Chapter17:RecoverySystemFailureClassificationTransactionStateAquickviewofLog-BasedRecovery浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第1頁!FailureClassificationTransactionfailure:Logicalerrors:transactioncannotpleteduetosomeinternalerrorconditionSystemerrors:thedatabasesystemmustterminateanactivetransactionduetoanerrorcondition(e.g.,deadlock)Systemcrash:apowerfailureorotherhardwareorsoftwarefailurecausesthesystemtocrash.Fail-stopassumption:non-volatilestoragecontentsareassumedtonotbecorruptedbysystemcrashDatabasesystemshavenumerousintegritycheckstopreventcorruptionofdiskdata

Diskfailure:aheadcrashorsimilardiskfailuredestroysallorpartofdiskstorageDestructionisassumedtobedetectable:diskdrivesusechecksumstodetectfailures浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第2頁!RecoveryAlgorithmsRecoveryalgorithmsaretechniquestoensuredatabaseconsistencyandtransactionatomicityanddurabilitydespitefailuresRecoveryalgorithmshavetwopartsActionstakenduringnormaltransactionprocessingtoensureenoughinformationexiststorecoverfromfailuresActionstakenafterafailuretorecoverthedatabasecontentstoastatethatensuresatomicity,consistencyanddurability浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第3頁!TransactionStateSinglestep浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第4頁!ImplementationofAtomicityandDurabilityTherecovery-managementponentofadatabasesystemimplementsthesupportforatomicityanddurability.Theshadow-databasescheme:assumethatonlyonetransactionisactiveatatime.apointercalleddb_pointeralwayspointstothecurrentconsistentcopyofthedatabase.allupdatesaremadeonashadowcopyofthedatabase,anddb_pointerismadetopointtotheupdatedshadowcopyonlyafterthetransactionreachespartialmitandallupdatedpageshavebeenflushedtodisk.incasetransactionfails,oldconsistentcopypointedtobydb_pointercanbeused,andtheshadowcopycanbedeleted.浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第5頁!RecoveryandAtomicityModifyingthedatabasewithoutensuringthatthetransactionwillmitmayleavethedatabaseinaninconsistentstate.ConsidertransactionTithattransfers$50fromaccountAtoaccountB;goaliseithertoperformalldatabasemodificationsmadebyTi

ornoneatall.SeveraloutputoperationsmayberequiredforTi(tooutputAandB).Afailuremayoccurafteroneofthesemodificationshavebeenmadebutbeforeallofthemaremade.Toensureatomicitydespitefailureswefirstoutputinformationdescribingthemodificationstostablestoragewithoutmodifyingthedatabaseitself.Thenmakethemodificationtodatabaseatmitpoint,orremoveallthemodificationifthetransactionfailed(rollback)浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第6頁!ImmediateDBModificationRecoveryExample

Belowweshowthelogasitappearsatthreeinstancesoftime.Recoveryactionsineachcaseaboveare:(a)undo(T0):Bisrestoredto2000andAto1000.(b)undo(T1)andredo(T0):Cisrestoredto700,andthenAandBaresetto950and2050respectively.(c)redo(T0)andredo(T1):AandBaresetto950and2050respectively.ThenCissetto600浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第7頁!TransactionState(Cont.)Active,theinitialstate;thetransactionstaysinthisstatewhileitisexecutingPartiallymitted,afterthefinalstatementhasbeenexecuted.Failed,afterthediscoverythatnormalexecutioncannolongerproceed.Aborted,afterthetransactionhasbeenrolledbackandthedatabaserestoredtoitsstatepriortothestartofthetransaction.Twooptionsafterithasbeenaborted:restartthetransaction–onlyifnointernallogicalerrorkillthetransactionCommitted,aftersuccessfulpletion.浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第8頁!ImplementationofAtomicityandDurability(Cont.)AssumesdiskstonotfailUsefulfortexteditors,butextremelyinefficientforlargedatabases:executingasingletransactionrequirescopyingtheentiredatabase.Willseebetterschemeslater.Theshadow-databasescheme:浙江大學數據庫系統概念第十七章共11頁，您現在瀏覽的是第9頁!Log-BasedRecoveryAlogiskeptonstablestorage.Thelogisasequenceoflogrecords,andmaintainsarecordofupdateactivitiesonthedatabase.WhentransactionTi

starts,itregistersitselfbywritinga

<Ti

start>logrecordBeforeTi

executeswrite(X),alogrecord<Ti,X,V1,V2>

iswritten,whereV1isthevalueofXbeforethewrite,and

V2

isthevaluetobewrittentoX.LogrecordnotesthatTihasperformedawriteondataitemXjX