Linked Data Tutorial

Linked Data Tutorial

@ Vlaams Theater Instituut
Brussels, 03/2009
bernhard.haslhofer@univie.ac.at
Bernhard Haslhofer, Linked Data Tutorial
Friday, March 6, 2009

Contents

• Motivation for Linked Open Data (LOD)

• Base Technologies

• The Linked Data Principles

• Publishing Solutions and Tools

• Vocabularies & Interoperability Issues

• Demos + Discussion

Bernhard Haslhofer, Linked Data Tutorial 2

How do we look up information about things?


... about this book?


... about this play?


... about these concepts?


... about things?

...that book? ...this play? ...these concepts?

we look up their names

Metamorphosis Efﬁ Briest Love / Hate

and see what we can ﬁnd out

An Example


The Hallstatt culture


formerly

Controlled Vocabulary


formerly

Metadata

Identifier


then

Identifier

Metadata

Controlled Vocabulary

now


If information is not available on the Web, people
tend to ignore it.


Current Situation
Web

Web Server
JDBC HTML

Database


The current Web is a Web of Documents intended
for human interpretation.


Database
Database
Database
The data are still locked in closed silos.

Database
Database Database

Other applications can not access and process
these data unless...


Existing Approaches
CD-ROM FTP
REST
OAI-PMH
Web Services
SRU/SRQ SOAP
OAI-ORE
RSS WSDL
Atom UDDI
JSON CD-ROM
RPC
RMI CORBA
Z39.50 DCOM

The linked data vision is to...


Documents

Data
Database

Bernhard Haslhofer, Linked Data Tutorial

• open the data silos and get rid of repository-centric
mindset

• publish data of public interest on the Web

• in a way that other applications can access and
interpret the data

• using common Web technologies


make it possible for applications to look up
(meta)data ...


... about things

...that book? ...this play? ...these concepts?

by looking up their names
http://dbpedia.org/resource/
Love
http://dbpedia.org/resource/ http://dbpedia.org/resource/
The_Metamorphosis Efﬁ_Briest
Hatred
on the Web

To do so, we need some technology


Uniform Resource Identiﬁer (URI)
• Names (Identiﬁers) for resources in an open
environment

• dereferencable URI = HTTP URI (URL)

Hallstatt_culture


Resource Description Framework (RDF)
• a model for representing metadata on the Web

• in the form of statements (triples)

“The Hallstatt culture
t
abstrac
: was ...”
p
dbppro
rdfs: label
“Hallstatt culture”
Hallstatt_culture
skos:s
ubject
http://
dbpedia.org/resource/
Category:Iron_Age_Europe


RDF/XML, N3, Turtle, etc.
• for exchanging RDF data

• serialization & de-serialization

<rdf:Description rdf:about=” http://dbpedia.org/resource/Hallstatt_culture”>
<dbpprop:abstract>The Hallstatt culture was...</dbpprop:abstract>
<skos:subject rdf:resource=”http://dbpedia.org/resource/
Category:Iron_Age_Europe”>
<rdfs:label>Hallstatt culture</rdfs:label>
</rdf:Description>


RDFS & OWL

• languages for describing vocabularies

rdf: Property

rdf: type rdf: type
rdf: type

dbpprop: abstract skos: subject rdfs: label


Simple Knowledge Organization System (SKOS)

• a language for describing controlled vocabularies

“Iron Age
“Hallstatt Period” “Hallstatt Culture”
Europe”

skos:prefLabel skos:prefLabel skos:altLabel
“La Tène Period”

skos:narrower
dbpedia:Category:
xyz: HallstattCulture
skos:prefLabel
Iron_Age_Europe

skos:narrower xyz: LateneCulture


SPARQL
• a query language & protocol for accessing RDF data
via the Web

SELECT ?uri
WHERE {
?uri skos:subject <http://dbpedia.org/resource/Category:Iron_Age_Europe>.
}


The vision is becoming reality...


http://en.wikipedia.org/wiki/Hallstatt_culture

<http://dbpedia.org/resource/Hallstatt_culture >
p:abstract “Die Hallstattzeit bezeichnet...” @de
p:abstract “The Hallstatt culture was...” @en
rdfs:label “Hallstatt culture”@en;
rdfs:label “Hallstattzeit” @de;
rdfs:label “Civilisation de Hallstatt” @fr;
rdfs:label “ ” @jp;

skos:subject dbpedia:Category:Iron_Age_Europe;
...

...
p:hasPhotoCollection <http://www4.wiwiss.fu-berlin.de/ﬂickrwrappr/
photos/Hallstatt_culture>;
...


