RDF: Building Block for the

looneyvillebiologistInternet and Web Development

Oct 21, 2013 (3 years and 11 months ago)

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RDF: Building Block for the
Semantic Web

Jim Ellenberger

UCCS CS5260

Spring 2011


2

RDF
-

Jim Ellenberger
-

May, 2011

S
emantic web


What is it?


A phrase coined by Tim Berners
-
Lee, inventor of the WWW, in a
2001
Scientific American
Article


Berners
-
Lee and others have described it as a major component
of “Web 3.0”



Wikipedia defines it well:


A “web of data” that enables machines to understand the
semantics, or meaning, of information on the WWW


Extends the network of hyperlinked human
-
readable web pages
by inserting machine
-
readable metadata


Enables automated agents to access the Web more intelligently
and perform tasks on behalf of users


3

RDF
-

Jim Ellenberger
-

May, 2011

Why do we need it?


Traditional web technologies like HTML are focused on
organizing, presenting and linking documents


Can’t directly access the
meaning

of information on the Web


Can’t provide consistent methods to aggregate and query
information on the Web



Semantic web technologies provide these missing
components


Information can be stored, aggregated and queried based on
its meaning


All of this can be automated, because the information is
available in machine
-
readable formats


4

RDF
-

Jim Ellenberger
-

May, 2011

How is the semantic web
implemented?


There is a need to encode and manipulate knowledge on the
web, but how can it be done?


Technologies that describe and manipulate information based
on meanings and relationships


Resource Description Framework (RDF)


Data interchange formats (RDF/XML, N3, Turtle, N
-
Triples)


Notations (RDFS, OWL)


Query languages (SPARQL)


My focus: RDF


Essentially, the building block for all semantic web technologies


Originally specified W3C as a metadata language; it was
extended to accommodate semantic web concepts


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


5

RDF
-

Jim Ellenberger
-

May, 2011

RDF: general structure


RDF is graph
-
based


Not hierarchical like XML and other data description formats


Single pieces of information are graph nodes and the
relationships between them are graph edges



Advantages of graph
-
based model


Virtually any kind and number of relationships can be
represented
-

no need to adhere to a hierarchy


Diverse graphs can be combined as simply as defining a
relationship between two nodes
-

no need for graphs to have
compatible hieracrchies



6

RDF
-

Jim Ellenberger
-

May, 2011

RDF statements


The basic unit of information in RDF is a
statement
or

triple

with three components


Subject


thing the statement is about


Predicate

or
property



a property or characteristic of the subject


Object



the value of the property or characteristic



Example, a statement about a camera:


The D300


subject of the statement


is manufactured by


predicate


Nikon


object of the predicate



This triple encodes a single piece of information: The D300 is
manufactured by Nikon





7

RDF
-

Jim Ellenberger
-

May, 2011

RDF URIs


Subjects and objects that make up RDF statements are called
resources


In order to be useful web wide, resources and the predicates
that link them need identifiers that are:


Unique


to avoid confusion


Universally accessible


to make useable web wide



These identifiers are called URIs
-

Uniform Resource Identifiers


The camera example in URIs:


http://dbpedia.org/page/Nikon_D300
-

subject


http://mywebpage.org/camera#manufactured_by
-

predicate


http://www.dbpedia.org/resource/Nikon
-

object

8

RDF
-

Jim Ellenberger
-

May, 2011

More abut URIs


URIs are not URLs (but URLs are URIs)


URLs represent things
retrievable

from the web


URIs represent things
identified on

the web, which may or may
not be retrievable



Where do URIs come from?


Use an existing URI if an appropriate one exists:
http://
dbpedia.org/page/Nikon_D300



If one doesn’t exist, make your own:
http
://mywebpage.org/camera#manufactured_by


If you create
your own,
it must be universally accessible and must
return data
to RDF
clients


9

RDF
-

Jim Ellenberger
-

May, 2011

Camera example in graph form



http://dbpedia.org/page/Nikon_
D300

http://www.dbpedia.org/resource/
Nikon

http://mywebpage.org/camera#manufact
ured_by

10

RDF
-

Jim Ellenberger
-

May, 2011

C
amera example linked to
other graphs



http://dbpedia.org/page/Nikon_
D300

http://www.dbpedia.org/resource/
Nikon

http://mywebpage.org/camera#manufact
ured_by

[URL: Review]

[URL of Stock Price]

[URL: review_of]

[URL: stock_price_of]

11

RDF
-

Jim Ellenberger
-

May, 2011

What Does RDF Look Like in
the Wild?



RDF statements need to be serialized to be used on the WWW
and processed by machines


There are many formats used for this:


RDF/XML


Turtle


N3


RDFa



RDF/XML is probably the most common

12

RDF
-

Jim Ellenberger
-

May, 2011

RDF/XML Example


RDF is not XML, but it can be encoded in XML


The camera example, in RDF/XML:

<?xml version="1.0"?>

<
rdf:RDF

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


xmlns:mypage
="
http://mywebpage.org#
">


<
rdf:Description

rdf:about
="
http://dbpedia.org/page/Nikon_D300
">


<
mypage:
manufacured_by

rdf:resource
="
http://www.dbpedia.org/resource/Nikon
"/>


</
rdf:Description
>

</
rdf:RDF
>


XML Tags


rdf:RDF

-

begin RDF document


rdf:Description



begin description of subject(s)


rdf:about



URI for the subject


mypage:manufactured_by



the predicate


rdf:resource



URI for the object

13

RDF
-

Jim Ellenberger
-

May, 2011

A
r
eal
w
orld
e
xample:
OpenCalais


OpenCalais

is a web service that automatically generates
semantic metadata in RDF/XML from text submitted to it


This is a portion of
OpenCalais
’ output when “D300” is
submitted:

<rdf:RDF xmlns:rdf=http://www.w3.org/1999/02/22
-
rdf
-
syntax
-
ns#


xmlns:c="http://s.opencalais.com/1/pred/">


<
rdf:Description

rdf:about
="
http://d.opencalais.com/
er
/product/electronics...
">


...


<
c:name
>
Nikon D300 Digital Camera
</
c:name
>


</
rdf:Description
>

</
rdf:RDF
>


Essentially, the edited RDF code contains the triple


electronics product (subject)


name (predicate)


Nikon D300 Digital Camera (object)


14

RDF
-

Jim Ellenberger
-

May, 2011

What else is happening?


DBPedia project


Publishes Wikipedia information in semantic web formats


http://dbpedia.org


FOAF
-

Friend of a Friend project


Uses RDF to describe relationships among people


http://www.foaf
-
project.org/


OpenPSI project


Publishes UK government data in semantic web formats


http://www.openpsi.org/


GoodRelations vocabulary


A means to publish product info in semantic web formats


http://www.heppnetz.de/projects/goodrelations/


15

RDF
-

Jim Ellenberger
-

May, 2011

Important Issues


The amount of information that could be encoded is
staggering


Encoding meaning isn’t always straightforward
--

e.g., what
does “young” mean?


Not everyone wants their information freely available


Information can be a commodity


Information can be a trade secret


Accuracy
--

how do we deal with information that is inaccurate
or deceptive


Performance
--

how will semantic web data stores perform
compared to more traditional datasets?


16

RDF
-

Jim Ellenberger
-

May, 2011

Conclusion


There is quite a bit more to RDF


RDF has more capabilities than described here


RDF has been expanded with other technologies to create still
more capabilities



There are also many related areas to explore


How can RDF data be created?


How can it be stored?


How can it be served and retrieved?


Once we retrieve RDF data, what should we do with it?