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Gray, N. (2015) RDF, the Semantic Web, Jordan, Jordan and Jordan. In: Moss, M. and Currall, J. (eds.) Transition to the Digital. Facet Publishing.

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RDF,theSemanticWeb, Jordan, JordanandJordanNormanGray1

24August2014

DRAFT chaptertoappearin‘TransitiontotheDigital’, MichaelMossandJamesCurrall(eds.) (2015?). Thiscollectionisaddressedtoarchivistsandlibraryprofessionals, andsohasaslightfocusonimplicationsimplicationsforthem. Thischapterisnonethelessintendedtobeamore-or-lessgenericintroductiontotheSemanticWebandRDF,whichisn'tspecifictothatdomain. CopyrightNormanGray2014.

ThischapterisaboutthekeynoveltiesoftheSemanticWeb–thenovelideas,andthenovelopportunities. ButwewilldiscussthesedigitalnoveltiesinthecontextoftheSemanticWeb’s continuities withotherfeaturesoftheinformationworld.

Ourmostobviousantecedentisnotthatold–the(non-semantic)Webdidn’texistbeforethe90s–andwewilllearnabouttheveryclosetechnicaloverlapbetweentheSemanticWebandthe‘textualWeb’ofournow-usualexperience.TheSemanticWebis, closerthanacousin, thesiblingofthetextualweb.

Otherantecedentshaveahistoryasoldasthefirstlibraryindex. TheSe-manticWebhas, wemightsay, a‘logicalwing’andan‘informationwing’. Thesearenotprimarilydistinguishedbytheirtechnicalororganisationalfeatures, butby the largely disjoint research questions they address, and by theirmotiva-tions. While the ‘logicalwing’ ischaracterisedbyaconcern for formal logicanditsimplementations, richinthetheoryofcomputingscience1, the‘informa-tionwing’, withasturdilypragmaticfocus, canberegardedascontinuouswiththeinformation-organisinggoalsoftheworldoflibraryscience, sharingitsaspi-rationtosystematizeandshareinformation, anditsacknowledgementthatsuchsharingisalwaysapproximateandneverunmediated, andthatonemustaimforabalancebetweenfaithfulness tosources, andwhat isactuallyusablebytheinformation’sactualaudience.

Below, wewillstartwithabroadintroductiontotheSemanticWeb. Fromtherewecanmovebrisklyontopractice, andthequestionofwhere, howandwhether thesemanticwebmightappear intechnological fact. Ourgoal is toindicatethecontinuitieswiththetextualweb, andthustoindicatethenovel-tiesoftheSemanticWeb, andsosuggestwhytheyareimportant. Afterabriefparenthesison‘Web2.0’(inSect. 2), wedescribe‘linkeddata’inSect. 3.

1 WhatistheSemanticWeb?

TheSemanticWebissimpleinsummary:

TheSemanticWebistheemergingnextstageoftheweb, designedtotransmitmachine-processablemeaning, through a logical framework

1PhysicsandAstronomy, UniversityofGlasgow, UK; http://nxg.me.uk1ThisstrandcanwithatleastalittlejusticeberegardedasthemostrecentlasthurrahofAI,a

traditionpronounceddeadmoreoftenthan SouthPark’s Kenny.

RDF,theSemanticWeb, Jordan, JordanandJordan

namedRDF,enhancedbymachineinferencebasedonOWL andotherontologies.

ThoughI assertthatthismodeststatementisplausible, andtheoutcomedesir-able, thereadermaybedisinclinedtoagree, onthegroundsthatthestatementisonthefaceofthingsgobbledegook. Overthecourseofthischapter, I intendtoexplaineachcomponentofthisremark, stepbystep, inthehopethatitisashorthopfromtheretoplausibility.

1.1 TheSemanticWeb…

First, agenerallament.The term ‘semanticweb’ is anunfortunateone, since itmakes the topic

soundmuchmorearcanethanitreallyis. Itisarguablysimple: thenextstageinalong-termvisionoftheweb, originallyformulatedbyTimBerners-Lee, andelaboratedbythe WorldWideWebConsortium(W3C) whichhewasinstrumen-talinfounding.

1.2 …istheemergingnextstageoftheweb…

Beforewecanfullyunderstandthe‘semanticweb’, weneedaclearideaofwhatthe‘WorldWideWeb’is, eventhough, nowadays, suchaquestionmayseemasoddasaskingwhat‘air’is.

Thewebis remarkably homogeneous: itconsistsof one protocol, one bitofglue, andmarkup.

Everythingon theweb is connectedby theHypertextTransport Protocol(HTTP) [1], andifyoulookatawebpage, updateapodcast, talkonJabber, ordownloadvideos, onacomputeroramobilephone, thebytescometoyourcomputerviaHTTP.Indeed, adebatablebutplausibledefinitionofthewebisas the setof things reachable throughanHTTP request. Theonlybitof thatprotocolyouevernoticeis‘404’, whichistheHTTP errorcodemeaning‘I don’thaveanythingbythatname’.

Thebitofglueisthe UniformResourceIdentifier(URI) [2]. That’sauniformnaming schemeforthingsonthewebandbeyond. EverythingonthewebhasaURI,andmostURIsrefertothingsontheweb.2

Themarkupismostly HypertextMarkupLanguage(HTML),theangle-bracketswhichindicatehowwebpagesshouldbeformatted. Butit’salsoRSS andAtomfeeds(ie, blogsandpodcasts), wikisyntax, PDFs, andafewothermoreobscurealternatives.

Thesecomponentscometogetherwhenyouclickonalinkinawebpage(thefollowingexplanationofhowthewebworksisprobablynotnewtoyou,butI'mspellingitoutinordertomakeclearhowthevariouscomponentsworktogether). Thewebbrowserprogramonyourcomputerortabletorphoneisaclient, whichdisplaysapagepreviouslysenttoitbyaprogramsittingona serversomewhereintheinternet(wewillusetheselattertermsrepeatedlybelow). Thepage(typically)arrivesatyourcomputerintheformofHTML whichthebrowserknowshowtoformatanddisplayasheadings, sidebars, images, andlinks. Thelinkassociates some texton thepagewitha URI,andclickingon it tells thebrowser‘goandlookatthispageinstead’. ThebrowserthenexaminestheURItodiscoverwhichwebserverisprovidingthatpage, thenimmediatelymakesanotherHTTP requesttothatservertoretrievethepage, displayittoyou, andstartthecycleoncemore.

2That’sasortof ‘mindspace’most: thisstatementprobably isn’tnumerically trueifyougobynumbersofobjectsorvolumeofdata.

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Beforewegoon, I should(parenthetically)becarefultodistinguishthewebfromthe internet. Theinternetisasetofprotocols, plural, forexchangingmate-rialbetweennetworkedcomputers. Whenyousendanemailorretrieveawebpage, thematerialtravelsovertheinternet, butunderthecontrolofdifferentpro-grams–anemailclientandawebbrowser–usingacombinationoflower-andhigher-levelprotocols, or languages.3 Internet telephony (suchasSkype)andtimeservicesaretworeasonablyvisibleinternetserviceswhicharedistinctfromtheweb. Thekeypointisthattheweb–andthisincludestheSemanticWeb–isanotablysimplestructuresittingatoptheinternet.

Thereareafewkeydatesinthehistoryoftheweb.

• 1990: TimBerners-LeeandRobertCaillaufirstproposedthesystemwhichbecametheWorldWideWeb [3].4 Thefirstserverandclientimplementa-tionsappearedasCERN-internalserviceslaterthatyear.

• August1991: Firstpublicweb server. Initially, users interactedwith theserverbyusingTelnet(anothernon-Webinternetprotocol)toconnectdi-rectlytoaclientprogramatCERN,andthencetotheserver.

• December1991: Firstweb serveroutsideEurope, at theStanfordLinearAcceleratorCenter(SLAC,likeCERN anexperimentalhigh-energyphysicslaboratory).

• April1993: Mosaic, fromthe(US) NationalCenterforSupercomputingAp-plications(NCSA),wasthefirstgraphicalbrowser. MosaicledtoMozilla,which led to today’s Firefox; Mozilla also led to the Netscape browser.Aboutthesametime, NCSA releasedtheir‘httpd’webserver, whicheven-tuallymutatedintothenow-ubiquitousApache.5

• October1994: TheWorldWideWebConsortium(W3C) wasfounded, withBerners-Lee as its director, to act as the standards bodyof the emergingsystem. EarlystandardsincludedHTML 3.2(1997),6 thefirstversionofXML(1998) [5], andthefirstmodelandsyntaxforRDF (1999) [6].

• 2006: Lolcats (andother ‘user-generated content’). We return to this inSect. 2

Although itmayseemaslight tangent, it isworthwhilebrieflydiscussingwhatitisthatmakesthewebsospecial, andsoverysuccessful, sincethese-manticwebisinprotocoltermsidenticaltothewebwearefamiliarwith, andsosharesthesamespecialfeatures.

Thewebisnotthefirsthypertextsystem(VannevarBush’shypotheticalMemexsystem [7], andTedNelson’s Xanadu system7 canprobablylayclaimtothat),and it’snot thefirstdistributedhypertext system (VAX NotesandLotusNotescanprobablyclaimthat), butitisthefirsttrulysuccessfulworldwidedistributedhypertextsystem.

Thewebgetssomethingsright:

3Thispointisrathermuddiedbytheobservationthatmanypeoplenowreademailmessagesinawebbrowser; nonethelesstheemailisstilltransportedbetweensystems, inthebackground, usingthedecades-oldemailprotocols.

