Chapter 16 : Concurrency Control

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Chopier 00 : Cowwrewy Ovdrol ‎Gystew Ovwepe Ot Cd.‏ لت ‎Steven nk wo Stern, ODS‏ © ‎oar‏ ا اك ‎ 

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+ Ohnper 06: Oounrreay Occercl Lock-Bosed Protocols ‏اس( هکس‎ Onthotce-Based Protects Outipte Grosutarty Outiversion Gchewes اك “| ‏جسم 0) ودوك () لحد نوصد‎ Coursey tt Tedex Grructures Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wo ©Sbervehnts, Cork ced Cnakershe 

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نیت سا + ۱ aovess too dota eo © ete tews can be locked to tur wards | 1 ‏ماه‎ 00 ods, Dot tooo con be bok read ae well tortion. Glock t& requested vein: bob X keirto. ©. shored (G) wards. Data te ox vay be read. Gotoh rexnesied usin: bok ® keiructes. Bo bock reqests ore wade ty ‏جاسكم" سجوددمب امدمصسرصج سحمون‎ cos proceed ‏رای‎ Per request is yrcaied. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wo ©Sbervehnts, Cork ced Cnakershe 

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+ ‏اس 6 لس‎ Protucks (Cou) مرا ۱ 5 x S | true | false X | false | false ۲ ‏ما مومس ما و لصوم با رو مش‎ the requested lock is cocwpatbe: woth locks dready held oo the tear by vier trocar © ‏انیت بو‎ oF ‏و‎ von hold shored locke vo oo tec, © bulP cop trocsuntiva holds on excheive vo he few ov viker trocsurios wo ok cay back oo the ites. BR ateck cased be yrovied, the requestiay ircczantiog food te uct tll opie locks belt by olser trassurtiows have b vet ‏.ل‎ "(۳ tots te th ‏اي‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 we ©Sbervehnts, Cork ced Cnakershe 

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+ ‏اس 6 لس‎ Protucks (Cou) ۴ ‏مرو ما دلوم‎ bachiey: 1 bok )0( read (0); ‏سید‎ (PB): boh-8(); read )0(: wcdook (DB). ‏بواج‎ )0+)©( 19 Lockey os ubove is oot ‏صا لمجاو ةنانك‎ uaruiee sertdtzabtiy — iP Pond @ yet updated ‏ام سا سل‎ oP (ocd O, the displved sue would be wrong. ۲ ۵ ‏ام پا‎ 0 set oP niles Polowed by ofl trowsariocs while requestor ord reteasten locks. Locktog protocols restrict the set oP possible schedules. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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+ PP db oP Look ‏با‎ Coxeter the port schedule lock-S(A) read(A) lock-S(B) © Oetker Tor Ty con che prowess — exer br 8((P) cower Nyt wrt Por De reese ‏جنا‎ beck oa @, white execurnt bok X(B) oases To wt ‏ما سا و( بوخ‎ ©. و ‎poe of DP, or Py. oust be rolled back‏ الم ‎Do kode o‏ و ‎vad tts locks released.‏ ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wo ©Sbervehnts, Cork ced Cnakershe 

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+ @ Pb oP book-Bwed Proivvols (Ova.) 18 Vhe poteutdl Por deadock exists to ‏اوم‎ ocho protocols. Deudechs ore o ‏تاره وی‎ او بلط عا موی ای وج ۶ ‎is dbp possible‏ و68 ۲ ‎(Por excnopte:‏ © O trocsuntivs way be waite Por oo Xoo oo oc tec, whie o secqueuse oP vier trocsanizes request oad are qroded oa Ook va the secre tec. © Dhe save treeourivs i repediedy rolled buck due ty deadoohs. © Cowrrewy ordi! woeger con be desigoed ty preved storveoticn. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wor ©Sbervehnts, Cork ced Cnakershe 

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لصاح <]) مارا نون ‎The‏ + Phe ts 9 proton whick eusures ‏ارو یبای‎ © Phe 0: Growiay Prose © trecsuniva way obtaia lochs Bl Dhe ‏.تهات امد اميد یج اس‎ con be proved tho ‏مت بط‎ mt ber ‏لبود‎ ia he order of thet bok potas (he. the potst where ‏مس و‎ canted te Fe loch). Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wo ©Sbervehnts, Cork ced Cnakershe 

