Introduction to RDF

Introduction to RDF

By: Sukhpal Singh Gill

PhD Research Scholar Thapar University, Patiala

1

Resource Description Framework

• A framework (not a language) for describing resources

• Model for data

• Syntax to allow exchange and use of information stored in various locations

• The point is to facilitate reading and correct use of information by computers, not necessarily by people

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w3c recommendation

• Find the official recommendation at http://www.w3.org/RDF/s

• Note the subtle difference between a standard and a recommendation

– w3c has no power to enforce compliance.

– Obeying the rules in the recommendation allows a site to participate in the world wide web cooperative enterprise.

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http://www.w3.org/RDF/

Identification and description

• RDF identifies resources with URIs – Often, though not always, the same as a URL

– Anything that can have a URI is a RESOURCE

• RDF describes resources with properties and property values – A property is a resource that has a name

• Ex. Author, Book, Address, Client, Product

– A property value is the value of the Property • Ex. “Joanna Santillo,” http://www.someplace.com/, etc.

• A property value can be another resource, allowing nested descriptions.

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Statements

• Resource, Property, Property Value

• Aka subject, predicate, object of a statement

• Predicates are not the same as English language verbs.

– Specify a relationship between the subject and the object

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Examples

• Statement: "The author of http://www.w3schools.com/RDF is Jan Egil Refsnes".

• Subject: http://www.w3schools.com/RDF

• Predicate: author

• Object: Jan Egil Refsnes

• Statement: "The homepage of http://www.w3schools.com/RDF is http://www.w3schools.com".

• Subject: http://www.w3schools.com/RDF

• Predicate: homepage

• Object: http://www.w3schools.com

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Binary predicates

• RDF offers only binary predicates.

• Think of them as P(x,y) where P is the relationship between the objects x and y.

• From the example,

• X = http://www.w3schools.com/RDF

• Y = Jan Egil Refsnes

• P = author

http://www.w3schools.com/RDF Jan Egil Refsnes author

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<?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:cd="http://www.recshop.fake/cd#"> <rdf:Description rdf:about="http://www.recshop.fake/cd/Empire Burlesque"> <cd:artist>Bob Dylan</cd:artist> <cd:country>USA</cd:country> <cd:company>Columbia</cd:company> <cd:price>10.90</cd:price> <cd:year>1985</cd:year> </rdf:Description> <rdf:Description rdf:about="http://www.recshop.fake/cd/Hide your heart"> <cd:artist>Bonnie Tyler</cd:artist> <cd:country>UK</cd:country> <cd:company>CBS Records</cd:company> <cd:price>9.90</cd:price> <cd:year>1988</cd:year> </rdf:Description> … </rdf:RDF>.

Root element of RDF documents

Source of namespace for elements with rdf prefix

Source of namespace for elements with cd prefix

Description element describes the resource identified by the rdf:about attribute.

Cd:country etc are properties of the resource.

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RDF validator

• Check the correctness of an RDF document:

• http://www.w3.org/RDF/Validator/

• Result shows the subject, predicate and object of each element of the document and a graph of the model.

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http://www.w3.org/RDF/Validator/



What is the Purpose of RDF?

• The purpose of RDF (Resource Description Framework) is to give a standard way of specifying data "about" something.

• Here's an example of an XML document that specifies data about China's Yangtze river:

<?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River>

"Here is data about the Yangtze River. It has a length of 6300 kilometers. Its startingLocation is western China's Qinghai-Tibet Plateau. Its endingLocation is the East China Sea."

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XML --> RDF

<?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River>

XML

Modify the following XML document so that it is also a valid RDF document:

<?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#"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River>

RDF

Yangtze.xml

Yangtze.rdf

"convert to"

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The RDF Format


RDF provides an ID attribute for identifying the resource being described.

The ID attribute is in the RDF namespace.

Add the "fragment identifier symbol" to the namespace.

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The RDF Format (cont.)


Identifies the type (class) of the resource being described.

Identifies the resource being described. This resource is an instance of River.

These are properties, or attributes, of the type (class).

Values of the properties

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Namespace Convention

xmlns="http://www.geodesy.org/river#"

Question: Why was "#" placed onto the end of the namespace? E.g.,

Answer: RDF is very concerned about uniquely identifying things - uniquely identifying the type (class) and uniquely identifying the properties. If we concatenate the namespace with the type then we get a unique identifier for the type, e.g.,

http://www.geodesy.org/river#River

If we concatenate the namespace with a property then we get a unique identifier for the property, e.g.,

http://www.geodesy.org/river#length

http://www.geodesy.org/river#startingLocation

http://www.geodesy.org/river#endingLocation

Thus, the "#" symbol is simply a mechanism for separating the namespace from the type name and the property name.