4This1990documentcomesabout18monthsafter[4], whichisabroaddiscussionofinforma-tionmanagement, inthecontextof“themanagementofgeneralinformationaboutacceleratorsandexperimentsatCERN.”Thisdocument–onwhichBerners-Lee’smanagerwrote“vague, butexciting”–isclearlyancestraltoboththewebandto[3], butitisparticularlyinterestingfromthepointofviewofthischapter, becauseinhindsightitmorecloselyprefiguresthestructureandpotentialofRDF andthe Semantic Web. Onecouldalmostcallthetextualweb‘SemanticWeb0.1’.

5Apacheoriginallyconsistedofasetofsoftware‘patches’toNCSA httpd, hencethe(unfortunatelyapocryphal, itseems)etymologyofitsnameas‘apatchyserver’.

6HTML 2.0wasanIETF standard; earlierversionswerenotformallystandardised.7http://xanadu.com/

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• It isdistributed, ordecentralised, so that there isno centre to theweb–thereisnosinglepointwhichcanfail, orbeco-opted, orwhichcangrantorrefusepermission.

• Itisnon-proprietary, oropen: theprotocolswhichdefinethewebarefreetoobtain, andmaybeimplementedwithoutanyinhibitionsfromlicences.Also, theweb’sgoverningbody(whichthe W3C is, ineffect)doesitsworkthroughanopenprocess.

• Thewebissimple, inthesensethatitisarchitecturallysimple(thenotionoftheweb, asserversplusbrowserspluslinks, iseasytocomprehend), andstraightforwardinprotocol terms(thecoreprotocolof theweb, HTTP, issuch thatacrudeserverorclient implementationcanbedeveloped inarelativelyshorttime).

• It’seasytojoinin: thisistoalargeextentaconsequenceofthesimplicityandopenness.

• YoucanwastehoursonWikipedia.

Onesenseof‘easytojoinin’isthatthewebhas always beenread-write:Berners-Leeconceiveditasaread-writemedium, thefirstbrowserscouldwritetothewebaswellasreadit, and‘anyone’canputupawebpage. Now, ‘anyone’heremeant‘anyonewithaccesstoaunixboxconnectedtotheinternet, whocanbuild, configureandinstallawebserver’; sonotquiteeveryone’s‘anyone’, butthekeypoint is that itwasusability that stoppedyou fromputtingupawebpage, nottheneedforpermission. Thusthe(genuinely)biginnovationofwhatbecameknownas‘Web2.0’, or‘theread/writeweb’, wasthatitwas‘Web1.0’foreveryoneelse.

Thelastpointinthelistisnotfrivolous, butisthepointthat, ontheweb,youcanlinkto anythingelseontheweb, andthatthisisbothculturallyaccept-ableandencouraged. Youcanwanderthroughalotofwebserversbystartingatonepageand‘followingyournose’. This isaconsequenceof thesimplic-ityandopenness, whichtogethermeanthatthereareverymanywebservers,fromveryheterogeneoussources, servingwebpageswrittenbyexpertsandnon-experts, andthatlinkingbetweentheseisnotinhibitedbyanyrequirementforpre-coordination.

Theweb’ssuccessisalsopartlyduetogettingsomethings wrong:

1. linkscanbreak, andpagesdisappear;

2. linksare all ‘seealso’(asopposedto‘parent’, ‘next’, ‘author’oranythingmoreinformative);

3. everythingisastring–theinformationonthewebis, fundamentally, com-municatedthroughtext.8

Onemightalsosaythat‘noqualitycontrol’isavice, butsinceonecansimulta-neouslyclaimthat‘nocontrolatall’isavirtue, thisisatleastdebatable.

Xanadu, forexample, guaranteedlinkintegrity, hadtypedlinks, andtriedtodevelopanewintellectualpropertymodel, allatthesametime. Berners-LeeandCaillau’sinsight, in [3], wasthatthesearesimplyignorableproblems–it’sacceptableforthingstobreak, andoccasional404sareapriceworthpayingtoavoidtheneedforpre-coordinationorregistration; andtheweb’stype-lessandone-directionallinkissuchapowerfulnotionthatitsintrinsicvaguenessisonly

8TheHTTP protocolcanbe, andis, usedtotransportdigitalmediaofalltypes–plaintext, images,PDFs, audio, andotherdata; forourpresentpurposes, I taketheterm‘theweb’torefertotheglobalhypertextsystemofBerners-Lee’soriginalconception; thisdoesnotaffecttheessentialpoint.

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adetail. Thepointofthesemanticwebisthatitstartstoaddressthesecondandthirdproblems.

Whyis‘everythingisastring’aproblem? Ifyousearchonthewebfor‘jor-dan’, yougetlinksaboutthecountry, theriver, theglamourmodel, thebreakfastcereal, thebrandofshoes, varioussmallbusinesses, thebasketballplayer, themathematician, andmore. Thesearenotallthesamething.

Thisdoesn’tmatter, however, becauseweashumansknowthey’redifferentthings, andwe’re not likely to get confused (and ifwe are, briefly, confusedwethinkit’sourfault, ratherthanGoogle’s). Wecan, inotherwords, addthesemanticsourselves, inexactlythesamewaythatwedosowhenreadingtextanywhereelse. Butthismeansthatcomputersareflyingblindwhentheytrytoperformactionsonthewebonourbehalf. Theyhavenoideawhatall thesestringsmean, nor(moreimportantly)howtheyrelatetooneanother.

Thewebworksverywellformanythings, andsearchenginesofcourseworkspookilywellinmanycases.9 Butitonlyreallyworkswhenthere’sahumanintheloop, orwherethere’salotofstatisticstobuildon, andyouwouldn’twanttoletyourcomputerunsupervisedontotheweb, withyourcreditcard.

Butinfactyou do wanttoletyourcomputerontothewebwithyourcreditcard. Inafamousandseminalpaper, Berners-Lee, HendlerandLassila [8]de-scribeanextendedscenarioinwhichcomputersareabletointeractwitheachothertoorganisetravelandotherappointments. Thisscenariohascomeaboutto someextent: price-comparisonwebsitesnow routinely interactwithotherwebsites to extract price data; and sites like tripit.com workbyparsing theconfirmationemailsfromtravelwebsitessuchasExpedia, toextractthedetailsofflightsandhotelstays. Theproblemisthattheseservicesworkbypainfullyscanningthecontentofwebpagesoremailsdirectedathumans(thisisknownas‘screenscraping’), heuristicallyparsingthem, andactingonwhatmayormaynotbetheintendedmeaning.

Thisishardworkforlimitedresults, but(atleastuntilcomputerssomehowmanagetounderstandtext)itisafundamentallimitationofthe webofstrings.

1.3 …designed…

I have stressed thatBerners-LeeandCaillau’soriginal conception is architec-turallystillveryclosetothewebweseenow, 25yearslater. Berners-Leewentontoformthe W3C,anditisthisbodywhichbygeneralconsent–fornocom-pulsionispossiblehere–stillshepherdsthedevelopmentofthecollectionofstandardswhichunderliestheweb(otherbodies, mostnotablytheIETF,arere-sponsible for the technicalgovernanceanddevelopmentof the internet, byasimilarprocessofgeneralconsent). Althoughthecorestandardsareeasilyiden-tified– HTTP, URIs and HTML –theseareaccompaniedbyablizzardofotheragreements, majorandminor, rangingfromthesyntaxofXML toconsensusontheformattingofdates. Theseagreementstaketheformof‘W3C Recommen-dations’,10 collaborativelyauthoredbyW3C WorkingGroups formallydrawnfromacademicandcommercialW3C memberorganisations, butwithwideandoccasionallynoisyparticipationfrominterestedindividualsworld-wide.

Muchof the developmentwork for the semanticweb, in particular, hascomefromuniversitiesandasmallnumberofresearch-activecommercialorgan-isations, oftenwithquitecloselinkstoacademia. Thereisastronginheritance

9Ifyouactuallysearchfor‘jordan’in, forexample, Google, Bing, andDuckDuckGo, youwillfindthat several ofthesedistinctsensesappearonthefirstpage, sothatthereismorevarietythanwouldresultfromsimplylistingthemostpopularpageswiththatstringinthem. Thisisbecausesearchenginescan statistically identifythattherearemultipleclustersofrelatedpagesandthus, forexample, displayonehitfromeachclusterinasituationlikethis. Thesearchenginesareunderstoodtoimprovetheirperformanceherewithsomelightweightsemantics, butthecoreworkofasearchengineisatpresentstillprobabilisticratherthanlogical.

10http://www.w3.org/standards/

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fromprecedingdecadesofresearchon ArtificialIntelligence(AI);thisinheritanceincludedcrucialexperienceofthelogicalunderpinningsofwhatbecameRDF(seeSect. 1.5 below), andexperiencewithrelatedtechnologies, whichmeantthatthefirstRDF standardswereremarkablyself-consistentandwell-developed.Howeverthissameprocessmeantthatthesefirststandardswereratherhermetic,whichwillhavecontributedtotheirearlyreputationforincomprehensibility.

Howevercomprehensiblethesestandardsare–andwewilltouchonthecoreideasbelow–itisimportanttostressthatitdoesnotrequiresomefoun-dationalunderstandingofthemathematico-logicalfoundationsofRDF inordertodescribe, forexample, abook’sbibliographicaldetails. Inrecentyears, the‘linkeddata’ paradigmhas emerged, againwith both academic and industrybacking, withafocusonthepracticalstepstobringabouttheimmediate-termpayoffsof thesemanticweb. Wediscussthis inalittlemoredetailbelow, inSect. 3.

