cs 4604: introducon to database management...
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CS4604:Introduc0ontoDatabaseManagementSystems
B.AdityaPrakashLecture#17:Transac0ons2:2PLand
Deadlocks
Announcement
§ HW6isout– DuenextWednesday(April13)
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Review(lastlecture)
§ DBMSssupportACIDTransac0onseman0cs.§ ConcurrencycontrolandCrashRecoveryarekeycomponents
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Review
§ ForIsola0onproperty,serialexecu0onoftransac0onsissafebutslow– Trytofindschedulesequivalenttoserialexecu0on
§ Onesolu0onfor“conflictserializable”schedulesisTwoPhaseLocking(2PL)
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Outline
§ 2PL/2PLC§ LockManagement§ Deadlocks– detec0on– Preven0on
§ SpecializedLocking
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SerializabilityinPrac0ce
§ DBMSdoesnottestforconflictserializabilityofagivenschedule– Imprac0calasinterleavingofopera0onsfromconcurrentXactscouldbedictatedbytheOS
§ Approach:– Usespecificprotocolsthatareknowntoproduceconflictserializableschedules
– Butmayreduceconcurrency
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Solu0on?
§ Onesolu0onfor“conflictserializable”schedulesisTwoPhaseLocking(2PL)
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Answer
§ (Fullanswer:)uselocks;keepthemun0lcommit(‘strict2phaselocking’)
§ Let’sseethedetails
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Lostupdateproblem-nolocks
T1Read(N)N=N-1Write(N) Prakash2016 VTCS4604 9
T2Read(N)N=N-1Write(N)
Solu0on–part1
§ withlocks:§ lockmanager:grants/denieslockrequests
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Lostupdateproblem–withlocks
0me
T1
lock(N)
Read(N)
N=N-1
Write(N)
Unlock(N)
T2
lock(N)
lockmanager
grantslock
denieslock
T2:waits
grantslocktoT2Read(N)...
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Locks
§ Q:Ijustneedtoread‘N’-shouldIs0llgetalock?
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Solu0on–part1
§ Locksandtheirflavors– exclusive(orwrite-)locks– shared(orread-)locks– <andmore...>
§ compa0bilitymatrix
X
S
XST2wantsT1has
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Solu0on–part1
§ Locksandtheirflavors– exclusive(orwrite-)locks– shared(orread-)locks– <andmore...>
§ compa0bilitymatrix
X
S
XST2wantsT1has
Yes
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Solu0on–part1
§ transac0onsrequestlocks(orupgrades)§ lockmanagergrantsorblocksrequests§ transac0onsreleaselocks§ lockmanagerupdateslock-table
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Solu0on–part2
locksarenotenough–eg.,the‘inconsistentanalysis’problem
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‘Inconsistentanalysis’
T1 Read(A) A=A-10 Write(A)
T2 Read(A) Sum = A
Read(B) Sum += B
Read(B) B=B+10 Write(B)
0me
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‘Inconsistentanalysis’–w/locks
0me T1
L(A)
Read(A)
...
U(A)
T2
L(A)
....
L(B)
....
theproblemremains!
T2readsaninconsistentDBstate
Solu0on??
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Generalsolu0on:
§ Protocol(s)§ Mostpopularprotocol:2PhaseLocking(2PL)
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2PL
X-lockversion:transac0onsissuenolockrequests,aoerthefirst‘unlock’
THEOREM:ifALLtransac0onsinthesystemobey2PL-->allschedulesareserializable
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2PL–example
§ ‘inconsistentanalysis’–howdoes2PLhelp?§ howwoulditbeunder2PL?
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2PL–X/Slockversion
transac0onsissuenolock/upgraderequest,aoerthefirstunlock/downgrade
Ingeneral:‘growing’and‘shrinking’phase
0me
#locks
growingphase shrinkingphase
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2PL–X/Slockversion
transac0onsissuenolock/upgraderequest,aoerthefirstunlock/downgrade
Ingeneral:‘growing’and‘shrinking’phase
0me
#locks
viola0onof2PL
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Two-PhaseLocking(2PL),cont.
§ 2PLonitsownissufficienttoguaranteeconflictserializability(i.e.,scheduleswhoseprecedencegraphisacyclic),but,itissubjecttoCascadingAborts.
time
# locks held release phase acquisition
phase
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2PL
§ Problem:CascadingAborts§ Example:rollbackofT1requiresrollbackofT2!
§ Solu0on:Strict2PL,i.e,§ keepalllocks,un0l‘commit’
T1:R(A),W(A), R(B),W(B),AbortT2: R(A),W(A)
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Strict2PL
§ Allowsonlyconflictserializableschedules,butitisactuallystrongerthanneededforthatpurpose.
