Linked data: a new alphabet for the semantic web

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by  Mauro Guerrini,Tiziana Possemato

Linked data: a new alphabet for the
semantic web
MauroGuerrini,TizianaPossemato
What is linked data
Thetermlinkeddataisenteringintocommonvocabularyand,as
mostinterestsusinthisinstance,intothespecificterminologyof
library and information science. The concept is complex; we can
summarize it as that set of best practices required for publishing
andconnectingstructureddataonthe webforusebya machine.
Itisanexpressionusedtodescribeamethodofexposing,sharing
andconnectingdataviaUniformResourceIdentifiers(URIs)onthe
web. Withlinkeddata,inotherwords,werefertodatapublishedon
thewebinaformatreadable,interpretableand,mostofall,useable
by machine, whose meaning is explicitly defined by a string of
wordsandmarkers. Inthiswayweconstitutealinkeddatanetwork
(hencelinkeddata)belongingtoadomain(whichconstitutesthe
initialcontext),connectedinturntootherexternaldatasets(thatis,
thoseoutsideofthedomain),inacontextofincreasinglyextended
relationships. NextispresentedtheLinkedOpenDatacloud(LOD),
which collects the open data sets available on the web, and the
paradigmofitsexponentialgrowthoccurringinaverybriefperiod
oftimewhichdemonstratesthelevelofinterestthatlinkeddatahas
garneredinorganizationsandinstitutionsofdifferenttypes.
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DOI:10.4403/jlis.it-6305
M.Guerrini,Linkeddata:anewalphabet
Figure1:Diagramofthelinkedopendatacloud(LOD)in2007.
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Figure2:Diagramofthelinkedopendatacloud(LOD)in2009.
Figure3:Diagramofthelinkedopendatacloud(LOD)in2011.
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M.Guerrini,Linkeddata:anewalphabet
Theconceptoflinkeddataiscloselyrelatedtothesemanticweb,
althoughthesemanticwebcannotbereducedtothemeretechni-
calityoflinkeddata,butrequires,foritsconstruction,thatcertain
importantrulesberespectedwhoseultimategoalisthecreationof
alayerofcontentaccessibletoautomatedprocesses. Linkeddata
makeexplicitthemeaningsandconnectionsimplicitlycontained(or
insomecases,absent)inwebresources(data,pages,programs,etc.).
Thetwoterms–linkeddataandsemanticweb–relatetothesame
semanticfieldandareaofapplication. Linkeddataisatechnology
usedtorealizethesemanticweb. Tobetterunderstandtheconcept
we are aided by the definition that Tim Berners-Lee, inventor of
theworldwideweb(www),providesforsemanticweb: ”Awebof
thingsintheworld,describedbydataontheweb”. Theconceptis
generic,butitcontainsimportantreferences:thenetwork,thethings
(the objects related), the data (no longer a record but individual
elements,atoms). Thisdifferentiatesthetraditionalweb(thehyper-
textweb)–constitutedofdocuments,HTMLobjects,connectedvia
unclassifiedhyperlinks–fromthewebconstitutedof”realthings”
(existingentities)describedviadata. Amorepreciseimagebegins
toemerge:
• the hypertextual web or web of documents as a flat, linear,
representationofobjects;theconcretenatureofthesemantic
webisinoppositiontotheabstractnatureofthetraditional
web;
• thesemanticweborwebofdataasacontainerofthings,of
objects,ratherthanasacontainerofrepresentationsofobjects:
an idea of concreteness, in the sense that the data relate to
theresourceandparticipateinitsnature,thatis,theyarean
integralpartofit,astheresourcewouldnotberepresentable
withoutthisdata.
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Thesemanticwebwasnotborn,therefore,toreplacethetraditional
web,butrathertoextenditspotential,realizingwhatTimBerners-
Leedescribesasaworldinwhich”thedailymechanismsofcom-
merce,ofbureaucracy,andofoureverydayliveswillbemanaged
bymachinesthatinteractwithothermachines,leavingtohuman
beingsthetaskofprovidingthemwithinspirationandintuition”
(Berners-LeeandFischetti).
Thewebofdatais, therefore, thenaturalevolutionofthewebof
documents. Letustrytoidentifythedistinctivefeaturesofeachof
them,comparingtheircharacteristics:
• webofdocuments(hypertextualweb):
– analogy with a global filesystem, an expression of ex-
tremerichnessbutalsoparticularlymonolithic;
– flatdescriptionofobjectsanddocuments;documentsas
primaryobjectsofdescription;
– networkofrelationshipsbetweenobjectsmadeupofrela-
tionshipsbetweendocumentswhichareneitherinherent
intheobjectsthemselves,norformpartoftheirstructure;
linksbetweendocuments;inconsequence:
* semantics of the content and of the links between
documentsisempirical,associatedwiththeobjects,
and thus not part of the object itself, created by a
humanagent;
* lowdegreeofstructureintheobjects;
* objectsrepresentedonthewebdesignedforhuman
consumption,notmachine-interpretableorreusable.
Thehypertextualwebissimpleinstructure,andhassparseconnec-
tionsbetweenthedata.Itcanbeimaginedasanenormousnotebook,
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M.Guerrini,Linkeddata:anewalphabet
inwhichinformationisnotedinalinearfashion, thatis, withlit-
tle structure and few relationships, and in which documents are
readableanduseableonlybyhumans.
Figure4:Representationofthewebofdocuments,17thInternationalWorld
WideWebConferenceW3CTrack@WWW2008,Beijing,China
23-24April2008-Linkeddata:principlesandstateoftheart.
