Linked Data on the Web

Linked Data on the Web

Olaf Hartig
http://olafhartig.de/foaf.rdf#olaf
Database and Information Systems Research Group
Humboldt-Universität zu Berlin

Outline

From a Web of Documents
to a Web of Data

Technical Foundations of Linked Data

Consuming Linked Data

Current Research Issues
Olaf Hartig - Linked Data on the Web

The Traditional Web

Traditional Web = Internet + Docs + Links


The Traditional Web

Traditional Web = Internet + Docs + Links
● HTML as shared content format
● HTTP to access documents on the Web
● URLs
● Globally unique identifiers for documents
● Retrieval mechanism
● Hyperlinks

● Single global information space

The Traditional Web

So what is the problem?


The Traditional Web

● Web content is only loosely structured
● Difficult for applications to do smart things


The Traditional Web


Solution:
● Increase the structure of Web content
● Publish data


The Traditional Web


Solution:
● Increase the structure of Web content
● Publish data
But wait…
don't we do that already?

The Traditional Web
● Content providers offer access via Web APIs

Web API
Web API Web API
Web API


The Traditional Web
● Mashups combine this data

Web API
Web API Web API
Web API


The Traditional Web
● Mashups combine this data

Shortcomings:
● APIs are proprietary
● Mashups are based on a fixed set of data sources
Web API
● YouWeb API
can not set hyperlinks between data object
Web API
Web API


● 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 URIs so
that they can discover more
things.
Tim Berners-Lee, July 2006

My Movie DB
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data

URI, provide useful
information.
things.
http://mymovie.db/movie1342



My Movie DB


http://m

ymovie

URI, provide useful
information.
?
.d
b/movie

2449

things.



My Movie DB


http://m

ymovie

URI, provide useful
information.
?
.d
b/movie

2449

things.


http://geo.db/country21

http://geo.db/country7

My Movie DB http://geo.db/cityCJ
http://geo.db/cityXA



Linked Data – An Example
http://data.linkedmdb.org/.../2014
rdf:type http://data.linkedmdb.org/.../film
mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425
_n

The Shining
ea
r

http://sws.geonames.org/2635167/


mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425
_n

The Shining
ea
r

n
atio
l
be

o pu l
n:p
:la

g
s
rdf

60943000
United Kingdom


mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425 dc:
title
_n

The Shining
ea
r

http://sws.geonames.org/2635167/ The Shining

sko
s:s
n
atio
l
be

pu l

ub
o
n:p
:la

g

je
s

ct
rdf

60943000
United Kingdom

http://www4.wi … /Fiction


mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425 dc:
title
_n

The Shining
ea
r

http://sws.geonames.org/2635167/ The Shining

sko
s:s
n
atio
l
be

pu l

ub
o
n:p
:la

g

je
s

ct
rdf

60943000
United Kingdom

http://www4.wi … /Fiction
http://www4.wi … /1571884029 t
skos:subjec


Properties of Linked Data
● Anyone can publish data to the Web of data
● Entities are connected by links
● Giant global data graph that spans data sources
● Data is self-describing
● Vocabulary terms are identified by URIs, too
● Look-up yields their RDFS or OWL definition
● The Web of data is open
● Applications can discover new data sources at run-time


Properties of Linked Data
● Entities are connected by links
● Giant global data graph that spans data sources
● Vocabulary terms are identified by URIs, too
● Look-up yields their RDFS or OWL definition
● The Web of data is open
● Applications can discover new data sources at run-time

Is this real?

W3C Linking Open Data Project

● Grassroots community effort
● Publish existing, open license datasets as Linked Data
● Interlink things between different data sources



As of July 2007

> 500M triples ca. 120,000 links



ca. 6.7B triples ca. 150M links



Media User generated
content Publications

Geographic

Cross-domain

Life Sciences

ca. 6.7B triples ca. 150M links


Linked Data Publishers
● UK government
● US government
● Thomson Reuters (Open Calais)
● MetaWeb (Freebase)
● BBC
● NY Times
● Best Buy
● CNET
etc.