# locks held
acquisition phase
time
release all locks at end of xact
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Strict2PL==2PLC(2PL0llCommit)
§ Ineffect,“shrinkingphase”isdelayedun0l– Transac0oncommits(commitlogrecordondisk),or
– Aborts(thenlockscanbereleasedaoerrollback).
# locks held
acquisition phase
time
release all locks at end of xact
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Lock_X(A) Read(A) A: = A-50 Write(A) Unlock(A)
Lock_S(A) Read(A) Unlock(A) Lock_S(B) Read(B) Unlock(B) PRINT(A+B)
Lock_X(B) Read(B) B := B +50 Write(B) Unlock(B)
Non-2PL, A= 1000, B=2000, Output =?
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Lock_X(A) Read(A) A: = A-50 Write(A)
Lock_X(B) Unlock(A)
Lock_S(A) Read(A)
Read(B) B := B +50 Write(B) Unlock(B)
Lock_S(B) Unlock(A) Read(B) Unlock(B) PRINT(A+B)
2PL, A= 1000, B=2000, Output =?
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Lock_X(A) Read(A) A: = A-50 Write(A)
Lock_X(B) Read(B) B := B +50 Write(B) Unlock(A) Unlock(B)
Lock_S(A) Read(A) Lock_S(B) Read(B) PRINT(A+B) Unlock(A) Unlock(B)
Strict 2PL, A= 1000, B=2000, Output =?
VennDiagramforSchedulesAll Schedules
Avoid Cascading Abort
Serial
Conflict Serializable
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Q:WhichschedulesdoesStrict2PLallow?
All Schedules
Avoid Cascading Abort
Serial
Conflict Serializable
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Q:WhichschedulesdoesStrict2PLallow?
All Schedules
Avoid Cascading Abort
Serial
Conflict Serializable
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AnotherVenndiagram
2PLschedules
serializableschedules
serialsch’s2PLC
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Outline
§ 2PL/2PLC§ LockManagement§ Deadlocks– detec0on– Preven0on
§ SpecializedLocking
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LockManagement
§ LockandunlockrequestshandledbytheLockManager(LM).
§ LMcontainsanentryforeachcurrentlyheldlock.
§ Q:structureofalocktableentry?
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LockManagement
§ LockandunlockrequestshandledbytheLockManager(LM).
§ LMcontainsanentryforeachcurrentlyheldlock.
§ Locktableentry:– Ptr.tolistoftransac0onscurrentlyholdingthelock
– Typeoflockheld(sharedorexclusive)– Pointertoqueueoflockrequests
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LockManagement,cont.
§ Whenlockrequestarrivesseeifanyotherxactholdsaconflic0nglock.– Ifnot,createanentryandgrantthelock– Else,puttherequestoronthewaitqueue
§ Lockupgrade:transac0onthatholdsasharedlockcanbeupgradedtoholdanexclusivelock
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LockManagement,cont.
§ Two-phaselockingissimpleenough,right?§ We’renotdone.There’sanimportantwrinkle…
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Example: Output = ?
Lock_X(A) Lock_S(B) Read(B) Lock_S(A)
Read(A) A: = A-50 Write(A) Lock_X(B)
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Example: Output = ?
Lock_X(A) Lock_S(B) Read(B) Lock_S(A)
Read(A) A: = A-50 Write(A) Lock_X(B)
lock mgr: grant grant
wait
wait
Outline
§ LockManagement§ Deadlocks– detec0on– Preven0on
§ SpecializedLocking
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Deadlocks
§ Deadlock:Cycleoftransac0onswai0ngforlockstobereleasedbyeachother.
§ Twowaysofdealingwithdeadlocks:– Deadlockpreven0on– Deadlockdetec0on
§ ManysystemsjustpuntanduseTimeouts– Whatarethedangerswiththisapproach?
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DeadlockDetec0on
§ Createawaits-forgraph:– Nodesaretransac0ons– EdgefromTitoTjifTiiswai0ngforTjtoreleasealock
§ Periodicallycheckforcyclesinwaits-forgraph
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DeadlockDetec0on(Con0nued)
Example: T1: S(A), S(D), S(B) T2: X(B) X(C) T3: S(D), S(C), X(A) T4: X(B)
T1 T2
T4 T3Prakash2016 VTCS4604 45
Anotherexample
T1 T2
T3 T4
• is there a deadlock?
• if yes, which xacts are involved?
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Anotherexample
T1 T2
T3 T4
• now, is there a deadlock?
• if yes, which xacts are involved?
