WarmGup#Exercise# CS#133:#Databases# TwoGPhase#Locking#(2PL)# Goals#for#Today#

Transcription

1 WarmGupExercise CS133:Databases Spring2017 Lec17 3/23 Transac:ons Prof.BethTrushkowsky (Seeexercisesheet.Youcanstartbeforeclass.) YesS1andS2areconflictGequivalent. Noneitherisconflictserializable. GoalsforToday Understandhowtomanagelocksand deadlockwhenimplemen:ng2plorstrict2pl Reasonaboutissuesthatcanarisewhendata isinsertedordeleted DiscussIsola:onlevelsinaDBMS 1)Protocol:eachtransac:onmust GetaS(shared)oranX(exclusive)lockonobjectbeforereading GetanX(exclusive)lockonobjectbeforewri:ng 2)!All!lock!requests!precede!all!unlock!requests! I.e.,a/transac2on/can/not/request/addi2onal/locks/once/it/releases/any/ locks!locks!held! TwoGPhaseLocking(2PL) Growing Phase CanupgradeaSharedlocktoan exclusivelock!(when/okay?) Shrinking Phase Time!

2 BasicLocking:Example(Take2Gabort) A=1000,B=2000,Output=? Lock_X(A)!!<granted>! T 1 T 2 Read(A)! Lock_S(A)! A&=&A,50& Write(A)! Lock_X(B)!!<granted>! Unlock(A)! Read(B)! B&=&B&+50& Write(B)! Unlock(B)! ABORT!!!!!!!!!!!!!!!!!!<granted>! Read(A)! Lock_S(B)!!!!!!!!!!!!!!<granted>! Unlock(A)! Read(B)! Unlock(B)! PRINT(A+B)! T 2 hasread uncommiged changes!it/must/ also/abort./ AvoidingCascadingAborts:Strict2PL Problemwith2PL:cascadingaborts Anotherexample: rollbackoft1requiresrollbackoft2 T1:R(A),W(A),R(B),W(B),Abort T2: R(A),W(A) Solu:on:!Strict!TwoKphase!Locking!(Strict2PL): Sameas2PL,except: Alllocksheldbyatransac:onarereleasedonly whenthetransac:oncompletes Consequence:awriterwillblockallother readersun:lthewritercommitsoraborts a) Yes2PL,NoStrict2PL Exercise2 b) Neither(schedulenotconflictGserializable) ViewSerializability SchedulesS1andS2areviewequivalentif: IfT1reads!iniMal!value!of!A!inS1, thent1alsoreadsini:alvalueofains2 IfT1reads!value!of!A!wriPenbyT2inS1, thent1alsoreadsvalueofawrigenbyt2ins2 Checkingforthis isnpgcomplete! IfT1writes!final!value!of!A!inS1, thent1alsowritesfinalvalueofains2 T1:R(A) W(A) T2: W(A) T3: W(A) T1:R(A),W(A) T2: W(A) T3: W(A)

3 Lock/unlockrequestsarehandledbytheLockManager Havetablewithentryforeachcurrently!held!lock! Whatobjectisbeinglocked? Possibili:es:table(s),row(s),page(s) ToocoarseGgrainedlimitsconcurrency! Moreongranularitylater Lock!table!entry LockManagement Objectidofobjectbeinglocked(e.g.,table,row,page) (Pointerto)listoftransac:onscurrentlyholdingthelock Typeoflockheld(sharedorexclusive) (Pointerto)!queue!of!lock!requests! LockManagement(cntd) Whenalockrequestarrives Checkifanyxactcurrentlyholdsaconflic:nglockonthe object Ifnot,createanentryandgrantthelock Else,putthereques:ngxactonthewaitqueue & Locking&and&unlocking&have&to&be&atomic&opera?ons!& TryExercise3 ObjectID! LockType! Xacts! XactsRequesMng!! A S T1 D S T1,T3 B X T2 T1,T4 C S T3 T2 BasicLocking:Example(Take3) Lock_X(A) <granted> Lock_S(B) <granted> Read(B) Lock_S(A) Read(A) A: = A-50 Write(A) Lock_X(B)