Linked Data Publishers
● UK government
● US government
● Thomson Reuters (Open Calais)
● MetaWeb (Freebase)
● BBC
● NY Times
● Best Buy
● CNET
etc. Can I become part?

Linked Data Publishing Tools
● Use HTTP URIs in your FOAF profile
● Legacy data in relational databases
● D2R Server, Triplify, Virtuoso, Ultrawrap, ...
● CMS
● Drupal
● Native RDF stores
● Sesame, AllegroGraph, Virtuoso
● Talis platform (Linked Data in the cloud)
● HTML with RDFa


Integrating the Traditional Web
● Annotate Web documents with Linked Data URIs

http://data.semanticweb.org/ … /eswc/2007/paper-69
dc
:su
bje
ct
http://dbpedia.org/resource/Machine_Learning

● Annotation services using named entity recognition
● Open Calais (Thomson Reuters) for news
● Zemanta for blog posts
● Epiphany


Technical Foundations
There is no magic – Linked Data is based
on well-established
(Semantic) Web technologies.

● HTTP
● URI
● RDF
● RDFS / OWL


URIs
● Hash URIs
● Slash URIs
http://data.linkedmdb.org/resource/film/2014


Looking up URIs
Give me data about

HTTP Request for http://olafhartig.de/foaf.rdf
GET /foaf.rdf HTTP/1.1
User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6 OpenSSL/0.9.8l zlib/1.2.3
Host: olafhartig.de
Accept: */*


Looking up URIs
HTTP Response:
HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 08:47:53 GMT
Server: Apache/2.2.6 (Unix) mod_ssl/2.2.6 OpenSSL/0.9.8g
Last-Modified: Fri, 05 Mar 2010 18:01:07 GMT
ETag: "72a16-1946-7fe53ec0"
Accept-Ranges: bytes
Content-Length: 6470
Content-Type: application/rdf+xml
Content-Language: de

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:foaf="http://xmlns.com/foaf/0.1/">

<foaf:PersonalProfileDocument rdf:about="">
<foaf:maker rdf:resource="http://olafhartig.de/foaf.rdf#olaf"/>
...

HTTP Content Negotiation
● Request the resource in a specific format (representation)
● Use the HTTP header Accept to specify a media type

Example:
GET /data/dbprofs HTTP/1.1
Host: researchersmap.informatik.hu-berlin.de
Accept: text/rdf+n3


HTTP Content Negotiation
HTTP Response:
HTTP/1.1 200 OK
Date: Thu, 11 Mar 2010 09:02:22 GMT
Server: Apache/2.2.13 (Linux/SUSE)
Content-Location: dbprofs.n3
Vary: negotiate,accept
TCN: choice
Last-Modified: Tue, 05 Jan 2010 14:46:17 GMT
ETag: "40e4d-2250-47c6be683f0e1;47c6be69482f5"
Accept-Ranges: bytes
Content-Length: 8784
Content-Type: text/rdf+n3

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix contact: <http://www.w3.org/2000/10/swap/pim/contact#> .

<> a foaf:Document ;
foaf:maker <http://www.informatik.hu-berlin.de/~hartig/foaf.rdf#olaf> .
...

Redirections
HTTP Request for http://data.linkedmdb.org/resource/film/2014
GET /resource/film/2014 HTTP/1.1
User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6
Host: data.linkedmdb.org
Accept: application/rdf+xml


Redirections
Accept: application/rdf+xml

Response:
HTTP/1.1 303 See Other
Date: Thu, 11 Mar 2010 08:15:50 GMT
Server: Jetty(6.1.4)
Location: http://data.linkedmdb.org/data/film/2014
Content-Length: 0
Via: 1.1 data.linkedmdb.org
Content-Type: text/plain


