浙江大学数据库系统概念PPT第十五课时,对应原版教材第五版.ppt

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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(BAtransactionisaunitofprogr

2、amexecutionthataccessesandpossiblyupdatesvariousdataitems.Atransactionmustseeaconsistentdatabase.Duringtransactionexecutionthedatabasemaybeinconsistent.Whenthetransactioniscommitted,thedatabasemustbeconsistent.Twomainissuestodealwith:Failuresofvariouskinds,suchashardwarefailuresandsystemcr

3、ashesConcurrentexecutionofmultipletransactionsACIDPropertiesAtomicity.Eitheralloperationsofthetransactionareproperlyreflectedinthedatabaseornoneare.CommitatransactionRollbackatransactionConsistency.Executionofatransactioninisolationpreservestheconsistencyofthedatabase.Isolation.Althoughmul

4、tipletransactionsmayexecuteconcurrently,eachtransactionmustbeunawareofotherconcurrentlyexecutingtransactions.Intermediatetransactionresultsmustbehiddenfromotherconcurrentlyexecutedtransactions.Durability.Afteratransactioncompletessuccessfully,thechangesithasmadetothedatabasepersist,evenift

5、herearesystemfailures.Topreserveintegrityofdata,thedatabasesystemmustensure:ExampleofFundTransferTransactiontotransfer$50fromaccountAtoaccountB:1.read(A)2.A:=A–503.write(A)4.read(B)5.B:=B+506.write(B)Consistencyrequirement–thesumofAandBisunchangedbytheexecutionofthetransaction.Atomicityreq

6、uirement—ifthetransactionfailsafterstep3andbeforestep6,thesystemshouldensurethatitsupdatesarenotreflectedinthedatabase,elseaninconsistencywillresult.FailurecouldbeduetosoftwareorhardwareExampleofFundTransfer(Cont.)Durabilityrequirement—oncetheuserhasbeennotifiedthatthetransactionhascomplet

7、ed(i.e.,thetransferofthe$50hastakenplace),theupdatestothedatabasebythetransactionmustpersistdespitefailures.Isolationrequirement—ifbetweensteps3and6,anothertransactionisallowedtoaccessthepartiallyupdateddatabase,itwillseeaninconsistentdatabase(thesumA+Bwillbe