Library of Congress Subject Headings


The LOD idea in brief

• expose data on the Web, not just documents

• interlink these data with those of other data sources


LOD beneﬁts

• other humans and applications can
• easily access your data using Web technologies

• follow the links in order to obtain further contextual
information

• links to your data and search engine indices can
increase the visibility of your data


“Stop hugging
your data”
Sir Tim Berners-Lee, 2009

(c) Paul Miller, http://www.slideshare.net/
cloudofdata/toward-the-data-cloud


Contents








The Semantic Web Layer Cake
User Interface & Applications

Trust

Proof

Unifying Logic

Vocabulary: Ontology: Rules:
SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S

Data Interchange: RDF

XML

URI Unicode


From a Database point of view

Database Domain Semantic Web

Relational Model
URI, RDF
(Tables)

SQL DDL
RDFS, OWL
(Create Table table-name ...)

SQL Query Language
SPARQL
(Select * from ...)


Uniform Resource Identiﬁers (URI)

• Unambiguous name for “something”
• for a digital resource
• for a concept within a vocabulary Hallstatt_culture

• etc..

• “A URI is a compact sequence of characters that
identiﬁes an abstract or physical resource”
[RFC 3986]
Hallstatt

• “Uniform”:
• different types of resource identifiers in the same context

• common syntactic conventions

• “Resource”:
• whatever might be identified by a URI

• “Identifier”:
• distinguish one resource from other ones


• An URI can be classiﬁed as locator (URL), name
(URN), or both

• Uniform Resource Locator (URL)
• a means for locating the resource by describing its
primary access mechanism (e.g., http://example.com)

• Uniform Resource Name (URN)
• a means for naming the resource (e.g.,
urn:example.com:animal)


• Generic syntax - a hierarchical sequence of
components
URI = scheme “:” hier-path [ “?” query ] [ “#” fragment ]

foo://example.com:8042/over/there?name=ferret#nose
_/ ________________/_________/ _________/ __/
| | | | |
scheme authority path query fragment
| ______________________|_
//
urn:example:animal:ferret:nose


• Example URIs:

ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix
tel:+1-816-555-1212
urn:oasis:names:speciﬁcation:docbook:dtd:xml:4.1.2


Internationalized Resource Identiﬁers (IRI)

• can we use non-ASCII characters in URIs (e.g.,
Umlaut, Chinese, Arabic, Hebrew, etc...)?

• extend the syntax of URI so that a much wider
repertoire of characters can be used

• uses Universal Character Set (ISO 10646)

• mapping from IRIs to URIs

• IRIs can be used wherever URIs are allowed



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode



• The basic structural element of RDF is the statement

rdfs: label
“Hallstatt culture”
Hallstatt_culture
skos:s
ubject

Category:Iron_Age_Europe

Subject Predicate Object
Subject Property Value

• The subject of a statement is always a (URI) resource

• The predicate of a statement is always a (URI)
resource

• The object of a statement can be a resource (URI) or
a typed literal

• An RDF statement forms a triple

• Triples can be merged into a set of triples, forming a
directed labeled graph


• Literals can be plain or typed using arbitrary datatypes

• It is recommended to use XML Schema datatypes
[XMLS2]

rdfs: label
“Hallstatt”
Hallstatt
geo:lo
ng

“13.646667”^^<http://
www.w3.org/2001/
XMLSchema#ﬂoat>


• Support for multilingual labels

• Plain literals may have a language tag, as deﬁned by
RFC 3066
“ (
l
: labe )”@jp
rdfs
rdfs: label
“Hallstatt”@de
Hallstatt
rdfs:la
bel

“Гальштат”@ru


• Blank nodes can be used to model structured
information that needs no URI by itself

• they must still be distinguishable within an RDF graph.
This is done using blank node identiﬁers

http://www.ex.com/
http://www.ex.com/#address
staff#85740

http://www.ex.com/#zipcode
http://www.ex.com/#city

http://www.ex.com/#street http://www.ex.com/#state
quot;01730quot;
quot;Bedfordquot;

quot;1501 Grant
quot;Massachusettsquot;
Avenuequot;


