Consuming Linked Data 4/5 Semtech2011
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This document discusses various approaches for building applications that consume linked data from multiple datasets on the web. It describes characteristics of linked data applications and generic applications like linked data browsers and search engines. It also covers domain-specific applications, faceted browsers, SPARQL endpoints, and techniques for accessing and querying linked data including follow-up queries, querying local caches, crawling data, federated query processing, and on-the-fly dereferencing of URIs. The advantages and disadvantages of each technique are discussed.
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Consuming Linked Data 4/5 Semtech2011
- 1. Consuming Linked DataJuan F. SequedaSemantic Technology ConferenceJune 2011
- 2. Now what can we do with this data?
- 3. Linked Data ApplicationsSoftware system that makes use of data on the web from multiple datasets and that benefits from links between the datasets
- 4. Characteristics of Linked Data ApplicationsConsume 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
- 5. Discover further information by following the links between different data sources: the fourth principle enables this.
- 6. Combine the consumed linked data with data from sources (not necessarily Linked Data)
- 7. Expose the combined data back to the web following the Linked Data principles
- 8. Offer value to end-usersGeneric Applications
- 10. Linked Data BrowsersNot actually separate browsers. Run inside of HTML browsersView the data that is returned after looking up a URI in tabular formUser can navigate between data sources by following RDF Links(IMO) No usability
- 13. Linked Data (Semantic Web) Search Engines
- 14. Linked Data (Semantic Web) Search EnginesJust like conventional search engines (Google, Bing, Yahoo), crawl RDF documents and follow RDF links.Current search engines don’t crawl data, unless it’s RDFaHuman focus SearchFalcons - KeywordSWSE – KeyworkdVisiNav – Complex QueriesMachine focus SearchSindice – data instancesSwoogle - ontologiesWatson - ontologiesUberblic – curated integrated data instances
- 15. (Semantic) SEO ++Markup your HTML with RDFaUse standard vocabularies (ontologies)Google VocabularyGood RelationsDublin CoreGoogle and Yahoo will crawl this data and use it for better rendering
- 20. Domain Specific ApplicationsGovernmentData.govData.gov.ukhttp://data-gov.tw.rpi.edu/wiki/DemosMusicSeevl.netDbpedia MobileLife ScienceLinkedLifeDataSportsBBC World Cup
- 21. Faceted Browsers
- 24. Query your data
- 25. Find all the locations of all the original paintings of Modigliani
- 26. Select all proteins that are linked to a curated interaction from the literature and to inflammatory responsehttp://linkedlifedata.com/
- 27. SPARQL EndpointsLinked 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/
- 28. Where can I find SPARQL Endpoints?Dbpedia: http://dbpedia.org/sparqlMusicbrainz: http://dbtune.org/musicbrainz/sparqlU.S. Census: http://www.rdfabout.com/sparqlhttp://esw.w3.org/topic/SparqlEndpoints
- 29. Accessing a SPARQL EndpointSPARQL endpoints: RESTful Web servicesIssuing SPARQL queries to a remote SPARQL endpoint is basically an HTTP GET request to the SPARQL endpoint with parameter queryGET /sparql?query=PREFIX+rd... HTTP/1.1 Host: dbpedia.orgUser-agent: my-sparql-client/0.1URL-encoded string with the SPARQL query
- 30. Query Results FormatsSPARQL 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)
- 31. Query Results FormatsPREFIX dbp: http://dbpedia.org/ontology/PREFIX dbpprop: http://dbpedia.org/property/SELECT ?name ?bdayWHERE { ?pdbp:birthplace <http://dbpedia.org/resource/Berlin> . ?pdbpprop:dateOfBirth ?bday . ?pdbpprop:name ?name .}
- 34. Query Result FormatsUse the ACCEPT header to request the preferred result format:GET /sparql?query=PREFIX+rd... HTTP/1.1 Host: dbpedia.orgUser-agent: my-sparql-client/0.1 Accept: application/sparql-results+json
- 35. Query Result FormatsAs an alternative some SPARQL endpoint implementations (e.g. Joseki) provide an additional parameter outGET /sparql?out=json&query=... HTTP/1.1 Host: dbpedia.orgUser-agent: my-sparql-client/0.1
- 36. Accessing a SPARQL EndpointMore convenient: use a librarySPARQL JavaScript Libraryhttp://www.thefigtrees.net/lee/blog/2006/04 sparql_calendar_demo_a_sparql.htmlARC for PHPhttp://arc.semsol.org/RAP – RDF API for PHPhttp://www4.wiwiss.fu-berlin.de/bizer/rdfapi/index.html
- 37. Accessing a SPARQL EndpointJena / 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/
- 38. Accessing a SPARQL EndpointExample with Jena/ARQimport com.hp.hpl.jena.query.*;String service = "..."; // address of the SPARQL endpoint String query = "SELECT ..."; // your SPARQL query QueryExecutione = QueryExecutionFactory.sparqlService(service, query)ResultSet results = e.execSelect(); while ( results.hasNext() ) {QuerySolutions = results.nextSolution(); // ...} e.close();
- 39. Querying a single dataset is quite boringcompared toIssuing queries over multiple datasets
- 40. Creating a Linked Data Application
