linked data on the web
DESCRIPTION
An comprehensive overview on Linked Data, including an introduction, technical foundations, consuming LD, and open research issues.TRANSCRIPT
Linked Data on the Web
Olaf Hartighttp://olafhartig.de/foaf.rdf#olaf
Database and Information Systems Research GroupHumboldt-Universität zu Berlin
Olaf Hartig - Linked Data on the Web
Outline
From a Web of Documentsto a Web of Data
Technical Foundations of Linked Data
Consuming Linked Data
Current Research Issues
Olaf Hartig - Linked Data on the Web
Traditional Web = Internet + Docs + Links
The Traditional Web
Olaf Hartig - Linked Data on the 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
Olaf Hartig - Linked Data on the Web
So what is the problem?
The Traditional Web
Olaf Hartig - Linked Data on the Web
So what is the problem?
● Web content is only loosely structured
● Difficult for applications to do smart things
The Traditional Web
Olaf Hartig - Linked Data on the Web
So what is the problem?
● Web content is only loosely structured
● Difficult for applications to do smart things
Solution:● Increase the structure of Web content● Publish data
The Traditional Web
Olaf Hartig - Linked Data on the Web
So what is the problem?
● Web content is only loosely structured
● Difficult for applications to do smart things
Solution:● Increase the structure of Web content● Publish data
The Traditional Web
But wait…don't we do that already?
Olaf Hartig - Linked Data on the Web
The Traditional Web
● Content providers offer access via Web APIs
Web APIWeb API
Web APIWeb API
Olaf Hartig - Linked Data on the Web
The Traditional Web
● Content providers offer access via Web APIs
● Mashups combine this data
Web APIWeb API
Web APIWeb API
Olaf Hartig - Linked Data on the Web
The Traditional Web
● Content providers offer access via Web APIs
● Mashups combine this data
Web APIWeb API
Web APIWeb API
Shortcomings:
● APIs are proprietary
● Mashups are based on a fixed set of data sources
● You can not set hyperlinks between data object
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
● 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
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB
http://mym
ovie.db/movie2449
?
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB
http://mym
ovie.db/movie2449
?
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB
http://mym
ovie.db/movie2449
?
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB http://geo.db/cityCJ
http
://ge
o.d
b/c
ou
ntry
7
http://geo.db/country21
http://geo.db/cityXA
http://mym
ovie.db/movie2449
?
Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
http://mymovie.db/movie0362
● 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
http://mymovie.db/movie2449
http://mymovie.db/movie5112
http://mym
ovi e.db
/movie
13 42
My Movie DB http://geo.db/cityCJ
http
://ge
o.d
b/c
ou
ntry
7
http://geo.db/country21
http://geo.db/cityXA
http://mym
ovie.db/movie2449
?
Olaf Hartig - Linked Data on the Web
http://www4.wi … /0743424425
movie:relatedBook
foaf:based_near
dc:title
rdf:type
Linked Data – An Example
http://data.linkedmdb.org/.../2014
The Shining
http://data.linkedmdb.org/.../film
http://sws.geonames.org/2635167/
Olaf Hartig - Linked Data on the Web
http://www4.wi … /0743424425
movie:relatedBook
foaf:based_near
dc:title
rdf:type
Linked Data – An Example
http://data.linkedmdb.org/.../2014
The Shining
http://data.linkedmdb.org/.../film
gn:population
60943000 rdfs
:labe
l
United Kingdom
http://sws.geonames.org/2635167/
Olaf Hartig - Linked Data on the Web
rdfs
:labe
l
United Kingdom
gn:population
movie:relatedBook
foaf:based_near
dc:title
rdf:type
Linked Data – An Example
http://data.linkedmdb.org/.../2014
The Shining
http://sws.geonames.org/2635167/
http://data.linkedmdb.org/.../film
60943000
dc:title
skos:subject
The Shining
http://www4.wi … /0743424425
http://www4.wi … /Fiction
Olaf Hartig - Linked Data on the Web
rdfs
:labe
l
skos:subject
skos:subject
dc:title
gn:population
movie:relatedBook
foaf:based_near
dc:title
rdf:type
Linked Data – An Example
http://data.linkedmdb.org/.../2014
The Shining
http://sws.geonames.org/2635167/
http://data.linkedmdb.org/.../film
60943000
United Kingdom
http://www4.wi … /0743424425
The Shining
http://www4.wi … /1571884029
http://www4.wi … /Fiction
Olaf Hartig - Linked Data on the Web
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
Olaf Hartig - Linked Data on the Web
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
Is this real?