• Containers allow to describe groups of things

• Bag (unordered set), Seq (ordered set), Alt (choice)
http://www.ex.com/ http://www.w3.org/1999/02/22-rdf-
courses#6.001 syntax-ns#Bag

http://www.ex.com/students#students http://www.ex.com/students#Amy
http://www.w3.org/1999/02/22-rdf-syntax-ns#type

http://www.w3.org/1999/02/22-rdf-syntax-ns#_1

http://www.ex.com/
http://www.w3.org/1999/02/22-rdf-syntax-ns#_2 students#Mohamed

http://www.w3.org/1999/02/22-rdf-syntax-ns#_3

http://www.w3.org/1999/02/22-rdf-syntax-ns#_4 http://www.ex.com/students#Johann

blank node

http://www.ex.com/students#Maria


• Collections allow the definition of closed containers

• define the exact set of items in a collection
http://www.ex.com/
courses#6.001

http://www.ex.com/students#students

http://www.w3.org/1999/02/22-rdf-syntax-ns#first
http://www.ex.com/students#Amy
http://www.w3.org/1999/02/22-rdf-syntax-ns#rest

http://www.ex.com/
students#Mohamed


http://www.ex.com/students#Johann

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



• RDF can be serialized using various syntax formats:
<?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#” xmlns:dbpprop=”http://dbpedia.org/property/” .... >

<rdf:Description rdf:about=” http://dbpedia.org/resource/Hallstatt_culture”>
<dbpprop:abstract>The Hallstatt culture was...</dbpprop:abstract>
<skos:subject rdf:resource=” http://dbpedia.org/resource/
Category:Iron_Age_Europe”>
<rdfs:label>Hallstatt culture</rdfs:label>
RDF/XML
</rdf:Description>



• RDF can be serialized using various syntax formats:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
@prefix rdfs: <http://dbpedia.org/about/html/http://www.w3.org/2000/01/rdf-schema#>
@prefix dbpprop: <http://dbpedia.org/property/>

<http://dbpedia.org/resource/Hallstatt_culture>
dbpprop:abstract “The Hallstatt culture was...” ;
skos:subject < http://dbpedia.org/resource/Category:Iron_Age_Europe > ;

Turtle / N3
rdfs:label “Hallstatt culture” .



• Reiﬁcation
• allows to describe other statements; i.e. make statements
about other statements

• rarely used in practice



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode


RDF Schema (RDFS)

• extends RDF with the possibility to deﬁne classes and
associated properties

• allows RDF applications to agree on a common data
description vocabulary

• is implemented on top of RDF - each valid RDFS
document is a valid RDF document


RDF Schema (RDFS)

• classes and subclass relationships

rdfs: Class
rdf:type
rdf:type

yago:
yago: City CitiesAndTownsInUpperAustria
rdfs: subClassOf

rdf:type

Hallstatt

RDF Schema (RDFS)

• properties and subPropertyOf relationships

rdf: Property
rdf:type
rdf:type

dcterms: description dbProp: abstract
rdfs: subPropertyOf

Note: in reality, dbpedia: abstract is no subject of
Bernhard Haslhofer, Linked Data Tutorial 63 dcterms; description

RDF Schema (RDFS)

• domain and range relationships rdfs: Class

rdf: type

xyz: Article
rdfs: domain

dbProp: abstract

rdfs: range xsd: string


RDF Schema (RDFS)

• comments: rdfs: comment

• human-readable names for resources: rdfs: label

• machine-readable information: rdfs: seeAlso, rdfs:
isDeﬁnedBy


RDF Schema (RDFS)

• Shortcomings:
• no cardinality constraints on properties

• no inverse / transitive / symmetric properties

• no union / disjoint / enumeration classes



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode


Web Ontology Language (OWL)
• An extension of RDFS

• logic based; provides automated reasoning support

• Three “sub-languages”
• OWL Full: maximum expressiveness and syntactic
freedom

• OWL DL: restricted to ﬁrst order logic; decidable

• OWL Lite: easy to implement; provides mechanisms for
creating classiﬁcation hierarchies with simple constraints


• owl:Class deﬁnes a group of individuals that belong
together because of shared properties

• owl:Thing is the class of all individuals, and a superclass
of all OWL classes
rdfs: Class
rdfs: subClassOf
owl: Class

owl: Thing
rdfs: subClassOf
rdf:type

rdf:type
dbpedia-owl: Place
Hallstatt


• DatatypeProperty: properties whose value is a literal

• ObjectProperty: properties whose value is an individual
owl:
rdf:type rdfs: range xsd: string
DatatypeProperty

dbPedia-owl: id
rdfs: subClassOf
rdfs: domain dbPedia-owl:
rdf: Property
WorldHeritageSite
dbPedia-owl:
rdf:type rdfs: domain
rdfs: subClassOf region
owl: dbpedia-owl:
rdfs: range
ObjectProperty PopulatedPlace


