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Linked Data on the Web

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An comprehensive overview on Linked Data, including an introduction, technical foundations, consuming LD, and open research issues.

An comprehensive overview on Linked Data, including an introduction, technical foundations, consuming LD, and open research issues.

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  • 1. Linked Data on the Web Olaf Hartig http://olafhartig.de/foaf.rdf#olaf Database and Information Systems Research Group Humboldt-Universität zu Berlin
  • 2. 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
  • 3. The Traditional Web Traditional Web = Internet + Docs + Links Olaf Hartig - Linked Data on the Web
  • 4. 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 Olaf Hartig - Linked Data on the Web
  • 5. The Traditional Web So what is the problem? Olaf Hartig - Linked Data on the Web
  • 6. The Traditional Web So what is the problem? ● Web content is only loosely structured ● Difficult for applications to do smart things Olaf Hartig - Linked Data on the Web
  • 7. The Traditional 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 Olaf Hartig - Linked Data on the Web
  • 8. The Traditional 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 But wait… don't we do that already? Olaf Hartig - Linked Data on the Web
  • 9. The Traditional Web ● Content providers offer access via Web APIs Web API Web API Web API Web API Olaf Hartig - Linked Data on the Web
  • 10. The Traditional Web ● Content providers offer access via Web APIs ● Mashups combine this data Web API Web API Web API Web API Olaf Hartig - Linked Data on the Web
  • 11. The Traditional Web ● Content providers offer access via Web APIs ● 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 Olaf Hartig - Linked Data on the Web
  • 12. ● 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
  • 13. ● 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/movie1342 http://mymovie.db/movie0362 http://mymovie.db/movie5112 My Movie DB http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 14. ● Use URIs as names for things ● Use HTTP URIs so that people can look up those names. http://m ● When someone looks up a ymovie URI, provide useful information. ? .d b/movie ● Include links to other URIs so that they can discover more 2449 things. Tim Berners-Lee, July 2006 http://mymovie.db/movie1342 http://mymovie.db/movie0362 http://mymovie.db/movie5112 My Movie DB http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 15. ● Use URIs as names for things ● Use HTTP URIs so that people can look up those names. http://m ● When someone looks up a ymovie URI, provide useful information. ? .d b/movie ● Include links to other URIs so that they can discover more 2449 things. Tim Berners-Lee, July 2006 http://mymovie.db/movie1342 http://mymovie.db/movie0362 http://mymovie.db/movie5112 My Movie DB http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 16. ● Use URIs as names for things ● Use HTTP URIs so that people can look up those names. http://m ● When someone looks up a ymovie URI, provide useful information. ? .d b/movie ● Include links to other URIs so that they can discover more 2449 things. Tim Berners-Lee, July 2006 http://mymovie.db/movie1342 http://mymovie.db/movie0362 http://mymovie.db/movie5112 My Movie DB http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 17. ● Use URIs as names for things ● Use HTTP URIs so that people can look up those names. http://m ● When someone looks up a ymovie URI, provide useful information. ? .d b/movie ● Include links to other URIs so that they can discover more 2449 things. Tim Berners-Lee, July 2006 http://mymovie.db/movie1342 http://mymovie.db/movie0362 http://geo.db/country21 http://geo.db/country7 http://mymovie.db/movie5112 My Movie DB http://geo.db/cityCJ http://geo.db/cityXA http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 18. ● Use URIs as names for things ● Use HTTP URIs so that people can look up those names. http://m ● When someone looks up a ymovie URI, provide useful information. ? .d b/movie ● Include links to other URIs so that they can discover more 2449 things. Tim Berners-Lee, July 2006 http://mymovie.db/movie1342 http://mymovie.db/movie0362 http://geo.db/country21 http://geo.db/country7 http://mymovie.db/movie5112 My Movie DB http://geo.db/cityCJ http://geo.db/cityXA http://mymovie.db/movie2449 Olaf Hartig - Executing SPARQL Queries over the Web of Linked Data
  • 19. 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/ Olaf Hartig - Linked Data on the Web
  • 20. 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/ n atio l be o pu l n:p :la g s rdf 60943000 United Kingdom Olaf Hartig - Linked Data on the Web
  • 21. 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 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 Olaf Hartig - Linked Data on the Web
  • 22. 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 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 Olaf Hartig - Linked Data on the Web
