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eventthatpassesfromtheapplicationtothelocalstore.Thepolicymanagerisresponsible
foracceptingeventsfromwhatevernetworkconnectionsareavailable,andstoringthemin
thelocalstore.Changeeventsfromothercollaboratinginstancesoftheapplicationaredis-
patchedindirectlyviathepolicymanager.Thisnotonlyhidesdatastructuremaintenance,
butitencapsulatestheimplementationofupdatepoliciesinasingleplace.Thegrayarrows
representcontroloperationsfromtheapplicationtoselectpolicyoptionsorrequestapar-
ticularlevelofactivity.

Policy ManagerNetwork
Application
Local Store
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ThisentirearchitectureisessentiallyseveralcomposedinstancesoftheObserverpattern
(Gamma,Helmetal.1995).Observerisawayofmodularizingupdatestostateinformation
bypassingthoseupdatestootherpartsofthesystemthatwouldbeaffectedbytheupdates.
Palimpsest’sdirectmanipulationofchangesmakesthispatternparticularlyusefulinthis
situation.ThepolicymanagercanalsobeseenasanapplicationoftheMediatorpattern,

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which“promotesloosecouplingbykeepingobjectsfromreferringtoeachotherexplicitly.”
(Gamma,Helmetal.1995),p.273.
7.1.1 Thelocalstore
Thelocalstore,likeGaul,canbedividedintothreeparts,thepublicinterface(anin-
stanceoftheFaçadepattern(Gamma,Helmetal.1995)),thePalimpsestdatastoreitself,
andthepolicymanager.ThedatastorewasdiscussedinChapter6,anddespitesignificant
internalcomplexityitpresentsaverysimpleinterface.Figure7.2showthemostciritical
operationsthatalocalstoreneedstosupport.Alloftheseoperationsaresuitablefordirect
usebyanapplicationthatiswillingtomanageitsownchangeconflictsdirectly,ortotake
advantageofbuilt-inresolutionstrategies.Thefunctionslistedareselfexplanatory,forthe
mostpart.Wewillseelaterthatmostapplicationswillprobablyuseawrappedinterfacethat
issimplerandmayprovidebetterperformanceinhighlyinteractiveapplications.
Item[]getSegmentAt(Addressa,ChangeSetIDset)
Item[]contentsOf(Addressa,ChangeSetIDset)
Item[]contentsOf(Addressstart,Addressend,ChangeSetIDset)
SegmentIteratorenumerateContents(Addressa,ChangeSetIDset)
ConflictSetaddChange(ChangeIDid,ChangeSetIDset)
voidremoveChangeFromSet(ChangeIDid,ChangeSetIDset)
ChangeIDinsert(Item[],Addressdest)
ChangeIDdelete(Addressstart,Addressend)
ChangeIDcopy(Addressstart,Addressend,Addressdest)
ChangeIDmove(Addressstart,Addressend,Addressdest)
voidsetResolveConflicts(Booleanflag)
ChangeSetIteratorgetChangeSets()
ChangeIteratorgetChanges(ChangeSetIDset)
CorrelationMapcompare(ChangeSetIDversion1,ChangeSetIDversion2)
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7.1.2 Thepolicymanager
Thepolicymanagermanagesapplicationconflicts,aswellasreplicationandeventpropa-
gation.Thepolicymanagerreceiveseverychangefromthelocalapplicationaswellasfrom