• owl: equivalentClass states that two classes are
equivalent, i.e., they have the same instances

owl: equivalentClass
dbpedia: Car dbpedia: Automobile

Note: DBpedia actually uses dbpedia: redirect

• owl: equivalentProperty states that two properties are
equivalent; they relate one individual to the same set
of other individuals

owl: equivalentProperty
myOnt: abstract dbpprop: abstract



• owl:sameAs states that two individuals are the same

owl: sameAs
fbase: Hallstatt culture
Hallstatt_culture



• owl: inverseOf (e.g., hasChild inverseOf hasParent)

• owl: transitiveProperty (e.g., hasAncestor)

• owl: symmetricProperty (e.g., knows)

• owl: functionalProperty (e.g. hasPrimaryEmployer)

• owl: inverseFunctionalProperty (e.g.,
hasSocialSecurityNumber)



• owl: allValuesFrom restricts properties to values of
classes

• owl: minCardinality restricts a property’s minimum
cardinality with respect to a class

• owl: maxCardinalty restricts a property’s maximum
cardinality with respect to a class

• owl: cardinality = shortcut for min/maxCardinality



• owl: intersectionOf (e.g., EmployedPerson = Person ∩
CorporateResources)

• owl: unionOf (e.g., NorthAmericanCitizen = USCitizen
∪ CanadaCitizen)

• owl: complementOf (e.g., JuniorResearcher =
Researcher - SeniorResearcher)

• owl: disjointWith (e.g., Woman ∩ Man = ∅)



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode



• a family of formal languages for the deﬁnition and
representation of controlled vocabularies on the Web
• thesauri

• classiﬁcation schemes

• taxonomies

• subject-heading systems

• an application of RDF/S & OWL



• vocabulary terms are represented as concepts
identiﬁed by URIs

skos: Concept

rdf: type rdf: type

dbpedia: Category:
Iron_Age_Europe



• concepts have preferred, alternate & hidden labels

• label is either preferred or alternate or hidden; one
per language
“Hallstatt Period@en”
bel
a
refL
:p
kos
s l
fLabe “Hallstatt Kultur@de”
s: pre
sko
skos:
altLab
el
“Hallstatt Culture@en”


• concepts can be organized in hierarchies by (the
inverse) skos: broader or skos: narrower relationships

dbpedia: Category:
dbpedia: Category: Iron_Age
Prehistoric_Europe

skos: broader skos: broader

skos: narrower
dbpedia: Category:
Iron_Age_Europe

Note: in DBPedia this narrower rel. does not exist


• concepts can be associated by non-hierarchical (non-
transitive) skos:related properties

skos: related
xyz: HallstattCulture xyz: HallstattCivilization



• concepts can have notes for general documentation
purposes
• scopeNote: an indication how the use of a concept is
limited in indexing practice

• deﬁnition: an explanation of the intended meaning

• example: supplies an example of the use of a concept

• historyNote: describes signiﬁcant changes to the meaning
of a concept



• concepts can be defined as part of well-defined
concept schemes (thesauri, classification schemes)
dc:title
“Archeology Thesaurus”
ex: archeologyThesaurus
dc:creator
“John Doe”

skos: inScheme
skos: inScheme

xyz: HallstattCulture xyz: HallstattCivilization


• Other features
• SKOS allows the mapping between different concept
schemes

• concepts can be organized in collections

• relationships between concept labels

• deﬁnition of broaderTransitive and narrowerTransitive
relationships

• ....



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode


SPARQL

• is a query language that allows to access RDF data
• currently read-only; no update or delete

• based on matching graph patterns

• is a protocol that deﬁnes how queries and results can
be transported over a network (over the Web)


SPARQL

• a simple query example
SELECT ?uri
WHERE {
}

uri
...

dbpedia: Hallstatt_culture

...


SPARQL

• querying multiple variables
SELECT ?uri ?abstract
WHERE {
?uri dbpprop:abstract ?abstract .
}

uri abstract
...

dbpedia: Hallstatt_culture The Hallstatt culture was the...

dbpedia: Hallstatt_culture Гальшта́тская культу́ра ...


