浙江大学数据库系统概念PPT第十五章

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1、Chapter15:TransactionsTransactionConceptConcurrentExecutionsSerializabilityTestingforSerializabilityRecoverabilityTransactionDefinitioninSQLTransactionConceptE.g.Transactiontotransfer$50fromaccountAtoaccountB:1.read(A)4.read(B)2.A:=A–505.B:=B+503.write(A)6.write(BAt

2、ransactionisaunitofprogramexecutionthataccessesandpossiblyupdatesvariousdataitems.Atransactionmustseeaconsistentdatabase.Duringtransactionexecutionthedatabasemaybeinconsistent.Whenthetransactioniscommitted,thedatabasemustbeconsistent.Twomainissuestodealwith:Failures

3、ofvariouskinds,suchashardwarefailuresandsystemcrashesConcurrentexecutionofmultipletransactionsACIDPropertiesAtomicity.Eitheralloperationsofthetransactionareproperlyreflectedinthedatabaseornoneare.CommitatransactionRollbackatransactionConsistency.Executionofatransact

4、ioninisolationpreservestheconsistencyofthedatabase.Isolation.Althoughmultipletransactionsmayexecuteconcurrently,eachtransactionmustbeunawareofotherconcurrentlyexecutingtransactions.Intermediatetransactionresultsmustbehiddenfromotherconcurrentlyexecutedtransactions.D

5、urability.Afteratransactioncompletessuccessfully,thechangesithasmadetothedatabasepersist,eveniftherearesystemfailures.Topreserveintegrityofdata,thedatabasesystemmustensure:ExampleofFundTransferTransactiontotransfer$50fromaccountAtoaccountB:1.read(A)2.A:=A–503.write(

6、A)4.read(B)5.B:=B+506.write(B)Consistencyrequirement–thesumofAandBisunchangedbytheexecutionofthetransaction.Atomicityrequirement—ifthetransactionfailsafterstep3andbeforestep6,thesystemshouldensurethatitsupdatesarenotreflectedinthedatabase,elseaninconsistencywillresu

7、lt.FailurecouldbeduetosoftwareorhardwareExampleofFundTransfer(Cont.)Durabilityrequirement—oncetheuserhasbeennotifiedthatthetransactionhascompleted(i.e.,thetransferofthe$50hastakenplace),theupdatestothedatabasebythetransactionmustpersistdespitefailures.Isolationrequi

8、rement—ifbetweensteps3and6,anothertransactionisallowedtoaccessthepartiallyupdateddatabase,itwillseeaninconsistentdatabase(thesumA+Bwillbe