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+ Ve Two-Phase bockiag Proovol (Ova) ۲ ‏مت و ناسا ام‎ eosure Preedow Prow deadlocks: هس بط له ۳ با سوم ی ام تالا مین ۲ د جما ضحد موصو ب مونلا" ‎beckon.‏ سسا ميو تجوت ‎oxded‏ ۳ و سبط با و کوب BO Rkprow two~phose boohiey is even stricter! here afllochs ore held a ‏ول موم‎ Te this protocol iracsucioas coc be sertdtzed to the order fet whirl they oval. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wo ©Sbervehnts, Cork ced Cnakershe 

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+ Ve Tu02-Pkwse bockiay Prowvol (Ova) © here con be coin setdizable schedules thot coca be obtoced P two-phase locker te used, BE Abowever, nike coeur of extra tPorwatin (es. ‏ص عه اد جلمد‎ cht), ‏ارم‎ beh & weeded ‏و تراسا مت نی و‎ he Pub see Cres 3 roms 7, tral does ot Polo two-phase lookin, we oo Pre ‏تور‎ hot wes twe-phuse lachiog, ued ‏ساره و‎ Por, aed frat wet vu bot pevniksab, Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 16.00 ©Sbervehnts, Cork ced Cnakershe 

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‎Cowersow‏ وم ‏مسجت جات مارا موی بت = ‎© con emir obo ‏مان‎ ‎© ‏ام الاخاطصط د شوم صن‎ © more able to a boleX (urate) ‎= Grooad Phe: ‏اه اه من ۰ ‏تساه ما مت © ‎© corer ab X walk (dour) ‎ie protec ‏مج‎ sertdtzobiliy. But stl retes oo the ‏بو ما موم‎ the various: lookiey icstruntiocs. ‎ ‎4 ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موه‎ 

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انا ان مراب مق + ۱ ۵ ‏جامد جيب لوحت 1 ما‎ ketruton, wihout expt ‏مایا‎ ‎ook, ‎۳ yeas ‏الس ا ا‎ BD bee obck oo D Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 هوه‎ 

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+ Dias Boqasion oF backs (od) 19 wrte(D) © provessed os: PD bea bokX oa O tea werte(D) vbr boda P cevessery uit vail oe ober trace. kar xy book oa O, PD as a bk os D fea ware lock oa D ۰ X ‏سر‎ ‎۱ ‎wwrte(D) ‎eed; ‎Bl Ollocks oe rebwerd Per crm or hort Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 16.00 ©Sbervehnts, Cork ced Cnakershe 

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رت اصورا اه وعتهزء عم عام ]|41 ار و و وب اما و ‎ook canner‏ 1 ‎cert beh seater erent‏ یسیو 9 Dhe book ‏اه وا اس مد‎ request by sear a book or =p (oro epee chin he reeearion ty rol back, rouse oP a decrdoch) اوه هط او لت ات و پم ‎Db‏ 8 ‎a lols fable to record gracied‏ ای مود و وم ما با 0 ‎locks ood prodey requests‏ BD he tock tobe & wndly kopleweoied os oo kewewory hook toble todexed vo he ‏اما با ما بل با اه سوم‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 10.00 ©Sbervehnts, Cork ced Cnakershe 

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۱ ‏یلصم سم ان‎ 0 BE ‏را‎ table ube records tee type oP looks ‏اه‎ ‎or requested Bh Dew request ty udded ty he word oP he queue oP rexnent> Por he dats teen, ood ‏وخ لهج‎ ‏مامتا اج ی این زمرت با‎ ۱ debted, cod hier requests are cheched tp see P they cos ow be grcnted BP rex@urtva boris, oll wrattery or graced ‏تاسسوم‎ oP ‏ای سس مات تس‎ ۱ held by eack expert, ip kopkwet they Peed Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 هوه‎ 

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او + ۳ ‏ام او ما‎ et © ‏رس یولع‎ of} oF oll dota tooo. © AP > of how ey ‏امسوم‎ wevevekn bok duel dwt woveny beta essary © Aeoples that he set O cry wow be viewed ‏جم‎ a drevted anicke graph, caked oe chet cp BL ‏اما اون سوه بط‎ oP ‏رو‎ proto. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 06.00 ©Sbervehnts, Cork ced Cnakershe 

