Consuming Linked Data

Juan F. SequedaDepartment of Computer Science

University of Texas at AustinSemTech 2010

How many people are familiar with

• RDF• SPARQL• Linked Data• Web Architecture (HTTP, etc)

History• Linked Data Design Issues by TimBL July 2006• Linked Open Data Project WWW2007• First LOD Cloud May 2007• 1st Linked Data on the Web Workshop WWW2008• 1st Triplification Challenge 2008• How to Publish Linked Data Tutorial ISWC2008• BBC publishes Linked Data 2008• 2nd Linked Data on the Web Workshop WWW2009• NY Times announcement SemTech2009 - ISWC09• 1st Linked Data-a-thon ISWC2009• 1st How to Consume Linked Data Tutorial ISWC2009• Data.gov.uk publishes Linked Data 2010• 2st How to Consume Linked Data Tutorial WWW2010• 1st International Workshop on Consuming Linked Data COLD2010• …

May 2007

Oct 2007

Nov 2007 (1)

Nov 2007 (2)

Feb 2008

Mar 2008

Sept 2008

Mar 2009 (1)

Mar 2009 (2)

July 2009

June 2010

YOU GET THE PICTURE

ITS BIG and getting

BIGGER and

BIGGER

Now what can we do with this data?

Let’s consume it!

The Modigliani Test

• Show me all the locations of all the original paintings of Modigliani

• Daniel Koller (@dakoller) showed that you can find this with a SPARQL query on DBpedia

Thanks Richard MacManus - ReadWriteWeb

Results of the Modigliani Test

• Atanas Kiryakov from Ontotext• Used LDSR – Linked Data Semantic Repository– Dbpedia– Freebase– Geonames– UMBEL– Wordnet

Published April 26, 2010: http://www.readwriteweb.com/archives/the_modigliani_test_for_linked_data.php

SPARQL QueryPREFIX fb: http://rdf.freebase.com/ns/PREFIX dbpedia: http://dbpedia.org/resource/PREFIX dbp-prop: http://dbpedia.org/property/PREFIX dbp-ont: http://dbpedia.org/ontology/PREFIX umbel-sc: http://umbel.org/umbel/sc/PREFIX rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns#PREFIX ot: http://www.ontotext.com/SELECT DISTINCT ?painting_l ?owner_l ?city_fb_con ?city_db_loc ?

city_db_citWHERE { ?p fb:visual_art.artwork.artist dbpedia:Amedeo_Modigliani ;

fb:visual_art.artwork.owners [ fb:visual_art.artwork_owner_relationship.owner ?ow ] ; ot:preferredLabel ?painting_l. ?ow ot:preferredLabel ?owner_l . OPTIONAL { ?ow fb:location.location.containedby [ ot:preferredLabel ?city_fb_con ] } .

OPTIONAL { ?ow dbp-prop:location ?loc. ?loc rdf:type umbel-sc:City ; ot:preferredLabel ?city_db_loc }

OPTIONAL { ?ow dbp-ont:city [ ot:preferredLabel ?city_db_cit ] }}

Let’s start by making sure that we understand what Linked Data is…

Do you SEARCH or do you FIND?

Search for

Football Players who went to the University of Texas at Austin, played for

the Dallas Cowboys as Cornerback

Why can’t we just FIND it…

Guess how I FOUND out?

I’ll tell you how I did NOT find it

Current Web = internet + links + docs

So what is the problem?

• We aren’t always interested in documents– We are interested in THINGS– These THINGS might be in documents

• We can read a HTML document rendered in a browser and find what we are searching for– This is hard for computers. – Computers have to guess (even though they are

pretty good at it)

What do we need to do?

• Make it easy for computers/software to find THINGS

How can we do that?

• Besides publishing documents on the web– which computers can’t understand easily

• Let’s publish something that computers can understand

RAW DATA!

But wait… don’t we do that already?

Current Data on the Web

• Relational Databases• APIs• XML• CSV• XLS• …• Can’t computers and applications already

consume that data on the web?

True! But it is all in different formats and data models!

This makes it hard to integrate data

The data in different data sources aren’t linked

For example, how do I know that the Juan Sequeda in Facebook is the same as Juan

Sequeda in Twitter

Or if I create a mashup from different services, I have to learn different APIs and I get different

formats of data back

Wouldn’t it be great if we had a standard way of publishing data on the Web?

