Semantic web: where are we now?

Semantic web: where are we Semantic web: where are we now? now?

ADLUG Users Group MeetingADLUG Users Group Meeting

Bilbao, 16-18 September, 2009Bilbao, 16-18 September, 2009

ÁdámÁdám HorváthHorváth

NSZLNSZL

2 Semantic web: where are we now? - Ádám Horváth - NSZL

ContentsContents

TutorialTutorialWhere are we now?Where are we now?


Linked data on the webLinked data on the web

Use RDF data modelUse RDF data modelUse RDF linksUse RDF linksWeb of Data or Semantic WebWeb of Data or Semantic WebLinked Data browsersLinked Data browsersLinked Data provides a single, Linked Data provides a single,

standardised access mechanismstandardised access mechanism (compared to APIs)(compared to APIs)


Advantage of Linked DataAdvantage of Linked Data

Linked Data provides a single, Linked Data provides a single, standardised access mechanismstandardised access mechanism– Easily crawlableEasily crawlable– Accessible using generic data browserAccessible using generic data browserss– Enables link between data from different Enables link between data from different

data sourcesdata sources


Web architecture : resourcesWeb architecture : resources

We have to identify the items of interest We have to identify the items of interest in our domainin our domain

All items of interest are called All items of interest are called resourcesresources– Information resources (everything in the Information resources (everything in the

traditional web)traditional web)– Non-information resources (real world Non-information resources (real world

objects, things)objects, things)


Web architecture : resources Web architecture : resources identifiersidentifiers

Uniform Resource Identifiers (URIs)Uniform Resource Identifiers (URIs)HTTP URIHTTP URI

– Not URN, DOI and so onNot URN, DOI and so onWhy HTTP URIWhy HTTP URI

– Simple way of creating globally unique Simple way of creating globally unique identifier without centralised managementidentifier without centralised management

– Can also be used for accessing human Can also be used for accessing human readable information (information on the readable information (information on the Web)Web)


Web architecture : representationWeb architecture : representation

Information resources can have Information resources can have representationsrepresentations

Representations are stream of bytesRepresentations are stream of bytesA resource can have many A resource can have many

representationsrepresentations


Web architecture : dereferencing Web architecture : dereferencing HTTP URIHTTP URI

This is the process of looking up URI on the This is the process of looking up URI on the WEB to get information about the referenced WEB to get information about the referenced resource resource

Information resourcesInformation resources– HTTP 200 HTTP 200 „OK”„OK” response code response code

Non-information resourcesNon-information resources– HTTP 303 HTTP 303 „„See See OOtherther”” reference code reference code– The client dereferences the new URI and gets The client dereferences the new URI and gets

representation describing the original non-representation describing the original non-information objectinformation object


Web architecture : content Web architecture : content negotiationnegotiation

For non-information resources it is a For non-information resources it is a good practice to create an HTML good practice to create an HTML representation describing the thing for representation describing the thing for human being and an RDF human being and an RDF representation for machinesrepresentation for machines

The different representations can be The different representations can be reached via the content negotiationreached via the content negotiation

Thing

http://nektar.oszk.hu/resource/manifestation/2645471

RDF

303 redirectand

Contentnegotiation

applicatoin/rdf+xml wins text/html wins

Content location:http://nektar.oszk.hu/data/manifestation/2645471 Content location:

http://nektar.oszk.hu/hu/manifestation/2645471

HTML

http://nektar.oszk.hu/resource/manifestation/2645471http://nektar.oszk.hu/resource/manifestation/2645471

http://nektar.oszk.hu/data/manifestation/2645471http://nektar.oszk.hu/data/manifestation/2645471


Web architecture : URI aliasesWeb architecture : URI aliases

Two data provider can assign different Two data provider can assign different URI for the same resourceURI for the same resource– http://dbpedia.org/resource/Berlinhttp://dbpedia.org/resource/Berlin– http://sws.geonames.org/2950159/ http://sws.geonames.org/2950159/

Information providers often set Information providers often set owl:sameAs owl:sameAs link to URI aliases they know aboutlink to URI aliases they know about