1.4 …totransmitmachineprocessablemeaning…

TheSemanticWebhassomefamilyrelationshipswiththeworldof AI,andin-deedhassuffered, inmarketingterms, fromitsassociationwiththatdiscipline’srepeatedpostponementsof itsgreatpromises. TheSemanticWebisnotcon-cernedwithmachineunderstanding, orwithmachines’creationofmeaning, butinsteadwiththemoremodestgoaloftransmittingmeaningacrosstheweb, inamorereliablewaythanispossiblewiththewebofstrings.

Ourconfusion, above, aboutthedifferentthingscalled‘jordan’arisesbe-causethesevariousverydifferentthingsallshareasinglelabel–‘jordan’–anditisonlyourabilitytounderstandthecontext, andourknowledgeofthestructureoftheworld, thatallowsusinpracticalfacttodistinguishthevariousdenota-tions(wedonotgetconfusedwhentheterm‘jordan’appearsonanewsweb-site’spoliticsandcelebritypages, denotingdifferentthings). Facedwiththesametext, computerscandolittlemorethan(beprogrammedto)relyonstatisticsandheuristics. Searchenginesshowthatthisapproachismoresuccessfulthanonemightexpect, butuntilcomputersdofinallymanageto‘understand’things(iftheyeverdo), thereislittlemorewecando, ifwestickwithjustthetext.

SemanticWebtechnologiesallowustotakeastepbeyondtheselimitations,by

• providinga foundationwithwhich todefinemore specific labels for theconceptsandcategorieswhichmakeupourworld; and

• allowingustomanipulatetheselabels(andbyanalogymanipulatethecon-ceptsandcategories)inamoreorlesssimplecalculusofrelationships.

Anexampleisusefulhere.Onename for theRiver Jordan is http://dbpedia.org/resource/Jordan_

River. ThisderivesfromDBPedia [9], whichisacollectionofSemanticWebnamesderiveddirectlyfromWikipedia. Thename http://sws.geonames.org/7874114/ isanotherone, whichderivesfromtheGeonamesdatabase.11 There-semblanceofthese‘names’toordinarywebURLsisnotacoincidence–allthenameswithintheSemanticWebaresyntacticallyofthisform. Oneoftherea-sonsforthisisthatitpreservesthe‘decentred’propertyofthe(text)web: theownersofthe geonames.org domaincancreatewhatnamestheylikewithinthatdomain, since‘creatinganame’inthiscontextconsistssolelyofdecidingthataparticular URI shouldactassuchapersistentname, andprovidingRDF (seeSect. 1.5)todescribeit. Thishastheimmediateattractivepropertythat(usually)onecanfindmoreaboutaparticularnamebytypingthenameintoanordinary

11http://www.geonames.org

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webbrowser; althoughit’snotarequirement, theusualgoodpracticeisforsuchaURI toproduceahuman-readabledescriptionofsometype.

Anyonecancreatesuchaname: I ownthedomain nxg.me.uk anddecided,bypersonal fiat, that http://nxg.me.uk/norman/ shouldbemy ‘name’ in thiscontext. Afterashortbitofwebsiteconfiguration, itwasso.

Oncewehavenames, wecanstarttodescribethethingssonamed.12

Sinceanyonecan–andmanydo–createonlinenamesforthings, oneofthefirstthingswemightwanttodoistoaddresstheapparentproblemofduplica-tion. So, forexample, wemightwanttosaythat http://dbpedia.org/resource/Jordan_River isthesamethingas http://sws.geonames.org/7874114/. Thiswillbetrueinsomecontexts, butfalseinothers. Inthiscase, itturnsoutthatthegeonames.orgdesignershavedecidedthat http://sws.geonames.org/7874114/referstotheRiverJordan inJordan, andthatthe‘same’riverinIsraelisnamedhttp://sws.geonames.org/294624/. Therefore, in some contexts, itwould besimplyfalse(andimportantlyfalse)tostatethat http://dbpedia.org/resource/Jordan_River is ‘the same as’ one or the other, and wemight find it bettertosaythat http://sws.geonames.org/294624/ is ‘partof’ http://dbpedia.org/resource/Jordan_River. Thissortof(oftenbarelyconsistent)subtletyissome-thingwhichwearecomfortablewithinhumanconversation, butwhichwehavetospelloutinpedanticdetailassoonascomputersareinvolved.13

Otherthingswemightwanttosayarethat http://dbpedia.org/resource/Jordan_River hasacertainlength, thatitisamemberofthecategoryof‘rivers’,that‘rivers’areatypeofgeographicalfeature, thattheriveris locatedwithin (asopposedtobeingacomponentpartof)bothIsraelandJordan; wemightwantacomputerto‘know’thatthecategoriesof‘rivers’and(forexample)‘cats’aredisjoint(computersare very ignorant).

The term ‘know’, here, doesnotof course refer to anycognitionon thepartofamachine. Instead, itreferstoacalculusofmanipulationswhichwouldallowittodetectthatastatementthat‘http://dbpedia.org/resource/Jordan_River isacat’isinconsistentwithwhatithasalreadystored, orthatasearchfor‘thingslabelled“jordan”whicharegeographicalfeatures’shouldnotreturnanybasketballplayers.

ThislastpointindicatesonefragmentofwheretheapplicationsoftheSe-manticWebmightlie. Weknowhowtostoreandmanipulate facts oncomputer–wecanmatchanemployeenumberwithanameanddepartment, orastaratparticularcoordinateswithbrightnessandcolour–butoftenthesefactslackthereal-worldstructureandinterrelationshipsthataresoimportanttohowweashumanswanttohandlethem. Evenwhensuchknowledgeisstructuredinsomecontexts, sothattheinternalstructureofalibrarycataloguemightreflectthereal-worldstructureoftheorganisationthatmaintainsit, thisstructuringisnotshareablewithoutclosepriorcooperation. IfI emailedyouacopyofsuchacatalogueasaspreadsheet, youasahumanwouldswiftlyworkoutwhatthisdocumentwasandwhatsomeofthecolumnsmeant, thoughyoumight(forex-ample)havedifficultytellingapartthecolumnswithpublicationyear, acquisitionyearand, say, theyearthebookwasfirstborrowed. I wouldhavetotalktoyoutotellyouwhichcolumnwaswhich; neitherofuswouldexpectthespreadsheet

12I noteparenthetically–forthereisa veryverylong distractionpossiblehere–thattheURIs http://dbpedia.org/resource/Jordan_River and http://nxg.me.uk/norman/ arenamesfor thephysicalthingsthemselves, andnotforanyonlineresourcedescribingthethings; inthesamesense, aDOI isadigitalidentifierforanobjectsuchasanarticle, andnotfortherecorddescribingthearticle. ThisistrueeventhoughbothofthemhavetheformofaURI whichisretrievable(or‘dereferenceable’)inthesensethatawebbrowsercanretrievesomethingfromthataddress. IfyouretrieveeitherofthoseURLsby typing it intoabrowser, youare redirected toanotherwebpagewhichdescribestheresource(thatis, aresourcewithadifferentname, which is awebpageratherthanapersonorconcept); inneithercasedoyougetme, orariver-fulofwater, deliveredtoyourcomputer. Ifyouareinterested, andhaveadayorsotoburn, searchfor‘httprange-14’.

13WemakesomeextraremarksaboutthelogicofRDF,andidentity, inSect. 1.5.

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tobeusableonacomputerwithoutthishuman-to-humaninteraction.ThegoaloftheSemanticWebistomakethissortofstructuredknowledge

–aboutgeography, stars, people, or retailopportunities–generallyshareableandmanipulable by computers, while preserving asmany as possible of the(text)web’spropertiesofdecentralisation(there isnopermissionorcoordina-tionrequired), openness(whatwecancommunicateisnotlimited apriori), andsimplicity(thereisstillonlyoneprotocol, HTTP).

Thatis, theSemanticWebisnotaboutmachine understanding, butaboutthetechnologyrequiredtoletcomputersmanipulatetheseURI-basednamesinwayswhichare, as faraspossible, not inconsistentwith thepropertiesof thereal-worldobjectsforwhichtheyareanalogues. Thisanalogy–thefunctionalconsistencybetweenthebehaviourofreal-worldobjectsandthedeclaredrulesformanipulatingtheirnamesinthemachine–isthe‘semantics’intheSemanticWeb.

1.5 …throughalogicalframeworknamedRDF…

Sofar, soabstract. Howdoweactually writedown thestatementthat‘http://dbpedia.org/resource/Jordan_River isa river’? For this, wemustexaminethe ResourceDescriptionFramework(RDF).

RDF isa framework fordescribingthings, andtheirmutualrelations. It’saratherabstractframeworkforthinkingaboutthemechanicshere, andisnot,itself, aspecificdata format orlanguage(thoughthereareformatsandlanguagesintimatelyassociatedwithit).

Inthelogicallanguageinwhichitwasfirstdescribed, RDF israthersim-ple(oratleastcompact, whichisnotnecessarilythesamething). Itseemsbest,therefore, tofollowtheoverallplanofthissectionandgivethecompactexpla-nationfirst, andsubsequentlyglossitatalittlemorelength. So(omittingafewdetailswhichareunimportantatthispoint):

• Theworldisdescribedbyamixtureof resources and literals.

• Literals aresimplystrings, suchas“RiverJordan”or“RíoJordán”.

• Resources arethingsintheworldsuchasindividuals, categoriesofthings(suchaspeopleorrivers), abstractconcepts(‘worldpeace’), thingsontheweboroffit, orindeed anythingthatcanbegivenaname.

• Resourcesaregiven names whichareallsyntacticallyURIs.

• Onecanmake statements aboutresources, allofwhichareoftheformofatriple ofsubject(theresourceinquestion), predicate, andobject. Subjectsandpredicatesarealwaysresources; objectsmayberesourcesorliterals.