We have a standardized way of publishing documents on the web, right?

HTML

Then why can’t we have a standard way of publishing data on the Web?

Good question! And the answer is YES. There is!

Resource Description Framework (RDF)

• A data model – A way to model data– i.e. Relational databases use relational data model

• RDF is a triple data model• Labeled Graph• Subject, Predicate, Object• <Juan> <was born in> <California>• <California> <is part of> <the USA>• <Juan> <likes> <the Semantic Web>

RDF can be serialized in different ways

• RDF/XML• RDFa (RDF in HTML)• N3• Turtle• JSON

So does that mean that I have to publish my data in RDF now?

You don’t have to… but we would like you to

An example

Document on the Web

Databases back up documents

Isbn Title Author PublisherID ReleasedData

978-0-596-15381-6

Programming the Semantic Web

Toby Segaran 1 July 2009

… … … … …

PublisherID PublisherName

1 O’Reilly Media

… …

This is a THING:A book title “Programming the Semantic Web” by Toby Segaran, …

THINGS have PROPERTIES:A Book as a Title, an author, …

Lets represent the data in RDF

book

Programming the Semantic Web

978-0-596-15381-6

Toby Segaran

Publisher O’Reilly

title

name

author

publisher

isbn

Isbn Title Author PublisherID ReleasedData

978-0-596-15381-6

Programming the Semantic Web

Toby Segaran

1 July 2009

PublisherID PublisherName

1 O’Reilly Media

Remember that we are on the web

Everything on the web is identified by a URI

And now let’s link the data to other data

http://…/isbn978

Programming the Semantic Web

978-0-596-15381-6

Toby Segaran

http://…/publisher1 O’Reilly

title

name

author

publisher

isbn

And now consider the data from Revyu.com

http://…/isbn978

http://…/

review1

Awesome Book

http://…/

reviewer

Juan Sequeda

hasReview

reviewer

description

name

Let’s start to link data

http://…/isbn978

Programming the Semantic Web

978-0-596-15381-6

Toby Segaran

http://…/publisher1 O’Reilly

title

name

author

publisher

isbn

http://…/isbn978

sameAs

http://…/

review1

Awesome Book

http://…/

reviewer

Juan Sequeda

hasReview

hasReviewer

description

name

Juan Sequeda publishes data too

http://juansequeda.

com/id

livesIn

Juan Sequedaname

http://dbpedia.org/Austin

Let’s link more datahttp://…/isbn978

http://…/

review1

Awesome Book

http://…/

reviewer

Juan Sequeda

http://juansequeda.

com/id

hasReview

hasReviewer

description

name

sameAs

livesIn

Juan Sequedaname

http://dbpedia.org/Austin

And more

http://…/isbn978

Programming the Semantic Web

978-0-596-15381-6

Toby Segaran

http://…/publisher1

O’Reilly

title

name

author

publisher

isbn

http://…/isbn978

sameAs

http://…/

review1

Awesome Book

http://…/

reviewer

Juan Sequeda

http://juansequeda.

com/id

hasReview

hasReviewer

description

name

sameAs

livesIn

Juan Sequedaname

http://dbpedia.org/Austin

Data on the Web that is in RDF and is linked to other RDF data is LINKED

DATA

Linked Data Principles

1. Use URIs as names for things

2. Use HTTP URIs so that people can look up (dereference) those names.

3. When someone looks up a URI, provide useful information.

4. Include links to other URIs so that they can discover more things.

Linked Data makes the web appear as ONE

GIANTHUGE

GLOBAL

DATABASE!

I can query a database with SQL. Is there a way to query Linked Data with a query language?

Yes! There is actually a standardize language for that

SPARQL

FIND all the reviews on the book “Programming the Semantic Web” by people who live in

Austin

http://…/isbn978

Programming the Semantic Web

978-0-596-15381-6

Toby Segaran

http://…/publisher1 O’Reilly

title

name

author

publisher

isbn

http://…/isbn978

sameAs

http://…/

review1

Awesome Book

http://…/

reviewer

Juan Sequeda

http://juansequeda.

com

hasReview

hasReviewer

description

name

sameAs

livesIn

Juan Sequedaname

http://dbpedia.org/Austin

This looks cool, but let’s be realistic. What is the incentive to publish Linked Data?