SPARQL
• querying and ﬁltering literal values
SELECT ?uri ?label
WHERE {
?uri rdfs:label ?label .
?uri skos:subject <http://dbpedia.org/resource/Category:Iron_Age_Europe> .
FILTER regex(?label, “culture”, “i”)
}
uri label
dbpedia: Basarabi_culture Basarabi Culture

... ...

dbpedia: Hallstatt_culture Hallstatt Culture


SPARQL
• optional graph patterns

SELECT ?uri ?label ?image
WHERE {
OPTIONAL { ?uri foaf:img ?image }.
}


SPARQL
• optional graph patterns with constraints
SELECT ?uri ?label ?image
WHERE {
OPTIONAL {
?uri foaf:img ?image .
FILTER regex(?image, “jpg”, “i”) .
}.
}


SPARQL
• matching alternatives

SELECT ?uri ?classification
WHERE {
{ ?uri rdf:type ?classification . }
UNION
{ ?uri skos:subject ?classification . }
}


SPARQL
• solution modiﬁers
SELECT ?uri ?name
WHERE {
?uri rdfs:label ?name
}
ORDER BY ?name
LIMIT 10
OFFSET 5


SPARQL

• Other SPARQL features:
• rich set of FILTER expressions (see xquery functions)

• CONSTRUCT, DESCRIBE, ASK queries

• Named Graphs



Trust

Proof

Unifying Logic

SKOS OWL RIF
Query:
SPARQL
Crypto
RDF-S


XML

URI Unicode


References
• General
• T. Berners-Lee, J. Hendler, O. Lassila: The Semantic Web. Scientific
American, May 2001.
• URI
• T. Berners-Lee, R. Fielding, L. Masinter: Uniform Resource Identifier (URI):
Generic Syntax (RFC 3986), January 2005.
• M. Duerst, M. Suignard: Internationalized Resource Identifiers (IRIs) (RFC
3987), January 2005.
• RDF/S
• F. Manola, E. Miller: RDF Primer. W3C Recommendation, February 2004.
• D. Brickley, R.V. Guha: RDF Vocabulary Description Language 1.0: RDF
Schema. W3C Recommendation, February 2004.


References
• OWL
• Deborah L. McGuiness, Frank van Harmelen (eds.): OWL Web Ontology
Language Overview. W3C Recommendation 10 February 2004. Available
at http://www.w3.org/TR/owl-features/
• Nataly F. Noy, Deborah L. McGuinness: Ontology Development 101: A
Guide to Creating Your First Ontology. Available at http://www-
ksl.stanford.edu/people/dlm/papers/ontology-tutorial-noy-mcguinness.pdf
• SKOS
• SKOS Primer: http://www.w3.org/TR/2008/WD-skos-primer-20080829/
• SPARQL
• Eric Prud’hommeaux, Andy Seaborne (eds.): SPARQL Query Language for
RDF. W3C Candidate Recommendation 14 June 2007, available at http://
www.w3.org/TR/rdf-sparql-query


Contents








The 4 principles

• Use URIs as names for things

• Use HTTP URIs so that people can look up those
names

• When someone looks up a URI, provide useful
information

• Include links to other other URIs, so that they can
discover more things


Use HTTP URIs....

Hallstatt

Hallstatt_culture

Category:World_Heritage_Sites_in_Austria


Provide useful information ... for humans


Provide useful information ... for machines
p:abstract “Die Hallstattzeit bezeichnet...” @de
p:abstract “The Hallstatt culture was...” @en
rdfs:label “Hallstatt culture”@en;
rdfs:label “Hallstattzeit” @de;
rdfs:label “Civilisation de Hallstatt” @fr;
rdfs:label “ ” @jp;

skos:subject dbpedia:Category:Iron_Age_Europe;
...


Provide useful information for humans and machines

• return different HTTP responses depending on HTTP-
Accept-Header


Provide useful information for humans and machines

• best practice: also assign names (URIs) to various
representations

Hallstatt_culture

Accept: application/rdf+xml Accept: text/html

http://dbpedia.org/data/ http://dbpedia.org/page/
Hallstatt_culture.rdf Hallstatt_culture


Include links

dbpprop: hasPhotoCollection http://www4.wiwiss.fu-berlin.de/
ﬂickrwrappr/photos/Hallstatt_culture

Hallstatt_culture

owl: sameAs
http://rdf.freebase.com/ns/guid.
9202a8c04000641f8000000000239c44


Include links

• other common properties
• rdfs: seeAlso

• foaf: knows

• foaf: based_near

• foaf: topic_interest


Include links

• manual link generation
• only for small data sets

• automatic link generation
• pattern-based algorithms (e.g., same ISBN number)

• more complex property-based algorithms

• see record-linkage problem in database domain


Beneﬁt
• Clients can easily look up names and retrieve
information and follow the links


References

• Berners-Lee 2006: Linked Data. Available at http://
www.w3.org/DesignIssues/LinkedData.html

