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DatabaseManagementSystem
Lecture9
Transaction,ConcurrencyControl
*SomematerialsadaptedfromR.Ramakrishnan,J.Gehrke andShawnBowers
BasicDatabaseArchitecture
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ConcurrencyControlandRecovery• Transactions• Awaytodefineasingle“allornothing”setofSQLactions
• Basedonasetofproperties(the“ACID”properties)
• Enableconcurrency
• Thebasisforcrashrecovery
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Transactions• A“transaction”isasetofSQLstatements(thatmodifyadatabase)chosenbyauser
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Transfer $100 from one account to another (MySQL): STARTTRANSACTION;SELECT@A1:=balanceFROMAccountWHEREacctno=500;
UPDATEAccountSETbalance:=@A1+100WHEREacctno=500;
SELECT@A2:=balanceFROMAccountWHEREacctno=501;
UPDATEAccountSETbalance:=@A2-100WHEREacctno=501;
COMMIT;
BEGINTRANSACTIONREADbalancefromaccount500ADD$100tobalanceofaccount500WRITEnewbalancetoaccount500READbalanceinaccount501VERIFYbalancetoseeifitcontainsatleast$100ABORTifbalanceislessthan$100SUBTRACT$100frombalanceofaccount501WRITEnewbalancetoaccount501COMMITTRANSACTION
Transactions• User(applicationdeveloper)must• Begintransaction
• Read,write,andmodifystatementsintermixedwithotherprogramminglanguagestatements(e.g.,verify)
• Pluseither• Committoindicatesuccessfulcompletionor
• Aborttoindicatethatthethetransactionshouldbe“rolledback”...i.e.,erasepreviousstepsoftransaction
• Toensuredatabasestaysinaconsistent/correctstate• TheDBMSandprogrammermustguarantee4propertiesoftransactions
• Thesearecalledthe“ACID”properties
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ACIDProperties• Atomicity
• Consistency
• Isolation
• Durability
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ACIDProperties
•WhatiftheOScrashedhalf-waythroughthetransaction...after$100wasremovedfromthefirstaccount?
• TherecoverymanageroftheDBMSmustassurethatthe$100isdepositedbacktothefirstaccount
• Oftencalled“rollback”
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AtomicityA transaction happens in its entirety or not at all
ACIDProperties
• Thenotionof“consistency”isspecifictotheapplicationconstraints(definedbytheuser)
• Thustheprogrammermustensuretransactionsareconsistent• TheDBMSensuresthetransactionisatomic
• E.g.,whatifthetransactiononlydeposited$100?• Probablynotconsistentaccordingtoourexampleapplication
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Consistency
If the DB starts in a consistent state, the transaction will transform it into a consistent state
ACIDProperties
•Whatifananothertransactioncomputedthetotalbankbalanceafter$100wasremovedfromthefirstaccount?
• Theconcurrencycontrolsubsystemmust• ensurethatalltransactionsruninisolation(i.e.,don’tmessupothertransactions)
• unlesstheprogrammerchoosesalessstrictlevelofisolation– similartoconcurrencycontrolinoperatingsystems
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Isolation
Each transaction is “isolated” from other transactions ... DB state is as if each transaction executed by itself
ACIDProperties
•WhatifafterthecommittheOScrashedbeforethedepositwaswrittentodisk?
• Therecoverymanagermustassurethatthedepositwasatleastlogged(e.g.,tomaketheDBconsistent)
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Durability
If a transaction commits, its changes to the DB state persist (changes are permanent)
Concurrency•Whyisconcurrencyimportant?• Betterutilizationofresources
• E.g.,whileoneuser/transactionisreadingthedisk,anothercanbeusingtheCPUorreadinganotherdisk
• Resultsinbetterthroughputandresponsetime
• Manyapplicationsrequireit(forperformance)
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Concurrency•We’lllookatconcurrencyintermsofisolationoftransactions• Isolationisaproblemwhenmultipletransactionsarerunning,usingthesamedata,andoperationsareinterleaved
• Isolationensuredbytheconcurrencycontrolsubsystem
• Thisshouldbefamiliarifyou’vetakentheOSclass...