Web architecture : Associated Web architecture : Associated descriptionsdescriptions

RDF description of a non-information RDF description of a non-information object that a client obtains by object that a client obtains by dereferencing the URI of the non-dereferencing the URI of the non-information objectinformation object


The RDF data modelThe RDF data model

RDF (Resource Description Framework) RDF (Resource Description Framework) is used to represent information about is used to represent information about resourcesresources

A description of a resource is A description of a resource is represented as a number of triplesrepresented as a number of triples

Triples contain subject, predicate, Triples contain subject, predicate, objectobject



ExampleExample– The bookThe book has the titlehas the title DekameronDekameron– SubjectSubject PredicatePredicate ObjectObject



SubjectSubject– URI identifying the described resourceURI identifying the described resource

ObjectObject– Literal value (string, number, date, etc)Literal value (string, number, date, etc)– URI that is related somehow to the subjectURI that is related somehow to the subject

PredicatePredicate– Indicates the relationship between the Indicates the relationship between the

subject and objectsubject and object– URI, and comes from vocabulariesURI, and comes from vocabularies



Two types of RDF triplesTwo types of RDF triples– Literal RDFLiteral RDF

• Used to describe the properties of resourcesUsed to describe the properties of resources– The title of a bookThe title of a book

– RDF linksRDF links• Three URIThree URI• Subject and Object identify the interlinked Subject and Object identify the interlinked

resourcesresources• Predicate tells the relationshipPredicate tells the relationship



ExampleExample 1 1– The bookThe book has the titlehas the title DekameronDekameron– SubjectSubject PredicatePredicate ObjectObject

Example 2Example 2– SubjectSubject

• http://nektar.oszk.hu/resource/manifestation/2645471http://nektar.oszk.hu/resource/manifestation/2645471

– PredicatePredicate• <dc:title><dc:title>

– ObjectObject• DekameronDekameron



The most valuable RDF links are those The most valuable RDF links are those that connect the resource to external that connect the resource to external data published by other data sourcesdata published by other data sources



Benefits of using the RDF data modelBenefits of using the RDF data model– Clients can lookup every URI in the RDF Clients can lookup every URI in the RDF

graph over the Web to retrieve additional graph over the Web to retrieve additional informationinformation

– Information from different sources merge Information from different sources merge naturallynaturally

– RDF links between data from different RDF links between data from different sources can be setsources can be set

– Information expressed in different schema Information expressed in different schema can be represented in a single modelcan be represented in a single model


Choosing URIsChoosing URIs

Good nameGood name– ExpressiveExpressive– Others can use confidentlyOthers can use confidently

Technical infrastructure to make them Technical infrastructure to make them dereferencabledereferencable



Good practice (Cool URIs)Good practice (Cool URIs)– Use HTTP URIUse HTTP URI

• Not URN, DOI Not URN, DOI

– Define URI in the namespace under your Define URI in the namespace under your controlcontrol



Good practice (Cool URIs)Good practice (Cool URIs)– Implementation specific things should not Implementation specific things should not

appearappear– Compare:Compare:

http://link.oszk.hu/libriurl.php?http://link.oszk.hu/libriurl.php?LN=hu&DB=OSZK&SRY=an&SRE=000002645471LN=hu&DB=OSZK&SRY=an&SRE=000002645471

http://nektar.oszk.hu/hu/manifestation/2645471http://nektar.oszk.hu/hu/manifestation/2645471



Good practice (Cool URIs)Good practice (Cool URIs)– Try to keep stable and persistentTry to keep stable and persistent