Redirections
Accept: text/html


Redirections
Accept: text/html

Response:
HTTP/1.1 303 See Other
Date: Thu, 11 Mar 2010 08:15:50 GMT
Server: Jetty(6.1.4)
Location: http://data.linkedmdb.org/page/film/2014
Content-Length: 0
Via: 1.1 data.linkedmdb.org
Content-Type: text/plain


Vocabularies and Ontologies
● Defined using RDFS or OWL
● A plenty of vocabularies exist:
● People
● Social media
● Commerce
● Events
● Radio and TV programmes
● Music
etc.


owl:sameAs

=
http://dbpedia.org/resource/United_Kingdom
=
http://rdf.freebase.com/ns/guid.9202a8c04000641f800000000003e30b
=
http://www4.wiwiss.fu-berlin.de/factbook/resource/United_Kingdom
=
http://www4.wiwiss.fu-berlin.de/eurostat/resource/countries/United_Kingdom


owl:sameAs
mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425
_n

The Shining
ea
r

ow l
:sam
n e As
atio
l
be

o pu l
n:p
:la

g
s

rdf

60943000
United Kingdom


owl:sameAs
mov
ie:re
dc

fo
late
d Bo
af
:t

ok
itle

:b
as
ed

http://www4.wi … /0743424425
_n

The Shining
ea
r

ow l
:sam
n e As
atio
l
be

o pu l
n:p
:la

g
s

rdf

60943000
r db
United Kingdom de :c
a all
p:le in
gC
db od
http://dbpedia.org/resource/Gordon_Brown e
44


… by Humans

● Linked Data browsers
● Faceted browsers
● On-the-fly Linked Data Mashups
● Linked Data based applications


Linked Data Browsers
● Provide a tabular view on retrieved RDF data
● Some integrate data from multiple sources
● Allow to follow RDF links
● Multiple options:
● Tabulator
● Disco
● OpenLink Data Explorer
● Zitgist Data Viewer
● Marbles
etc.


Faceted Browsers

http://dbpedia.neofonie.de

On-the-fly Mashups

http://sig.ma

Linked Data based Applications
[SFSW'09]


New Kind of Applications

● Users retain full control over their data
● Users manage and publish data on their own
● All that is needed for the application is a URI

http://researchersmap.informatik.hu-berlin.de/data/dbprofs

…
<http://www.dbis.informatik.hu-berlin.de/ … /freytag.rdf#me>
rdf:type :DBProfessor .
…


Users Really Own their Data
http://www.dbis.informatik.hu-berlin.de/ ... /freytag.rdf

…
<http://www.dbis.informatik.hu-berlin.de/ … /freytag.rdf#me>
contact:fullName "Prof. Johann-Christoph Freytag, Ph.D." ;
contact:office [ contact:address
[ contact:street "Rudower Chaussee 25" ;
contact:city "Berlin"^^xsd:string ;
contact:postalCode "12489"^^xsd:string ] ] ;
foaf:topic_interest
<http://dbpedia.org/resource/Query_optimization> ,
<http://dbpedia.org/resource/Privacy> ,
<http://dbpedia.org/resource/Data_quality> ,
<http://dbpedia.org/resource/Data_warehouse> ;
owl:sameAs
<http://dblp.l3s.de/d2r/resource/authors/Johann_Christoph_Freytag> .
…


… in Applications

● Look up URIs and process the retrieved data
● Query with SPARQL


Brief Introduction to SPARQL
● Query language for RDF data
● Main idea: pattern matching
● Describe subgraphs of the queried RDF graph
● Subgraphs that match your description yield a result
● Mean: graph patterns (i.e. RDF graphs with variables)

?v rdf:type
http://.../Volcano


Brief Introduction to SPARQL
Queried
graph:
rdf:type
http://.../Mount_Baker http://.../Volcano
p:lastEruption rdf:type
"1880" htp://.../Mount_Etna

?v rdf:type
http://.../Volcano
Results:
?v
http://.../Mount_Baker
http://.../Mount_Etna