RDF version1.1isformallydescribedin [10].14

Tosaymore, wewillhavetowritedownRDF.ThereisnosingleRDF syntax;wewillpickthesyntaxTurtle [11]fromtheavailableoptions. Inthissyntax, wecanwrite

<http://nxg.me.uk/norman/><http://www.w3.org/1999/02/22-rdf-syntax-ns#type>

<http://xmlns.com/foaf/0.1/Person>.

<http://nxg.me.uk/norman/>14Thisreplacestheoriginalversion1.0; therearefurtherdocuments, includinglinkstoprimersat

differenttechnicallevels, at http://www.w3.org/standards/techs/rdf, andabroadercollectionofresourcesat http://www.w3.org/RDF/. TheRDF suiteofdocumentswascomprehensivelyrefreshedinFebruary2014, withthereleaseofnewversionsofmanyofthestandardswhichhadaccumulatedovertheprevious15years.

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<http://xmlns.com/foaf/0.1/name>"Norman Gray".

Therearetwostatements (ie, triples)here, onestatingthat http://nxg.me.uk/norman/ isaPerson, andtheotherstatingthatresource’sname. Ineachcase,wecanseethestructureofthesubject-predicate-object triple, withtheresourcehttp://nxg.me.uk/norman/ being the subject in both cases, and the resourcehttp://xmlns.com/foaf/0.1/Person andliteral "Norman Gray" beingtwo objects.Thefirstpredicate–statingthetypeoftheresource–isoneofthepredicatesde-fined in theW3C‘sRDF ‘Schema’ standard (RDFS) [12]; the type inquestionisonedefinedbytheindependentlydefined FriendofaFriend(FOAF) schema(whichwewillreturntobelow). TheFOAF schemaalsodefinesa‘name’predi-cate, whichgivestheliteral, conventional, nameoftheresourcethatisitssubject.

Thisnotationisobviouslyrathercumbersome. TheTurtlesyntaxstatesthatwecanabbreviatethe‘type’predicatebyjust‘a’, thatwecancollapserepeatedsubjects, andthatwecanprovideabbreviationsforcumbersomelylongURIs; asaresult, themoreidiomaticwayofrepresentingthestatementsaboveisjust

@prefix foaf: <http://xmlns.com/foaf/0.1/>.

<http://nxg.me.uk/norman/>a foaf:Person;foaf:name "Norman Gray".

Thereareafewmoresubtletiestothisnotation, whichcanbefoundin [11], buttheyneednotdetainus, sinceourgoalhereissimplytoconcretizethegeneral-itiesofearliersections, andtoallowustousethisnotationinexamplesbelow.

Youmayobjectthatthisseemsaterriblylong-windedwaytoindicatesome-one’sname, andyouwouldberight. ThereisanothernotationcalledRDF/XML [13]whichisevenmorelong-winded, whichonlyitsmothercouldlove, andwhichwewillpassoverinsilence. AndthereisyetanotherRDF notationcalledRDFa15

whichisconcernedwithembeddingRDF statementswithinotherdocuments,andmostparticularlywithinhuman-readable HTML pages; theintentionisthatthesamedocumentthatahumanreadsdescribing, forexample, alibrarybookora‘retailopportunity’, isatthesametimereliablyinterpretablebymachine.Butwemustnotallowourselvesbedistractedbysyntax.

ThecentralgoalofRDF isnottoprovideadata-transportformat(andhereisagoodpointtoemphasisethatthe‘F’inRDF standsfor‘framework’, andnot‘format’), buttoprovideasetofprimitivenotionswithwhichwecanrepresentawidebreadthofknowledge. Thosenotionsaresimpleenoughthatitisfeasibletotranslateawidevarietyofother, possiblymorenatural, formatsintoRDF –forexampletherowsinadatabasetable, ortheelementsinanXML file; andtheyarewell-enoughdefinedthatwecanuselogicaltoolstoprocesstheresultsanddrawout their implications, whilestandingagoodchanceofpreservingtheirreal-worldmeanings(cf. Sect. 1.6).

Saying thatweneednot storeor transportknowledge in this form isnotsayingthatweshouldnotdoso, anditisperfectlyreasonabletostoreandtrans-portRDF ifthatiswhatisconvenient. ThedatabasesinwhichonestoresRDFiscalleda‘triple-store’, after thesubject-predicate-object triples theycontain;theyarearchitecturallydifferent from the relationaldatabasesof tables, rowsandcolumns, withwhichyoumightbemorefamiliar, andalthoughtheyarenotquiteastechnicallymatureasrelationaldatabases, theyaresteadilyimproving.

Asafinalpoint, thestatementsofRDF arenotrequiredtobetrue, consistent,orevenmeaningful(youcansay‘Truthsmellsofmuesli’ifyouwantto, or‘thepresentKingof France isbald’) – anxieties about such statements are for thehigherlevelsofinferenceandontologiesthatwewillcometoshortly.

15https://www.w3.org/standards/techs/rdfa

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A brieflogicalexcursion, fortheenthusiast: RDF byitselfhasanexceed-inglysimplecoresemantics. Thelinkbetweenaname(intheformofaURI)anditsreference(inRDF terms, a‘resource’)ismadeinnaturallanguage, andisnotexpectedtobemeaningfultoamachine. RDF doesnotdistinguishbetween,forexample, senseanddenotation(inFrege’s terminology), andisuncommit-tedabouttheidentityorotherwiseofresources; thusitispossibletonamebothMarkTwainandSamuelClemenswithURIs, anditisnotpossible, withinRDFalone, tomakestatementsabouttheirmutualidentityorotherwise. OWL on-tologies(seebelow)areabletomakestatementsaboutequivalence, butevenheretheframeworkisuncommittedaboutwhatequivalencemeans(itdoesnot,forexample, distinguishsenseanddenotation), andinonecontextitmay, andinanothermaynot, beuseful for theauthorofastatement/triple tostate thatMarkTwainandSamuelClemens, are‘equivalentresources’, orthemodelJor-danandtheécrivaineKatiePrice, ortheGeonamesandDBPedianamesfortheRiverJordan. Onceanassociationhasbeenmadebetweenanameandarefer-ent, thegoalofRDF istoallowthisassociationtobepreservedacrossmachinesandnetworks, andtoallowmachinesprovidedwithbothRDF statementsandontologies(whichmaycomefromdifferentsources)todiscoverthestatementsentailedbytheontologies, whichhavethesametruthvaluesastheinitialRDF.Forfurtherdetailedlogicaldiscussion, see [14].

1.6 …enhancedbymachineinference…

IntheTurtleexampleabove, I ‘described’theresource http://nxg.me.uk/norman/bysaying that the thingwith thatnameisa foaf:Person, whose foaf:name is“NormanGray”. Theseare two terms froma simple ontology (seeSect. 1.7)called FOAF.16 TheFOAF ontologyhasa namespace http://xmlns.com/foaf/0.1/, meaningthatallofitstypesandpredicatesstartwiththat URI.Itincludesa number of types such as ‘Person’ and ‘Project’, and a number of relationssuchas‘name’, ‘mbox’(emailaddress), ‘publications’andsoon. Anotheron-tology, well-knowninthelibrarycommunity, is DublinCore(DC).This isde-scribedauthoritativelyin http://dublincore.org/documents/dcmi-terms/, andtheRDF expressionof thisvocabularydescribesthenamespace http://purl.org/dc/terms/, sothattheDC ‘title’predicateis, infull, http://purl.org/dc/terms/title, which ismostoftenseenabbreviated to just dc:title. TheDContologyisfocused, atleastinitially, onmetadataforbibliographicandarchivalresources, andincludesrelationssuchas‘title’, ‘creator’, andsoon.

Importantly, thesevarioustermshavenointrinsicmeaning, andRDF placesnorestrictionsonthepredicatesattachedtoaresource, norwhoattachesthem.Thusifyouwishtosay

<http://nxg.me.uk/norman/>foaf:name "Capitania General Bernardo O'Higgins";myprops:shirtType "class C".

thenyouareatperfectlibertytodoso, evenalthoughthefirststatement(statingthatmynameisBernardoO’Higgins)isfalse, andthesecond(apparentlysay-ingsomethingaboutmyshirt)ismeaninglesstome, thoughdoubtlesssomehowusefultoyou.

Theprimarygoalofthesevariousrelationsistodescribearelationshipsuf-ficientlyunambiguouslythatevenamachinecanprocessit. Thusa‘title’intheDC sense–thatis, theobjectofthe dc:title predicate–isalwaysabibliographictitle, suchas dc:title "Transition to the Digital". TheFOAF ontologyalsohasa‘title’predicate, butinthiscaseitisalwaysandonlyaperson’shonorific:<http://nxg.me.uk/norman/> foaf:title "Dr".

16The‘FriendofaFriend’ontologywasoriginallyconceivedofasawayofcreatingapeer-to-peersocialnetwork.

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<http://nxg.me.uk/norman/>foaf:name "Norman Gray";foaf:mbox <mailto:norman@astro.gla.ac.uk>.

<http://example.org/a.n.other>a foaf:Person;foaf:name "Aloysius Naismythe Other".

<http://someone.org/ping>foaf:mbox <mailto:norman@astro.gla.ac.uk>.

<isbn:????> # XXX INSERT BOOK'S ISBN HEREdc:title "Transition to the Digital;dc:contributor <http://someone.org/ping>.

Figure 1: Somestatementsaboutpeople.

