Chapter 7: Resource Description Framework (RDF)

Chapter 7:Resource Description Framework (RDF)

Service-Oriented Computing: Semantics, Processes, Agents– Munindar P. Singh and Michael N. Huhns, Wiley, 2005

Chapter 7 2Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and

Michael Huhns

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


Michael Huhns

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


Michael Huhns

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


Michael Huhns

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.


Michael Huhns

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)


Michael Huhns

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


Michael Huhns

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 Service is 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.


Michael Huhns

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 Egil Refsnes".

The subject of the statement above is: http://www.w3schools.com/RDF

The predicate is: author The object is: Jan Egil Refsnes

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


Michael Huhns

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


Michael Huhns

RDF Main Elements The <rdf:RDF> element is 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.


Michael Huhns

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


Michael Huhns

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.


Michael Huhns

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


Michael Huhns

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


Michael Huhns

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.


Michael Huhns

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


Michael Huhns

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


Michael Huhns

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


Michael Huhns

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>


Michael Huhns

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.


Michael Huhns

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

Chapter 7: Resource Description Framework (RDF)

Documents

Transcript of Chapter 7: Resource Description Framework (RDF)