An Introduction to RDF and the Semantic Web

An Introduction to RDF and the Semantic

WebDr. Randy Kaplan

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Resource Description Framework

RDF

Least Understood standard to come from the W3C

May be the most powerful

In order that the web achieve its potential

May be the most important

In order that the web achieve its potential

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Resource Description Framework

Why RDF?

With HTML and XML we can swap our documents easily

No meaning is attached to them - they are just data

RDF addresses the problem of meaning in the data on the web

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What We Need To Know

When we exchange data we need to know things like,

Who wrote the data

When was the data written

When was the data last updated

These pieces of data are not data per se but the data about the data or meta data

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XML

Promised to deliver us from the unstructured data that makes up the Internet

XML brings structure to the data

Because HTML combined the appearance of the document with the content of the document it, the content was extremely hard to extract

XML separated content from presentation

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XML

XML specifically dealt with the data of the content

<music genre =”classical”><title>Eine Kleine Nacht Muzik</title><composer>Mozart</composer><key>E Flat</key><tempo>2/4</tempo></music>

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XML

We could convey some of the same information with different data

<document type =”classical music”><name>Eine Kleine Nacht Muzik</name><author>Mozart</author></document>

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XML

What if we wanted to find all pieces of music composed by Mozart?

We would have to find all documents where the <composer> element had a value of ‘Mozart’.

We would also have to find all documents where the <author> element had a value of ‘Mozart’.

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XML

If there was another element used to denote the creator of the music then that term would have to be searched for also

In order to be able to find all compositions written by Mozart without having to identify all elements designating the creator of the music then the same term would have to be used to identify the creator

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XML

This problem could also be solved by indicating that when the term composer is used, it means the same when another document says written by, and another says created by

This would be quite an undertaking though as it involves identifying all words and phrases in all languages having this meaning

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Missing

Our ability to know that one or more terms mean the same thing is the thing that is missing from the Internet

If we can build this layer into the Internet, it will take the information to a fundamentally different level

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Dublin Core

1995

Conference in Dublin, Ohio

Discussed issues of semantics

Agreed to a core set of themes common to all documents

Set of properties became known as the Dublin Core (DC) initiative

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Dublin Core

3 Core Properties

DC.Title

DC.Creator

DC.Subject

15 core properties were defined in the Dublin core (originally)

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Dublin CoreThe Dublin Core can be applied to XML

<music genre =”classical”><title>Eine Kleine Nacht Muzik</title><Creator>Mozart</Creator><key>E Flat</key><tempo>2/4</tempo></music>

<document type =”classical music”><name>Eine Kleine Nacht Muzik</name><Creator>Mozart</Creator></document>

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Dublin Core

Even though we now have used the same element to identify the entity responsible for creating the we don’t know if the meaning of “Creator” is the same in both of these instances

The only way to be sure is to use a very precise mechanism to identify the element being used

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Dublin CoreThe Dublin Core can be applied to XML

<music genre =”classical”><title>Eine Kleine Nacht Muzik</title><dc.Creator xmlns:dc=”http://purl.org/dc/elements/1.1/”>Mozart</dc.Creator><key>E Flat</key><tempo>2/4</tempo></music>

<document type =”classical music”><name>Eine Kleine Nacht Muzik</name><dc.Creator xmlns:dc=”http://purl.org/dc/elements/1.1/”>Mozart</dc.Creator></document>

Now we can see that these elements refer to exactly the same concept

http://purl.org/dc/elements/1.1/


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CD DatabaseSuppose you keep a small database of CDs on your computer

There is a table in the database as below

Primary Key

Album Name Artist

1The Ecleftic: Two Sides II a Book

Wyclef Jean

2 Eine Kleine Nacht Muzik Mozart

3 SoultraneJohn Coltrane

4 The Real Eminem

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Another CD DatabaseThere is a second database kept by another person who has a CD collection

A table in the database is shown below

Key Title Performer


2 The Ecleftic Wyclef Jean

3 Kind of Blue Miles Davis

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Comparing Databases

Exchanging Information

If we wanted to share information there would be a problem since the tuple names are different

The same solution we used in the XML can be used in the database - the unique identifier

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PrimaryKey

http://purl.org/dc/elements/1.1/Title http://purl.org/dc/elements/1.1/Creator

1 The Ecleftic: Two Sides II a Book Wyclef Jean


3 Soultrane John Coltrane

4 The Real Eminem

http://purl.org/dc/elements/1.1/Title

http://purl.org/dc/elements/1.1/Creator



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Key http://purl.org/dc/elements/1.1/Title http://purl.org/dc/elements/1.1/Creator