What was your incentive to publish an HTML page in 1990?

1) Share data in documents2) Because you neighbor was doing it

So why should we publish Linked Data in 2010?

1) Share data as data2) Because you neighbor is doing it

And guess who is starting to publish Linked Data now?

Linked Data Publishers

• UK Government• US Government• BBC• Open Calais – Thomson Reuters• Freebase• NY Times• Best Buy• CNET• Dbpedia• Are you?

How can I publish Linked Data?

Publishing Linked Data• Legacy Data in Relational Databases– D2R Server– Virtuoso– Triplify– Ultrawrap

• CMS– Drupal 7

• Native RDF Stores– Databases for RDF (Triple Stores)

• AllegroGraph, Jena, Sesame, Virtuoso

– Talis Platform (Linked Data in the Cloud)• In HTML with RDFa

Consuming Linked Data by Humans

HTML Browsers

Links to other URIs

<span rel="foaf:interest"><a href="http://dbpedia.org/resource/Database"

property="dcterms:title">Database</a>, <a href="http://dbpedia.org/resource/Data_integration" property="dcterms:title">Data Integration</a>, <a href="http://dbpedia.org/resource/Semantic_Web" property="dcterms:title">Semantic Web</a>, <a href="http://dbpedia.org/resource/Linked_Data" property="dcterms:title">Linked Data</a>, etc.</span>

HTML Browsers

• RDF can be serialized in RDFa• Have you heard of– Yahoo’s Search Monkey– Google Rich Snippets?

• They are consuming RDFa• But WHY?

Because there is life beyond ten blue links

Google and Yahoo are starting to crawl RDFa!

The Semantic Web is a reality!

The Reality

• Yahoo is crawling data that is in RDFa and Microformats under a specific vocabularies – FOAF– GoodRelations– …

• Google is crawling RDFa and Microformats that use the Google vocabulary

Linked Data Browsers

Linked Data Browsers

• Not actually separate browsers. Run inside of HTML browsers

• View the data that is returned after looking up a URI in tabular form

• (IMO) UI lacks usability

Linked Data Browsers

• Tabulator– http://www.w3.org/2005/ajar/tab

• OpenLink– http://ode.openlinksw.com/

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

• Marbles– http://www5.wiwiss.fu-berlin.de/marbles/

• Explorator– http://www.tecweb.inf.puc-rio.br/explorator

Faceted Browsers

http://dbpedia.neofonie.de

http://dev.semsol.com/2010/semtech/

On-the-fly Mashups

http://sig.ma

What’s next?

Time to create new and innovative ways to interact with Linked Data

This may be one of the Killer Apps that we have all been waiting for

http://en.wikipedia.org/wiki/File:Mosaic_browser_plaque_ncsa.jpg

It’s time to partner with HCI community

Semantic Web UIs don’t have to be ugly

Consume Linked Data with SPARQL

SPARQL Endpoints

• Linked Data sources usually provide a SPARQL endpoint for their dataset(s)

• SPARQL endpoint: SPARQL query processing service that supports the SPARQL protocol*

• Send your SPARQL query, receive the result

* http://www.w3.org/TR/rdf-sparql-protocol/

Where can I find SPARQL Endpoints?

• 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

• http://esw.w3.org/topic/SparqlEndpoints

Accessing a SPARQL Endpoint

• SPARQL endpoints: RESTful Web services• Issuing SPARQL queries to a remote SPARQL

endpoint is basically an HTTP GET request to the SPARQL endpoint with parameter query

GET /sparql?query=PREFIX+rd... HTTP/1.1 Host: dbpedia.org User-agent: my-sparql-client/0.1

URL-encoded string with the SPARQL query

Query Results Formats

• SPARQL endpoints usually support different result formats:– XML, JSON, plain text

(for ASK and SELECT queries)– RDF/XML, NTriples, Turtle, N3

(for DESCRIBE and CONSTRUCT queries)

Query Results Formats

PREFIX dbp: http://dbpedia.org/ontology/PREFIX dbpprop: http://dbpedia.org/property/SELECT ?name ?bday WHERE { ?p dbp:birthplace <http://dbpedia.org/resource/Berlin> . ?p dbpprop:dateOfBirth ?bday . ?p dbpprop:name ?name .}

