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SchoolofComputerScience

PrinciplesofSo3wareConstruc9onSerializabilityandTransac9onsJoshBloch CharlieGarrod

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Administrivia

•  Homework6checkpointdueFriday5p.m.•  FinalexamFriday,Dec16th5:30-8:30p.m.,GHC4401

–  ReviewsessionWednesday,Dec14th7-9:30p.m.,DH1112

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“MadMax”

publicclassMax{publicstaticdoublemax(double...vals){if(vals.length==0)thrownewIllegalArgumentException("Novalues!");doubleresult=Double.MIN_VALUE;for(doubleval:vals)if(val>result)result=val;returnresult;}publicstaticvoidmain(String[]arguments){System.out.println(max(-1,0,-2.718281828));}}

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Whatdoesitprint?

publicclassMax{publicstaticdoublemax(double...vals){if(vals.length==0)thrownewIllegalArgumentException("Novalues!");doubleresult=Double.MIN_VALUE;for(doubleval:vals)if(val>result)result=val;returnresult;}publicstaticvoidmain(String[]arguments){System.out.println(max(-1,0,-2.718281828));}}

(a)  0.0 (b)  4.9E-324 (c)  Throwsexcep:on(d)  Noneoftheabove

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(a)  0.0 (b)  4.9E-324 (c)  Throwsexcep9on(d)  Noneoftheabove Double.MIN_VALUEisverydifferentfromInteger.MIN_VALUE

Whatdoesitprint?

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AnotherlookpublicclassMax{publicstaticdoublemax(double...vals){if(vals.length==0)thrownewIllegalArgumentException("Novalues!");doubleresult=Double.MIN_VALUE;for(doubleval:vals)if(val>result)result=val;returnresult;}publicstaticvoidmain(String[]arguments){System.out.println(max(-1,0,-2.718281828));}}

Integer.MIN_VALUEismostnega+veint.Double.MIN_VALUEisthesmallestposi+vedouble.

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Youcouldfixitlikethis...

publicclassMax{publicstaticdoublemax(double...vals){if(vals.length==0)thrownewIllegalArgumentException("Novalues!");doubleresult=Double.NEGATIVE_INFINITY;//Mindoublevalfor(doubleval:vals)if(val>result)result=val;returnresult;}publicstaticvoidmain(String[]arguments){System.out.println(max(-1,0,-2.718281828));}}

Prints0.0

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Butthisfixismuchbe`er

publicclassMax{publicstaticdoublemax(doublefirst,double...rest){doubleresult=first;for(doubleval:rest)if(val>result)result=val;returnresult;}publicstaticvoidmain(String[]arguments){System.out.println(max(-1,0,-2.718281828));}}

Prints0.0

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•  TheleastdoublevalisDouble.NEGATIVE_INFINITY,notDouble.MIN_VALUE–  ThesameistrueofFloat

•  Ifamethodrequiresoneormorearguments,declarewith(Tfirst,T...rest)–  Thetechniquegeneralizestonormorevalues,foranyn

•  ForAPIdesigners–  Don’tviolatetheprincipleofleastastonishment–  Useconsistentnames

Themoral

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Last9me:MapReduce

•  Master–  Assigntaskstoworkers–  Pingworkerstotestforfailures

•  Mapworkers–  Mapforeachkey/valuepair–  Emitintermediatekey/valuepairs

•  Reduceworkers

–  Sortdatabyintermediatekeyandaggregatebykey

–  Reduceforeachkey

The shuffle:

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• Foreachpairofpeopleinasocialnetwork,countmutualfriends§  ForMap:key1isaperson,valueisthelistoftheirfriends§  ForReduce:key2isapairofpeople,valuesisalistof1s,foreachmutualfriendthatpairhas

MapReducetocountmutualfriendsandetc…

f1(String key1, String value): for each pair of friends

in value: EmitIntermediate(pair, 1);

f2(String key2, Iterator values): int result = 0; for each v in values: result += v; Emit(key2, result);

MapReduce: (person, friends)* à (pair of people, count of mutual friends)*

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Today:Serializabilityandtransac9ons

•  Aformaldefini9onofconsistency•  Introduc9ontotransac9ons•  Concurrencycontrolandserializability•  Distributedconcurrencycontrol(9mepermikng)

–  Two-phasecommit

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Anaside:Double-entrybookkeeping

•  Astyleofaccoun9ngwhereeveryeventconsistsoftwoseparateentries:acreditandadebit

voidtransfer(AccountfromAcct,AccounttoAcct,intval){fromAccount.debit(val);toAccount.credit(val);}staticfinalAccountBANK_LIABILITIES=…;voiddeposit(AccounttoAcct,intval){transfer(BANK_LIABILITIES,toAcct,val);}booleanwithdraw(AccountfromAcct,intval){if(fromAcct.getBalance()<val)returnfalse;transfer(fromAcct,BANK_LIABILITIES,val);returntrue;}