• webofdata(semanticweb):
– analogywithaglobaldatabaseconceivedasarelational
database,consistingofindividualobjectsrichlyrelated
toeachother,whichinturnformlargerentities;
– articulateddescriptionoftheobject,adescriptionwhich
itselfbecomesanobjectintheweb,becauseitisreusable;
things (or descriptions of things) as primary objects of
description;
– networkofrelationshipsbetweenobjectsinherentinthe
objectsthemselves;linksbetweenthings(includingdocu-
ments);inconsequence:
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* semanticsofthecontentandofthelinksisexplicit,
expressive;
* highdegreeofstructurein(thedescriptionsof)things;
* entitiesdesignedformachinesfirst, humanbeings
second.
Figure5:Representationofthewebofdata17thInternationalWorldWide
WebConferenceW3CTrack@WWW2008,Beijing,China23-24
April2008-Linkeddata:principlesandstateoftheart.
Thecomparisonwithrelationaldatabasesisabasicconceptinthe
literatureonthistopic. WecanreadonthesiteoftheW3C:
”Thesemanticwebandrelationaldatabases. Thesemantic
webdatamodelisverydirectlyconnectedwiththemodelof
relationaldatabases. Arelationaldatabaseconsistsoftables,
whichconsistsofrows,orrecords.Eachrecordconsistsofaset
offields.Therecordisnothingbutthecontentofitsfields,just
asanRDFnodeisnothingbuttheconnections: theproperty
values.Themappingisverydirect
• arecordisanRDFnode;
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M.Guerrini,Linkeddata:anewalphabet
• thefield(column)nameisRDFpropertyType;and
• therecordfield(tablecell)isavalue.”
A strong point of the semantic web has always been the expres-
sion,ontheweb,ofalargequantityofinformationintherelational
databaseformulatedinamachine-processableformat. Theserial-
ization format RDF – with its syntax XML – is a format suitable
forexpressingtheinformationinrelationaldatabases. Theanalogy
is appropriate as the central point of linked data is precisely the
“predicates”thatexpressthetypesofrelationshipsthroughwhich
ontologiesandnetworkscanberepresented.
Figure6:Representationofarelationaldatabase.
Theatomizationofthestructureofinformationexpressesthechar-
acteristicsofthewebofdata;onenolongerhasamonolithicobject,
ratherasetofindividualdatapoints,minimalparticles–atoms–
thatcanbereaggregatedindifferentwaysandfordifferentpurposes;
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eachattributeoftheobjecthasavalueinitself,andparticipatesin
itsnature,throughexpressive,self-explanatory,relationships. The
entitiesconstitutedbytheensembleofatomsareassembledintoa
setofstructureddata,eachindividuallyindependent,butabletobe
logicallycombinedwithotherdatatoproducenewentities. Having
giventheimageofthenotebooktoillustratethewebofdocuments,
wecannowtaketheimageofthemechanism(reminiscentofRan-
ganathan), in which every element, independent in itself, can be
combinedandreusedinaninfinitevarietyofsolutions. Theweb
ofdatais,therefore,aglobalnetworkofstatements(orsentences)
connectedthroughqualifiedandself-expressivelinkswhichbecome
acollectionofknowledge,whichisreadableandunderstandableby
amachine,onlysecondarilyforaperson.
Linked data: the world of the internet and
the role of libraries, archives and museums
Why is the world of networked information so interested in the
legacy data produced by libraries, archives and museums? Why
are libraries, archives and museums equally interested in linked
data? Theinterestisactuallyreciprocal. Librarieshavealwayspro-
ducedqualitydatainhighly-structuredbibliographicandauthority
records,accordingtosharedandwidelydisseminatedrules,avast
quantityofdata. Theworldoflibrariesandtheworldoftheinternet
arebothinterestedinintegrationintothenet;theformertoensure
thevisibilityandusabilityofitsdata,thelattertoexploitinformation
andcreateincreasinglylargeandsignificantnetworks. Thequan-
tityandqualityoftheinformationthatpopulatesthenetaretwo
aspectswhichareofteninverselyproportional: muchinformation
isofpoorquality. Theincreaseinnetworkedinformation(through
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publicationmethodsthatareincreasinglywidely-knownandused,
suchasforexample,self-publishing,socialnetworks)isnot,infact,
alwayssynonymouswithquality. Theexponentialgrowthanduse
ofinformationavailableonthenetdoesnotcoincidewithincreasing
trustworthinessoftherecordseither: theirdegreeofreliabilityis
low.Usersmustselectfromtheseaofinformationretrievedtoarrive
atacrediblerecord. Onwhichcriteriontobasetheselection? The
authoritativenessofthesourcebecomesthekeyfactor,theselection
takesplaceattheoutset,preferringtoselectaresourceonthebasis
oftheauthoritativenessofitscreator,insteadoflateron,choosing
uncriticallyonthebasisoftherankingoftherecordsthatappearon
thepage. Thequalityofthesource,thecertaintyoftheprovenance
become, therefore, crucial elements in the searcher’s exploratory
process. Theroleoflibraries,archivesandmuseumsthusbecomes
relevant,duetotheirtraditionofattentiontothequalityoftheinfor-
mationtheyproduce. Libraries,archives,museumsassume,thus,
theroleofgeneratorsofqualityinformationforthenet. Itisforthis
reasonthattheirdataaresoughtafter.
Legacy metadata in libraries: still functional?
The history of library catalogues demonstrates early widespread
useofmetadata,understoodasinformationservingasasurrogate
fortheresource. Theevolutionofdataintoevermorestructured
anddetailedrecordscoincidedwiththerenewedcentralityofthe
catalogueonwhicheveryserviceofthelibraryisbased,theprolif-
erationofformatsofbibliographicresourcesandthecentralroleof
automationinlibrarysystems. Themaincharacteristicsofmetadata
areits:
1. nature: itiscreated,formedfromtheresource;
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