• Bizer et al.: How to Publish Linked Data on the Web.
Available at: http://www4.wiwiss.fu-berlin.de/bizer/pub/
LinkedDataTutorial/


Contents








Relational Databases

Triplify D2R Server Virtuoso RDF Views

Scripting languages
Java Middleware Solution
Technology (PHP)

- Yes Yes
SPARQL Endpoint

SQL RDF based RDF based
Mapping Language

Manual Semi-automatic Manual
Mapping Generation

Medium-high
medium high
Scalability (but no SPARQL)

(c) Sören Auer, http://www.slideshare.net/soeren1611/linked-data-
Bernhard Haslhofer, Linked Data Tutorial 112 tutorial-presentation-955375

Triplify

• Goal: expose semantics available in RDBMS as simple
as possible

• Available for most popular Web app languages
• PHP (ready), Ruby/Python (under dev.)

• Works with most popular Web app databases
• MySQL, PHP-PDO DBs (SQLite, Oracle, DB2, MS SQL,
PostgreSQL)


Triplify
• Conﬁguration
• number of SQL queries selecting information, which
should be made publicly available

SELECT id, name AS ‘foaf:name’ FROM users


Triplify
!quot;quot;#$%&'()'*()+,-).&'*)/0&12(3')4.#quot;'5)6%'*)&)3728(0)#/)
9:-).&;(03quot;<)
!quot;#$%%&'()*+,-.*(/)%0/1#'123%#(-0%455567
SELECT id, post_author AS 'sioc:has_creator->user',
post_title AS 'dc:title',
post_content AS 'sioc:content', post_date AS
'dcterms:modified^^xsd:dateTime‘,
post_modified AS 'dcterms:created^^xsd:dateTime'
FROM posts
WHERE post_status='publish‘ (AND id=xxx)

SELECT post_id id, tag_label AS 'tag:taggedWithTag‘
FROM post2tag INNER JOIN tag ON(post2tag.tag_id=tag.tag_id)
(WHERE id=xxx)

SELECT post_id id, category_id AS 'belongsToCategory->category‘
FROM post2cat
(WHERE id=xxx)


D2R Server
• a tool for publishing the data from relational databases
on the Web


D2R Server
• D2RQ mapping excerpt

map:Conference a d2rq:ClassMap;
d2rq:dataStorage map:Database1.
d2rq:class :Conference;
d2rq:uriPattern quot;http://conferences.org/comp/confno@@Conferences.ConfID@@quot;;
.
map:eventTitle a d2rq:PropertyBridge;
d2rq:belongsToClassMap map:Conference;
d2rq:property :eventTitle;
d2rq:column quot;Conferences.Namequot;;
d2rq:datatype xsd:string;
.

D2R Server

• supports auto-generation of mapping files
• extracted from database structure

• good starting point

• requires adaption for domain-specific vocabulary

• supports dumping databases to RDF files


Virtuoso RDF Views
• transforms the result of SQL SELECT statements into
RDF

• mapping steps
• define RDFS class IRIs for each table

• define construction of subject IRIs from primary key
column values

• define construction of predicate IRIs from each non-key
column


OAI2LOD Server
• publishes metadata from arbitrary OAI-PMH
endpoints as linked data on the Web

• provides a simple linking framework


OAI2LOD Server

• Sample harvesting conﬁguration:
<> a oai2lod:Server;

rdfs:label quot;Example OAI2LOD Serverquot;;

oai2lod:port 2020;

oai2lod:baseURI <http://localhost:2020/>;

oai2lod:publishes <oai1>;

oai2lod:linkedWith <link1>;

.

<oai1> a oai2lod:OAIServer;

oai2lod:serverURL <http://oai-bdb.onb.ac.at/Script/oai2.aspx>;

oai2lod:metadataPreﬁx quot;oai_dcquot;;

oai2lod:styleSheet quot;xsl/oai_dc2rdf_xml.xslquot;;

oai2lod:maxRecords 50;

.


OAI2LOD Server

• Sample linking conﬁguration:
<link1> a oai2lod:LinkedSPARQLEndpoint;

oai2lod:sparqlService <http://DBpedia.org/sparql>;

oai2lod:maxResults 5000;

oai2lod:linkingRule <lrule1>;

.

<lrule1> a oai2lod:LinkingRule;

oai2lod:sourceType <http://www.mediaspaces.info/vocab/oai-pmh.rdf#Item>;

oai2lod:sourceProperty <http://purl.org/dc/elements/1.1/subject>;

oai2lod:targetType <http://dbpedia.org/class/yago/Capital108518505>;

oai2lod:targetProperty <http://www.w3.org/2000/01/rdf-schema#label>;

oai2lod:linkingProperty <http://www.w3.org/2000/01/rdf-schema#seeAlso>;

oai2lod:similarityMetrics quot;uk.ac.shef.wit.simmetrics.similaritymetrics.Levenshteinquot;;

oai2lod:minSimilarity 1.0;

.