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The RDF Format

<?xml version="1.0"?> <Class rdf:ID="Resource" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="uri"> <property>value</property> <property>value</property> ... </Class>

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Advantage of using the RDF Format • You may ask: "Why should I bother designing my XML to be in the RDF

format?"

• Answer: there are numerous benefits: – The RDF format, if widely used, will help to make XML more

interoperable: • Tools can instantly characterize the structure, "this element is a type (class),

and here are its properties”.

• RDF promotes the use of standardized vocabularies ... standardized types (classes) and standardized properties.

– The RDF format gives you a structured approach to designing your XML documents. The RDF format is a regular, recurring pattern.

– It enables you to quickly identify weaknesses and inconsistencies of non-RDF-compliant XML designs. It helps you to better understand your data!

– You reap the benefits of both worlds: • You can use standard XML editors and validators to create, edit, and validate

your XML.

• You can use the RDF tools to apply inferencing to the data.

– It positions your data for the Semantic Web!

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Disadvantage of using the RDF Format

• Constrained: the RDF format constrains you on how you design your XML (i.e., you can't design your XML in any arbitrary fashion).

• RDF uses namespaces to uniquely identify types (classes), properties, and resources. Thus, you must have a solid understanding of namespaces.

• Another XML vocabulary to learn: to use the RDF format you must learn the RDF vocabulary.

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The Semantic Web “Layer Cake”

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RDF Basics

• RDF is based on the idea of identifying resources using Web identifiers and describing resources in terms of simple properties and property values.

• To identify resources, RDF uses Uniform Resource

Identifiers (URIs) and URI references (URIrefs). • Definition: A resource is anything that is identifiable

by a URIref.

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Example

• Consider the following information:

“there is a Person identified by

http://www.w3.org/People/EM/contact#me,

whose name is Eric Miller, whose email

address is [email protected], and whose title is

Dr.”

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Example (cont’d)

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Basics (cont’d) • Forget the long URIs for the moment! • RDF is based on the idea that the resources being described

have properties which have values, and that resources can be described by making statements, similar to the ones above, that specify those properties and values.

• Terminology:

– The part that identifies the thing the statement is about is called the subject.

– The part that identifies the property or characteristic of the subject that the statement specifies is called the predicate.

– The part that identifies the value of that property is called the object.

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Uniform Resource Identifiers

• The Web provides a general form of identifier, called the Uniform Resource Identifier (URI), for identifying (naming) resources on the Web.

• Unlike URLs, URIs are not limited to identifying things that have network locations, or use other computer access mechanisms. A number of different URI schemes (URI forms) have been already been developed, and are being used, for various purposes.

• Examples:

– http: (Hypertext Transfer Protocol, for Web pages) – mailto: (email addresses), e.g., mailto:[email protected] – ftp: (File Transfer Protocol) – urn: (Uniform Resource Names, intended to be persistent location-

independent resource identifiers), e.g., urn:isbn:0-520-02356-0 (for a book)

• No one person or organization controls who makes URIs or how they can be used.

While some URI schemes, such as URL's http:, depend on centralized systems such as DNS, other schemes, such as freenet:, are completely decentralized.

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URIs (cont’d) • A URI reference (or URIref) is a URI, together with an optional fragment identifier

at the end.

Example: the URIref http://www.example.org/index.html#section2

consists of the URI http://www.example.org/index.html and (separated

by the "#" character) the fragment identifier section2. • URIrefs may be either absolute or relative. • An absolute URIref refers to a resource independently of the context in which the

URIref appears, e.g., the URIref http://www.example.org/index.html. • A relative URIref is a shorthand form of an absolute URIref, where some prefix of

the URIref is missing, and information from the context in which the URIref appears is required to fill in the missing information.

Example: the relative URIref otherpage.html, when appearing in a resource http://www.example.org/index.html, would be filled out to the

absolute URIref http://www.example.org/otherpage.html.

http://www.example.org/index.html#section2

http://www.example.org/index.html#section2

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URIrefs in RDF (cont’d)

• Another difference is in the way URIrefs with fragment identifiers are handled. Consider the following URIrefs: http://www.example.org/index.html

http://www.example.org/index.html#Section2

• In normal HTML usage, these URIrefs are related (they both refer to the same document, the second one identifying a location within the first one).