Butwecandomorethanthis.ConsiderFig. 1: thisprovidesaFOAF nameandemailaddress for http:

//nxg.me.uk/norman/, statesthat http://example.org/a.n.other isa(FOAF) Per-son, and provides an email address for an otherwise unidentified somethingnamed http://someone.org/ping. Youmightalreadyhaveapictureoftheenti-tiesbeingdescribedhere, intermsoftheirnumberandtype.

Ifyouwere toput thisdescriptions inFig. 1 intoa triple-store, and thenqueryittoretrieveallofthePersonsdescribed, youwouldobtainonlyasinglePerson, namely http://example.org/a.n.other, becausethisistheonlyresourcewhichhasbeenexplicitlystatedtobeoftype foaf:Person. Itisobvioustousashumans, however, thatifsomethinghasanameandanemailaddress, thenit’salmostcertainlyhuman, andtheFOAF specificationindeedmakesthisstip-ulation, thata foaf:mbox propertycanbeattachedonly toa foaf:Person. Informallanguage, wecansaythat foaf:Person isthe‘domain’ofthe foaf:mboxand foaf:name predicates, meaningthatonlythingsoftype foaf:Person areper-mittedtohavethosepredicates(thisisdistinctfromtheuseof‘domain’tonameacollectionofmachinesontheinternet, asinforexample www.w3.org); simi-larly, wecansaythat dc:Agent isthe‘range’ofthe dc:contributor predicate,meaningthatanyobjectofthispredicatecanbededucedtobea dc:Agent. Inconsequence, anRDF triple-store withthisextrainformation(moreprecisely, atriple-store possessedofbasicinferencingcapabilities)can deduce that /norman/and /ping arePersons, andsowouldgivethreeanswers, whenaskedtolistthePersonsitknowsabout.

A furtherthingthathumansknowisthat, whileindividualsmayhavemul-tipleemailaddresses, anaddress isusuallyownedbyasingleperson. FOAFagrees: onlyasingle foaf:Person canbethesubjectofa foaf:mbox property.Butboth /norman/ and /ping havethispropertywiththesamevalue. Thereso-lutionisstraightforward: thesetwoURIscanbededucedtobemerelydifferentnamesforthesamePerson. Primedwiththisextrainformationaboutthedomainof foaf:mbox, a triple-store, askedhowmanyPersonswererepresentedinFig. 1,wouldanswer‘two’. Sucha triple-store cananswermorecomplicatedquerieswithoutgettingconfused. Askedforthetitlesofthingstowhichthepersonnamed"Norman Gray" hascontributed, a triple-store providedwithFig. 1 couldanswer"Transition to the Digital", byfollowingthechainofallowed inferences –itsynthesisesstatements, suchas“http://nxg.me.uk/norman/ isa foaf:Person”,and“/norman/ sameAs /ping”, whichareonlyimplicitintheoriginalsetofstate-

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ments.Theend result, asmaynowbeclear, is that a triple-store canaggregate

informationfrommultiplesources: theinformationinFig. 1 mighthavebeengatheredfromapublisher’swebsite, amembershipdatabase, andanindividual’shomepage, andmayhavestartedoffinanyorallofRDFa, XML,orarelationaldatabase. Onceitisgathered, the triple-store candrawtheconclusionswhicharenotapparentfromanydatasourcebyitself.

Beforegoingontodescribebrieflyjusthowthesevariousrelationsarear-ticulated, wehavea fewfinal remarks tomake. Firstly, whenwesay, above,that“a foaf:mbox propertycanbeattachedonlytoa foaf:Person”, wearesay-ingsomethingdifferentfromtheapparentlysimilarstatementwemightfindinanXML schema. InXML,suchastatementisasyntacticalone, sayingthatan‘mbox’element(inthatexample)is permitted tobeattachedonlytoaPerson, sothatifitisattachedtosomethingwhichisn’taPerson, oratleastisn’tknowntobeaPerson, thenthisisanerror. InRDF,incontrast, thestatementmeansthatanythingtowhichthe foaf:mbox propertyisattachedmustbe deduced tobeaPerson. Itisnotevenillegitimate(althoughitisinfactuntrue)tosaythat

<http://nxg.me.uk/norman/> foaf:name "Norman Gray";dc:bibliographicCitation "Gray (2014)".

even though thedomainof dc:bibliographicCitation allowsa triple-store todeducethat /norman/ isof type dc:BibliographicResource (that is, somethinglike a book, that canhave ‘Gray (2014)’ as its citation). Only if the store issubsequentlytoldthat foaf:Person and dc:BibliographicResource are disjointwillitraiseanyobjection, andthatobjectionwillnotbeasyntacticone, buttheannouncementofalogicalinconsistency.17

Secondly, wemightreasonablyobjectthat, intherealworld, alibrary(whichisnota foaf:Person)mighthaveanemailaddressforenquiries, andthatarole-basedemailaddressmightinfactbesharedbymultiplepeople. Thisisaccurate,buttherestrictionsweplacedonthe foaf:mbox propertyabovearepartoftheapproximationtotherealworldthatwemustmakewhenwedescribethatworldintermssimpleenoughforacomputer. Thelibrary’semailaddressisthereforenottheobjectofany foaf:mbox property, forpreciselythereasonthatalibraryisnota foaf:Person; andsimilarlyfortherole-basedaddress, forpreciselythereasonthatitisincompatiblewiththedefinitionof foaf:mbox foranemailad-dresstohavemultipleowners. A different‘people’ontologymighthavedifferentrestrictions, andsopermitdifferentimplications.

Anotherlogicalexcursion: Above, wementionedtheideathat(someof)theRiverJordanislocatedwithinJordan, asopposedtobeingpartofit. Thetruthofthisstatementsdependsinpartonwhat, precisely, ‘ispartof’means, andsopotentiallydrags inasetofphilosophicalconsequences, andindeedpoliticalandculturalones; buttheonlyonesthatmatterstothecomputerarethelogicalconsequences: giventheinformationthat‘A ispartofB’, whatotherstatementsisitpermittedtosynthesise? ThatisalargelytechnicalproblemconcerningwhatisPartOf or sameAs isintendedtomeaninaparticularsystem, anditcorrespondstothehumanproblemofarticulatingwhatoneknowswithinaformalsystem,inawaywhichallowsthemachinetodrawtheconclusionsoneexpects. Itispossibletospendagooddealofwork-dayeffortarguingaboutjustwhat‘is’is.Thesequestionsarewhatpermitanontologytohavesufficientstructuretomakeinferencespossibleanduseful, andworkingoutthenecessarybalancebetweenapproximationandexpressivenessiswhatmakesthedesignofontologieschal-lenging. Allthatsaid, suchexoticquestionsarelargelyirrelevanttothe users of

17Saying that the twoclassesare disjoint is to say that therearenoobjectswhichare inbothclasses; thisisincompatiblewiththededuction, above, that /norman/ is inbothclasses, anditisthisinconsistencythatmachinewillobjectto.

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anontology, aslongastheyarebroadlyawareofthepotentialdislocationbe-tweenstatementsintheformallanguageandtimelessstatementsabouttherealworld.

ThuswecanseethatRDF statements, andtheontologieswhichstructurethem, arenotreally, ornotonly, about truth, otherthanintheabstractlogicalsensethatvalidargumentsmustpreservethetruthvaluesoftheirinputs. Simi-larly, wearenotconcernedwithdevelopingasingletrueontologyoftheworld,orevenwiththeclaimthatanontologywhichisusefulinonecontext, willbeusefulorevenmeaningfulinanother, orthattwoontologiesdescribingoneob-jectwillnecessarilybecommensurable. Developingontologiesisatypeofpro-grammingactivity, andthetypesandpredicates, andtherelationshipsbetweenthem, mustbechosentomatchthethingsbeingmodelled, toacceptthepreci-sionofwhatcanbesaidaboutthem, andtosupporttherangeofconclusionsonemayhopetodraw.

FOAF andDC arerelativelysimpleontologies, withratherlightweightcon-straints; theyarethereforeverywidelyapplicablefordescribingpeopleandarte-facts, respectively, and, inconsequence, areveryusefulforintegratingotherwiseratherdisconnectedinputdatasets. Incontrast, alargeontologysuchastheGeneOntology [15, 16]isconcernedwithalogicalstructurewhichissufficientlyin-tricate, andsufficientlycloselymappedtonature, that theontologycandrawscientificallyvalidconclusions, atthecostofbeingconsiderablyhardertouse.

Intheexamplesabove, wehavetalkedofvariouswaysofaddingstructuretothelistoftypesandpredicatesinanontology; nowisthetimetodiscussbrieflyhowthisactuallyhappens.

1.7 …basedonOWL andotherontologies.

Theinternalstructureofontologiesisthemosttechnicallyintricatepartofourstoryhere. Fortunately, itisnotnecessarytodiscussthatstructureindeepdetail:ourgoalinthissectionissimplytoconcretizetherathergeneralstatementsaboutontologystructureabove, and togivegeneralpointers towardsmoredetailedadvice.

There’sawell-knowndescriptionofanontologybyThomasGruber[17],18

inwhichhedeclaresthatanontologyis

“aformalspecificationofasharedconceptualization”

Thatappearsopaqueatfirstglance, butinfactit very conciselypullstogetherallofthekeyconcepts. Thatistosay, anontologyis

conceptualization asetofconcepts

shared …whichatleasttwopeopleagreeabout

specification …andwhichhasbeenwrittendown

formal …inamachine-readableway.

Like‘semantics’, theterm‘ontology’hasaforbiddingaspect. Inthiscontext,however, itsimplylabelsoneendofaspectrumofwaysofstructuringinforma-tion. InDeborahMcGuinness’s ‘semanticspectrum’ (in [18], andredrawninFig. 2)sheillustratesarangeof ‘sharedconceptualizations’, andsuggests thattheterm‘ontology’ismostnaturallyrestrictedtothosemoreformalstructurestotherightofthecentralline, dividinginformalfromformal‘is-a’relations; Gru-ber’sdefinitionappliestomostofthisspectrum.