Olaf Hartig - Linked Data on the Web
W3C Linking Open Data Project
● Grassroots community effort
● Publish existing, open license datasets as Linked Data
● Interlink things between different data sources
Olaf Hartig - Linked Data on the Web
As of July 2007
> 500M triples ca. 120,000 links
W3C Linking Open Data Project
Olaf Hartig - Linked Data on the Web
ca. 6.7B triples ca. 150M links
W3C Linking Open Data Project
Olaf Hartig - Linked Data on the Web
ca. 6.7B triples ca. 150M links
W3C Linking Open Data Project
Media User generatedcontent Publications
Life Sciences
Geographic
Cross-domain
Olaf Hartig - Linked Data on the Web
Linked Data Publishers
● UK government
● US government
● Thomson Reuters (Open Calais)
● MetaWeb (Freebase)
● BBC
● NY Times
● Best Buy
● CNET
etc.
Olaf Hartig - Linked Data on the Web
Linked Data Publishers
● UK government
● US government
● Thomson Reuters (Open Calais)
● MetaWeb (Freebase)
● BBC
● NY Times
● Best Buy
● CNET
etc. Can I become part?
Olaf Hartig - Linked Data on the Web
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
Olaf Hartig - Linked Data on the Web
Integrating the Traditional Web
● Annotate Web documents with Linked Data URIs
● Annotation services using named entity recognition● Open Calais (Thomson Reuters) for news● Zemanta for blog posts● Epiphany
dc:subject
http://data.semanticweb.org/ … /eswc/2007/paper-69
http://dbpedia.org/resource/Machine_Learning
Olaf Hartig - Linked Data on the Web
Outline
From a Web of Documentsto a Web of Data
Technical Foundations of Linked Data
Consuming Linked Data
Current Research Issues
Olaf Hartig - Linked Data on the Web
Technical Foundations
● HTTP
● URI
● RDF
● RDFS / OWL
There is no magic – Linked Data is based on well-established
(Semantic) Web technologies.
Olaf Hartig - Linked Data on the Web
URIs
● Hash URIs
http://olafhartig.de/foaf.rdf#olaf
● Slash URIs
http://data.linkedmdb.org/resource/film/2014
Olaf Hartig - Linked Data on the Web
Looking up URIs
GET /foaf.rdf HTTP/1.1User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6 OpenSSL/0.9.8l zlib/1.2.3Host: olafhartig.deAccept: */*
HTTP Request for http://olafhartig.de/foaf.rdf
Give me data abouthttp://olafhartig.de/foaf.rdf#olaf
Olaf Hartig - Linked Data on the Web
Looking up URIs
HTTP/1.1 200 OKDate: Thu, 11 Mar 2010 08:47:53 GMTServer: Apache/2.2.6 (Unix) mod_ssl/2.2.6 OpenSSL/0.9.8gLast-Modified: Fri, 05 Mar 2010 18:01:07 GMTETag: "72a16-1946-7fe53ec0"Accept-Ranges: bytesContent-Length: 6470Content-Type: application/rdf+xmlContent-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 Response:
Olaf Hartig - Linked Data on the Web
HTTP Content Negotiation
GET /data/dbprofs HTTP/1.1Host: researchersmap.informatik.hu-berlin.deAccept: text/rdf+n3
● Request the resource in a specific format (representation)
● Use the HTTP header Accept to specify a media type
Example:
Olaf Hartig - Linked Data on the Web
HTTP Content Negotiation
HTTP/1.1 200 OKDate: Thu, 11 Mar 2010 09:02:22 GMTServer: Apache/2.2.13 (Linux/SUSE)Content-Location: dbprofs.n3Vary: negotiate,acceptTCN: choiceLast-Modified: Tue, 05 Jan 2010 14:46:17 GMTETag: "40e4d-2250-47c6be683f0e1;47c6be69482f5"Accept-Ranges: bytesContent-Length: 8784Content-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> . ...