SILK - Link Discovery Framework

• Supports data publishers in linking their data sets with
other

• Provides a declarative Link Speciﬁcation Language
• specify which links to be discovered

• specify conditions data items must fulﬁll in order to be
linked

• implemented in Python


SILK - Link Discovery Framework
<Interlink id=quot;countriesquot;>
<LinkType>owl:sameAs</LinkType>
<SourceDataset dataSource=quot;dbpediaquot; var=quot;aquot;>
<RestrictTo> ?a rdf:type dbpedia:Country </RestrictTo>
</SourceDataset>
<TargetDataset dataSource=quot;factbookquot; var=quot;bquot;/>
...
<LinkCondition>
<AVG>
<Compare id=quot;jaroSimilarityquot; weight=quot;2quot;>
<Param name=quot;string1quot; path=quot;?a/rdfs:labelquot; />
<Param name=quot;string2quot; path=quot;?b/rdfs:labelquot; />
</Compare>
<Compare id=quot;numSimilarityquot;>
<Param name=quot;num1quot; path=quot;?a/dbpedia:populationquot; />
<Param name=quot;num2quot; path=quot;?b/fb:totalPopulationquot; />
<Param name=quot;factorquot; value=quot;2quot; />
</Compare>
</AVG>
</LinkCondition>
<Thresholds accept=quot;0.9quot; verify=quot;0.7quot; />

References

• A Survey of current approaches for mapping of relational databases to RDF:
http://esw.w3.org/topic/Rdb2RdfXG/StateOfTheArt

• Triplify: http://triplify.org/Overview

• D2R Server: http://www4.wiwiss.fu-berlin.de/bizer/d2r-server/

• OpenLink Virtuoso: http://virtuoso.openlinksw.com/Whitepapers/html/
rdf_views/virtuoso_rdf_views_example.html

• OAI2LOD Server: http://www.mediaspaces.info/tools/oai2lod

• SILK: http://www4.wiwiss.fu-berlin.de/bizer/silk/


Contents








Vocabularies & Interoperability Issues

• LOD is about publishing data on the Web

• vocabularies deﬁne the terms that describe the
semantics of these data (e.g., creator, title, abstract,
etc.)

• we can apply the LOD principles and expose
vocabularies on the Web



• Use URIs as names for vocabularies and terms

• Use HTTP URIs so that people (and machines) can
look up those names

• When someone looks up a URI, provide useful
information

• Include links (mappings) to other vocabularies


• Example: Dublin Core terms


• Example: Friend of a Friend (FOAF)



• Publishing vocabularies on the Web recipe



• Step 1: create a complete RDF/XML serialization of the
vocabulary (e.g., example.rdf)

• Step 2: copy the serialization into Web Server directory

• Step3: add a .htaccess directive to the Web Server
directory



• LOD and the Semantic Web provide the technologies
to expose data on the Web

• exposing data in RDF and describing vocabularies in
RDF/S or OWL provides data interoperability on a
technical / structural level

• semantic interoperability is not given by default


rule-of-thumb to establish semantic interoperability
1. whenever possible, use terms from widely used vocabularies that are
published on the Web in a structured, machine-readable format (RDFS
or OWL)

2. if there is no such term that reflects the required semantics, re-use a
semantically broader term by establishing e.g. a rdfs:subProperty
relationship, refine its semantics for the purpose of your application
within a new namespace, and publish it on the Web

3. if (1) and (2) are not feasible, create your own vocabulary and publish
it on the Web in order to make it accessible for other users and
applications; consider to define mappings to other vocabularies


Concepts/
Name Purpose Base
Properties
Dublin Core General /
RDFS 22 / 55
(http://purl.org/dc/terms) Documents

Friend of a Friend (FOAF) Contacts /
OWL 12 / 54
(http://xmlns.com/foaf/0.1/) Communication

VCard Ontology Contacts /
OWL 5 / 54
(http://www.w3.org/2006/vcard/ns#) Communication

Description of a Project (DOAP)
Projects RDFS 7 / 30
(http://usefulinc.com/ns/doap# )