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۲ ‏اس و اما ماه رن‎ Bl Dhe Prot bok by Pc be oa ow ch . Cabsecnenty, 0 chia Q rf locked by Pod B ke pared of Gis mreciy locked ‏و‎ 17 © Oot tews woy be unioched of oop eve. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 جووه‎ 

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+ @rapk-Bwed Procol (Orc) Bb. tee prod ewswres oodPint seridizoblliy os well os Preedow Prow deudock. BE Oddeckie, ay poor rater fa he treetockin protocol thor he tpl ‏ام مایا‎ © shorter writes fkver, oad Rreae kt ey ‏ی ماما ره اس اس‎ required 0 ‏روارک جر ی بت سر اس‎ or cusvute Breede ١ ‏المصصت «سلحماها صا لج(‎ depeudeures to eerure recovery ۶ ‏و رون مه‎ to look dota tiews that hey do oot ‏یت‎ ‏رهم ای امه امه ماما لو‎ Reve ۱9 ‏بصصعصجمم ما و‎ ۱ Gchedkies wt poosble under two-phese backiny ore possible wander tree proc, ood vice versa. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 هوه‎ 

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‘Picpesiawp(Dosed (Procol Bl Gk erection & bed 0 keeoknop wheal euler he syste. ‏ومتسخصمة لامج خا"‎ 1١ kee hoes TO(), ace innearton 7۱ ۰ ‏موی له‎ NE(T) suck trot DO(P) <PE(T). 0h peer seers seeruneg mae re oink Ben be recep eee a serndiublay ‏بساحن‎ ۳ ‏اوه وچ و و و‎ behav, be pried cerca Bor park ‏مه مدا‎ inkes! ۶ ‏خر رس مها بط تا( )سین‎ oxy ‏نكا مسمس‎ exerted wore(@) © (Ci) he kent tex-oknoy oP oxy trexeuntia frat exerted rerd(C2) ۳ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 06.00 ©Sbervehnts, Cork ced Cnakershe 

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+ Vrowskrop-Dosed Prowools (Ovw.) Bh Dhe tkoeskop onderkny procond exsures thot oxy cocci, ‏صمي حب لمحم‎ pyres ‏او سم ام و بو‎ Bl Capper o ixxewion 1 eer area) ARDEP) < O-cwestanp(Q), hea 7 weeds to read ouch oP © thot wos dred overumtien. B Aewe, he read ‏لمم رت ام‎ 17١ ‏عامجا لجا جا‎ AE PC(P)= O-Roesknop(Q), hea the read operctioa te exerted, orn (R- ikoeeknop(Q) et to be conten of R-terestry(@) ont DEP). Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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+ ‘Pramekrop-(Daset (Proeods (od) ۱ ‏مس با یت‎ 7) ioouee wrte(@). بمصطسم ط ‎1١‏ ادا © خم ساس جد ‎Racesknop(Q), fea‏ > (00) 10089 1۳ ۱ ‎th thot cher wml ever‏ تسه مرو سا اجه ‎wre weeded previa),‏ ام با بجاصصا ما لمج له مه رت ‎ewe,‏ 8 ‎choot‏ جه جمد جا مجه ‎thea te‏ | ‎inhe of Q.‏ جامجا لام جا ‎1١‏ اج لح مه رت ‎ewe,‏ 8 مسد ها (© )رمعم 0 ‎operaiog te exerted, od‏ رت ۲ ,شمان )۵ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 16.00 ©Sbervehnts, Cork ced Cnakershe 

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+ Cbathapls (Dew bP be (Bromoct © port schedule Por severd deta ews ‏ای و‎ uh: ikvestawps 0, ©, ©, 0, © سا0 لح 0 لا سواه 1 موم ©00© , 09 جنا ‎rare,‏ "© - بسي مسي ساي 