HTTP Response:
Olaf Hartig - Linked Data on the Web
URIs
● Hash URIs
http://olafhartig.de/foaf.rdf#olaf
● Slash URIs
http://data.linkedmdb.org/resource/film/2014
Olaf Hartig - Linked Data on the Web
Redirections
GET /resource/film/2014 HTTP/1.1User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6Host: data.linkedmdb.orgAccept: application/rdf+xml
HTTP Request for http://data.linkedmdb.org/resource/film/2014
Olaf Hartig - Linked Data on the Web
GET /resource/film/2014 HTTP/1.1User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6Host: data.linkedmdb.orgAccept: application/rdf+xml
HTTP/1.1 303 See OtherDate: Thu, 11 Mar 2010 08:15:50 GMTServer: Jetty(6.1.4)Location: http://data.linkedmdb.org/data/film/2014Content-Length: 0Via: 1.1 data.linkedmdb.orgContent-Type: text/plain
HTTP Request for http://data.linkedmdb.org/resource/film/2014
Response:
Redirections
Olaf Hartig - Linked Data on the Web
Redirections
GET /resource/film/2014 HTTP/1.1User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6Host: data.linkedmdb.orgAccept: text/html
HTTP Request for http://data.linkedmdb.org/resource/film/2014
Olaf Hartig - Linked Data on the Web
GET /resource/film/2014 HTTP/1.1User-Agent: curl/7.19.6 (i686-pc-linux-gnu) libcurl/7.19.6Host: data.linkedmdb.orgAccept: text/html
HTTP/1.1 303 See OtherDate: Thu, 11 Mar 2010 08:15:50 GMTServer: Jetty(6.1.4)Location: http://data.linkedmdb.org/page/film/2014Content-Length: 0Via: 1.1 data.linkedmdb.orgContent-Type: text/plain
HTTP Request for http://data.linkedmdb.org/resource/film/2014
Response:
Redirections
Olaf Hartig - Linked Data on the Web
Vocabularies and Ontologies
● Defined using RDFS or OWL
● A plenty of vocabularies exist:● People● Social media● Commerce● Events● Radio and TV programmes● Music
etc.
Olaf Hartig - Linked Data on the Web
owl:sameAs
http://sws.geonames.org/2635167/
=
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
Olaf Hartig - Linked Data on the Web
http://www4.wi … /0743424425
movie:relatedBook
foaf:based_near
dc:title
rdf:typehttp://data.linkedmdb.org/.../2014
The Shining
http://data.linkedmdb.org/.../film
gn:population
owl:sameAs
60943000
owl:sameAs
rdfs
:labe
l
United Kingdom
http://sws.geonames.org/2635167/
http://dbpedia.org/resource/United_Kingdom
Olaf Hartig - Linked Data on the Web
owl:sameAs
gn:population
rdfs
:labe
l
60943000
United Kingdom
http://sws.geonames.org/2635167/
http://www4.wi … /0743424425
movie:relatedBook
foaf:based_near
dc:title
rdf:typehttp://data.linkedmdb.org/.../2014
The Shining
http://data.linkedmdb.org/.../film
owl:sameAs
44
db:callingCode dbp:le
ader
http://dbpedia.org/resource/Gordon_Brown
http://dbpedia.org/resource/United_Kingdom
Olaf Hartig - Linked Data on the Web
Outline
From a Web of Documentsto a Web of Data
Technical Foundations of Linked Data
Consuming Linked Data
Current Research Issues
Olaf Hartig - Linked Data on the Web
Consuming Linked Data
… by Humans
● Linked Data browsers
● Faceted browsers
● On-the-fly Linked Data Mashups
● Linked Data based applications
Olaf Hartig - Linked Data on the Web
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.