Concepts/
Name Purpose Base
Properties
Semantically Interlinked Online Communities
Contacts /
(SIOC) OWL 11 / 53
Communication
(http://rdfs.org/sioc/ns#)
Nepomuk Message Ontology (NMO)
Contacts /
(http://www.semanticdesktop.org/ontologies/ RDFS 7 / 23
Communication
2007/03/22/nmo# )

GeoNames Ontology
Locations OWL 7 / 18
(http://www.geonames.org/ontology#)

Music Ontology
Multimedia OWL 53 / 131
(http://purl.org/ontology/mo/ )


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!quot;#$%&304$$5$), !$,+6!quot;#$%&304$$5$),
'$(quot;)(*%*+,*quot;)

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!quot;#$#%&'()*# 4)quot;#%#quot;#*4)quot;#:
+#$,-#,) 4;%)< +#$,-#,)*
-$.*)*,*quot;)& 4)quot;#*4;%)<
.)/,/*01+2*34+2* .)/,/*@A4B* 4#CC'$,
.)/,/*40=9
/+)01+0$2
54+*!67)8#9 >?(quot;&#6quot;*4;%)<9

H<;?#<*A;$6)Cquot;-#<*4;%)<
!quot;#$%#&%'()%*
.)/,/*AB@0A:AQ42*=QRQ9
4)quot;#%#quot;#*!67)8#
.)/,/*@A2*KGB2* 4)quot;#%#quot;#*
!.#$#!(,*-*,*# 4)quot;#%#quot;#*=&#8)D;&E
40@!9 A&;((D#<E2*
MN?&'%*4)quot;#%#quot;#*
.)/,/*4FGH:IJ2*4GK!2*0>B!9
7(8$5$2
!67)8#*4#CC'$,
!N(quot;)8
.)/,/*4FGH:O2
>CC<'6#quot;';$*F&;L'<)
KP:>$Nquot;'8)9
.)/,/*@A*A;<<)6quot;';$*F&;L'<)9

P#<-)*G$6;%'$,*!67)8#
.)/,/*B!0:S;&8(2*Q=A:!C)6'L'6#quot;';$(9

A;$quot;&;<<)%*P;6#?-<#&N B$(quot;#$6)*
!+#$#!(,*-*,*
.)/,/*+A!M2*@AA2*4)!M9 K&#$(L;&8#quot;';$
>-quot;7;&'quot;N*Q)6;&%
.)/,/*+0A*>-quot;7;&'quot;')(2*@)-quot;(67)*F)&(;$)$$;&8%#quot;)'*
.FS@99


References
• Miles et al.: Best Practices Recipes for Publishing RDF Vocabularies, Available
at: http://www.w3.org/TR/swbp-vocab-pub/

• Heery and Patel 2004: Application proﬁles: mixing and matching metadata
schemes. Available at: http://www.ariadne.ac.uk/issue25/app-proﬁles/

• Haslhofer and Klas 2009: A Survey of approaches for achieving metadata
interoperability. To be published in ACM Computing Surveys Q4/2009.
Available on request.

• Haslhofer and Schandl 2009: Interweaving OAI-PMH metadata with the
Linked Data Cloud. To be published in International Journal of Metadata,
Semantics, and Ontologies. Available on request.


Contents








Demo 1 - browsing and querying DBPedia

• http://dbpedia.org/resource/Hallstatt_culture

• http://dbpedia.org/snorql

• http://lookup.dbpedia.org


Demo 2 - cURLing the LIBRIS catalogue

• LIBRIS = The Swedish Union Catalog

• Starting point: http://libris.kb.se/bib/10432900


Demo 3 - exposing MySQL DB using D2R

• sample database: http://dev.mysql.com/doc/world-
setup/en/world-setup.html


Demo 4 - sample OAI2LOD instances

• http://www.mediaspaces.info/tools/oai2lod/


Demo 5 - RDF Viewers

• Zitgist: http://dataviewer.zitgist.com/

• Disco: http://www4.wiwiss.fu-berlin.de/rdf_browser/

• OpenLink Data Explorer: http://demo.openlinksw.com/
rdfbrowser2


Known / Open Issues
• Automatic linking is non-trival and domain speciﬁc
• how to deal with false positives?

• precision / recall of existing approaches?

• Nobody can guarantee the (long-term) availability of
LOD resources
• what to do when a resource disappears?

• annoying for humans / problematic for applications

• Licensing

Summary

• Linked Data is about publishing and interlinking public
interest data on the Web

• Other applications can access these data with common
Web technologies

• It is still research, with lots of construction areas, but
within its two year (!!!) history it has attracted quite a lot
of interest

• It is an exciting research ﬁeld to work on...


Summary


Questions / Discussion


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