1 Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart







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URI’s

Uniform Resource Identifiers (URI’s) give us a way to insure that the meaning of the column of data between databases is the same so long as the column is labeled with the same URI

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Other ProblemsUnfortunately when we look at the databases we notice some other problems

PrimaryKey

http://purl.org/dc/elements/1.1/Title http://purl.org/dc/elements/1.1/Creator

1 The Ecleftic: Two Sides II a Book Wyclef Jean


3 Soultrane John Coltrane

4 The Real Eminem

Key http://purl.org/dc/elements/1.1/Title http://purl.org/dc/elements/1.1/Creator

1 Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart










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Other Problems

Problem 1

Albums which may be the same have different names

Problem 2

Different names are used to denote the same composers

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Taxonomies

These problems can be solved through the use of taxonomy

A taxonomy is a -

Controlled vocabulary of words

Usually about a constrained topic

Unique identifiers are key to developing taxonomies

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Taxonomies

If we were to devise a controlled classification list so we could tell which CD’s were which genre then we would avoid problems like having one CD labeled as classical and another CD labeled as classic

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Taxonomies

CD Taxonomy

Jazz

Classical

Soul

Pop

Hip Hop

Folk

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Taxonomies

We are not limited to taxonomies of of music

We could have type of performance, i.e., play, movie, live performance, etc.

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Moving the Problem

We really didn’t solve the problem we described earlier

We only moved the problem up a level

We now have the problem with having more than one taxonomy for the same thing

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Moving the Problem

Consider

http://taxonomies.org/Plays/PorgyAndBess

http://taxonomies.org/Albums/PorgyAndBess

We do not know whether the PorgyAndBess in the first reference is the same as the PorgyAndBess in the same reference

http://taxonomies.org//Plays/PorgyAndBess

http://taxonomies.org//Plays/PorgyAndBess

http://taxonomies.org/Albums/PorgyAnd





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We Need An Authority Figure

Let us imagine that there is some authority that keeps track of al CDs that are released

This is similar to books and their ISBN numbers which are unique

We will call the fictitious authority MuzicBiz.org

MuzicBiz.org maintains a central database of CDs that have been released

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Tables Now ...Key http://purl.org/dc/elements/1

.1/Titlehttp://purl.org/dc/elements/1.1/Creator

http://MuzicBiz.org/Album/1011234

Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart


The Ecleftic Wyclef Jean


Kind of Blue Miles Davis

Key http://ebiz.org/Stock http://ebiz.org/Cost


5 $16.00


4 $19.00


10 $12.00











http://ebiz.org/Stock








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Unique Identifiers

Since we are guaranteed that these identifiers ALWAYS refer to the same CD any table row having a specific key will ALWAYS refer to the same CD - there is NO reason to doubt this

Data validity is enforced

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Meta-Data

Meta-Data

Data that describes data

Creator, Type, Date are all kinds of meta-data

So far the meta-data we have described consists of two values - an attribute name and an attribute value

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Meta-Data

To be precise we need to add one more piece of meta-data to complete any meta-data we might have

Since it is entirely possible to have as Creator, the value Mozart, we need to identify what/where Mozart is the creator of - the so-called DOCUMENT

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Triples

The combination of Source, Attribute name, and Value makes what is called in the RDF-biz a TRIPLE and that constitutes a fundamental element in RDF

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Transporting Triples

We will assume the following -

Meta-data can be expressed as a set of triples

Key to sharing meta-data is the URI

Now given that we accept this representation, the next challenge is to decide how we will share this information (transport)

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Sharing Meta-Data and Data

The database contains the information as organized in the table above

We need to transform this data into the accepted form, i.e., triples

Key http://ebiz.org/Stock http://ebiz.org/Cost


5 $16.00


4 $19.00


10 $12.00









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Sharing Data and Meta-Data

Document Name Value


http://ebiz.org/Stock 5


http://ebiz.org/Cost $16.00
















http://ebiz.org/Cost













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We have adequately represented the meta-data and it is “ready” for transport via XML

But this table only represents the meta-data and does not relate to any data described by it

Document Name Value

































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We need a way to identify the document that the meta-data describes