Olaf Hartig - Linked Data on the Web
Faceted Browsers
http://dbpedia.neofonie.de
Olaf Hartig - Linked Data on the Web
On-the-fly Mashups
http://sig.ma
Olaf Hartig - Linked Data on the Web
Linked Data based Applications
[SFSW'09]
Olaf Hartig - Linked Data on the Web
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://www.dbis.informatik.hu-berlin.de/ … /freytag.rdf#me> rdf:type :DBProfessor . …
http://researchersmap.informatik.hu-berlin.de/data/dbprofs
Olaf Hartig - Linked Data on the Web
Users Really Own their Data
…
<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> . …
http://www.dbis.informatik.hu-berlin.de/ ... /freytag.rdf
Olaf Hartig - Linked Data on the Web
Consuming Linked Data
… in Applications
● Look up URIs and process the retrieved data
● Query with SPARQL
Olaf Hartig - Linked Data on the Web
● 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)
?vhttp://.../Volcano
rdf:type
Brief Introduction to SPARQL
Olaf Hartig - Linked Data on the Web
Queriedgraph:
?vhttp://.../Volcano
rdf:type
http://.../Mount_Baker http://.../Volcanordf:type
"1880"
p:lastEruption
htp://.../Mount_Etna
rdf:type
?v
http://.../Mount_Bakerhttp://.../Mount_Etna
Results:
Brief Introduction to SPARQL
Olaf Hartig - Linked Data on the Web
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
Olaf Hartig - Linked Data on the Web
Querying Linked Data with SPARQL
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
Olaf Hartig - Linked Data on the Web
Traditional approach 1: data centralization
● Querying a collection ofcopies from all relevantdatasets
Querying Linked Data with SPARQL
Olaf Hartig - Linked Data on the Web
Traditional approach 2: federated query processing
● Querying a mediator whichdistributes subqueries torelevant sources andintegrates the results
Querying Linked Data with SPARQL
???
?
Olaf Hartig - Linked Data on the Web
Main drawback:
You have to know the relevantdata sources in advance.
You restrict yourself tothe selected sources.
You do not tap thefull potential of
the Web !
Olaf Hartig - Linked Data on the Web
A novel approach:
Link Traversal Based Query Execution[ISWC'09]
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocation
http://.../movie2449
statistics
?loc
Query unemp_rate?ur
?stat
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocation
http://.../movie2449
statistics
?loc
Query unemp_rate?ur
?stat
http://.../movie2449
?
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocation
http://.../movie2449
statistics
?loc
Query unemp_rate?ur
?stat
http://.../movie2449
?
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocation
http://.../movie2449
statistics
?loc
Query unemp_rate?ur
?stat
http://.../movie2449
?
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
filmingLocationhttp://geo.../Italyhttp://.../movie2449
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
Queried data
http://geo.../Italy
?loc
filmingLocationhttp://.../movie2449
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
http://geo.../Italy
Olaf Hartig - Linked Data on the Web
Main Idea
Queried data
http
://ge
o.../
Italy
?
● 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 datato the queried data set
http://geo.../Italy
?loc
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
Main Idea
Queried data
http
://ge
o.../
Italy
?
● 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 datato the queried data set
http://geo.../Italy
?loc
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
Main Idea
Queried data
http
://ge
o.../
Italy
?