VocabulariesVocabularies Well-known vocabulariesWell-known vocabularies

– Friend-of-a-Friend (FOAF), vocabulary for describing people.Friend-of-a-Friend (FOAF), vocabulary for describing people.– Dublin Core (DC) defines general metadata attributes. See also their Dublin Core (DC) defines general metadata attributes. See also their

new domains and ranges draft.new domains and ranges draft.– Semantically-Interlinked Online Communities (SIOC), vocabulary Semantically-Interlinked Online Communities (SIOC), vocabulary

for representing online communities.for representing online communities.– Description of a Project (DOAP), vocabulary for describing projects.Description of a Project (DOAP), vocabulary for describing projects.– Simple Knowledge Organization System (SKOS), vocabulary for Simple Knowledge Organization System (SKOS), vocabulary for

representing taxonomies and loosely structured knowledge.representing taxonomies and loosely structured knowledge.– Music Ontology provides terms for describing artists, albums and Music Ontology provides terms for describing artists, albums and

tracks.tracks.– Review Vocabulary, vocabulary for representing reviews.Review Vocabulary, vocabulary for representing reviews.– Creative Commons (CC), vocabulary for describing license terms.Creative Commons (CC), vocabulary for describing license terms.


Serving information as Linked DataServing information as Linked Data

RequirementRequirement– Things must be identified with dereferenceable Things must be identified with dereferenceable

HTTP URIsHTTP URIs– Data source must return an RDF/XML Data source must return an RDF/XML

description of the identified resourcedescription of the identified resource– Links to external resourcesLinks to external resources– Links from external resources (ensure that there Links from external resources (ensure that there

are external RDF links pointing at URIs from are external RDF links pointing at URIs from your dataset)your dataset)



How to create external links pointing to your How to create external links pointing to your data?data?– In your FOAF file point the central resources of In your FOAF file point the central resources of

your datasetyour dataset• If one of your friends has a FOAF file and points to If one of your friends has a FOAF file and points to

you your dataset is now part of the Web of Datayou your dataset is now part of the Web of Data

– Convince the owners of related data sets to Convince the owners of related data sets to auto-generate links to your datasetauto-generate links to your dataset



Serving the dataset as static RDF fileServing the dataset as static RDF file– Which cases?Which cases?

• RDF files are created manuallyRDF files are created manually• RDF files are created programs that produces file RDF files are created programs that produces file

outputoutput

– HowHow• Give .rdf filename extensionGive .rdf filename extension• In httpd.conf add this lineIn httpd.conf add this line

– AddType application/rdf+xml .rdfAddType application/rdf+xml .rdf

– Problem: 303 redirect does not workProblem: 303 redirect does not work



Serving relational databasesServing relational databases– D2R serverD2R server

• Provides a SPARQL endpoint to the relational Provides a SPARQL endpoint to the relational database by the means of mapping filedatabase by the means of mapping file



Wrappers around existing applications and Wrappers around existing applications and WEB APIWEB API– RDF Book MashupRDF Book Mashup

• The RDF Book Mashup assigns a HTTP URI to each The RDF Book Mashup assigns a HTTP URI to each book that has an ISBN numberbook that has an ISBN number

• Whenever the HTTP URI is dereferenceWhenever the HTTP URI is dereferencedd RDF Book RDF Book Mashup requires data from Amazon API and Google Mashup requires data from Amazon API and Google Base APIBase API



RDF databaseRDF database– JenaJena

• SPARQL endpoint is JoSPARQL endpoint is Josekiseki


Testing and debuggingTesting and debugging

RDF browsersRDF browsers– TabulatorTabulator– MarblesMarbles– Open Link RDF BrowserOpen Link RDF Browser– DiscoDisco

Testing dereferencingTesting dereferencing– curlcurl

W3C RDF validation serviceW3C RDF validation service


Discovering linked data on the webDiscovering linked data on the web

Ping the semantic webPing the semantic web– Registry service for RDF documentsRegistry service for RDF documents

HTML link auto-discoveryHTML link auto-discovery– Set links from existing web pages to RDF dataSet links from existing web pages to RDF data– HTML <link> element in the <head> of your HTML <link> element in the <head> of your

HTML pageHTML page– <link rel="alternate" type="application/rdf+xml" <link rel="alternate" type="application/rdf+xml"

href="link_to_the_RDF_version"/>href="link_to_the_RDF_version"/>


Discovering linked data on the webDiscovering linked data on the web

Semantic Web Crawling: a Sitemap Semantic Web Crawling: a Sitemap ExtensionExtension– Data publishers can state where RDF is located Data publishers can state where RDF is located – Robot.txtRobot.txt

Dataset List on the ESW WikiDataset List on the ESW Wiki


Web of data search enginesWeb of data search engines

FalconsFalcons developed by IWS China developed by IWS ChinaSindiceSindice developed by DERI Ireland developed by DERI IrelandWatsonWatson developed by KMi, UK developed by KMi, UKSemantic Web Search Engine (SWSE)Semantic Web Search Engine (SWSE)

developed by DERI Irelanddeveloped by DERI IrelandSwoogleSwoogle developed by ubiquity group at developed by ubiquity group at

UMBC USAUMBC USA


Semantic web: where are we now? Semantic web: where are we now?