For this purpose we add a name/value pair that names the URL of the document

Document Name Value

































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<documenttype="News Item"url="http://www.ePolitix.com/Articles/0000005a4787.htm"xmlns:dc="http://purl.org/dc/elements/1.1/">

<dc:Title>I will stand says Portillo</dc:Title><dc:Creator>Craig Hoiy</dc:Creator><dc:Subject>Tory leadership contest</dc:Subject>

</document>

http://www.ePolitix.com/Articles/0000005a4787.htm



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RDF: Model and Syntax

RDF Model

In this case the model we are speaking of are the triples

The definition of RDF is representation independent

This means that XML is only one way of writing RDF

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RDF Terminology

In RDF terminology a STATEMENT is used to describe a triple

This term arises from using a triple to make a statement about a document

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RDF Terminology

Triples

Resources and Properties

In the RDF specification the name part of the name/value pair is regarded as a PROPERTY

The subject of the meta data is regarded as a RESOURCE

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RDF Terminology

Triples

A triple is the combination of the three parts - a resource with a property and a value

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RDF Terminology

A triple can express a relationship between resources

Resource Property Value

http://MuzicBiz.org/Albums/7655432

http://MuzicBiz.org/Prop/Track

http://MuzicBiz.org/Tracks/1667653



Track











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RDF Terminology

The terminology for this model is the SUBJECT of our statement is the album and the track is the OBJECT

The two resources are joined by a PREDICATE

The predicate specifies the nature of the relationship between the two resources



Track





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RDF Terminology

Notation

When writing about RDF it is useful to be able to show statements or sets of triples for discussion

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Notation

English

English is simplist

Craig Hoy is the author of http://www.ePolitix.com/Articles/0000005a4787.htm



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Notation

SUBJECT has a PREDICATE of OBJECT

Example

http://www.ePolitix.com/Articles/0000005a4787.htm has an author of Criag Hoy



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NotationDirected labeled graphs

.../Articles/.../Articles/000000005a4787.htm000000005a4787.htm Craig HoyCraig Hoy

author

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Notation

Three parts of a triple{[http://MuzicBiz.org/Review],[http://MuzicBiz.org/Albums/101234],“A relaxing album to prune to.”}

{[http://MuzicBiz.org/Review],[http://MuzicBiz.org/Albums/7655432],“Lively! Perfect when mowing the lawn.”}

{[http://MuzicBiz.org/Review],[http://MuzicBiz.org/Albums/8997654],“Very moody. Great when planning your next planting.”}

http://MuzicBiz.org/Review%5D


















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Notation

Complex sets of data can most compactly be represented in a graph

.../Articles/.../Articles/000000005a4787.htm000000005a4787.htm .../Authors/Craig%20Hoy.../Authors/Craig%20Hoy

.../companynumber/.../companynumber/39356443935644 EditorEditor

<dc:Creator>

<dc:Publisher>

<dc:Creator>

<xyz:JobTitle>

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RDF Syntax

So far we’ve seen how RDF models meta data

Now we need to look at how these models are expressed in XML

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RDF/XML

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How is a statement formed?

Statement begins -

Reference to the resource that the statement is about (SUBJECT)

This is in the rdf:about attribute of the <rdf:Description> element

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How is a statement formed?

The statement is located inside the <rdf:Description> element

Says there is a property of this resource - dc:Creator that has a value of “Craig Hoy”

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Many Namespaces

When there are many namespaces to be defined in an RDF document grouping them in one place makes them stand out

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RDF Elements

<rdf:Description> Element

Contains the URI for the resource being described

The <rdf:Description> element identifies the subject

A child element defines a predicate/object pair

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<rdf:Description>

More detail about this element -

Multiple properties for the same resource

String literals and resource URI’s

Nesting statements

rdf:about attribute

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<rdf:Description>

More detail about this element -

The rdf:ID attribute

Anonymous resources

The rdf:type attribute

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<rdf:Description>

The <rdf:Description> element is actually a container for as many predicate/object pairs are you want

.../Articles/.../Articles/000000005a4787.htm000000005a4787.htm Craig HoyCraig Hoy

dc:Creator

ePolitixePolitixdc:Publisher

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<rdf:Description>

One or more properties may be specified for the same resource

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<rdf:Description>

An alternative syntax

Attributes take the place of child elements

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<rdf:Description>

In order that a resource not be confused with a string literal, there is an RDF attribute

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<rdf:Description>

Supposing we wanted to add some information to the description

../Articles/../Articles/000000005a4787.htm000000005a4787.htm

../companynumber/../companynumber/3935644.htm3935644.htm

../Authors/Craig%20Hoy../Authors/Craig%20Hoy

EditorEditor

<dc:Publisher>

<dc:Creator>

<>

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<rdf:Description>

One way to code this in RDF is to simply add a statement that contains the new information

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<rdf:Description>

RDF allows for the <rdf:Description> element to be nested

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<rdf:Description>

Both representations are correct and the underlying model is the same in both cases

Which to use depends on context?

If there are many articles, the nested information would be repeated

Therefore the first representation would be preferable in this case

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<rdf:Description>

Attributes

We know about the rdf:about attribute

The contents of the rdf:about attribute are a URI

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<rdf:Description>

rdf:ID

This attribute allows a resource in a document to be named and then referred to with this name

The ID attribute and the about attribute ARE EXCLUSIVE - only one or the other can be used

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rdf:ID

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Anonymous Resources

An option for the <rdf:Description> element would be to NOT specify an rdf:about or rdf:ID attribute

This would be the way to introduce anonymous resources as part of an RDF description

The description element would exist for no other reason then to be given properties

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Anonymous Resources

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Anonymous Resources

<dc:creator>

<v.name>

<v.email>

<xyz:jobtitle>

../Articles/000000005a4787.htm

Craig Hoy

[email protected]

editor

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Anonymous Resources

Back to Mozart

Assume that some authority has given the piece “Eine Kleine Nacht Muzik” the URL:

http://MuzikBiz.org/233456

We can also give this piece of music an assigned code from the Dewey Decimal Classification code

781.68


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Anonymous Resources

The resulting statement describing this would be:

http://MuzikBiz.org/233456 has a dc:Subject of 781.68

The RDF is shown following


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Anonymous Resources

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Anonymous Resources

If we want now to identify the source of this classification we can do so with the RDF value tag (shown following)

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Anonymous Resources

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Anonymous Resources

When representing an anonymous resource like this one, we know that there is some resource we are representing, we just don’t know how to name it

This is why we introduce an <rdf:Description> tag into the RDF without an rdf:about tag

The result is a graph with an anonymous node

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Anonymous Resources

<dc:Subject>

<rdf:value>

<xya:Classification>

http://muzicBiz.com/233456

781.68

DDC

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rdf:type Attribute

Applies to <rdf:Description>

Powerful

Links worlds of knowledge representation to object orientation (ooh ... aah)

Allows us to specify that the resource being referred to is of a particular class

Allows parsers to understand more about the meta data

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rdf:Type Attribute

Assume that an organization named the International Press Telecommunications Council (IPTC) is responsible for the XML format used in the ePolitix articles we have been using

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rdf:Type Attribute

IPTC has defined a URI that allows us to indicate that the article being referred to is in their NITF format

NITF refers to News Industry Text Format

This format is used widely to transfer news between organizations

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rdf:type Attribute

The URL for all object types that belong to the NITF group of objects is something like -

http://www.iptc.org/schema/NITF#

http://www.iptc.org/schemeeeeeee%5D

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rdf:type Attribute

This information could be used to enhance the RDF used in the ePolitix XML

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rdf:Type Attribute

Now the rdf:type attribute gives us a very powerful capability akin to one that we would find in object-oriented programming

Once we know that a particular resource is of a particular type then we can use that information to check its meta-tags to insure that the correct meta-tags are used

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rdf:type Attribute

For example if we are referring to a person resource AND we have said that a person has a FORMAT then this is probably incorrect

(The dc:Format property is used to specify the type of MIME documet)

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rdf:type Attribute

But we know nothing more at this point about the resource

http://www.ePolitix.com/Authors/Craig%20Hoy

By specifying an rdf:type we can give the RDF processor more information

http://www.ePolitix.com/Craig%20Hoy

http://www.ePolitix.com/Craig%20Hoy

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rdf:type Attribute

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Typed Elements

An alternate syntax to use to express the same type of information are known as TYPED ELEMENTS

In this notation the resource that would be used in the rdf:type attribute would be turned into a namespace qualified element

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Typed Elements

We assumed a namespace prefix for objects created by the IPTC for their NITF stnadard

The namespace prefix was:

http://www.iptc.org/schema/NITF

It is now possible to create object types or references to schemas by specifying a URI as in:

http://www.iptc.org/schema/NITF#NewsArticle

http://www.iptc.org/schema/NITF



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Typed Elements

By assigning the prefix that was just defined to a namespace paceholder, and use the classname as the name of an element the <rdf:Description> element can be replaced

<rdf:type rdf:resource=”http://www.itpc.org/schema/NITF#NewsArticle ==>

<rdf:RDF xmlns:nitf=”http://www.iptc.org/schema/NITF#”

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Typed Elements

becomes

Namespace definitions

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Typed Elements

This feature is very important to RDF

Anything which can appear in an RDF description tag,

is valid when used as a typed element

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Typed Elements

Observe the change to attributes

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Typed Elements

Being able to do this allows you to extract data from existing XML documents in the form of triplese

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Property Elements

Property Information can be expressed through -

String literals

value for a predicate defined by the name of the element containing the literal

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Property Elements

Said a lot about the <rdf:Description> elements so far

Recap

String Literals

Value for a predicate that is defined by the name of the element containing the literal

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Property Elements

Example

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Property Elements

Resources

Express properties of a resources

The value of the predicate is actually another resource

Use a URI to specify which resource it is

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Property Elements

Example

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Property Elements

Yet another way to accomplish this is to nest RDF statements one within another

This says that the value of the property <dc:Creator> is itself a resource

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Property Elements

Type information can also be specified in the content of a property element

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Property Elements

Taking a type resource and turning it into a namespace-qualified element name could abbreviate this

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parseType=”Literal”

Sometimes it is necessary to tell the parser that it should NOT parse a particular part of the RDF

The RDF should be stored as is

Consider the following example

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parseType=”Literal”

We are writing a mathematical paper entitled “Ramifications of (a+b)2 to World Peace”

We would like to create a MathML to specify the title since it can help us format the various symbols properly

If we place the MathML inside the <dc:Title> tag we need a way to tell the RDF parser that the MathML is not RDF

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Ramifications of ...

The contents of this element are not simply a string

The text must be well-formed XML otherwise the parser will fail

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parseType=”Resource”

There are times when the parser cannot tell the difference between a property value and a resource

Property values are usually inside an rdf:Description element

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If this were all there is to it then all would be well. Unfortunately RDF allows us to make statements about the author as follows

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parseType=”resource”

What is all we wanted to do was to provide the email of the author?

We really don’t care about identifying the author

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This still seems too elaborate

We could simply express this information as follows

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So if we were to interpret this we would come up with two different interpretations making its meaning ambiguous

On the one hand if we evaluated the representation from the inside out we would have an anonymous <dc:Creator> element which has a <v:Email> property

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<dc:Creator>

<v:email>

../Articles.0000005a4787.htm

[email protected]

First Interpretation “Inside Out”

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If you interpret the RDF representation from the outside in you would say you had a resource of a web page that had a <dc:Creator> property and that this <dc:Creator> property refers to an anonymous resource of rdf:type v:Email

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<dc:Creator>

<rdf:value>

<<rdf:type>>

../Articles.0000005a4787.htm

[email protected]

http://www.vCard.org/Schemas#Email

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parseType =”Resource”

This second interpretation of the RDF/XML is the one that we would prefer but the parser cannot distinguish which of these two models it should create

The problem is we need the <dc:Creator> element to be interpreted as both a web page and also as an anonymous resource so properties can attach to it

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parseType = “Resource”

RDF/XML does allow us to force the <dc:Creator> to be interpreted as

a predicate

an anonymous resource

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parseType = “Resource”

which is exactly the same as specifying the anonymous resource explicitly

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Containers

Containers

list of resources

collection of resources

Example

List of articles that make up a web site

List of authors who have contributed to an article

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Containers

RDF

Three types of containers

bag

sequence

alternative

can be used anywhere the <rdf:description> element can be used

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<rdf:Bag>

simplest container

used to contain multiple values for a property

no significance to the order of the values

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<rdf:Bag>

Example

The elements in a bag may also be literals

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<rdf:Seq>

Whereas a bag does not impose any order on the elements in the list that is associated with the element, <rdf:Seq> does require that the list attached to it will be in a specific order

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<rdf:Seq>

Example

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<rdf:Alt>

<rdf:Alt> provides us with a way to select from a list of resources, a specific resource

In other words <rdf:Alt> provides a way of specifying alternative options

An rdf processor could choose a resource based on some desirable property

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<rdf:Alt>

Example

Documents

An Introduction to RDF and the Semantic Web