18ThisisthebynowapparentlytraditionalslightmisquotationofGruber’s‘explicitspecification’as‘formalspecification’.

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Catalogue/ID

Terms, glossary,folksonomy

Thesauri(‘narrower’

relation)

Informal is-a

Formal is-aValue

restrictions

Generallogical

constraintsProperties

Figure 2: Thesemanticspectrum, simplifiedfrom[18]

First, attheextremeleft, simplylistingidentifiersforasetofobjectsisasortofprimitivesharedconceptualization.

Next, andmarginallymore formally, a controlledvocabulary is simplyadeliberaterestrictionofthetermsweusetodescribetheworldinsomecontext.Inthesamegeneralcategory, a folksonomymightbedefinedasa‘collaboratively’or‘loosely’controlledvocabulary, inwhich, ideally, someconsensusemergesonthetermstousetodescribesomeuniverseofresources. Inbothcases, though,thereisnorealstructureto(therelationshipbetween)theterms.

A thesaurus is acontrolledvocabulary representingconcepts, plus somedeclaredrelationshipbetweentheconcepts. Mosttypically, athesaurusisusedfor, oratleastassociatedwith, informationretrieval: youmightgointoalibraryandasktobedirectedtobooksaboutcats, orwhatwemightcalltheconceptof‘Cats’, andalibrarianmightbeabletotakeyoutoashelfofusefulbooks. Ifyou’reinterestedinspecifically‘domesticcats’, they’reonasmallerpartoftheshelf; ifyou’reactuallyinterestedin‘mammals’ingeneral, theymightbespreadoverafewshelves. Therelationshipsbetweenthese broader and narrower terms–forthisisthemostcommonstructuralrelationinthesauri–isapracticalone:any information retrievedbyuseofagivenconceptwillalsobe retrievedbyuseofthe‘broader’concept. Theseare‘is-a’relations, inthesensethateverydomesticcat isa cat, andeachcat isa mammal; butthisisan‘informalis-a’, inthesenseofFig. 2, sinceinthelayoutofapet-shop, forexample, orthelayoutofitssupplier’scatalogue, ‘catcollars’mayreasonablybea‘narrower’conceptwithrespectto‘cat’withoutanysuggestionthatacatcollarisatypeofcat, andalibrarybookaboutcatsmaybelabelled‘cat’(orforexamplewiththeDeweynotation636.8)withoutanyoneoranythingbeingentitledtoconcludethatthebookitselfwouldbeimprovedbytheinsertionoffish.

The W3C standardforthesauriistheSimpleKnowledgeOrganizationSys-tem(SKOS),19 whichformalisesthesebroader/narrowerrelationsandaverysmallsetoffurtherones, inadeliberatelysimpleframeworkwhichisdesignedtobebroadlydeployable.

Next, andsteppingoverthedividinglineofFig. 2, thesimplest ontologiesarethosewherethishierarchyof‘is-a’relationsisindeedexpectedtoholdinalogicallyusefulsense, andwherelabellingsomethingwithanontologytermis indeedanassertion that that thing is an instanceof the labelled type, andthereforeofanysupertype. Forexample, theLinnaeannameforcatsis Feliscatus;thisisaspeciesofthegenus Felis inthefamily Felidae, intheorder Carnivora, allthewayuptothekingdom Animalia; andthishierarchyisexplictlyintendedtoallowmetoconclude, onbeingpresentedwithsomethinglabelled Feliscatus,thatthatthingisindeedacatratherthanabookaboutcats, andthatthe(bynowindignantlywriggling)subjectisacarnivorousanimal, withclaws.

This‘formalis-a’istheintendedinterpretation–thatis, theintended‘seman-

19http://www.w3.org/2004/02/skos/intro

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tics’–ofthestatementinFig. 1 thatthethingnamedby http://example.org/a.n.other isof‘type’ foaf:Person. Theusualwordforthetypeinthiscontextisclass, andthe(FOAF) URI http://xmlns.com/foaf/0.1/Person (usuallyabbrevi-atedtojust foaf:Person)isthenameoftheabstractclassofpersons; individualpeopleare instances ofthisclass.

Thenextstepalongthesemanticspectrumistobeabletodeclarethatin-stancesofsomeclassesmaypossess properties (theterm predicates isgenerallyinterchangeable)suchas foaf:name. Anontologyofthistypemaydeclarethedomainandrangeofapredicate, restrictingthetypeofsubjectorobjectthatthepredicatemayhave. A triple-store canthenusethisinformationtomakecertaindeductions, asweillustratedinSect. 1.6 onp.11. However, thereisnoway,inthistypeofsimpleontology, to require thataninstanceofaclassmusthaveaparticularproperty(I canbea foaf:Person withouthavinganemailaddress,forexample, bizarrelyvictorianthoughthatnotionmayseem), andareasonerwhichdoesnotknowmyemailaddress, becauseithasn’tbeenprovidedwithanfoaf:mbox propertyforme, isnotentitledtoconcludethatI thereforedon’thaveone.20

Thesimpleontologicalframeworkrepresentedby RDF Schemas(RDFS) iscapableofexpressingnomorethanthis: thatclassesexistandthatsomeclassesaresubclassesofothers, thatresourcescanbeinstances(ormembers)ofclasses,thatpropertiesexistandhaveclassesasdomainandrange. Thepayoffisthatreasoners21 whichimplementthislogiccanbesimplerandfasterthanareasonercapableofmoreintricacy. ReferringbacktothediscussionaroundFig. 1, suchareasonerwouldbecapableofconcludingthat http://nxg.me.uk/norman/ isa foaf:Person (becausethatresourceisassertedtohavea foaf:name, implyingthat /norman/ is in thatproperty’sdomain), and it couldconclude that http://someone.org/ping isa dc:Agent; butitcouldnotusetheidentityofthe foaf:mbox properties toconclude that theseare the sameperson, and it couldnotdetectanyinconsistencyinasubsequentassertionordiscoverythat /norman/ isa dc:BibliographicResource.

Togetsuchextrareasoningpower, itisnecessarytogobeyond RDFS totheright-handsideoftheontologyspectrum, towardsmoreelaborateframeworkssuchasOWL.TheWebOntologyLanguage(OWL22) comesinseveralstandardvarieties–OWL Lite, OWL DL,andOWL Full–andfurthervarietiesassociatedwithparticularimplementations(see [19]andreferencesat http://www.w3.org/standards/techs/owl). Thedifferencebetweenthesevarietiesisthatsomearemore expressive, inthesensethatitispossibletoarticulatemorecomplicatedrelationshipsbetweenresources; thepracticaltradeoffisthatthemoreexpressivevariantsareharderormoreexpensivetoimplement. ItisinOWL,andnotRDFS,thatitispossibletosay

dc:Agent owl:disjointWith dc:BibliographicResource.

A reasoner, onceprogrammedtocalculatethelogicalimplicationsofthe owl:disjointWith predicate, canthereafterconcludethatifaresourcesuchas http://nxg.me.uk/norman/ hasbeenassertedordiscoveredtobeinbothclasses, thenithasdetectedalogicalinconsistency. Thismightformpartofalongerchainof reasoning, or itmaybepracticallyuseful fordiscovering latent errors ina

20This‘permissionforignorance’isknownasthe‘openworldassumption’. Itisterriblyimportantforthemathematicallogicwhichareasonerimplements, butmostnon-specialistusersofRDF canbeforgivenforfeelingittobearatherabstractdetail. Itisincontrasttothe‘closedworldassumption’builtintomoretraditionalrelationaldatabases: ifthereisnosalarybesidemynameinthepersonneldepartment’sdatabase, thenthesystemwillconcludethatI amapersonwithoutasalary, asopposedtoapersonwhosesalaryisunknown(whichistheonlyconclusionpossibleinanRDF analogueofthisdatabase); inthiscase, thepersonnelofficeiscertainthey’renotgoingtopayme.

21A ‘reasoner’ in thiscontext is apieceof softwarewhichcan implement the logicalcalculusindicatedbyanontology.

22Yes, thestandardisindeedcrackingaWinnie-the-Poohjokehere.

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database: ifamappingdatabasediscoversa feature that ismarkedasbothariverdeepandmountainhigh, thenitscuratorscantherebydiscovertheyhavesomerepairingtodo.

1.8 Sowhat, briefly, istheSemanticWeb?

Whatwehaveendedupwithisthis: inRDF wehaveaflexiblemodelforrep-resenting relatively simplestatementsabout theworld, inaprimitive subject-predicate-objectpattern. Thisframeworkhasanumberofsyntaxeswhicharemoreorlesssuitableinparticularcontexts; thesearemostprominentlyRDF/XML,TurtleandRDFa, buttherearealsosomeemergingalternativessuitableforJSON.Despitethisabundanceofsyntax, itisthesimplicityandverybroadapplicabilityoftheprimitive triples thatiskey, sincealmostanyreasonablystructureddatasourcecanbewrangledintoRDF form, onewayoranother, andthismakesRDFanexcellentmechanismforcombiningheterogeneousdatasources.

Animportantstepinthiscombiningmechanismisthecarefully-struckbal-ancebetweenthelogicalprecisionofthecalculusofresourceswhichontologiesexpress(givenasetofRDF triples, whatfurthertriplesarelogicallyentailed?),andboththeflexible imprecisionofthenatural-languagelinkbetweenURIsandtheirdenotations, andthevariableprecisionofontologicaldesigns, whichmayrangefromthebroad-brushutilityofFOAF (‘only Personshaveemailaddresses’)totheintricacyoftheGeneOntology.