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SerialSchedules• Considerthesetransactions:
• DeposittoAandwithdrawfromB
• ComputethebalanceofAandB
• ApplyinteresttoAandB
• Ascheduleisaninterleavingoftheactionsofthetransactionssothateachtransaction’sorderispreserved
• Ascheduleoftransactionsisserialifitstransactionsoccurconsecutively,oneafteranother
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T1:BEGINA=A+100;B=B– 100;END
T2:BEGINC=A+B;END
T3:BEGINA=1.06*A;B=1.06*A;END
SerialSchedules•Whichoftheseisaschedule?Whichisserial?
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S1T1 T3A=A+100B=B-100
A=1.06*AB=1.06*B
S2T1 T3A=A+100
B=B-100A=1.06*A
B=1.06*B
S3T1 T2A=A+100
B=B-100C=A+B
S4T1 T3B=B-100A=A+100
C=A+B
SerialSchedule!
Non-serialSchedule!
Non-serialSchedule!
NotaSchedule!
AllowableConcurrency•WhatiswrongwithS3?• Itdoesnotgivethesameresultasanyserialschedule
• TheDBMSshould• AllowserialscheduleslikeS1
• ForbidinterleavedscheduleslikeS3
• ButwhataboutS2?• NotethatitgivesthesameresultasS1!
• TheDBMSshouldalsoallowthisschedule!
• IftheDBMSonlyallowsserialschedules,thenitbecomesabatchsystem(whereistheconcurrency?)
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SerializableSchedules• Ascheduleis“serializable”ifitseffectontheDBisthesameastheeffectonsomeserialschedule• Serialschedulesarealwaysserializable
• S2isserializable,butS3isnot
• Serializability isthesameastheisolationcondition
• Thegoaloftheconcurrencycontrolsubsystemistoensureserializability
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SchedulesasReadsandWrites• AnexpressionA=1.06*Ameans– ReadAfromdisk• SetAequalto1.06*A
• WriteAtodisk
• Onlytheread andwrite todiskmattertotheDBMS!• We’llusethenotation...
• R(A) forReadA
• W(A) forWriteA
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SchedulesasReadsandWrites• Theseareequal(fromDBMS/concurrencyperspective)
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S2T1 T3A=A+100
B=B-100A=1.06*A
B=1.06*B
S2T1 T3R(A),W(A)
R(B),W(B)R(A),W(A)
R(B),W(B)
ConflictSerializability• S2hasaspecialstructurethatmakesitpossibletoshowthatitisserializable...
• Twoactionsare“nonconflicting”iftheyareindifferenttransactionsandeitherthey• Accessdifferentdatatimes(resources)
• Orbotharereads
• Ifnonconflicting actionsarecommutedthenthenewschedulegivesthesameresult
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ConflictSerializability•woschedulesare“conflict equivalent”if• Onecanbetransformedintotheotherbycommuting(swapping)nonconflicting actions
• Ascheduleis“conflict serializable”ifitisconflictequivalenttoatleastoneserialschedule
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Thus, every conflict serializable schedule is serializable! (... but not necessarily the other way around)
Serializability• Nonconflicting showninRed
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S2T1 T3R(A),W(A)
R(B),W(B)R(A),W(A)
R(B),W(B)
S2T1 T3R(A),W(A)R(B),W(B)
R(A),W(A)R(B),W(B)
This is a serial schedule! This means S2 is “conflict serializable” (... and thus is serializable)
commute
Ais usedinT3,BinT1
PrecedenceGraphs• Verifying conflict serializability is tedious
• There is an easier way!
