Atransactionisanaction,oraseriesofactions,carriedoutbyasingleuseror(71),thataccessesorchangesthecontentsofthedatabase.
Atransactionshouldpossessthefourbasicorso-calledACIDproperties:
Atomicity.The“allornothing”property.Atransactionisan(72)unitthatiseitherperformedinitsentiretyorisnotperformedatall.
Consistency.Atransactionmusttransformthedatabasefromoneconsistentstatetoanotherconsistentstate.
Isolation.Transactionsexecuteindependentlyofoneanother.Inotherwords,thepartialeffectsof(73)transactionsshouldnotbevisibletoothertransactions.
Durability.Theeffectsofasuccessfullycompleted(committed)transactionarepermanentlyrecordedinthedatabaseandmustnotbelostbecauseofasubsequentfailure.
Thefourpropertiesabove,only(74)propertyistheresponsibilityoftheconcurrencycontrolsubsystemtoensure.Itistheresponsibilityofboththe(75)andtheapplicationdeveloperstoensureconsistency.ToguaranteetheAtomicityandDurabilitypropertiesafterafailure,alogfileisneeded.
71、A.applicationdeveloper
B.allusers
C.applicationprogram
D.DBMS
72、A.Indivisible
B.permanent
C.infrequent
D.infinite
73、A.completed
B.incomplete
C.aborted
D.committed
74、A.atomicity
B.consistency
C.isolation
D.durability
75、A.DBMS
B.DBA
C.enduser
D.DBS