Query Result Formats

• Use the ACCEPT header to request the preferred result format:

GET /sparql?query=PREFIX+rd... HTTP/1.1 Host: dbpedia.org User-agent: my-sparql-client/0.1 Accept: application/sparql-results+json

Query Result Formats

• As an alternative some SPARQL endpoint implementations (e.g. Joseki) provide an additional parameter out

GET /sparql?out=json&query=... HTTP/1.1 Host: dbpedia.org User-agent: my-sparql-client/0.1

Accessing a SPARQL Endpoint

• More convenient: use a library• SPARQL JavaScript Library

– http://www.thefigtrees.net/lee/blog/2006/04 sparql_calendar_demo_a_sparql.html

• ARC for PHP– http://arc.semsol.org/

• RAP – RDF API for PHP– http://www4.wiwiss.fu-berlin.de/bizer/rdfapi/index.html

Accessing a SPARQL Endpoint

• Jena / ARQ (Java)– http://jena.sourceforge.net/

• Sesame (Java)– http://www.openrdf.org/

• SPARQL Wrapper (Python)– http://sparql-wrapper.sourceforge.net/

• PySPARQL (Python)– http://code.google.com/p/pysparql/

Accessing a SPARQL Endpoint

Example with Jena/ARQimport com.hp.hpl.jena.query.*;

String service = "..."; // address of the SPARQL endpoint String query = "SELECT ..."; // your SPARQL query QueryExecution e =

QueryExecutionFactory.sparqlService(service, query)

ResultSet results = e.execSelect(); while ( results.hasNext() ) {

QuerySolution s = results.nextSolution(); // ...

}

e.close();

• Querying a single dataset is quite boringcompared to:

• Issuing SPARQL queries over multiple datasets

• How can you do this?1. Issue follow-up queries to different endpoints2. Querying a central collection of datasets3. Build store with copies of relevant datasets4. Use query federation system

Follow-up Queries

• Idea: issue follow-up queries over other datasets based on results from previous queries

• Substituting placeholders in query templates

String s1 = "http://cb.semsol.org/sparql"; String s2 = "http://dbpedia.org/sparql";

String qTmpl = "SELECT ?c WHERE{ <%s> rdfs:comment ?c }";String q1 = "SELECT ?s WHERE { ..."; QueryExecution e1 = QueryExecutionFactory.sparqlService(s1,q1); ResultSet results1 = e1.execSelect(); while ( results1.hasNext() ) {

QuerySolution s1 = results.nextSolution(); String q2 = String.format( qTmpl, s1.getResource("s"),getURI()

); QueryExecution e2=

QueryExecutionFactory.sparqlService(s2,q2); ResultSet results2 = e2.execSelect(); while ( results2.hasNext() ) {

// ... }e2.close();

}e1.close();

Find a list of companies Filtered by some criteria and return Dbpedia URIs from them

Follow-up Queries

• Advantage– Queried data is up-to-date

• Drawbacks– Requires the existence of a SPARQL endpoint for

each dataset– Requires program logic– Very inefficient

Querying a Collection of Datasets

• Idea: Use an existing SPARQL endpoint that provides access to a set of copies of relevant datasets

• Example:– SPARQL endpoint over a majority of datasets from

the LOD cloud at:

http://lod.openlinksw.com/sparql

http://uberblic.org

Querying a Collection of Datasets

• Advantage:– No need for specific program logic

• Drawbacks:– Queried data might be out of date – Not all relevant datasets in the collection

Own Store of Dataset Copies

• Idea: Build your own store with copies of relevant datasets and query it

• Possible stores:– Jena TDB http://jena.hpl.hp.com/wiki/TDB– Sesame http://www.openrdf.org/– OpenLink Virtuoso http://virtuoso.openlinksw.com/– 4store http://4store.org/– AllegroGraph http://www.franz.com/agraph/ – etc.