• RDF assumes no particular relationship between these two URIrefs. As far as RDF is concerned, they are syntactically different URI references, and hence may refer to unrelated things.

Uniquely Identify the Resource

• Earlier we said that RDF is very concerned about uniquely identifying the type (class) and the properties. RDF is also very concerned about uniquely identifying the resource, e.g.,


This is the resource being described. We want to uniquely identify this resource.

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rdf:ID

• The value of rdf:ID is a "relative URI".

• The "complete URI" is obtained by concatenating the URL of the XML document with "#" and then the value of rdf:ID, e.g.,


Suppose that this RDF/XML document is located at this URL: http://www.china.org/geography/rivers. Thus, the complete URI for this resource is:

Yangtze.rdf

http://www.china.org/geography/rivers#Yangtze 27

xml:base • On the previous slide we showed how the URL of the document provided the base URI.

• Depending on the location of the document is brittle: it will break if the document is moved, or is copied to another location.

• A more robust solution is to specify the base URI in the document, e.g.,

<?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#" 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>

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

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rdf:about

• Instead of identifying a resource with a relative URI (which then requires a base URI to be prepended), we can give the complete identity of a resource. However, we use

rdf:about, rather than rdf:ID, e.g.,

<?xml version="1.0"?> <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>

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Triple -> resource/property/value

http://www.china.org/geography/rivers#Yangtze has a http://www.geodesy.org/river#length of 6300 kilometers

resource property value

http://www.china.org/geography/rivers#Yangtze has a http://www.geodesy.org/river#startingLocation of western China's ...


http://www.china.org/geography/rivers#Yangtze has a http://www.geodesy.org/river#endingLocation of East China Sea


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The RDF Format = triples! • The fundamental design pattern of RDF is to structure your XML data

as resource/property/value triples!

The value of a property can be a literal (e.g., length has a value of 6300 kilometers). Also, the value of a property can be a resource, as shown above (e.g., property-A has a value of Resource-B, property-B has a value of Resource-C). We will see examples of properties having a resource value in a little bit.

<?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

Notice that the RDF design pattern is an alternating sequence of resource-property. This pattern is known as "striping".

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Naming Convention

• The convention is to use a capital letter to start a type (class) name, and use a lowercase letter to start a property name.

– This helps the eye quickly discern the striping pattern.


uppercase

lowercase

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RDF Model (graph)

Legend: Ellipse indicates "Resource" Rectangle indicates "literal string value"

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rdf:Description + rdf:type • There is still another way of representing the XML. This way makes it

very clear that you are describing something, and it makes it very clear what the type (class) is of the thing you are describing:

<?xml version="1.0"?> <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>

This is read as: "This is a Description about the resource http://www.china.org/geography/rivers#Yangtze. This resource is an instance of the River type (class). The http://www.china.org/geography/rivers#Yangtze resource has a length of 6300 kilometers, a startingLocation of western China's Qinghai-Tibet Plateau, and an endingLocation of the East China Sea."

Note: this form of describing a resource is called the "long form". The form we have seen previously is an abbreviation of this long form. An RDF Parser interprets the abbreviated form as if it were this long form.

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Alternative

• Alternatively we can use rdf:ID rather than rdf:about, as shown here:

<?xml version="1.0"?> <rdf:Description 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"> <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>

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Equivalent Representations!

<?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#" 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>


<?xml version="1.0"?> <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.

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RDF Namespace

http://www.w3.org/1999/02/22-rdf-syntax-ns#

ID

about

type

resource

Description

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Terminology

• As you read the RDF literature you may see the following terminology: – Subject: this term refers to the item that is playing the role of the

resource.

– predicate: this term refers to the item that is playing the role of the property.

– Object: this term refers to the item that is playing the role of the value.

Subject Object predicate

Resource Value property

Equivalent!

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RDF Parser

• There is a nice RDF parser at the W3 Web site:


This RDF parser will tell you if your XML is in the proper RDF format.