- 41. Linked Data ArchitecturesFollow-up queriesQuerying Local CacheCrawlingFederated Query ProcessingOn-the-fly Dereferencing
- 42. Follow-up QueriesIdea: issue follow-up queries over other datasets based on results from previous queriesSubstituting placeholders in query templates
- 43. 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 DbpediaURIs from them
- 44. Follow-up QueriesAdvantageQueried data is up-to-dateDrawbacksRequires the existence of a SPARQL endpoint for each datasetRequires program logicVery inefficient
- 45. Querying Local CacheIdea: Use an existing SPARQL endpoint that provides access to a set of copies of relevant datasetsUse RDF dumps of each datasetSPARQL endpoint over a majority of datasets from the LOD cloud at:http://uberblic.orghttp://lod.openlinksw.com/sparql
- 46. Querying a Collection of DatasetsAdvantage:No need for specific program logicIncludes the datasets that you wantComplex queries and high performanceEven reasoningDrawbacks:Depends on existence of RDF dumpRequires effort to set up and to operate the store How to keep the copies in sync with the originals?Queried data might be out of date
- 47. CrawlingCrawl RDF in advance by following RDF linksIntegrate, clean and store in your own triplestoreSame way we crawl HTML todayLDSpider
- 48. CrawlingAdvantages:No need for specific program logic Independent of the existence, availability, and efficiency of SPARQL endpointsComplex queries with high performanceCan even reason about the dataDrawbacks:Requires effort to set up and to operate the store How to keep the copies in sync with the originals?Queried data might be out of date
- 49. Federated Query ProcessingIdea: Querying a mediator which distributes sub-queries to relevant sources and integrates the results
- 50. Federated Query ProcessingInstance-based federationEach thing described by only one data source Untypical for the Web of DataTriple-based federationNo restrictions Requires more distributed joinsStatistics about datasets required (both cases)
- 51. Federated Query ProcessingDARQ (Distributed ARQ)http://darq.sourceforge.net/Query engine for federated SPARQL queriesExtension of ARQ (query engine for Jena)Last update: June 2006Semantic Web Integrator and Query Engine(SemWIQ)http://semwiq.sourceforge.net/Last update: March 2010Commercial…
- 52. Federated Query ProcessingAdvantages:No need for specific program logic Queried data is up to dateDrawbacks:Requires the existence of a SPARQL endpoint for each datasetRequires effort to set up and configure the mediator
- 53. In any case:You have to know the relevant data sourcesWhen developing the app using follow-up queriesWhen selecting an existing SPARQL endpoint over a collection of dataset copiesWhen setting up your own store with a collection of dataset copiesWhen configuring your query federation system You restrict yourself to the selected sources
- 54. In any case:You have to know the relevant data sourcesWhen developing the app using follow-up queriesWhen selecting an existing SPARQL endpoint over a collection of dataset copiesWhen setting up your own store with a collection of dataset copiesWhen configuring your query federation system You restrict yourself to the selected sourcesThere is an alternative: Remember, URIs link to data
- 55. On-the-fly DereferencingIdea: Discover further data by looking up relevant URIs in your application on the flyCan be combined with the previous approachesLinked Data Browsers
- 56. Link Traversal Based Query ExecutionApplies the idea of automated link traversal to the execution of SPARQL queriesIdea:Intertwine query evaluation with traversal of RDF linksDiscover data that might contribute to query results during query executionAlternately:Evaluate parts of the query Look up URIs in intermediate solutions
- 57. Link Traversal Based Query Execution
- 58. Link Traversal Based Query Execution
- 59. Link Traversal Based Query Execution
- 60. Link Traversal Based Query Execution
- 61. Link Traversal Based Query Execution
- 62. Link Traversal Based Query Execution
- 63. Link Traversal Based Query Execution
- 64. Link Traversal Based Query Execution
- 65. Link Traversal Based Query Execution
- 66. Link Traversal Based Query Execution
- 67. Link Traversal Based Query ExecutionAdvantages:No need to know all data sources in advanceNo need for specific programming logicQueried data is up to dateDoes not depend on the existence of SPARQL endpoints provided by the data sourcesDrawbacks:Not as fast as a centralized collection of copiesUnsuitable for some queriesResults might be incomplete (do we care?)
- 68. ImplementationsSemantic Web Client library (SWClLib) for Javahttp://www4.wiwiss.fu-berlin.de/bizer/ng4j/semwebclient/SWIC for Prologhttp://moustaki.org/swic/
- 69. ImplementationsSQUIN http://squin.orgProvides SWClLib functionality as a Web serviceAccessible like a SPARQL endpointInstall package: unzip and startLess 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()
- 71. What else?Vocabulary Mappingfoaf:namevsfoo:nameIdentity Resolutionex:Juanowl:sameAsfoo:JuanProvenanceData QualityLicense
- 72. Getting Started Finding URIsUse search enginesFinding SPARQL Endpoints