4 Deadlocks Deadlock:Cycleoftransac:onswai:ngforlockstobe releasedbyeachother. CanseecycleinawaitsKfor!graph: Nodesaretransac:ons ThereisanedgefromTitoTjifTiiswai:ngforTjto releasealock DeadlockPreven:on Assignpriori:esbasedon:mestamps SupposeTiwantsalockthatTjholds Twopossiblepolicies: WaitKDie:IfTiisolder,TiwaitsforTj;otherwiseTi aborts WoundKwait:IfTiisolder,Tjaborts(gets wounded ); otherwisetiwaits Inboth,theolderneveraborts TwomainwaysofdealingwithdeadlocksinDBMS: DeadlockprevenMon! DeadlockdetecMon! Ifatransac:onreGstarts,makesureitgetsits original:mestamp Why? DeadlockExercise:4(a) RecallWait>Die: TiwantsalockthatTjholds IfTiisolder,TiwaitsforTj;otherwiseTiaborts Schedule1: 1) T1requestsforandgetsS(A) 2) T2requestX(A),can tgetit.abortsbecauset2youngerthant1 3) T3requestsandgetsX(B) 4) T1requestsX(B),can tgetit.waitsbecauset1olderthant3 5) T3commits,releaseslockonB 6) T1getsX(B),thencommits (T2restarted) Schedule2: (sameasschedule1,exceptt2makesabitmoreprogressbeforeabor:ng) DeadlockDetec:on Alterna:veistoallowdeadlockstohappenbutto checkforthemandfixthemiffound. Periodicallycheck!for!cycles!inthewaitsGforgraph Ifcycledetected findatransac:onwhose removalwillbreakthecycleandkillit

5 Example: DeadlockDetec:on(Cntd) 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 T3 DeadlockExercise:4(b) Schedule1: T2blocksonT1onobjectA T1blocksonT3onobjectB WhenT3finishes,T1resumesandgetsB WhenT1finishes,T2resumesandgetsA(and thenb) Schedule2: T2blocksonT1onobjectA T3blocksonT2onobjectB T1blocksonT2onobjectB DEADLOCK!WaitsGforGgraphhascycle betweent1andt2 Dynamic Databases Relaxassump:onthatdatabaseisasta:cset ofobjects WithInsertandDeletepossible,evenStrict 2PL(onindividualobjects)willnotassure serializability The Phantom Problem Example1 ConsiderT1 Findoldestsailor T1locksallSailorrecords, findsoldestsailor(age=71) T1checksfortheoldestsailor, findsoldestsailor(age=96) T2insertsanew!sailor;age=96 commits! Noserialexecu:onwhere T1 sresultcouldhappen! Thesailorwithage96isa phantomtuple from T1 spointofviewgggfirstit snotthere,thenitis

6 The Phantom Problem Example2 ConsiderT3 Findoldestsailorforeachra:ng T3locksallpagescontaining sailorrecordswithra2ng=1 findsoldestsailor(age=71) T4insertsanewsailor;ra2ng=1,age=96 T4alsodeletesoldestsailorwithra:ng=2,age=80 commits T3nowlocksallpagescontainingsailorrecords withra2ng=2,andfindsoldest(age=63). T3sawonlypartofT4 seffects! Noserialexecu:onwhere T3 sresultcouldhappen! TheProblem Howdoyoulock!something!that!does!not!yet!exist?? T1andT3implicitlyassumedthattheyhadlockedthe setofallsailorrecordssa:sfyingapredicate. Assump:ononlyholdsifnosailorrecordsareaddedwhile theyareexecu:ng! Needsomemechanismtoenforcethisassump:on,e.g., index!locking! Conflictserializabilityonreadsandwritesofindividual objectsguaranteesserializabilityonlyifthesetof objectsisfixed Isola:onLevelsinSQLStandard Idea:giveuserscontroloverlockingoverheadincurred bytheirxacts XactscanbespecifiedwithdesiredIsola:onLevel Also,accessmodelike readgonly onlygetsslocks Isola:onLevel Dirty/ Read/ Unrepeatable/ Read/ Phantom// Problem/ Read!UncommiPed! Read!CommiPed! Doesnotgetreadlocks, (notallowedtowriteobjects) Writelocksheldtocommit. Getreadlocks,butrelease thoserightaway Repeatable!Read! Serializable! No No No No Maybe No Strict2PL.Locksbeforeread& write,onindividualobjects Strict2PL.Getslocksbefore read/write,includingon setsofobjects(indexlocks)