  • 23. 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
  • 24. 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
  • 25. 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
  • 26. W3C Linking Open Data Project As of July 2007 > 500M triples ca. 120,000 links Olaf Hartig - Linked Data on the Web
  • 27. W3C Linking Open Data Project ca. 6.7B triples ca. 150M links Olaf Hartig - Linked Data on the Web
  • 28. W3C Linking Open Data Project Media User generated content Publications Geographic Cross-domain Life Sciences ca. 6.7B triples ca. 150M links Olaf Hartig - Linked Data on the Web
  • 29. 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
  • 30. 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
  • 31. 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
  • 32. 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 Olaf Hartig - Linked Data on the Web
  • 33. 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
  • 34. Technical Foundations There is no magic – Linked Data is based on well-established (Semantic) Web technologies. ● HTTP ● URI ● RDF ● RDFS / OWL Olaf Hartig - Linked Data on the Web
  • 35. 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
  • 36. Looking up URIs Give me data about http://olafhartig.de/foaf.rdf#olaf 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: */* Olaf Hartig - Linked Data on the Web
  • 37. 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"/> ... Olaf Hartig - Linked Data on the Web
  • 38. 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 Olaf Hartig - Linked Data on the Web
  • 39. 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> . ... Olaf Hartig - Linked Data on the Web
  • 40. 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
  • 41. 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 Olaf Hartig - Linked Data on the Web
  • 42. 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 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 Olaf Hartig - Linked Data on the Web
  • 43. 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: text/html Olaf Hartig - Linked Data on the Web
  • 44. 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: 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 Olaf Hartig - Linked Data on the Web
  • 45. 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
  • 46. 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
  • 47. owl:sameAs 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/ ow l :sam n e As atio l be o pu l n:p :la g s http://dbpedia.org/resource/United_Kingdom rdf 60943000 United Kingdom Olaf Hartig - Linked Data on the Web
  • 48. owl:sameAs 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/ ow l :sam n e As atio l be o pu l n:p :la g s http://dbpedia.org/resource/United_Kingdom rdf 60943000 r db United Kingdom de :c a all p:le in gC db od http://dbpedia.org/resource/Gordon_Brown e 44 Olaf Hartig - Linked Data on the Web
  • 49. 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
  • 50. 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
  • 51. 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
  • 52. Faceted Browsers Olaf Hartig - Linked Data on the Web http://dbpedia.neofonie.de
  • 53. On-the-fly Mashups Olaf Hartig - Linked Data on the Web http://sig.ma
  • 54. Linked Data based Applications [SFSW'09] Olaf Hartig - Linked Data on the Web
  • 55. 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 . … Olaf Hartig - Linked Data on the Web
  • 56. 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> . … Olaf Hartig - Linked Data on the Web
  • 57. Consuming Linked Data … in Applications ● Look up URIs and process the retrieved data ● Query with SPARQL Olaf Hartig - Linked Data on the Web
  • 58. 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 Olaf Hartig - Linked Data on the Web
  • 59. 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 Olaf Hartig - Linked Data on the Web
  • 60. 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
  • 61. 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
  • 62. Querying Linked Data with SPARQL Traditional approach 1: data centralization ● Querying a collection of copies from all relevant datasets Olaf Hartig - Linked Data on the Web
  • 63. Querying Linked Data with SPARQL Traditional approach 2: federated query processing ? ● Querying a mediator which distributes subqueries to relevant sources and integrates the results ? ? ? Olaf Hartig - Linked Data on the Web
  • 64. 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 ! Olaf Hartig - Linked Data on the Web
  • 65. A novel approach: Link Traversal Based Query Execution [ISWC'09] Olaf Hartig - Linked Data on the Web
  • 66. 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 Olaf Hartig - Linked Data on the Web