• Precedence graphs
• One node per transaction
• Edge from Ti to Tj if an action in Ti occurs before an action in Tj and the
actions conflict
• Theorem
• A schedule is conflict serializable if an only if its precedence graph is acyclic
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T1 T2
SERIAL NOTSERIAL NOTSERIAL
S1T1 T3R(A)W(A)R(B)W(B)
R(A)W(A)R(B)W(B)
S3T1 T2R(A)W(A)
R(B)W(B)
R(A)W(A)
R(B)W(B)
S4T1 T3R(A)W(A)
R(B)W(B)
R(A)R(B)W(C)
Exercise•Withapartner,drawtheprecedencegraphsforthepreviousschedules(S1-S3)• You’llfirsthavetoconvertthemtoR’sandW’s
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S1T1 T3A=A+100B=B-100
A=1.06*AB=1.06*B
S3T1 T2A=A+100
B=B-100C=A+B
S4T1 T3B=B-100A=A+100
C=A+B
Serializable,but..
Exercise•Withapartner,drawtheprecedencegraphsforthepreviousschedules(S1-S3)
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T1 T2
T2
T1 T2
T2NotSerializable
S6 is actually serializable: whichever transaction writes A last “wins” Serializable but not Conflict Serializable
T1 T2 T3R(A)
R(B)
W(B)
W(A)
W(A)
R(B)
S5T1 T2 T3R(A)
W(B)W(A)
W(A)
S6
Relationships
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Serializable
ConflictSerializable
AcyclicPrecedenceGraph
Serial
Serializability inPractice• Precedencegraphsgiveusasimplewaytoprovethatascheduleis(conflict)serializable• Buttheydonotworkforallschedules
• Intheory,thiscouldbeusedbyaDBMStocheckforserializability ...
• Inpractice• ADBMSisnotpresentedwithschedules...itonlyseesastreamoftransactions
• Instead,locking isusedtoachieve“isolation”
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Locking• Transactionsmustobtainalockbeforereadingorupdating(writing)data
• Twokindsoflocks:• Shared(S)locks
• Exclusive(X)locks
• ToreadarecordyouMUSTgetanSlock
• Towrite(modify/delete)arecordyouMUSTgetanXlock
• Lockinformationismaintainedbya“lockmanager“
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HowLocksWork• IfanobjecthasanS(shared)lock• newtransactionscanobtainS(shared)locks
• butnot X(exclusive)locks
• IfanobjecthasanXlock• noothertransactioncanobtainany lockonthatobject
• Ifatransactioncannotobtainalock• Itisblocked (i.e.,waitsinaqueue)
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StrictTwoPhaseLockingProtocol(Strict2PL)• InStrict2PL• TransactionTobtains(SandX)locksgradually,asneeded
• Tholdsalllocksuntilendoftransaction(commit/abort)
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time
#ofLocksheldbya
transactionT
01234 Alllocksarereleased
attheend,uponcommitorabort
This guarantees serializability! • Still permits interleaved schedules• But can lead to deadlock
StrictTwoPhaseLockingProtocol(Strict2PL)• Examples
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S6T1 T2 T3R(A)
W(A)W(A)
W(A)
S6(S2PL)T1 T2 T3X(A)R(A)W(A)Commit
X(A)W(A)Commit
X(A)W(A)Commit
S7(S2PL)T1 T2
X(B)X(B)
X(C)W(C)Commit
X(A)W(A)
X(C)W(C)Commit
StrictTwoPhaseLockingProtocol(Strict2PL)• Yetanotherexample• T1:W(A),W(B)• T2:W(B),W(A)
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S7(S2PL)T1 T2X(A)W(A)
WaitingforX(B)…
X(B)W(B)
WaitingforX(A)…
DEADLOCK!!
DeadlocksinDBMS•WhatisaDeadlock?• Acycleoftransactions,e.g.,T1...Tn whereeachTi iswaitingforTi-1toreleasealock!
• Causesthesetransactionstosleep/waitforever
• Deadlockscanoccurinstrict2PL
• Solutions• Alwaysaccessresourcesinthesameorder(impractical)
• TheDBMSwilltypicallydetect deadlocks
• ...andthenabort thetransaction(itthinks)hasusedtheleastresources
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