数据库系统概念第六版课后习题部分答案26s

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1、CHAPTER26AdvancedTransactionProcessingPracticeExercises26.1Likedatabasesystems,workflowsystemsalsorequireconcurrencyandrecoverymanagement.Listthreereasonswhywecannotsimplyapplyarelationaldatabasesystemusing2PL,physicalundologging,and2PC.Answer:a.Thetasksinaworkflowhavedependenciesbasedon

2、theirstatus.Forexamplethestartingofataskmaybeconditionalontheoutcome(suchascommitorabort)ofsomeothertask.Allthetaskscannotexecuteindependentlyandconcurrently,using2PCjustforatomiccommit.b.Onceataskgetsover,itwillhavetoexposeitsupdates,sothatothertasksrunningonthesameprocessingentitydon

3、’thavetowaitforlong.2PListoostrictaformofconcurrencycontrol,andisnotappropriateforworkflows.c.Workflowshavetheirownconsistencyrequirements;thatis,failure-atomicity.Anexecutionofaworkflowmustfinishinanacceptableterminationstate.Becauseofthis,andbecauseofearlyexposureofuncommittedupdates,the

4、recoveryprocedurewillbequitediffer-ent.Someformoflogicalloggingandcompensationtransactionswillhavetobeused.Alsotoperformforwardrecoveryofafailedworkflow,therecoveryroutinesneedtorestorethestateinforma-tionoftheschedulerandtasks,notjusttheupdateddataitems.ThussimpleWALcannotbeused.26.2Co

5、nsideramain-memorydatabasesystemrecoveringfromasystemcrash.Explaintherelativemeritsof:12Chapter26AdvancedTransactionProcessing•Loadingtheentiredatabasebackintomainmemorybeforeresumingtransactionprocessing.•Loadingdataasitisrequestedbytransactions.Answer:•Loadingtheentiredatabaseintomem

6、oryinadvancecanprovidetransactionswhichneedhigh-speedorrealtimedataaccesstheguar-anteethatoncetheystarttheywillnothavetowaitfordiskaccessestofetchdata.Howevernotransactioncanruntilltheentiredatabaseisloaded.•Theadvantageinloadingondemandisthattransactionprocessingcanstartrightaway;howe

7、vertransactionsmayseelongandunpre-dictabledelaysindiskaccessuntiltheentiredatabaseisloadedintomemory.26.3Isahigh-performancetransactionsystemnecessarilyareal-timesystem?Whyorwhynot?Answer:Ahigh-performancesystemisnotnecessarilyareal-timesys-tem.Inahighperformancesystem,themainaimisto