Populating Your Store

• Get RDF dumps provided for the datasets• (Focused) Crawling

• ldspider http://code.google.com/p/ldspider/– Multithreaded API for focussed crawling– Crawling strategies (breath-first, load-balancing)– Flexible configuration with callbacks and hooks

Own Store of Dataset Copies

• Advantages:– No need for specific program logic – Can include all datasets– Independent of the existence, availability, and

efficiency of SPARQL endpoints• Drawbacks:– Requires effort to set up and to operate the store – Ideally, data sources provide RDF dumps; if not? – How to keep the copies in sync with the originals?– Queried data might be out of date

Federated Query Processing

• Idea: Querying a mediator which distributes sub-queries to relevant sources and integrates the results

Federated Query Processing

• Instance-based federation– Each thing described by only one data source – Untypical for the Web of Data

• Triple-based federation– No restrictions – Requires more distributed joins

• Statistics about datasets required (both cases)

Federated Query Processing

• DARQ (Distributed ARQ)– http://darq.sourceforge.net/ – Query engine for federated SPARQL queries– Extension of ARQ (query engine for Jena)– Last update: June 28, 2006

• Semantic Web Integrator and Query Engine(SemWIQ)– http://semwiq.sourceforge.net/– Actively maintained

Federated Query Processing

• Advantages:– No need for specific program logic – Queried data is up to date

• Drawbacks:– Requires the existence of a SPARQL endpoint for

each dataset– Requires effort to set up and configure the

mediator

In any case:

• You have to know the relevant data sources– When developing the app using follow-up queries– When selecting an existing SPARQL endpoint over

a collection of dataset copies– When setting up your own store with a collection

of dataset copies– When configuring your query federation system

• You restrict yourself to the selected sources

In any case:

• You have to know the relevant data sources– When developing the app using follow-up queries– When selecting an existing SPARQL endpoint over

a collection of dataset copies– When setting up your own store with a collection

of dataset copies– When configuring your query federation system

• You restrict yourself to the selected sourcesThere is an alternative:

Remember, URIs link to data

Automated Link Traversal

• Idea: Discover further data by looking up relevant URIs in your application

• Can be combined with the previous approaches

Link Traversal Based Query Execution

• Applies the idea of automated link traversal to the execution of SPARQL queries

• Idea:– Intertwine query evaluation with traversal of RDF links– Discover data that might contribute to query results

during query execution• Alternately:– Evaluate parts of the query – Look up URIs in intermediate solutions

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

Link Traversal Based Query Execution

• Advantages:– No need to know all data sources in advance– No need for specific programming logic– Queried data is up to date– Does not depend on the existence of SPARQL

endpoints provided by the data sources• Drawbacks:– Not as fast as a centralized collection of copies– Unsuitable for some queries– Results might be incomplete (do we care?)

Implementations

• Semantic Web Client library (SWClLib) for Javahttp://www4.wiwiss.fu-berlin.de/bizer/ng4j/semwebclient/• SWIC for Prologhttp://moustaki.org/swic/

Implementations

• SQUIN http://squin.org – Provides SWClLib functionality as a Web service– Accessible like a SPARQL endpoint– Install package: unzip and start• Less than 5 mins!

– Convenient access with SQUIN PHP tools:

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

Real World Example

Getting Started

• Finding URIs• Finding Additional Data• Finding SPARQL Endpoints

What is a Linked Data application

• Software system that makes use of data on the web from multiple datasets and that benefits from links between the datasets

Characteristics of Linked Data Applications

• Consume data that is published on the web following the Linked Data principles: an application should be able to request, retrieve and process the accessed data

• Discover further information by following the links between different data sources: the fourth principle enables this.

• Combine the consumed linked data with data from sources (not necessarily Linked Data)

• Expose the combined data back to the web following the Linked Data principles

• Offer value to end-users

Examples

• http://data-gov.tw.rpi.edu/wiki• http://dbrec.net/• http://fanhu.bz/• http://data.nytimes.com/schools/schools.html• http://sig.ma • http://visinav.deri.org/semtech2010/

Hot Research Topics• Interlinking Algorithms• Provenance and Trust• Dataset Dynamics• UI• Distributed Query• Evaluation– “You want a good thesis? IR is based on precision and recall.

The minute you add semantics, it is a meaningless feature. Logic is based on soundness and completeness. We don’t want soundness and completeness. We want a few good answers quickly.” – Jim Hendler at WWW2009 during the LOD gathering

Thanks Michael Hausenblas

THANKS

Juan Sequedawww.juansequeda.com

@juansequeda#cold

www.consuminglinkeddata.org

Acknowledgements: Olaf Hartig, Patrick Sinclair, Jamie Taylor

Slides for Consuming Linked Data with SPARQL by Olaf Hartig