Querying Linked Data with SPARQL
● Linked Data sources usually provide a SPARQL service
● Send your query, receive the result

Data Source Endpoint Address

DBpedia http://dbpedia.org/sparql

Musicbrainz http://dbtune.org/musicbrainz/sparql

U.S. Census http://www.rdfabout.com/sparql

Semantic Crunchbase http://cb.semsol.org/sparql

More complete list: http://esw.w3.org/topic/SparqlEndpoints

Querying a single dataset is quite boring
compared to:
Issuing SPARQL queries over multiple datasets

How can you do this?
● Issue follow-up queries to different endpoints
● Query a central collection of datasets
● Build store with copies of relevant datasets
● (Use query federation system)
● Use a link traversal based query system

Traditional approach 1:
data centralization

● Querying a collection of
copies from all relevant
datasets


Traditional approach 2:
federated query processing ?
● Querying a mediator which
distributes subqueries to
relevant sources and
integrates the results
?
? ?


Main drawback:

You have to know the relevant
data sources in advance.
You restrict yourself to
the selected sources.
You do not tap the
full potential of
the Web !


A novel approach:

Link Traversal Based Query Execution
[ISWC'09]


Main Idea
● Intertwine query evaluation with traversal of RDF links
● Alternately:
● Evaluate parts of the query on a
continuously augmented set of data
● Look up URIs in intermediate
solutions and add retrieved data
to the queried data set

Queried data


Main Idea
● Alternately:

Queried data

http://.../movie2449 s ?stat unem Query
filmin tis t ic p_ r a
g Loca sta te
t io n ?loc ?ur

Main Idea
Alternately:

htt
●

p:/
/.
Evaluate parts of the query on a

../m ?
●


ov
ie2
44

9

Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
● Alternately:

filmingLocation
http://.../movie2449 http://geo.../Italy
Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy

filmingLocation
http://.../movie2449 http://geo.../Italy
Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy

? aly

./I t
..
g eo
Look up URIs in intermediate

://
●

p
htt

Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy

Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy

tics http://stat.db/.../it
statis
http://geo.../Italy
Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy
● Look up URIs in intermediate ?loc ?stat
solutions and add retrieved data http://geo.../Italy http://stats.db/../it

tics http://stat.db/.../it
statis
http://geo.../Italy
Queried data

g Loca sta te
t io n ?loc ?ur

Main Idea
?loc
● Alternately:
http://geo.../Italy
● Look up URIs in intermediate ?loc ?stat
solutions and add retrieved data http://geo.../Italy http://stats.db/../it

Queried data

g Loca sta te
t io n ?loc ?ur

In a Nutshell

● Link traversal based query execution:
● Evaluation on a continuously augmented dataset
● Discovery of potentially relevant data during execution
● Discovery driven by intermediate solutions

● Main advantage:
● No need to know all data sources in advance


Real-World Example
SELECT DISTINCT ?author ?phone WHERE {
?pub swc:isPartOf
<http://data.semanticweb.org/conference/eswc/2009/proceedings> .
?pub swc:hasTopic ?topic . ?topic rdfs:label ?topicLabel .
FILTER regex( str(?topicLabel), "ontology engineering", "i" ) .

?pub swrc:author ?author .
{ ?author owl:sameAs ?authorAlt }
Return phone numbers of
authors of ontology engineering papers
UNION
at ESWC'09.
{ ?authorAlt owl:sameAs ?author }

?authorAlt foaf:phone ?phone # of query results 2
} # of retrieved graphs 297
# of accessed servers 16
avg. execution time 1min 30sec

Application
● Researchers Map implemented with SQUIN
● Query interface to the whole Web of Data

SELECT DISTINCT ?i ?label
WHERE {

?prof rdf:type <http://res ... data/dbprofs#DBProfessor> ;
foaf:topic_interest ?i .