Do Lab1

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Example #2

<?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river"> <length>6300 kilometers</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>

Yangtze2.xml

Modify the following XML document so that it is RDF-compliant:

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Note the two types (classes)

River Dam

Instance: Yangtze Properties: length startingLocation endingLocation

Instance: ThreeGorges Properties: name width height cost

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Dam - out of place <?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river"> <length>6300 kilometers</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>

Dam

Types (classes) contain properties . Here we see the River type containing the properties - length, startingLocation, and endingLocation. It also shows River containing a type - Dam. Thus, there is a Resource that contains another Resource. This is inconsistent with RDF design pattern. (We are seeing one of the benefits of using the RDF format - to identify inconsistencies in an XML design.) 42

Property value must be a Literal or a Resource

<length>6300 kilometers</length>

property

Value is a Literal

<obstacle> <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> </obstacle>

property

Value is a Resource

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Modified XML (to make it consistent) <?xml version="1.0"?> <River id="Yangtze" xmlns="http://www.geodesy.org/river"> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> <obstacle> <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> </obstacle> </River>

Yangtze2,v2.xml

"The Yangtze River has an obstacle that is the ThreeGorges Dam. The Dam has a name - The Three Gorges Dam. It has a width of 1.5 miles, a height of 610 feet, and a cost of $30 billion."

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RDF Format <?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#" 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> <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>

Changed id to rdf:ID Added the '#' symbol

As always, the other representations using rdf:about and rdf:Description are available. 45

RDF Model (graph)

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<?xml version="1.0"?> <Dam rdf:ID="ThreeGorges" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/dam#" xml:base="http://www.china.org/geography/rivers"> <name>The Three Gorges Dam</name> <width>1.5 miles</width> <height>610 feet</height> <cost>$30 billion</cost> </Dam>

<?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#" 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> <obstacle rdf:resource="http://www.china.org/geography/rivers#ThreeGorges"/> </River>

Three-Gorges-Dam.rdf

Alternatively, suppose that someone has already created a document containing information about the Three Gorges Dam:

Yangtze.rdf

Then we can simply reference the Three Gorges Dam resource using rdf:resource, as shown here:

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Note: reference is to a resource, not to a file

<obstacle rdf:resource="http://www.china.org/geography/rivers #ThreeGorges"/>

Why was this the reference:

<obstacle rdf:resource="http://www.china.org/geography/rivers /Three-Gorges-Dam.rdf"/>

and not this:

That is, why wasn't the reference to a "file"? Answer: 1. What if the file moved? Then the reference would break. 2. By using an identifier of the Three Gorges Dam, and keeping a particular file unspecified, then an "aggregator tool" will be able to collect information from all the files that talk about the Three Gorges Dam resource (see next slide).

Do Lab2

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Anyone, Anywhere, Anytime Can Talk About a Resource

• In all of our examples we have provided a unique identifier to resources, e.g.,

http://www.china.org/geography/rivers#Yangtze

• Consequently, if another RDF document identifies the same resource then the data that it specifies gives additional data about that resource.

• An aggregator tool will be able to collect all data about a resource and present a consolidated set of data for the resource. That's powerful!

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rdf:ID versus rdf:about

• When should rdf:ID be used? When should rdf:about be used?

– When you want to introduce a resource, and provide an initial set of information about a resource use rdf:ID

– When you want to extend the information about a resource use rdf:about • The RDF philosophy is akin to the Web philosophy. That is,

anyone, anywhere, anytime can provide information about a resource.

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http://www.china.org/geography/rivers/yangtze.rdf <?xml version="1.0"?> <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#"> <name>Dri Chu - Female Yak River</name> <name>Tongtian He, Travelling-Through-the-Heavens River</name> <name>Jinsha Jiang, River of Golden Sand</name> </River>

http://www.encyclopedia.org/yangtze-alternate-names.rdf

<?xml version="1.0"?> <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> <name>Dri Chu - Female Yak River</name> <name>Tongtian He, Travelling-Through-the-Heavens River</name> <name>Jinsha Jiang, River of Golden Sand</name> </River>

Aggregated Data!

Aggregator tool collects data about the Yangtze

A distributed network of data!

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<?xml version="1.0"?> <Dam rdf:ID="ThreeGorges" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.geodesy.org/dam#" xml:base="http://www.china.org/geography/rivers"> <name>The Three Gorges Dam</name> <width>1.5 miles</width> <height>610 feet</height> <cost>$30 billion</cost> </Dam>

<?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#" 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> <obstacle rdf:resource="http://www.china.org/geography/rivers #ThreeGorges"/> </River>

http://www.china.org/geography/rivers/yangtze.rdf

http://www.encyclopedia.org/three-gorges-dam.rdf

<?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#" 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> <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>

Aggregate!

