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Understanding the Resource Description Framework (RDF) in Service-Oriented Computing

The Resource Description Framework (RDF) serves as a foundational model for knowledge representation on the web. It allows for the description of resources through a simple syntax, enabling independent parties to exchange and utilize data effectively. RDF combines traditional knowledge representation concepts, such as frames and object-oriented models, while leveraging the web's capabilities to enhance its application. Designed for machine readability rather than human display, RDF utilizes XML and facilitates expressing semantics, making it essential for the development of the Semantic Web.

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Understanding the Resource Description Framework (RDF) in Service-Oriented Computing

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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 • RDF stands for Resource Description Framework • Provides a basis for knowledge representation • Supports inferencing • Simple language to capture assertions (statements), which help capture knowledge, e.g., about resources • Combines old Knowledge Representation (KR) ideas (frames, Object-Oriented (OO) modeling) but uses the Web to enhance their range and avoid some longstanding problems Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  3. RDF • RDF is a framework for describing resources on the web • RDF provides a model for data, and a syntax so that independent parties can exchange and use it • RDF is designed to be read and understood by computers • RDF is not designed for being displayed to people • RDF is written in XML • RDF is a part of the W3C's Semantic Web Activity • RDF is a W3C Recommendation Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  4. RDF - Examples of Use • Describing properties for shopping items, such as price and availability • Describing time schedules for web events • Describing information about web pages, such as content, author, created and modified date • Describing content and rating for web pictures • Describing content for search engines • Describing electronic libraries Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  5. Why RDF? • XML can present the content but not meaning. • 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 • The motivation of RDF is to provide a language to present semantics of a resource. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  6. RDF Rules • RDF identifies things using Web identifiers (URIs), and describes resources with properties and property values. • Explanation of Resource, Property, and Property value: • A Resource is anything that can have a URI, such as "http://www.w3schools.com/RDF" • A Property is a Resource that has a name, such as "author" or "homepage" • A Property value is the value of a Property, such as "Jan Egil Refsnes" or "http://www.w3schools.com" (note that a property value can be another resource) Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  7. 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 (Universal Resource Identifier) • A literal is something simpler • A value, e.g., string or integer • Cannot be given a description • RDF is based on a simple grammar • An RDF document is just a set of statements or triples Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  8. Statements or Triples • Each statement consists of • Subject: a resource • Object: a resource or a literal • Predicate: a resource, the property of the given triple • Resource examples are entities on the Web such as documents, images, video clips, and services. • RDF properties are two-place predicates. Predicates with two arguments (subject, object) form a statement. • Comes with RDFS, a vocabulary to create vocabularies • W3C's RDF Validation Serviceis useful when learning RDF. Here you can experiment with RDF files. • The Dublin Core Metadata Initiative (DCMI) has created some predefined properties for describing documents. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  9. RDF Statements • The combination of a Resource, a Property, and a Property value forms a Statement (known as the subject, predicate and object of a Statement). • Let's look at some example statements to get a better understanding: • Statement: "The author of http://www.w3schools.com/RDF is Jan EgilRefsnes". • The subject of the statement above is: http://www.w3schools.com/RDF • The predicate is: author • The object is: Jan EgilRefsnes • Statement: "The homepage of http://www.w3schools.com/RDF is http://www.w3schools.com". • The subject of the statement above is: http://www.w3schools.com/RDF • The predicate is: homepage • The object is: http://www.w3schools.com Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  10. 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:Description rdf: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 Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  11. RDF Main Elements • The <rdf:RDF> elementis the root element of an RDF document. It defines the XML document to be an RDF document. • The <rdf:Description> element identifies a resource with the about attribute. • The property elements can also be defined as attributes. • The property elements can also be defined as resources. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  12. RDF Container Elements • RDF statements can involve three components. To allow involving more than 3 components, RDF provides a means to define structured containers of resources. • Containers enable the representation of a relationship where more than one resource may be required to participate. • Function as containers • rdf:Bag is an unodered collection. • rdf:Sequence is an ordered collection. • rdf:Alt (choice) captures a disjoint union. • Accompanied by properties to extract elements • Schematically represented as rdf:li, and so on Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  13. RDF Container Elements <rdf:Bag ID = "group1"> <rfd:li>One</rdf:li> <rfd:li>Two</rdf:li> <rfd:li>Three</rdf:li> </rdf:Bag> <rdf:Seq ID = "sequence1"> <rfd:li>First</rdf:li> <rfd:li>Second</rdf:li> <rfd:li>Third</rdf:li> </rdf:Seq> <rdf:Alt ID = "alt1"> <rfd:li>default choice</rdf:li> <rfd:li>another choice</rdf:li> <rfd:li>yet another choice</rdf:li> </rdf:Alt> • RDF collections are used to describe groups that contains ONLY the specified members. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  14. Rendering RDF in XML • RDF is not about the surface syntax but the content it captures • We will use 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

  15. Example in N-Triples Notation <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

  16. Key Primitives: Reification, Model • To reify a concept is to enable it to be referenced. The process is called reification. • The essential idea behind reification is to covert statements into a resource. • RDF provides an information model based on graphs. • Each edge corresponds to a statement. • The label of the edge is a predicate. • The source vertex of the edge is a subject. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  17. 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

  18. 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

  19. RDF Schema • RDF statements can be readily stored in a relational database or in an object database. • 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 (Web Ontology Language), which greatly enhances the above Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  20. RDF Schema <?xml version="1.0"?> <rdf:RDF xmlns:rdf= "http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xml:base= "http://www.animals.fake/animals#"> <rdfs:Class rdf:ID="animal" /> <rdfs:Class rdf:ID="horse"> <rdfs:subClassOf rdf:resource="#animal"/> </rdfs:Class> </rdf:RDF> Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  21. RDF Schema versus XML Schema • Both help define custom vocabularies • An XML Schema document gives us syntactic details • An RDF Schema document gives us meaning • An OWL document (next chapter) captures richer meaning • Jena (http://jena.sourceforge.net/) is an open source RDF and RDFS toolkit. Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

  22. 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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