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+ Correvicess oP Mevesiucmp-Orderiey Crotscel 0 ‏تطلس صصص دا‎ proiool quersoiees serttzobiiiy store oll he orcs ia he prevedeure yrupk ure oF the Porc? wits saree ‏اس‎ Dhue, there ull be odes in he prevedeue yropk BP kvesknrp ‏مه ام‎ Preedow Prow deudock ‏.كسد و متا مت و‎ But the ‏تا موی سا من روت لاه‎ ced way oot eve be revovercble, Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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را را + © Croblew wit ‏اس روم‎ © Guppose TP cboris, bu Thos read a dota ew writes by TP ۶ ۳ 1 ‏سه‎ ober) PDP hod bees olowed tp coset corer, he schedule ‏ع‎ wet reo ۰ ee ei efor ei © This cod lead to cascada rolback --- thot is, a choc of rolbacks: ۰ مسبت ۲ ۱ suck thot is writes ore oll perPorwed of the ead oF ts processiny © @luories oP o tronsuntios Por ‏مات و ود‎ op ‏وه رو متس‎ hie o trocerontiog ts betes usrities © 0 treceuntivn tot uborts is restated wih ‏مت وه و‎ © Gokiica ©: Licted Porn of leche uvat Por dota te be ooercvited bePore recor © Goktod 9: Ose cow depeudeuwies ty usw recovery Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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+ Vows’ Drie Rie © QodPed versiva of the teestecop-orderteny prized ta which obsvlete uate ‏مومت اه ی لصو با بو اه‎ ۵ ‏ما‎ 1 otewps burt hts te Q, FDEP) < O-tereokaep(Q), hea Ti ‏وا واه‎ Warts on obookte vor of {GQ}. © Roker foc roti bark Pc he tkoeskney orderien proton wud hve chew, this {wrt} operation von be kore. ۲ Otherwise this protocol ie the ‏سود‎ or ‏بط مه‎ 1 Dhoune! rte Que ‏سم لورت واه‎ © ‏اوه وه نم موه رای مور وه وا‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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+ Odkktoe-Orewd Protoel B Gxenadics of trexeuntion Dis door to three proses. 0. Red cad period phee: Prosustoa T writes oly to tewwporary lod variables: ©. Orders phase: Trocuntios TP perPorns a volrdaticg test" to detente Pood votobles ca be writes uvthout vicki, ‏.بهاحامد ام جد‎ 9. Orte prose: 1۳ 1 ۰ ‏ول سا ,الا‎ are upphed to the database; vherwise, T, te roled back. “he tree phoses of ‏شمسا بلج رسمه‎ cot be ‏فا راما‎ puck ‏اما مب امه وتو‎ the three phases is thot order. © seve Por skopiniy thot the voidaiog cod wrt phase pour toyeher, iowicdly ocd serial * Le, caly ove trowsuntiva exevuies vokdaicad warts of a fie. © ‏وت اوه هجوت تون جهن لایس تا‎ trocsuniivg executes Puy ic the ‏ما تخس سب جيب لس له فا سا‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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+ Orkkior-Bwed Protval (Ov.) ۱ ‏و سس ی‎ keeoknops © ‏له 7 مات بط : ( )نم8‎ execution ‏سم ملس ات ۱ ایب با ناسون و‎ © Prish(D) 2 he teoe when P, Protshed te ware phrase © Gertazobliy order deterwiced by tkoestrep ‏مرف‎ ot valde toe, to free AEE. © Dhue DET) & que he che of Oukkaoa(T). Die protwonl is wseP ul ond ques ‏موجه اه ول یی‎ iP probability oF oP hits ts bow. © beoner the sertatzobiliy order ts ot pre-devided, ord © rebel Pew trowrartoes wil hove to be voted back. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 جوم‎ 

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+ Odkdatva Tesi Por ‏موم‎ 1“ BP Por ‏اس‎ TG (1) < DG (7) ether cae oP the Polo wer carditis hobs: ۶ ۳)( > staa(?) © ‏)نسي‎ 7) < Prtsk(T) < vekdeaoa( 7) ‏با سمب صصح صمل خات اعد جا جه‎ 1١ ‏السصحانا اجن سحل‎ wih he set ‏ممصم صمح مكل خإه‎ by 10 thea ‏لحي طلسصصصت طايه‎ 11١ ou be exewited. Otkeritoe, vation Paks ond De domed. ‎Biker he Prot coadioa & soishied, cdl here te a7 overkpped‏ بسو كاوس ل ا ‎or tee erred erantics re same orl‏ رجو موسر ‎Ske artes of Ps do cot PP ent reads oD, skoe they ooo Per, keer‏ ‎Frteches rhs‏ ‎artes of Pde ot PH‏ بط ۳ ‎rears PP, oko 1١ 4 ۱ ‏با مرن‎ T,. ‎read coy # ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏موم‎ ‎ 

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+ Ocho Proceed by ‏مت‎ لا ‎schedule produced usta‏ و لا read((P) @:= 0-80 read() @:= @t8O (vakckate) ