RDF
Dr. Alexandra I. Cristea
http://www.dcs.warwick.ac.uk/~acristea/
RDF Definition
• The Resource Description Framework
(RDF) is a W3C standard for describing
resources on the Web.
• RDF is a framework for describing Web
resources,
– e.g., title, author, modification date,
content, and copyright information of a
Web page.
2
RDF original purpose & the metadata
• originally created in 1999
• as a standard on top of XML, for encoding
metadata
– literally, data about data.
– e.g., who authored a web page, what date a
blog entry was published, etc.,
– secondary info
3
RDF general information on
standard evolution
• http://www.w3.org/standards/techs/rdf
4
What (else) is RDF?
• RDF provides a model for data, and a syntax so
that independent parties can exchange and use it
• RDF is designed to be read and understood by
computers
• RDF is not designed for being displayed to people
• RDF (can be) written in XML
• RDF is a part of the W3C's Semantic Web Activity
• RDF is a W3C Recommendation (so a standard!)
• http://www.w3.org/RDF/
5
RDF Usage Examples
• Describing properties for shopping items,
such as price and availability
• Describing time schedules for web events
• Describing information about web pages,
such as content, author, created and modified
date
• Describing content and rating for web
pictures
• Describing content for search engines
• Describing electronic libraries
6
RDF is Designed to be Read by
Computers
• RDF was designed to provide a
common way to describe information so
it can be read and understood by
computer applications.
• RDF descriptions are not designed to
be displayed on the web.
7
RDF and "The Semantic Web"
• The RDF language is a part of the
W3C's Semantic Web Activity. W3C's
"Semantic Web Vision" is a future
where:
– Web information has exact meaning
– Web information can be understood and
processed by computers
– Computers can integrate information
from the web
8
RDF Statements
• RDF is built of Statement s
• known as the
– subject,
– predicate and
– object
(of a Statement).
9
RDF Statements
Subject
Predicate
Object
10
Informal graph of sample triples
11
Subjects and Objects
•
•
•
•
Resources and values
IRI
Literals
Blank nodes
• Let’s understand them:
12
IRI, URI, URL
• International Resource Identifier
• Uniform Resource Identifier
• Uniform Resource Locator
• The IRI for an INA video about the Mona Lisa entitled
'La Joconde à Washington' in Europeana is:
http://data.europeana.eu/item/04802/243FA8618938
F4117025F17A8B813C5F9AA4D619
13
IRI
• are global identifiers, so other people can
re-use this IRI to identify the same thing.
• IRI used as an RDF property to state
acquaintance relationship :
– http://xmlns.com/foaf/0.1/knows
• DBpedia uses IRIs for articles:
– http://dbpedia.org/resource/Name
14
Literals
• Basic values that are not IRI
• Examples
– "La Joconde",
– "the 4th of July, 1990"
– "3.14159“
• associated with a datatype
• Strings optionally assoc. with language tag.
– "Léonard de Vinci" - "fr" language tag
– "李奥纳多·达·文西" - "zh" language tag.
15
Blank nodes
• When mentioning resources without global
identifier
• ~ variables in algebra
16
Multiple
graphs
17
Example using URIs
• Let's look at some example statements to get a
better understanding:
– Statement: "The author of
http://www.w3schools.com/RDF is Jan Egil Refsnes".
– Statement: "The homepage of
http://www.w3schools.com/RDF is
http://www.w3schools.com".
18
Example – cont.
• Statement: "The author of
http://www.w3schools.com/RDF is Jan Egil
Refsnes".
– The subject of the statement above is:
http://www.w3schools.com/RDF
– The predicate is: author
– The object is: Jan Egil Refsnes
• Statement: "The homepage of
http://www.w3schools.com/RDF is
http://www.w3schools.com".
– The subject of the statement above is:
http://www.w3schools.com/RDF
– The predicate is: homepage
– The object is: http://www.w3schools.com
19
RDF Statements
Subject
Predicate
http://www.w3schools.com/RDF
Object
author
Jan Egil Refsnes
20
RDF Resource, Property, and
Property Value
• RDF identifies things using Web identifiers (URIs), and
• describes resources with properties and property
values.
• A Resource is anything that can have a URI,
– such as "http://www.w3schools.com/RDF"
• A Property is a Resource that has a name,
– such as "author" or "homepage"
• A Property value is the value of a Property,
– such as "Jan Egil Refsnes" or "http://www.w3schools.com"
(note that a property value can be another resource)
21
RDF Statements
Subject
Predicate
http://www.w3schools.com/RDF
Resource
Object
author
Property
Jan Egil Refsnes
Property Value
Can be another resource !!
22
Example
23
RDF vocabulary examples
• "Friend of a Friend" (FOAF)
• Dublin Core – web resources
• schema.org – mark up by search
providers
• SKOS – classification schemes for
libraries
24
So, RDF ..
• Different world view
• Triples (instead of pairs)
• More natural?
• This is the concept … how to implement it?
25
Writing RDF graphs
• Turtle family of RDF languages (NTriples, Turtle, TriG and N-Quads);
• JSON-LD (JSON-based RDF syntax);
• RDFa (for HTML and XML embedding);
• RDF/XML (XML syntax for RDF).
26
RDF can be Written in XML
• RDF documents can be written in XML
language : RDF/XML.
• information exchange between
– different types of computers
– types of operating systems and
– application languages.
27
RDF/XML
http://www.w3.org/TR/2014/REC-rdfsyntax-grammar-20140225/
28
RDF 1.1 XML Syntax
• Original version 1999; most recent
recommendation 25th of February 2014
• Encodes RDF graphs
• Nodes and predicates need represented in XML
terms
• element names,
• attribute names,
• element contents,
• attribute values
29
RDF/XML is XML
30
XML Rules
1.
2.
3.
4.
Every start-tag must have a matching end-tag.
Tags cannot overlap. Proper nesting is required.
XML documents can only have one root element.
Element names must obey the following XML
naming conventions:
a) Names must start with letters or the "_" character.
Names cannot start with numbers or punctuation
characters.
b) After the first character, numbers and punctuation
characters are allowed.
31
XML Rules (cont.)
c) Names cannot contain spaces.
d) Names should not contain the ":" character as it is a
"reserved" character.
e) Names cannot start with the letters "xml" in any
combination of case.
f) The element name must come directly after the "<" without
any spaces between them.
5. XML is case sensitive.
6. XML preserves white space within text.
7. Elements may contain attributes. If an attribute is
present, it must have a value, even if it is an empty
string "".
32
Example RDF/XML
<?xml version="1.0"?>
Resource
Property
<RDF>
<Description about="http://www.w3schools.com/RDF">
<author>Jan Egil Refsnes</author>
<homepage>http://www.w3schools.com</homepage>
</Description>
Property value
</RDF>
33
RDF/XML
• Is not the only solution,
• but the recommended one
• No one-to-one mapping
–
–
–
–
RDF doesn’t have ordering
XML does!
Serialization of a graph!
So is this the price to pay?
• Others: Notation 3
http://www.w3.org/DesignIssues/Notation3.html ,
precursor to Turtle http://www.dajobe.org/2004/01/turtle/
34
XML has namespaces:
Why namespaces?
35
Naming ambiguity
36
The Idea to Solve it
• Assign a URI (~ URL) to every sublanguage:
– E.g., for XHTML 1.0:
http://www.w3.org/1999/xhtml
• Qnames: Qualify element names with
URIs:
– {http://www.w3.org/1999/xhtml}head
Web Naming and Addressing Overview (URIs, URLs, ...)
37
The actual solution
• Namespace declarations bind URIs to
prefixes:
• Default namespace (no prefix) declared
with: xmlns=“…”
• Lexical Scope
• Attribute names can also be prefixed
38
Applying namespaces
39
RDF/XML namespace
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdfsyntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">
40
Other namespaces
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdfsyntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">
41
Example: Mona Lisa
<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF xmlns:dcterms="http://purl.org/dc/terms/" xmlns:foaf="http://xmlns.com/foaf/0.1/"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:schema="http://schema.o
<rdf:Description rdf:about="http://example.org/bob#me">
<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
<schema:birthDate rdf:datatype="http://www.w3.org/2001/XMLSchema#date">1990-0704</schema:birthDate>
<foaf:knows rdf:resource="http://example.org/alice#me"/>
<foaf:topic_interest rdf:resource="http://www.wikidata.org/entity/Q12418"/>
</rdf:Description>
<rdf:Description rdf:about="http://www.wikidata.org/entity/Q12418">
<dcterms:title>Mona Lisa</dcterms:title>
<dcterms:creator rdf:resource="http://dbpedia.org/resource/Leonardo_da_Vinci"/>
</rdf:Description>
<rdf:Description
rdf:about="http://data.europeana.eu/item/04802/243FA8618938F4117025F17A8B813C5F9AA
19">
<dcterms:subject rdf:resource="http://www.wikidata.org/entity/Q12418"/>
</rdf:Description>
42
</rdf:RDF>
RDF Online Validator
• W3C's RDF Validation Service is useful when learning
RDF. Here you can experiment with RDF files.
• The online RDF Validator parses your RDF document,
checks your syntax, and generates tabular and
graphical views of your RDF document.
43
The <rdf:RDF> Element
• <rdf:RDF> is the root element of an RDF document. It
defines the XML document to be an RDF document.
• It also contains a reference to the RDF namespace.
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdfsyntax-ns#"> . . Description goes here
</rdf:RDF>
44
The <rdf:Description> Element
•
The <rdf:Description> element identifies a resource
with the about attribute.
• The <rdf:Description> element contains elements
that describe the resource.
<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>
45
A Resource and its Description
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">
<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire
Burlesque">
…
</rdf:Description> . . .
</rdf:RDF>
46
Another example
Title
Artist
Country Company
Price Year
Empire Burlesque Bob Dylan
USA
Columbia
10.90 1985
Hide your heart
Bonnie Tyler UK
CBS Records 9.90 1988
...
47
RDF Declaration
<?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> . . .
48
</rdf:RDF>
Properties as Attributes
• The property elements can also be defined as
attributes (instead of elements):
<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire
Burlesque" cd:artist="Bob Dylan"
cd:country="USA" cd:company="Columbia"
cd:price="10.90" cd:year="1985" />
Can you identify the triples here? (resource, property, value)
49
Properties as Resources
• The property elements can also be defined as
resources:
<rdf:Description
rdf:about="http://www.recshop.fake/cd/Empire
Burlesque">
<cd:artist
rdf:resource="http://www.recshop.fake/cd/dylan" />
...
</rdf:Description>
50
Compare: properties as
• elements
<rdf:Description rdf:about="http://www.recshop.fake/cd/Empire
Burlesque">
<cd:artist>Bob Dylan</cd:artist>
</rdf:Description>
• attributes
<rdf:Description rdf:about="http://www.recshop.fake/cd/Empire
Burlesque" cd:artist="Bob Dylan" />
• resources
<rdf:Description rdf:about="http://www.recshop.fake/cd/Empire
Burlesque">
<cd:artist rdf:resource="http://www.recshop.fake/cd/dylan" />
</rdf:Description>
51
RDF Containers
• RDF containers are used to describe
groups of things.
• e.g, to list the authors of a book or to list
the members in a band.
• The following RDF elements are used to
describe such groups:
<Bag>, <Seq>, and <Alt>.
52
RDF Bag
• The <rdf:Bag> element is used to describe a list of values that
is intended to be unordered.
• The <rdf:Bag> element may contain duplicate values.
<rdf:Description rdf:about="http://www.recshop.fake/cd/Beatles">
<cd:artist>
<rdf:Bag>
<rdf:li>John</rdf:li>
<rdf:li>Paul</rdf:li>
<rdf:li>George</rdf:li>
<rdf:li>Ringo</rdf:li>
</rdf:Bag>
</cd:artist>
</rdf:Description>
53
Student example
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:s="http://example.org/students/vocab#">
<rdf:Description rdf:about="http://example.org/courses/6.001">
<s:students>
<rdf:Bag>
<rdf:li rdf:resource="http://example.org/students/Amy"/>
<rdf:li rdf:resource="http://example.org/students/Mohamed"/>
<rdf:li rdf:resource="http://example.org/students/Johann"/>
<rdf:li rdf:resource="http://example.org/students/Maria"/>
<rdf:li rdf:resource="http://example.org/students/Phuong"/>
</rdf:Bag>
</s:students>
</rdf:Description>
</rdf:RDF>
54
RDF Bag
55
The <rdf:Seq> Element
• The <rdf:Seq> element is used to describe a list of values that is
intended to be ordered (For example, in alphabetical order).
• The <rdf:Seq> element may contain duplicate values.
<rdf:Description rdf:about="http://www.recshop.fake/cd/Beatles">
<cd:artist>
<rdf:Seq>
<rdf:li>George</rdf:li>
<rdf:li>John</rdf:li>
<rdf:li>Paul</rdf:li>
<rdf:li>Ringo</rdf:li>
</rdf:Seq>
</cd:artist>
</rdf:Description>
56
The <rdf:Alt> Element
• The <rdf:Alt> element is used to describe a list of alternative
values (the user can select only one of the values).
<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>
57
RDF Alt
58
The rdf:parseType="Collection"
Attribute
• collection can be closed (container cannot )
• RDF collections are used to describe a group
that contains ONLY the specified members.
• A collection is described by the attribute
rdf:parseType="Collection".
59
Collection attribute example
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntaxns#" xmlns:cd="http://recshop.fake/cd#">
<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>
</rdf:RDF>
60
Collection: a graphical representation
61
Collection
is a short
notation
<rdf:Description rdf:about="http://e.org/family/349">
<s:familyMembers rdf:nodeID="sch1"/>
</rdf:Description>
<rdf:Description rdf:nodeID="sch1">
<rdf:first rdf:resource="http://e.org/person/Paolo"/>
<rdf:rest rdf:nodeID="sch2"/>
</rdf:Description>
<rdf:Description rdf:about="http://e.org/family/349">
<rdf:Description rdf:nodeID="sch2">
<s:familyMembers rdf:parseType="Collection">
<rdf:first rdf:resource="http://e.org/person/Emanuele"/>
<rdf:Description rdf:about="http://e.org/person/Paolo"/>
<rdf:rest rdf:nodeID="sch3"/>
<rdf:Description rdf:about="http://e.org/person/Emanuele"/>
</rdf:Description>
<rdf:Description rdf:about="http://e.org/person/Maria"/>
<rdf:Description rdf:about="http://e.org/person/Franco"/>
<rdf:Description rdf:nodeID="sch3">
</s:familyMembers> <rdf:first rdf:resource="http://e.org/person/Maria"/>
</rdf:Description>
<rdf:rest rdf:nodeID="sch4"/>
</rdf:Description>
<rdf:Description rdf:nodeID="sch4">
<rdf:first rdf:resource="http://e.org/person/Franco"/>
<rdf:rest rdf:resource="http://www.w3.org/1999/02/22-rdfsyntax-ns#nil"/>
</rdf:Desciption>
62
RDF Reification
• Statements about statements
– E.g., record info about when statements
were made, who made them, etc.
("provenance" information).
63
Example
• example.com could record the fact that:
John Smith made the original statement
about the tent's weight
• By:
– original statement URIref (exproducts:triple12345),
describing it w. reification, then
– adding an additional statement that
exproducts:triple12345 was written by John Smith
(w. URIref for John Smith)
64
Example figure: reification
original
statement
John
65
Reification Rules
• For the given IRI event r and the statement with terms s, p and o
corresponding to the N-Triples:
• spo.
• add the following statements to the graph:
r.string-value <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> s .
r.string-value <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> p .
r.string-value <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> o .
r.string-value <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement> .
66
RDF Schema and Application Classes
• RDF describes resources with classes,
properties, and values
• RDF needs to define applicationspecific classes and properties
– with extensions to RDF
• One such extension is RDF Schema
67
RDF Schema
http://www.w3.org/TR/2014/REC-rdfschema-20140225/
68
RDF Schema
• W3C recommendation since 2004;
newest 25th February 2014
• RDF Schema 1.1
• provides a data-modelling vocabulary
for RDF data.
• is an extension of the basic RDF
vocabulary.
69
RDF Schema (RDFS)
• does not provide actual applicationspecific classes and properties.
• … but framework to describe them
• Classes ~ OO programming languages.
• Resources:
– instances of classes, or
– subclasses of classes.
70
RDFS Example
The following example demonstrates some of the RDFS facilities:
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf= "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xml:base=
"http://www.animals.fake/animals#">
<rdf:Description rdf:ID="animal">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
</rdf:Description>
<rdf:Description rdf:ID="horse">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
<rdfs:subClassOf rdf:resource="#animal"/>
</rdf:Description>
</rdf:RDF>
In the example above, the resource "horse" is a subclass of the class "animal".
71
Example of class structure
72
<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://example.org/schemas/vehicles">
<rdf:Description rdf:ID="MotorVehicle">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
</rdf:Description>
<rdf:Description rdf:ID="PassengerVehicle">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
<rdfs:subClassOf rdf:resource="#MotorVehicle"/>
</rdf:Description>
<rdf:Description rdf:ID="Van">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
<rdfs:subClassOf rdf:resource="#MotorVehicle"/>
</rdf:Description>
<rdf:Description rdf:ID="MiniVan">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
<rdfs:subClassOf rdf:resource="#Van"/>
<rdfs:subClassOf rdf:resource="#PassengerVehicle"/> Multiple inheritance!
</rdf:Description>
</rdf:RDF>
73
Example Abbreviated
• Since an RDFS class is an RDF
resource we can abbreviate it using
rdfs:Class instead of rdf:Description,
and drop the rdf:type information
74
Original Class declaration
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf= "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base= "http://www.animals.fake/animals#">
<rdf:Description rdf:ID="animal">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
</rdf:Description>
<rdf:Description rdf:ID="horse">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
<rdfs:subClassOf rdf:resource="#animal"/>
</rdf:Description>
</rdf:RDF>
75
Example Class Abbreviated
<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf= "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base= "http://www.animals.fake/animals#">
<rdfs:Class rdf:ID="animal" />
<rdfs:Class rdf:ID="horse">
<rdfs:subClassOf rdf:resource="#animal"/>
</rdfs:Class>
</rdf:RDF>
76
Original and abbreviated class
So we replace:
<rdf:Description rdf:ID="animal">
<rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/>
</rdf:Description>
with
<rdfs:Class rdf:ID="animal" />
77
Classes
•
rdfs:Resource
rdfs:Class
rdfs:Literal
rdfs:Datatype
rdf:XMLLiteral
rdf:Property
78
Properties
•
rdfs:range
rdfs:domain
rdf:type
rdfs:subClassOf
rdfs:subPropertyOf
rdfs:label
rdfs:comment
79
RDF Conclusion
• We have learned:
– RDF
– Usage scenarios, basics
– RDFS and examples
– Comparison w. XML and graph languages
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Homework
• Lists 10 URI that identify things in your environment.
• Give several examples of how to extend the definition of literals in RDF. Why
would this be useful?
• The RDFS specification allows more than one domain to be defined for a
property and uses the intersection of these domains. Discuss the pros and
cons of taking the union versus taking the intersection of domains.
• In an older version of the RDFS specfication, rdfs:subClassOf was not
allowed to have cycles. Try to imagine situations where a cyclic clas
relationship would be beneficial. (Hint: Think of equivalences between
classes)
• Discuss the differences between the following statements, and draw graphs
to illustrate the difference:
• X supports the proposal; Y supports the proposal; Z supports the proposal.
• The group of X, Y and Z supports the proposal.
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