Furthermore, theveryclosetechnicalanalogybetweentheSemanticWeband theWebof Strings reassuresus that thepropertieswhichmade thewebsuccessful (itsopenness, fault-tolerance, andsoon, asdiscussed inSect. 1.2)willsupporttheSemanticWeb, too.

Onceinformationis inRDF form, andstoredina ‘triple-store’, itcanbequeried23 eitherdirectly, orwiththeassistanceofa‘reasoner’, whichsynthesisesextratriplesbasedontheinferencesobtainedfromtheoriginallyassertedtriples,inthepresenceoftheextrastructureprovidedbyasimpleoracomplex ontology.

Wecouldsaymoreaboutthe‘SemanticWeb’. Wecouldtalkaboutsomeofthedetailsof triple-stores andqueryengines, butthatwouldveryquicklyim-merseus ina levelof technicaldetailwhichcanvergeon the impenetrable,andwhichisinanycasestillinactiveresearch-leveldevelopment. Wecouldintroducesomeof thefeaturesofontologydesign: thoughthis isnowlargelystable, itwouldtakeseveralmorechapterstocover, withoutimmediateintellec-tualprofit. Wecouldtalkaboutdeployment, butmanyofthemaindeploymentsofSemanticWebtechnologyareinback-endsystemssupportingcomplicateddata-integrationprojects, anddonotmakeforvividillustration.

Instead, thegoalofthischapterhasbeentoprovideanintroductiontothecoreconceptsofthe‘SemanticWeb’, atalevelandtoanextentthatwillallowthe reader tounderstand thegoalsand starting-pointof aprojectusing thesetechnologies, andtounderstandthewayinwhichthe‘Semantic’WebrelatestotheWebofStringsofourlasttwodecades’familiarexperience. Formoredetails,the W3C’s‘DataActivity’24 providesusefullinkstofurtherreading; andforsomepracticaldetails, includingarathermorescepticalaccountoftheSemanticWebthanI haveproducedhere, see [21].

1.9 WhereistheSemanticWeb?

ItishardertopointtotheSemanticWebthantothetextualweb. AlthoughweencounterthetextwebwheneverwelookatWikipedia, bookaflight, orsearchforcatvideos, thesemanticversionremainsinthebackground.

23ThestandardW3C querylanguageforRDF iscalledSPARQL [20], whichistoa triple-store whatSQL istoarelationaldatabase.

24http://www.w3.org/2013/data/

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TheFrenchNationalLibrary(BnF) hasalargecatalogue, splitintomultipleheterogeneousparts, usingvariousstandards(suchasEAD andMARC),andmak-inglinkstoadozenorsoexternalsites. A SemanticWebapproachhasmadethisinformationavailablethroughahomogeneousinterfacewhichprovidesintelligi-bilityandsyntacticalconsistency, withoutsacrificingtheopen-endednesswhichisnecessaryifsuchacomplicatedandlong-accumulatedinformationsourceistobemadefullyavailable [22]. Thiscatalogueisnowinusebyvariousapplica-tionsandsmallerlibraries, orenhancedandexpandedbymorespecialisedcata-logueselsewhere. Goingfromthecataloguetotheinstance, [23]describesasys-temwhichmakesavailable, inmachine-readableform, elementsofthesemanticcontentofpapersinPubMedCentral. Reference [22], isinthe‘in-use’trackofthatyear’sESCW conferenceproceedings, whereyoumightfindotherillustra-tiveexamples; thereare further illustrations in thecollectionofopendatasetsmaintainedbytheSemanticWebJournal.25

Atthetimeofwriting(2014), it isnotyetclear, atleasttome, justwhentheSemanticWebwillbemanifestly‘working’, norhowitwillgetthere; itisnoteventransparentlyclearwhat‘working’wouldmean. Thehighly-integratedscenarioof [8]isstillinthefuture, notbecauseitisinfeasible, butmostlikelybecauseitispredicatedonagooddealoflarge-scalecoordinationwhichisdiffi-culttoforceonadecentredweb. LinkedData(seeSect. 3, below)showsapathintothefuture, butisnotanend-point. Butperhapsweareaskingtoomuchforagranddesign. Theevolutionfromwebpages, toblogs, toTwitterisobviousonlyin retrospect; the fact that (for example) Instagram, Facebook, Twitter, Flickr,andup-to-dateconferenceorganisersallusehashtagsisexplicable, butcouldneverhavebeenpredicted; conversely, itisrathersurprisingthatwearestillus-ingSMTP-basedemail32yearsafteritwasfirststandardised, andinthefaceofnumerousclaimsforkilleralternatives. PerhapsthemostlikelyfuturefortheSe-manticWebmatchesthatofArtificialIntelligenceresearch, eventhoughthefor-mercommunityhasalwaysbeennervousofthecomparison: AI neverobviouslydelivereditsgrandpromises–itnever‘worked’–butinaworldofself-drivingcars, face-recognition, andpocket-sizedmachine-translationdevices, it'sclearthatit'sbeenquietlyengineeringitswayintosuccessfuldeploymentfordecades.

Thetwofinalremarkstobemadeconcern, inthefirstplace, Web 2.0, whichissometimeslinkedtotheSemanticWeb, butwhichinfacthasalmostnothingtodowithit; andinthesecondplacetheLinkedDataparadigm, whichisarguablythefirstappearanceoftheSemanticWebinmorenearlyeverydayexperience.

2 Web2.0

Theterm‘Web2.0’hasdisorientinglymanymeanings. Tosome, itreferstotheclusterofmid-leveltechnologieswhichletservicessuchasGoogleMapsorFace-bookactasan‘applicationinsideawebpage’; toothersithasmeaningswhichcanbegroupedundertheslogan‘theread/writeweb’; anothercampseesitasamarketingexpressionandisdividedaboutwhetherthisisanobviouslygoodoranobviouslybadthing; andforyetothersitisaproxyforabroadchangeinsocietytowardsamoredecentred, orengaged, orempowered, oropen-licensed,orotherwiseutopianfuture. Thesetermsarenotremotelyequivalent: insomecasestheymaynotevenintersect, sosomethingasobviouslynext-generationasWikipediaisWeb 2.0undersomedefinitionsbutnotothers.

Thetermisreallyonlyusefulasanadjective, todenotesomething–anything–thatismorethanacollectionofstaticwebpages. However, evensayingthat

25http://www.semantic-web-journal.net/accepted-datasets

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muchbegsthequestion: whatisitthat’sso wrong withstaticwebpages(whichwe’renowobligedtocallWeb1.0)? Indeed, dependingonyourconceptionofwhat‘Web2.0’means, podcastsarefirmlyWeb1.0, andifyouturnoffvisitoreditingonyourwikipages, orcommentingonyourblog, thenthosebecomeWeb1.0, aswell.

The term ‘Web 2.0’ appeared as the title of a conference series run byO’ReillyMediain2004, andthetermspreadfromtheretowiderandstillva-guerusage. Thereisnotechnicaldifferencebetween‘Web 2.0’andtheoriginalversionoftheweb; instead, thetermreferstoaloosecollectionoftechniquesandevenattitudes, whichbecameincreasinglypopularinthefirsthalfofthatdecade, whichwereheavilyorientedtowardssupporting, encouraging, andex-ploitinguser interactivity ontheweb.

Above, I characterisedthewebasaspacewhereanyonecanjoinin, butadmittedthatinitiallythiswasmoretrueinprinciplethaninfact. Thewebofthe90swas, formost, aread-onlymedium: thewebwasfulloftextandimages,buttheonlywaytotalkbackwasthroughemail, orafewnow-arcanespacessuchasUsenet. Thoughbothwereeffective, andthoughUsenetisstill, in2014,NotQuiteDeadYet, theyarehardlyafree-flowingworld-wideconversation. Inthebroadest senseof the term, ablog is aWeb 2.0 thing: it’seasy to setupforoneself, andifeventhatis tooinconvenient, it’seasytouseablogsetupbya service suchasWordpress. With thatstep taken, it’smerelyamatterofcreativediligencetobroadcasttotheworld, andtodispute, declaim, decryandgenerallybickerincommentthreads. Broadlyblog-like thingssuchasTwitterandFacebookbecomemucheasiertoconceive, asbothproviderandconsumer,oncetheideaofablogservicepluscommentshasbecomepartoftheculture.

Withthatsteptaken, othertechnologieswereabletocollide; ‘AJAX’wasonesuch. The‘J’standsforJavascript, whichoriginallyappearedasalanguagesupportingsimplescriptingwithinawebbrowser–providinganelementofdy-namisminwebpages, sothattheycouldadjustthemselvestothebrowser’sen-vironment rather thanappearingasa simpleblockof text sent froma server.Thislanguage, andthesetofpracticeswhichAJAX represents, havegrowntothepointwhereawebbrowsermaynowbeseenasaseparateprogrammingecology. Somewebpages, suchasaGoogleMailpage, arriveasaminimalamountof HTML,enclosingaJavascriptprogramwhich, runningentirelywithinthebrowser, interrogatesaserver, andsynthesisesfromscratchtheHTML whichthebrowserfinallydisplays. Fromaprotocolpointofview, thereareminimaldifferencesbetweentheearliestwebpagesat http://info.cern.ch/hypertext/WWW/TheProject.html (witha‘lastmodified’dateof3December1992), andthetwitter.com homepagewhichcreatesthepageprogrammaticallyandontheflyin thebrowser: bothuseHTTP to transportmaterials fromaserver, andbothofferHTML forthebrowsertorender.26

Oneoftheotherlong-latentpossibilitieswhichwasexploredintheearly2000swastheideaofuploadedandsharedcontent. Flickr(https://www.flickr.com)wasoneofthefirstbroadly-knownservicestosupportusers’uploadingandsharingimages. Atatechnicallevel(again), thisisnotmuchdifferentfromblog-ging, butFlickrandthesocial-bookmarkingsite delicious.com areinterestingforthischapter’spurposesbecausetheywereamongthefirstwidespreadservicestosupport tagging. Flickrallowedusers toaddsimple labels topictures, andDeliciousalloweduserstosimilarlylabelwebpages; thereispresumablysomeindirectlinkherewiththerapidadoptionofTwitterhashtags. Thesetags–whichwereintendedtosupportbothcategorisationandsearch–weretakenfroman

26Indeedthereissomethingofaparadoxhere: thewebof2010andthewebof2000lookhugelydifferent, totheextentthattheyseemtocomefromdifferentworlds. Butthesedifferencesaretodowithnewsocialpossibilities, andwiththebrowser’snewandalmostentirelyunpredictedprogram-mingecology, ratherthanarisingfromanychangeintheunderlyingnutsandbolts; andthecloserwelookatWeb 2.0, themoreoutoffocusitseemstobecome.