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Someproper9esofdouble-entrybookkeeping

•  Redundancy!•  Sumofallaccountsissta9c

–  Canbe0

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Dataconsistencyofanapplica9on

•  SupposeDisthedatabaseforsomeapplica9onandϕ isafunc9onfromdatabasestatesto{true,false}–  Wecallϕ an integrity constraint for the application if ϕ(D)is

trueifthestateDis"good" –  WesayadatabasestateDisconsistentifϕ(D)istrueforallintegrity

constraintsϕ–  WesayDisinconsistentifϕ(D)isfalseforanyintegrityconstraintϕ

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Dataconsistencyofanapplica9on

•  SupposeDisthedatabaseforsomeapplica9onandϕ isafunc9onfromdatabasestatesto{true,false}–  Wecallϕ an integrity constraint for the application if ϕ(D)is

trueifthestateDis"good" –  WesayadatabasestateDisconsistentifϕ(D)istrueforallintegrity

constraintsϕ–  WesayDisinconsistentifϕ(D)isfalseforanyintegrityconstraintϕ

•  E.g.,forabankusingdouble-entrybookkeepingonepossibleintegrityconstraintis:def IsConsistent(D): If sum(all account balances in D) == 0: Return True Else: Return False

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Database transactions

•  Atransac+onisanatomicsequenceofreadandwriteopera9ons(alongwithanycomputa9onalsteps)thattakesadatabasefromonestatetoanother–  "Atomic"~indivisible

•  Transac9onsalwaysterminatewitheither:–  Commit:completetransac9on'schangessuccessfully–  Abort:undoanypar9alworkofthetransac9on

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Database transactions

•  Atransac+onisanatomicsequenceofreadandwriteopera9ons(alongwithanycomputa9onalsteps)thattakesadatabasefromonestatetoanother–  "Atomic"~indivisible

•  Transac9onsalwaysterminatewitheither:–  Commit:completetransac9on'schangessuccessfully–  Abort:undoanypar9alworkofthetransac9on

booleanwithdraw(AccountfromAcct,intval){begin_transaction();if(fromAcct.getBalance()<val){abort_transaction();returnfalse;}transfer(fromAcct,BANK_LIABILITIES,val);commit_transaction();returntrue;}

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Afunc9onalviewoftransac9ons

•  Atransac9onT isafunc9onthattakesthedatabasefromonestateD toanotherstateT(D)

•  Inacorrectapplica9on,ifDisconsistentthenT(D)isconsistentforalltransac9onsT

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Afunc9onalviewoftransac9ons

•  Atransac9onT isafunc9onthattakesthedatabasefromonestateD toanotherstateT(D)

•  Inacorrectapplica9on,ifDisconsistentthenT(D)isconsistentforalltransac9onsT–  E.g.,inacorrectapplica9onanyserialexecu9onofmul9pletransac9ons

takesthedatabasefromoneconsistentstatetoanotherconsistentstate

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Databasetransac9onsinprac9ce

•  Theapplica9onrequestscommitorabort,butthedatabasemayarbitrarilyabortanytransac9on–  Applica9oncanrestartanabortedtransac9on

•  Transac9onACIDproper9es:–  Atomicity: Allornothing–  Consistency: Applica9on-dependentasbefore–  Isola9on: Eachtransac9onrunsasifalone–  Durability: Databasewillnotabortorundoworkof

atransac9ona3eritconfirmsthecommit

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Concurrenttransac9onsandserializability

•  Forgoodperformance,databaseinterleavesopera9onsofconcurrenttransac9ons

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Concurrenttransac9onsandserializability

•  Forgoodperformance,databaseinterleavesopera9onsofconcurrenttransac9ons

•  Problemstoavoid:–  Lostupdates

•  Anothertransac9onoverwritesyourupdate,basedonolddata–  Inconsistentretrievals

•  Readingpar9alwritesbyanothertransac9on•  Readingwritesbyanothertransac9onthatsubsequentlyaborts

•  Ascheduleoftransac9onopera9onsisserializableifitisequivalenttosomeserialorderingofthetransac9ons

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Concurrencycontrolforadatabase

•  Two-phaselocking(2PL)–  Phase1:acquirelocks–  Phase2:releaselocks

•  E.g.,–  Lockanobjectbeforereadingorwri9ngit–  Don'treleaseanylocksun9lcommitorabort

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Summary

•  Distributedsystemsareagreatsourceofcomplexity–  Abstrac9onstoreducecomplexity:

•  Protocols•  RPCandcomputa9onalframeworks•  Commonbuildingblocks