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
2

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.
3

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.
4

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
5

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
6

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
7

<?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.
8

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.
9

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:
<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."
10

XML --> RDF
<River id="Yangtze"
</River>
XML
Modify the following XML document so that it is also a valid RDF document:
<River rdf:ID="Yangtze"
xmlns="http://www.geodesy.org/river#">
</River>
RDF
Yangtze.xml
Yangtze.rdf
"convert to"
11

The RDF Format
</River>
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.
1
2
3
12

The RDF Format (cont.)
</River>
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
1
2
3
4
13

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.
BestPractice
BestPractice
14

The RDF Format
<Class rdf:ID="Resource"
xmlns="uri">
<property>value</property>
<property>value</property>
...
</Class>
15

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!
NetworkeffectInteroperability
16

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.
17

18
The Semantic Web “Layer Cake”

19
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.

20
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 em@w3.org, and whose title is
Dr.”

22
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.

23
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:em@w3.org
– 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.

24
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.

25
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.,
</River>
This is the resource being described. We want to uniquely
identify this resource.
26

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.,
</River>
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:base="http://www.china.org/geography/rivers">
</River>
Resource URI = concatenation(xml:base, '#', rdf:ID)
= concatenation(http://www.china.org/geography/rivers, '#', "Yangtze")
= http://www.china.org/geography/rivers#Yangtze
28

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.,
<River rdf:about="http://www.china.org/geography/rivers#Yangtze"
</River>
29

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
30

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.
<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".
31

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.
</River>
uppercase
lowercase
32

RDF Model (graph)
Legend:
Ellipse indicates "Resource"
Rectangle indicates "literal string value"
33

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:
<rdf:Description rdf:about="http://www.china.org/geography/rivers#Yangtze"
<rdf:type rdf:resource="http://www.geodesy.org/river#River"/>
</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
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. 34

Alternative
• Alternatively we can use rdf:ID rather than rdf:about, as
shown here:
<rdf:Description rdf:ID="Yangtze"
</rdf:Description>
35

Equivalent Representations!
</River>
</River>
<rdf:Description rdf:about="http://www.china.org/geography/rivers#Yangtze"
</rdf:Description>
Note: In the RDF literature the examples
are typically shown in this form.
36

RDF Namespace
http://www.w3.org/1999/02/22-rdf-syntax-ns#
ID
about
type
resource
Description
37

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!
38

RDF Parser
• There is a nice RDF parser at the W3 Web site:
http://www.w3.org/RDF/Validator/
This RDF parser will tell you if your XML
is in the proper RDF format.
Do Lab1
39

Example #2
<River id="Yangtze"
<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:
40

Note the two types (classes)
River Dam
Instance: Yangtze
Properties:
length
startingLocation
endingLocation
Instance: ThreeGorges
Properties:
name
width
height
cost
41

Dam - out of place
<River id="Yangtze"
</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
property
Value is a Literal
<obstacle>
</Dam>
</obstacle>
property
Value is a Resource
43

Modified XML (to make it consistent)
<River id="Yangtze"
<obstacle>
</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."
44

RDF Format
<obstacle>
<Dam rdf:ID="ThreeGorges"
xmlns="http://www.geodesy.org/dam#">
</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

xmlns="http://www.geodesy.org/dam#"
</Dam>
<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:
47

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 unspecifie
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
48

Anyone, Anywhere, Anytime Can
Talk About a Resource
• In all of our examples we have provided a unique
identifier to resources, e.g.,
• 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!
49

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.
50

</River>
http://www.china.org/geography/rivers/yangtze.rdf <?xml version="1.0"?>
<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
</River>
Aggregated Data!
Aggregator tool collects
data about the Yangtze
A distributed network of data
51

xmlns="http://www.geodesy.org/dam#"
</Dam>
<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
<obstacle>
xmlns="http://www.geodesy.org/dam#">
</Dam>
</obstacle>
</River>
Aggregate!
Note that the reference to the
ThreeGorges Dam resource has bee
replaced by whatever information th
aggregator could find on this resour
Another Example of Aggregation
52

Example #3
<River xmlns="http://www.geodesy.org/river#">
<name>Yangtze</name>
</River>
Notice that in this XML document there is no unique identifier:
Yangtze3.xml
XML
</River>
Yangtze3.rdf
RDF
The RDF is
identical to the
XML!
53

Interpreting the RDF
</River>
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
</River>
http://www.china.org/geography/rivers/yangtze.rdf
</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
<River id="Yangtze"
xmlns="http://www.geodesy.org/river"
xmlns:uom="http://www.measurements.org/units-of-measure">
<length uom:units="kilometers">6300</length>
</River>
XML
Yangtze4.xml
Yangtze4.rdf
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>
</River>
RDF
56

<River id="Yangtze"
</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:
<River id="Yangtze"
<length>
<value>6300</value>
</length>
</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
<length>
<rdf:Description>
</rdf:Description>
</length>
</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: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"
<length rdf:parseType="Resource">
</length>
</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:Description>
</length>
<length rdf:parseType="Resource">
</length>
Do Lab3
63

Summary
Modify the following XML document so that it is also a valid RDF document:
<River id="Yangtze"
</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>
<rdf:RDF
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

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