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Deadlockdetec0on
§ howooenshouldwerunthealgo?§ howmanytransac0onsaretypicallyinvolved?
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Deadlockhandling
T1 T2
T3 T4
• Q: what to do?
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Deadlockhandling
T1 T2
T3 T4
• Q0: what to do?
• A: select a ‘victim’ & ‘rollback’
• Q1: which/how to choose?
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Deadlockhandling
• Q1: which/how to choose?
• A1.1: by age
• A1.2: by progress
• A1.3: by # items locked already...
• A1.4: by # xacts to rollback
• Q2: How far to rollback?
T1 T2
T3 T4
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Deadlockhandling
• Q2: How far to rollback?
• A2.1: completely
• A2.2: minimally
• Q3: Starvation??
T1 T2
T3 T4
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Deadlockhandling
• Q3: Starvation??
• A3.1: include #rollbacks in victim selection criterion.
T1 T2
T3 T4
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Outline
§ LockManagement§ Deadlocks– detec0on– Preven0on
§ SpecializedLocking
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DeadlockPreven0on
§ Assignpriori0esbasedon0mestamps(older->higherpriority)
§ Weonlyallow‘old-wait-for-young’§ (oronlyallow‘young-wait-for-old’)§ androllbackviolators.Specifically:§ SayTiwantsalockthatTjholds-twopolicies:– Wait-Die:IfTihashigherpriority,TiwaitsforTj;otherwiseTiaborts(ie.,oldwaitforyoung)
– Wound-wait:IfTihashigherpriority,Tjaborts;otherwiseTiwaits(ie.,youngwaitforold)
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DeadlockPreven0on
Wait-Die Wound-Wait Ti wants Tj has Ti wants Tj has
Priori0es
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DeadlockPreven0on
§ Q:Whydotheseschemesguaranteenodeadlocks?
§ A:§ Q:Whenatransac0onrestarts,whatisits(new)priority?
§ A:
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DeadlockPreven0on
§ Q:Whydotheseschemesguaranteenodeadlocks?
§ A:onlyone‘type’ofdirec0onallowed.§ Q:Whenatransac0onrestarts,whatisits(new)priority?
§ A:itsoriginal0mestamp.--Why?
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SQLstatement
§ usually,conc.controlistransparenttotheuser,but
§ LOCK<table-name>[EXCLUSIVE|SHARED]
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Quiz:
§ isthereaserialschedule(=interleaving)thatisnotserializable?
§ isthereaserializableschedulethatisnotserial?
§ can2PLproduceanon-serializableschedule?(assumenodeadlocks)
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Quiz-cont’d
§ isthereaserializableschedulethatcannotbeproducedby2PL?
§ axactobeys2PL-canitbeinvolvedinanon-serializableschedule?
§ allxactsobey2PL-cantheyendupinadeadlock?
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Outline
§ LockManagement§ Deadlocks– detec0on– Preven0on
§ SpecializedLocking
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Thingswewillnotstudy§ Weassumed0llnowDBobjectsarefixedandindependent---nottrueinmanycases!§ Mul0-levellocking
– Lockdborfileorpagesorrecord?§ Whataboutlockingindexes?
– E.g.B+-trees– CrabbingAlgorithm
§ Whataboutdynamicdatabases?– ‘phantom’problem– Solu0on:predicatelocking
§ Non-lockingbasedTechniques– TimestampbasedConcurrencyControl
§ Alltheseareinthetextbookthough
SKIP
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Transac0onSupportinSQL-92
§ SERIALIZABLE–Nophantoms,allreadsrepeatable,no“dirty”(uncommited)reads.
§ REPEATABLEREADS–phantomsmayhappen.§ READCOMMITTED–phantomsandunrepeatablereadsmayhappen
§ READUNCOMMITTED–allofthemmayhappen.
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Transac0onSupportinSQL-92
§ SERIALIZABLE:obtainsalllocksfirst;plusindexlocks,plusstrict2PL
§ REPEATABLEREADS–asabove,butnoindexlocks
§ READCOMMITTED–asabove,butS-locksarereleasedimmediately
§ READUNCOMMITTED–asabove,butallowing‘dirtyreads’(noS-locks)
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Transac0onSupportinSQL-92
§ SETTRANSACTIONISOLATIONLEVELSERIALIZABLEREADONLY
§ Defaults:§ SERIALIZABLE§ READWRITE
isolation level
access mode
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Conclusions
§ 2PL/2PL-C(=Strict2PL):extremelypopular§ Deadlockmays0llhappen– detec0on:wait-forgraph– preven0on:abortsomexacts,defensively
§ philosophically:concurrencycontroluses:– locks– andaborts
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