12
canalsogainrealvaluebyprovidingdurableeditinghistories,somethingthatmanydata-
basescanlivewithout.Authoringhistoriesalsomustbeabletorepresentandhandlecon-
flictingandbranchinglinesofdevelopment,sincetheyareinherenttotheauthoringproc-
ess,whiletheanalogoussituationsindatabasesareruledoutaprioribytheconsistency
requirements.Isolationisaconceptthatreallyonlyappliesinthecontextofatransaction
basedsystem.Infact,wewillseethatforcollaborationwewanttoincreasetheabilityto
mergechangesproductively,somethingthatwillactuallyincreasethepotentialforinterac-
tionsbetweenchanges.
Theforgoingpointsaremoregenerallytrueincollaborativesystems,wheremoreliberal
optimisticmethodsaresometimesusedtocompensateorproceed,eveninthepresenceof
non-serializable(conflicted)executionhistories.Thisispossiblebecauseofthepresenceof
thehumanauthors,acriticalfactorinvolvedincollaborationthatisnotavailableintypical
databasesystems.Beyondautomaticpolicies,thereisalwaysalastresortincollaboration:a
humanbeingthatcanbetrustedtocorrectlyresolveaconflict(ifsheiswillingandauthor-
izedtodoso).Furthermore,formostapplications,theneedforabsoluteconsistencyiscon-
siderablyless.Dourish(Dourish1996)hasproposedtwolevelsofconsistencyforcollabora-
tionsupportsystems:syntacticconsistencyandapplicationconsistency.Syntactic
consistencyisconsistencyinthedatastructuresthatsupporttheimplementationofapplica-
tions;iftheseareinconsistent,thereisabuginthesystemandthesoftwarewillfailto
workcorrectlyoratall.Applicationinconsistencyisdifferent.Itreferstotheconsistencyof
thedatamodelimpliedbyanapplication,whetherthatisamodelforatext,astructured
drawing,orsomeotherapplicationobject.Aninfrastructurethatcansupportdivergence
fromacompletelyconsistentdatamodelisessentiallyanoptimisticconcurrencycontrol
model,wherehumancollaboratorsareresponsibleforresolvingconflicts.Theadvantageof
suchdatamodelsisthesameasthatforoptimisticconcurrencycontrol:greateravailability
toupdates,atthecostofgreatertimeexpendedresolvingconflicts.
canalsogainrealvaluebyprovidingdurableeditinghistories,somethingthatmanydata-
basescanlivewithout.Authoringhistoriesalsomustbeabletorepresentandhandlecon-
flictingandbranchinglinesofdevelopment,sincetheyareinherenttotheauthoringproc-
ess,whiletheanalogoussituationsindatabasesareruledoutaprioribytheconsistency
requirements.Isolationisaconceptthatreallyonlyappliesinthecontextofatransaction
basedsystem.Infact,wewillseethatforcollaborationwewanttoincreasetheabilityto
mergechangesproductively,somethingthatwillactuallyincreasethepotentialforinterac-
tionsbetweenchanges.
Theforgoingpointsaremoregenerallytrueincollaborativesystems,wheremoreliberal
optimisticmethodsaresometimesusedtocompensateorproceed,eveninthepresenceof
non-serializable(conflicted)executionhistories.Thisispossiblebecauseofthepresenceof
thehumanauthors,acriticalfactorinvolvedincollaborationthatisnotavailableintypical
databasesystems.Beyondautomaticpolicies,thereisalwaysalastresortincollaboration:a
humanbeingthatcanbetrustedtocorrectlyresolveaconflict(ifsheiswillingandauthor-
izedtodoso).Furthermore,formostapplications,theneedforabsoluteconsistencyiscon-
siderablyless.Dourish(Dourish1996)hasproposedtwolevelsofconsistencyforcollabora-
tionsupportsystems:syntacticconsistencyandapplicationconsistency.Syntactic
consistencyisconsistencyinthedatastructuresthatsupporttheimplementationofapplica-
tions;iftheseareinconsistent,thereisabuginthesystemandthesoftwarewillfailto
workcorrectlyoratall.Applicationinconsistencyisdifferent.Itreferstotheconsistencyof
thedatamodelimpliedbyanapplication,whetherthatisamodelforatext,astructured
drawing,orsomeotherapplicationobject.Aninfrastructurethatcansupportdivergence
fromacompletelyconsistentdatamodelisessentiallyanoptimisticconcurrencycontrol
model,wherehumancollaboratorsareresponsibleforresolvingconflicts.Theadvantageof
suchdatamodelsisthesameasthatforoptimisticconcurrencycontrol:greateravailability
toupdates,atthecostofgreatertimeexpendedresolvingconflicts.