  • 67. 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 http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 68. Main Idea ● Intertwine query evaluation with traversal of RDF links Alternately: htt ● p:/ /. Evaluate parts of the query on a ../m ? ● continuously augmented set of data ov ie2 44 ● Look up URIs in intermediate 9 solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 69. Main Idea ● Intertwine query evaluation with traversal of RDF links Alternately: htt ● p:/ /. Evaluate parts of the query on a ../m ? ● continuously augmented set of data ov ie2 44 ● Look up URIs in intermediate 9 solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 70. Main Idea ● Intertwine query evaluation with traversal of RDF links Alternately: htt ● p:/ /. Evaluate parts of the query on a ../m ? ● continuously augmented set of data ov ie2 44 ● Look up URIs in intermediate 9 solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 71. 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 http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 72. 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 filmingLocation http://.../movie2449 http://geo.../Italy Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 73. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● 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 filmingLocation http://.../movie2449 http://geo.../Italy Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 74. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● Evaluate parts of the query on a ? aly continuously augmented set of data ./I t .. g eo Look up URIs in intermediate :// ● p htt solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 75. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● Evaluate parts of the query on a ? aly continuously augmented set of data ./I t .. g eo Look up URIs in intermediate :// ● p htt solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 76. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● Evaluate parts of the query on a ? aly continuously augmented set of data ./I t .. g eo Look up URIs in intermediate :// ● p htt solutions and add retrieved data to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 77. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● 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 http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 78. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● 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 http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 79. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● 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 tics http://stat.db/.../it statis http://geo.../Italy Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 80. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● Evaluate parts of the query on a continuously augmented set of data ● Look up URIs in intermediate ?loc ?stat solutions and add retrieved data http://geo.../Italy http://stats.db/../it to the queried data set tics http://stat.db/.../it statis http://geo.../Italy Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 81. Main Idea ● Intertwine query evaluation with traversal of RDF links ?loc ● Alternately: http://geo.../Italy ● Evaluate parts of the query on a continuously augmented set of data ● Look up URIs in intermediate ?loc ?stat solutions and add retrieved data http://geo.../Italy http://stats.db/../it to the queried data set Queried data http://.../movie2449 s ?stat unem Query filmin tis t ic p_ r a g Loca sta te t io n ?loc ?ur Olaf Hartig - Linked Data on the Web
  • 82. 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
  • 83. 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 Olaf Hartig - Linked Data on the Web
  • 84. 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 Olaf Hartig - Linked Data on the Web
  • 85. 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 Olaf Hartig - Linked Data on the Web
  • 86. 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 Olaf Hartig - Linked Data on the Web
  • 87. 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
  • 88. 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 Olaf Hartig - Linked Data on the Web
  • 89. Olaf Hartig - Linked Data on the Web
  • 90. Olaf Hartig - Linked Data on the Web
  • 91. 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" ) . } Olaf Hartig - Linked Data on the Web
  • 92. 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 Olaf Hartig - Linked Data on the Web
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  • 95. Olaf Hartig - Linked Data on the Web
  • 96. 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 Olaf Hartig - Linked Data on the Web
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  • 99. 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 Olaf Hartig - Linked Data on the Web
  • 100. 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
  • 101. 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 Olaf Hartig - Linked Data on the Web
  • 102. 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? Olaf Hartig - Linked Data on the Web
  • 103. 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
  • 104. Take-away Summary The traditional Web of documents evolves into a Web of data. ● 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 Olaf Hartig - Linked Data on the Web
  • 105. 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/) Olaf Hartig - Linked Data on the Web