Thing

http://nektar.oszk.hu/resource/manifestation/2645471

RDF

303 redirectand

Contentnegotiation

applicatoin/rdf+xml wins text/html wins

Content location:http://nektar.oszk.hu/data/manifestation/2645471 Content location:

http://nektar.oszk.hu/hu/manifestation/2645471

HTML


The model in detailThe model in detail

The thing URIThe thing URIhttp://nektar.oszk.hu/resource/manifestation/2645471http://nektar.oszk.hu/resource/manifestation/2645471– The 303 redirection code indicates that this URI is for thingThe 303 redirection code indicates that this URI is for thing

The RDF document URIThe RDF document URIhttp://nektar.oszk.hu/data/manifestation/2645471 http://nektar.oszk.hu/data/manifestation/2645471

The WEB (LibriVision) document URIThe WEB (LibriVision) document URIhttp://nektar.oszk.hu/http://nektar.oszk.hu/huhu/manifestation/2645471/manifestation/2645471http://nektar.oszk.hu/http://nektar.oszk.hu/enen/manifestation/2645471/manifestation/2645471


The model in detailThe model in detail

Content negotiation rulesContent negotiation rules

– If application/rdf+xml is accepted the xml is given If application/rdf+xml is accepted the xml is given from this address via content negotiation and 303 from this address via content negotiation and 303 redirect:redirect:http://nektar.oszk.hu/data/manifestation/2645471http://nektar.oszk.hu/data/manifestation/2645471

– If text/html is acceptedIf text/html is accepted• Depending on the language of the browser either the Depending on the language of the browser either the

Hungarian or the English interface of LibriVision is given. Hungarian or the English interface of LibriVision is given. The default is Hungarian (again via content negotiation):The default is Hungarian (again via content negotiation):http://nektar.oszk.hu/hu/manifestation/2645471http://nektar.oszk.hu/hu/manifestation/2645471


The working modelThe working model


The record in LibriVisionThe record in LibriVision


















SKOSSKOS

SKOS is now a W3C recommendationSKOS is now a W3C recommendation– Last year it just was a proposed Last year it just was a proposed

recommendationrecommendationNSZL is among the first implementersNSZL is among the first implementers


Useful linksUseful links How to Publish Linked Data on the WebHow to Publish Linked Data on the Web

– http://www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/http://www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/ Cool URIs for the Semantic WebCool URIs for the Semantic Web

– http://www.w3.org/TR/2008/NOTE-cooluris-20080331/http://www.w3.org/TR/2008/NOTE-cooluris-20080331/ SKOS Simple Knowledge Organization System SKOS Simple Knowledge Organization System

ReferenceReference– http://www.w3.org/TR/skos-reference/http://www.w3.org/TR/skos-reference/

SKOS Simple Knowledge Organization System PrimerSKOS Simple Knowledge Organization System Primer– http://www.w3.org/TR/skos-primer/http://www.w3.org/TR/skos-primer/

SKOS implementation reportSKOS implementation report– http://www.w3.org/2006/07/SWD/SKOS/reference/20090315/http://www.w3.org/2006/07/SWD/SKOS/reference/20090315/

implementation.htmlimplementation.html

http://www4.wiwiss.fu-berlin.de/bizer/pub/LinkedDataTutorial/

http://www.w3.org/TR/2008/NOTE-cooluris-20080331/

http://www.w3.org/TR/skos-reference/

[email protected]@oszk.hu

Thank you for your attentionThank you for your attention

Technology

Semantic web: where are we now?