OPTIONAL {
?i rdfs:label ?label

?
FILTER( LANG(?label)="en" || LANG(?label)="")
}
}
ORDER BY ?label

SQUIN
SemWeb
Client
Lib

Application

SELECT DISTINCT ?i ?label
WHERE {

?prof rdf:type <http://res ... data/dbprofs#DBProfessor> .
?prof foaf:topic_interest ?i .

OPTIONAL {
?i rdfs:label ?label
FILTER( LANG(?label)="en" || LANG(?label)="")
}
}
ORDER BY ?label

Application
● Implementation of Researchers Map was very easy due to:
● SQUIN / SemWeb Client Lib
● Approx. 700 LOC JavaScript (incl. 100 for the queries)
● Approx. 50 LOC PHP (Mainly to set up server side proxy
due to same origin policy)

● Convenient access to SQUIN with SQUIN PHP tools

$s = 'http:// …'; // address of the SQUIN service
$q = new SparqlQuerySock( $s, '… SELECT ...' );
$res = $q->getJsonResult(); // or getXmlResult()

● Try it: http://squin.org

… getting started

Issues people have when they want to start:
● Finding URIs
● Finding additional data
● Finding SPARQL endpoints


Finding URIs
Problem: What URIs exist that identify
the thing I'm interested in?
Two options:
● Data source specific solutions
● Some Linked Data sources provide a keyword based search
for things in their dataset(s)
● Search Engines for the Web of data


Finding URIs

What if there is no search possibility?

You may try a SPARQL query:

SELECT DISTINCT ?s WHERE {
?s rdfs:label ?label .
FILTER regex( str(?label), "Berlin", "i" ) .
}


Finding URIs

● Search engines for the Web of data provide keyword
based search for things in different datasets)
● Falcons http://iws.seu.edu.cn/services/falcons/
● Sindice http://sindice.com
● SWSE http://www.swse.org
● Watson http://watson.kmi.open.ac.uk
● They have also APIs


Finding Additional Data
Problem: Given a URIs, where do I find
more data as what is available
by looking it up?
Three options:
● Follow links (e.g. rdfs:seeAlso, owl:sameAs)
● Use a search engine for the Web of data
● Use a co-reference service
● Co-reference services find different
URIs that refer to the same thing
● They may also provide an API

Finding SPARQL Endpoints
Problem: What relevant endpoints exist?
Where is the SPARQL endpoint
for a dataset?
What is the data provided via a
SPARQL endpoint about?

● Look at: http://esw.w3.org/topic/SparqlEndpoints
● Still an open issue


Linked Data Fusion
Applications want an integrated view on
all data that is available about a thing

Requirements:
● Schema mapping: map data into a single schema
● Identity resolution: smush data from all sources
● Conflict resolution: resolve inconsistencies in the data


User Interfaces and Interaction

● How do we build interfaces that operate over such
a large amount of data?
● What will be their interaction paradigm?
● How to explain data provenance and data fusion?


Provenance, Quality, and Trust
● There are no facts on the Web – everything is a claim
● Increasing amount of research in this area
● W3C provenance incubator group

● Our contributions so far:
● A provenance model for the Web of data [LDOW'09]
● A provenance based Information Quality assessment method
[SWPM'09]
● tSPARQL – a trust aware extension for SPARQL [ESWC'09]


Take-away Summary
The traditional Web of documents
evolves into a Web of data.
● Entities are connected by data links
● Linked Data holds an enormous potential: users may
benefit from a virtually unbound set of data sources

● Learn more about Linked Data:
● “Linked Data – The Story So Far”
by C. Bizer, T. Heath, T. Berners-Lee
● On consuming Linked Data: http://consuminglinkeddata.org

These slides have been created by
Olaf Hartig
http://olafhartig.de

Some slides are based on slide sets provided by
● Christian Bizer

● Juan Sequeda

This work is licensed under a
Creative Commons Attribution-Share Alike 3.0 License
(http://creativecommons.org/licenses/by-sa/3.0/)


Linked Data on the Web

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