● 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 datato the queried data set
http://geo.../Italy
?loc
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
http://geo.../Italy
?loc
Queried data
filmingLocation
http://.../movie2449
statistics unemp_rate?ur
?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
http://geo.../Italy
?loc
Queried data
filmingLocation
http://.../movie2449 unemp_rate?ur
statistics ?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
http://geo.../Italy
?loc
Queried data
filmingLocation
http://.../movie2449 unemp_rate?ur
statistics ?stat
?loc
Query
statistics http://stat.db/.../it
http://geo.../Italy
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
http://geo.../Italy
?loc
Queried data
http://geo.../Italy http://stats.db/../it
?stat?loc
statistics http://stat.db/.../it
http://geo.../Italy
filmingLocation
http://.../movie2449 unemp_rate?ur
statistics ?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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 datato the queried data set
http://geo.../Italy
?loc
Queried data
http://geo.../Italy http://stats.db/../it
?stat?loc
filmingLocation
http://.../movie2449 unemp_rate?ur
statistics ?stat
?loc
Query
Olaf Hartig - Linked Data on the Web
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
Olaf Hartig - Linked Data on the Web
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 }
UNION
{ ?authorAlt owl:sameAs ?author }
?authorAlt foaf:phone ?phone
}
Return phone numbers ofauthors of ontology engineering papers
at ESWC'09.
2
297
161min 30sec
# of query results
# of retrieved graphs
# of accessed servers
avg. execution time
Olaf Hartig - Linked Data on the Web
Application
SELECT DISTINCT ?i ?labelWHERE {
?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 ?
SQUINSemWeb
ClientLib
● Researchers Map implemented with SQUIN● Query interface to the whole Web of Data
Olaf Hartig - Linked Data on the Web
SELECT DISTINCT ?i ?labelWHERE {
?prof rdf:type <http://res ... data/dbprofs#DBProfessor> . ?prof foaf:topic_interest ?i .
OPTIONAL {?i rdfs:label ?labelFILTER( LANG(?label)="en" || LANG(?label)="")
}}ORDER BY ?label
Application
Olaf Hartig - Linked Data on the Web
● 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
● Try it: http://squin.org
$s = 'http:// …'; // address of the SQUIN service$q = new SparqlQuerySock( $s, '… SELECT ...' );$res = $q->getJsonResult(); // or getXmlResult()
Application
Olaf Hartig - Linked Data on the Web
Consuming Linked Data
… getting started
Issues people have when they want to start:
● Finding URIs
● Finding additional data
● Finding SPARQL endpoints
Olaf Hartig - Linked Data on the Web
Finding URIs
Problem:
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
What URIs exist that identifythe thing I'm interested in?
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Finding URIs
You may try a SPARQL query:
What if there is no search possibility?
SELECT DISTINCT ?s WHERE { ?s rdfs:label ?label . FILTER regex( str(?label), "Berlin", "i" ) .}
Olaf Hartig - Linked Data on the Web
● 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 URIs
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Finding Additional Data
Problem:
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
Given a URIs, where do I find more data as what is available by looking it up?
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Olaf Hartig - Linked Data on the Web
Finding SPARQL Endpoints
Problem:
● Look at: http://esw.w3.org/topic/SparqlEndpoints
● Still an open issue
What relevant endpoints exist?Where is the SPARQL endpointfor a dataset?What is the data provided via a SPARQL endpoint about?
Olaf Hartig - Linked Data on the Web
Outline
From a Web of Documentsto a Web of Data
Technical Foundations of Linked Data
Consuming Linked Data
Current Research Issues
Olaf Hartig - Linked Data on the Web
Linked Data Fusion
Requirements:
● Schema mapping: map data into a single schema
● Identity resolution: smush data from all sources
● Conflict resolution: resolve inconsistencies in the data
Applications want an integrated view onall data that is available about a thing
Olaf Hartig - Linked Data on the Web
User Interfaces and Interaction
● How do we build interfaces that operate over sucha large amount of data?
● What will be their interaction paradigm?
● How to explain data provenance and data fusion?
Olaf Hartig - Linked Data on the Web
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]
Olaf Hartig - Linked Data on the Web
Take-away Summary
● Entities are connected by data links
● Data is self-describing
● Anyone can publish data to the Web of data
● 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
The traditional Web of documents evolves into a Web of data.
Olaf Hartig - Linked Data on the Web
These slides have been created byOlaf Hartig
http://olafhartig.de
Some slides are based on slide sets provided by● Christian Bizer● Juan Sequeda
This work is licensed under aCreative Commons Attribution-Share Alike 3.0 License
(http://creativecommons.org/licenses/by-sa/3.0/)