linked data on the web

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

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

The Traditional Web

But wait…don't we do that already?

The Traditional Web

● Content providers offer access via Web APIs

Web APIWeb API

The Traditional Web

● Mashups combine this data

Web APIWeb API

The Traditional Web

● Mashups combine this data

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

http://mymovie.db/movie0362

http://mym

ovi e.db

/movie

My Movie DB

http://mym

ovi e.db

/movie

My Movie DB

http://mym

ovie.db/movie2449

http://mym

ovi e.db

/movie

My Movie DB

http://mym

ovie.db/movie2449

http://mym

ovi e.db

/movie

My Movie DB

http://mym

ovie.db/movie2449

http://mym

ovi e.db

/movie

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

http://geo.db/country21

http://geo.db/cityXA

http://mym

ovie.db/movie2449

http://mym

ovi e.db

/movie

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

http://geo.db/country21

http://geo.db/cityXA

http://mym

ovie.db/movie2449

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/

http://www4.wi … /0743424425

movie:relatedBook

foaf:based_near

dc:title

rdf:type

The Shining

gn:population

60943000 rdfs

United Kingdom

gn:population

movie:relatedBook

foaf:based_near

dc:title

rdf:type

The Shining

60943000

dc:title

skos:subject

The Shining

http://www4.wi … /0743424425

http://www4.wi … /Fiction

skos:subject

dc:title

gn:population

movie:relatedBook

foaf:based_near

dc:title

rdf:type

The Shining

60943000

United Kingdom

http://www4.wi … /0743424425

The Shining

http://www4.wi … /1571884029

http://www4.wi … /Fiction

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

● 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?

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 generatedcontent Publications

Life Sciences

Geographic

Cross-domain

Linked Data Publishers

● UK government

● US government

● Thomson Reuters (Open Calais)

● MetaWeb (Freebase)

● BBC

● NY Times

● Best Buy

● CNET

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

● 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

Outline

Technical Foundations

● HTTP

● URI

● RDF

● RDFS / OWL

There is no magic – Linked Data is based on well-established

(Semantic) Web technologies.

● Hash URIs

http://olafhartig.de/foaf.rdf#olaf

● Slash URIs

http://data.linkedmdb.org/resource/film/2014

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

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:

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:

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:

● Hash URIs

http://olafhartig.de/foaf.rdf#olaf

● Slash URIs

http://data.linkedmdb.org/resource/film/2014

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

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

Response:

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/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

Response:

Redirections

Vocabularies and Ontologies

● Defined using RDFS or OWL

● A plenty of vocabularies exist:● People● Social media● Commerce● Events● Radio and TV programmes● Music

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

http://www4.wi … /0743424425

movie:relatedBook

foaf:based_near

dc:title

rdf:typehttp://data.linkedmdb.org/.../2014

The Shining

gn:population

owl:sameAs

60943000

owl:sameAs

United Kingdom

owl:sameAs

gn:population

60943000

United Kingdom

http://www4.wi … /0743424425

movie:relatedBook

foaf:based_near

dc:title

rdf:typehttp://data.linkedmdb.org/.../2014

The Shining

owl:sameAs

db:callingCode dbp:le

http://dbpedia.org/resource/Gordon_Brown

Outline

… 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

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

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

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

… in Applications

● Look up URIs and process the retrieved data

● Query with 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)

?vhttp://.../Volcano

rdf:type

Brief Introduction to SPARQL

Queriedgraph:

?vhttp://.../Volcano

rdf:type

http://.../Mount_Baker http://.../Volcanordf:type

"1880"

p:lastEruption

htp://.../Mount_Etna

rdf:type

http://.../Mount_Bakerhttp://.../Mount_Etna

Results:

Brief Introduction to SPARQL

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 ofcopies from all relevantdatasets

Traditional approach 2: federated query processing

● Querying a mediator whichdistributes subqueries torelevant sources andintegrates the results

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 !

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 datato the queried data set

Queried data

Main Idea

Queried data

filmingLocation

http://.../movie2449

statistics

Query unemp_rate?ur

Main Idea

Queried data

filmingLocation

statistics

Query unemp_rate?ur

Main Idea

Queried data

filmingLocation

statistics

Query unemp_rate?ur

Main Idea

Queried data

filmingLocation

statistics

Query unemp_rate?ur

Main Idea

Queried data

filmingLocation

statistics unemp_rate?ur

Main Idea

Queried data

filmingLocationhttp://geo.../Italyhttp://.../movie2449

filmingLocation

Main Idea

Queried data

http://geo.../Italy

filmingLocationhttp://.../movie2449

filmingLocation

http://geo.../Italy

Main Idea

Queried data

http://geo.../Italy

filmingLocation

Main Idea

Queried data

http://geo.../Italy

filmingLocation

Main Idea

Queried data

http://geo.../Italy

filmingLocation

Main Idea

http://geo.../Italy

Queried data

filmingLocation

Main Idea

http://geo.../Italy

Queried data

filmingLocation

http://.../movie2449 unemp_rate?ur

statistics ?stat

Main Idea

http://geo.../Italy

Queried data

filmingLocation

statistics ?stat

statistics http://stat.db/.../it

http://geo.../Italy

Main Idea

http://geo.../Italy

Queried data

http://geo.../Italy http://stats.db/../it

?stat?loc

statistics http://stat.db/.../it

http://geo.../Italy

filmingLocation

statistics ?stat

Main Idea

http://geo.../Italy

Queried data

http://geo.../Italy http://stats.db/../it

?stat?loc

filmingLocation

statistics ?stat

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 }

{ ?authorAlt owl:sameAs ?author }

?authorAlt foaf:phone ?phone

Return phone numbers ofauthors of ontology engineering papers

at ESWC'09.

161min 30sec

# of query results

# of retrieved graphs

# of accessed servers

avg. execution time

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

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

● 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

… getting started

Issues people have when they want to start:

● Finding URIs

● Finding additional data

● Finding SPARQL endpoints

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?

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" ) .}

● 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

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?

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?

Outline

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

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?

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

● Entities are connected by data links

● Data is self-describing

● 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.

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/)

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