Note that the reference to the ThreeGorges Dam resource has been replaced by whatever information the aggregator could find on this resource!

Another Example of Aggregation

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Example #3

<?xml version="1.0"?> <River xmlns="http://www.geodesy.org/river#"> <name>Yangtze</name> <length>6300 kilometers</length> <startingLocation>western China's Qinghai-Tibet Plateau</startingLocation> <endingLocation>East China Sea</endingLocation> </River>

Notice that in this XML document there is no unique identifier:

Yangtze3.xml

XML


Yangtze3.rdf

RDF

The RDF is identical to the XML!

53

Interpreting the RDF


Yangtze3.rdf

This is read as: "This is an instance of the River type (class). The River has a name of Yangtze, a length of 6300 kilometers, a startingLocation of western China's Qinghai-Tibet Plateau, and an endingLocation of the East China Sea."

In this document the resource is anonymous - it has no identifier.

54

Disadvantage of anonymous resources


http://www.china.org/geography/rivers/yangtze.rdf

<?xml version="1.0"?> <River xmlns="http://www.geodesy.org/river#"> <name>Yangtze</name> <name>Dri Chu - Female Yak River</name> <name>Tongtian He, Travelling-Through-the-Heavens River</name> <name>Jinsha Jiang, River of Golden Sand</name> </River>

http://www.encyclopedia.org/yangtze-alternate-names.rdf

An aggregator tool will not be able to determine if these documents are talking about the same resource.

Aggregate

55

Example #4 <?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>

XML

Yangtze4.xml

Yangtze4.rdf

<?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>

RDF

56

<?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>

Yangtze4.xml

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 only binary relations i.e., a single value for a property.

57

rdf:value

length

6300

kilometers

length has two values - 6300 and kilometers. RDF provides a special property, rdf:value, to be used for specifying the "primary" value. In this example, 6300 is the primary value, and kilometers is a value which provides additional information about the primary value.

58

RDF Format

<?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>

Yangtze4.rdf

An anonymous resource

Read this as: "The Yangtze River has a length whose value is a resource which has a value of 6300 and whose units is kilometers.

59

Advantage of anonymous resources

<rdf:Description> <rdf:value>6300</rdf:value> <uom:units>kilometers</uom:units> </rdf:Description>

This is an anonymous resource. Its purpose is solely to provide a context for the two properties. Other RDF documents will have no need to amplify this resource. So, in this case, there is no reason for giving the resource an identifier. In this case it makes good sense to use an anonymous resource.

60

RDF Model (graph)

An anonymous resource (also called a "blank node"). That is, a resource with no identifier. (Note: RDF Parsers will typically generate a unique identifier for anonymous resources, to distinguish one anonymous resource from another.)

Legend:

61

rdf:parseType="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: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>

Yangtze4,v2.rdf

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:

The meaning of this is identical to that shown on the previous slide.

62

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>

Do Lab3

63

Summary

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>

Yangtze.xml

See next slide --> 64

Containers

• Groups of things: <bag> <seq> <alt>

• <bag> unordered list; duplicates allowed

• <seq> ordered list; duplicates allowed

• <alt> list of alternatives; one will be selected

65

Example <alt>

<?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:cd="http://www.recshop.fake/cd#"> <rdf:Description rdf:about="http://www.recshop.fake/cd/Beatles"> <cd:format> <rdf:Alt> <rdf:li>CD</rdf:li> <rdf:li>Record</rdf:li> <rdf:li>Tape</rdf:li> </rdf:Alt> </cd:format> </rdf:Description> </rdf:RDF>

Exactly one of these formats

66

Limiting the scope

• Collection - describes a group that contains only the specified members, no others.

<rdf:Description rdf:about="http://recshop.fake/cd/Beatles"> <cd:artist rdf:parseType="Collection"> <rdf:Description

rdf:about="http://recshop.fake/cd/Beatles/George"/> <rdf:Description

rdf:about="http://recshop.fake/cd/Beatles/John"/> <rdf:Description

rdf:about="http://recshop.fake/cd/Beatles/Paul"/> <rdf:Description

rdf:about="http://recshop.fake/cd/Beatles/Ringo"/> </cd:artist> </rdf:Description>

67

Introduction to RDF

Education