RDF), emphasizing its role in knowledge representation within Service-Oriented Computing. RDF enables the representation of resources and their relationships through simple statements known as triples—comprising subjects, predicates, and objects. It supports inferencing and facilitates incremental knowledge acquisition by organizing content effectively, using URIs to clarify meanings. The chapter also discusses RDF's syntax, relationship modeling, and practical examples in XML and N-Triples notation, highlighting the significance of RDF in semantic web technologies.

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Chapter 7: Resource Description Framework (RDF)

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  1. Chapter 7:Resource Description Framework (RDF) Service-Oriented Computing: Semantics, Processes, Agents– Munindar P. Singh and Michael N. Huhns, Wiley, 2005

  2. RDF • A basis for knowledge representation • Supports inferencing • Simple language to capture assertions (statements) • Elements of knowledge about a resource • Facilitate incremental acquisition of knowledge • Consolidates old KR ideas • Frames, OO modeling • Applies URIs to clarify meanings and handle vocabulary differences Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  3. Why RDF? • Whereas XML • Gives us a document tree • Doesn’t identify the content represented by a document, where content means • Concepts the document is about • Relationships among them • Enables multiple representations for the same content • RDF expresses the content itself Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  4. Resources and Literals • RDF captures descriptions of resources • A resource is an “addressable” object • Of which a description can be given • Identified via a URI • Worth talking about and possible to talk about • A literal is something simpler • A value, e.g., string or integer • Cannot be given a description Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  5. Statements or Triples • RDF is based on a simple grammar • An RDF document is simply a set of statements also known as triples • Each statement consists of • Subject: a resource • Object: a resource or a literal • Predicate: a resource • Comes with RDFS, a vocabulary to create vocabularies Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  6. Rendering RDF • RDF is not about the surface syntax but about the underlying content • Using the XML serialization of RDF • RDF is not tied to XML • Standard XML namespace syntax • Namespaces defined by the RDF standard • Typically abbreviated rdf and rdfs Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  7. Example in XML (Using Dublin Core) <?xml version='1.0' encoding='UTF-8'?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/"> <rdf:Descriptionrdf:about="http://www.wiley.com/SOC"> <dc:title>Service-Oriented Computing</dc:title> <dc:creator>Munindar</dc:creator> <dc:creator>Michael</dc:creator> <dc:publisher>Wiley</dc:publisher> </rdf:Description> </rdf:RDF> • rdf:Description gathers statements about one subject • Distinguish rdf:ID from rdf:about Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  8. Example in N-Triples NotationSubject-Predicate-Object <http://www.wiley.com/SOC> <http://purl.org/dc/elements/1.1/title> "Service-Oriented Computing" . <http://www.wiley.com/SOC> <http://purl.org/dc/elements/1.1/creator> "Munindar" . <http://www.wiley.com/SOC> <http://purl.org/dc/elements/1.1/creator> "Michael" . <http://www.wiley.com/SOC> <http://purl.org/dc/elements/1.1/publisher> "Wiley" . • Could also write individual statements in the XML syntax, but the rdf:Description element simplifies the notation Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  9. Exercise • Graphs represent binary relationships naturally • The vendor ships SKU-99 • Express a three-party relationship • The vendor ships SKU-99 quickly • Hint: think of gerunds from natural language grammar Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  10. Multiparty Relationships • An edge has two terminals, so limited to binary relationships • To represent a multiparty relationship, introduce a resource corresponding to the relationship itself • That’s what a gerund does in NL • Analogous to an association entity • Include edges originating or targeting this resource Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  11. Exercise • Express a quotation • Alice says the vendor ships SKU-99 • Hint(?): In RDF, we can only talk about resources • And literals, but literals are points of last resort Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  12. Reification of Statements • Reify: to make referenceable, essential for quoting statements to • Agree or disagree with them • Assert modalities: possible, desirable, … • Make a statement into a resource; then talk about it • rdf:Statement is the class whose rdf:type the given statement (object) is; additional properties such as rdf:subject, rdf:object, and rdf:predicate Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  13. RDF Schema • Analogous to an object-oriented type system built on top of RDF. Defines • rdfs:Class, rdfs:subClassOf • rdfs:Resource, rdfs:Literal • rdfs:Property, rdfs:subPropertyOf • rdfs:range, rdfs:domain • rdfs:label, rdfs:comment, rdfs:seeAlso • Applications of RDF Schema deferred to OWL, which greatly enhances the above Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  14. RDF Schema versus XML Schema Both help define custom vocabularies • An XML Schema document gives us syntactic details • An RDF Schema document gives us a way to capture part of the meaning through a standard vocabulary (rdfs) • An OWL document (next chapter) captures richer meaning Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  15. Collections • Function as containers • rdf:Bag • rdf:Sequence • rdf:Alt (choice) • Accompanied by properties to extract elements • Schematically represented as rdf:_1, and so on [Not emphasized in this course, though they are applied within OWL] Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  16. Chapter 7 Summary • RDF captures deeper structure than XML • RDF captures graphs in general • Meaning depends on the graph, not the document that represents a graph • RDF is based on an simple linguistic representation: subject, predicate, object • But “webified” via URIs • RDF comes with RDF Schema • In essence, an object-oriented type system: a vocabulary to create new vocabularies • Used for important vocabularies (FOAF, DC, Mozilla extensions) • Provides a basis for OWL Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

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