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unrestrictedvocabularybutwiththeexpectationthatuserswouldspontaneouslycooperatetochoosebroadlycompatibletags, inalightweightsharedsemanticenvironmentwhichwassoonlabelleda‘folksonomy’(aswebrieflymentionedabove). HoweverthisisasfartowardstheSemanticWebasWeb 2.0hasevermoved, andso, despitethesuggestionofthe‘Web 2.0’nameandsomeoftheaccompanyingrhetoric, Web 2.0hasratherlittletodowiththeSemanticWeb.There'smoretoexploreintheprogrammingecology, butinwebterms, Web 2.0isratheradeadend.

3 LinkedData

EvenacknowledgingthattheSemanticWebwillprobablyremainsomewhatob-scure, onecanaskhowbroadlyitisdeployed, ortowhatextentitsdeploymentsarereachable. Aswediscussedabove, theSemanticWebisstilllargelyusedin‘server-side’applications, ratherthanbecomingpartofthecommerceoftheopenweb, aswasoriginallyanticipated. PartofthereasonforthisisthattheSemanticWebtechnologystackischallengingtouse: thetoolsareofvaryingmaturity,anduseabroadrangeofunderlyingtechnologies, sothatdeploymentsarestilltoalargeextentbespoke. Evenwhensimplymakingdataavailable, ratherthanattemptingtoprocessit, makingdataweb-readymayrequireaquiteprofoundengagementwiththeSemanticandtextualwebs’conceptualfoundations; manysuchdeploymentsarestill, evennow, worthyofanacademicpaperdescribingtheexperience. Fewnon-specialistprojectscanmakesuchinvestments.

Analternativeapproach is touse the LinkedData paradigm.27 Atheart,the linkeddataparadigm is the SemanticWeb, butwithapractical focusontheirreducibleminimumrequiredtogetdataontothewebinawaywhichiscompatiblewiththegoalsandpracticesdescribedinSect. 1.

Quoting [24], thelinkeddataprinciplesare:

1. “UseURIsasnamesforthings.

2. UseHTTP URIssothatpeoplecanlookupthosenames.

3. Whensomeonelooksupa URI,provideusefulinformation, usingthestan-dards(RDF*, SPARQL).

4. IncludelinkstootherURIs. sothattheycandiscovermorethings.”

Notably, theseprincipleshavemoretosayaboutHTTP andURIs–thatis, aboutthe mechanics ofretrievingthisinformation, andbyextensionthemechanicsofmakingitavailable–thantheysayaboutthesemanticsoftheinformationbeingdistributed.

Together, thesesay(ineffect)thatthe‘linkeddataweb’isjustlikethewebofstrings, familiar tousall, except that therawmaterialsarenot HTML files,butfiles inoneorotherRDF syntax (soRDF/XML,Turtle, orRDFa). All fourpointsareessentiallyadaptationsofthedesignprinciplesofSect. 1.2 andbestpracticessuchas [25], whichwere, asitturnedout, soveryeffectiveinmakingtheweb(ofstrings)soverysuccessful. ThethirdpointstatesthatlookingupaURI shouldprovideuseful(RDF) information toamachine, butisalsotakentosuggestthatputtingaresource’sURI intoawebbrowsershouldprovidehuman-readable(HTML) informationaswell–thisprovidesonebridgebetweenthetwowebs.

27Theword‘paradigm’isprobablythebesttermingeneral, totheextentthatitrepresentstheresultofacceptingtheinsightofasetofratherhigh-levelprinciples. Howevermakingtheseprinciplesconcrete is intricateenough that ‘LinkedDataBestPractices’sometimesseemsmoredescriptive;andwhendefendingtheprinciplesagainstotherapproaches, theword‘dogma’springsquicklytomind.

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Figure 3: TheLinkingOpenDataclouddiagram, asofApril2014. Althoughthe text andmost of the interconnections are too small tomakeout at thisscale, thetwolargestcentralblobs–whichactasrichsourcesofconsensusnamesforobjectsintheworld–representDBPediaandGeoNames. Thegreen-colouredclusters representgovernment (left)andbibliographicdata (right);otherclustersrepresentprovidersoflifesciences, geographic, socialnetwork-ing, media, user-generated, and linguistic data. For details and credits, seehttp://lod-cloud.net.

Althoughthetraditionalwebisgenerallythoughtofasdeliveringhuman-readablepages, ithasfromtheverybeginningbeenusedfordeliveringmachine-readablecontent, suchasdatafilesforanalysisorPDF filesforprinting. Thisisnot themachine-readabilitywemean in thiscontext. Justasahumanmightread aWikipedia article, internalise the results, andclickona link formoreinformation, alinkeddataclient–typicallyacomponentofalargerapplication– isexpected toprocess thesemanticcontentof thedata itfindsatoneURI,andthenfollowfurtherlinksforfurtherinformation. GiventheRDF ofFig. 1,forexample, a linkeddataclientmight store the information listed, and thenfollowthelink http://someone.org/ping toseeifthereisanyfurtherinformationavailablethere. Itisthisfinalstep, whichletsthemachine‘followitsnose’thatrepresentsthekeyinsightofthelinkeddataparadigm, andwhichisthemachineanalogueof‘wastinghoursonWikipedia’.

Thenetworkoflinksbetween‘linked-data’-styledatasourcesisillustratedinFig. 3. A largefractionofthedatainthiscloudisintheformofSKOS vocab-ularies, orusesthesimpleFOAF orDC ontologies; butwhiletheseareusefulforcoordinationbetweendatasources, thereisnorestrictionontheontologieswhichareusedinfact.

ThearchetypalLinkedData(client-side)applicationisasimpleapplication–perhapsevenrunningentirelywithinabrowser–whichreadsthecontentsofaURI,interpretsandperhapsdisplayswhatpredicatesitrecognises(whichwillprobablyincludeatleastFOAF),andletsausermoveontofurtherrelatedinfor-mation. ThearchetypalSemanticWebapplication(ifsuchathingexists)mightuseSPARQL toqueryalargeandsemanticallyrichdatabase, possiblyrequiringafairamountofinferencingonthepartoftheserviceandwithinitself, andre-quiringsignificanteffortonthepartofitsauthor, tounderstandtheontologyorontologiesinwhichtheinformationisencoded. Thoughthesewillbevisiblydifferentapplications, andoneisaloteasiertoimplementthantheother, itishardtoarticulateafundamentaldifferencebetweenthem.

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Therearefurthercompactdetailsin [26], abook-lengthdiscussionwithapractical focus in [27], andpointers tocurrent informationat http://www.w3.org/standards/semanticweb/data.

4 Conclusion

Onecouldderivetheimpression, fromthelongdescriptionofSect. 1, thattheSemanticWebiscomplicatedandarcane. Itcanbe, buttherudehealthofthelinkeddatacloudshowsthatthatcomplicationisinthebackgroundofasimpleandrobustmeansofdistributingmachine-intelligibleinformation. Whilewemaynotbequiteatthestageoflettingourcomputersrunamokwithourcreditcard,ororganiseourlivesasthevisionof[8]imagines, wehavenowconfidently, ifstillratherdiscreetly, steppedintoawebofdata.

Acknowledgements

I ammostgratefultoSusanStuart forexactinganddetailedcommentsonthedraftsof this chapter, tomembersof the semantic-web@w3.org list for ‘in-use’references, andtoChrisBizerformakingavailableanearlycopyofthe2014LinkedDatacloud.

Glossary

AI ArtificialIntelligence.

DC DublinCore: asetofcoremetadatatermsdevelopedbyandforthelibrarycommunity; see http://dublincore.org/.

FOAF FriendofaFriend: avocabularyforattributesof, andrelationshipsbe-tween, people; see http://www.foaf-project.org/.

HTML HypertextMarkupLanguage; see http://www.w3.org/html/wg/.

HTTP HypertextTransportProtocol: theunderlyinglanguageoftheweb; see [1].

RDFS RDF Schemas: lightweightontologiesforRDF;see [12].

RDF ResourceDescriptionFramework: see [10].

URI UniformResourceIdentifier: theformofaresourcename(almostthesameasaURL);see [2].

W3C WorldWideWebConsortium: theorganisationwhichdevelopsandstan-dardisesprotocolsfortheweb; see http://www.w3.org.

triple Thetripleofsubject, predicateandobjectwhichisattheheartoftheRDFmodel.

triple-store A databasedesignedtostoreRDF triples, ratherthanthetabulardataofamorecommon‘relational’database.

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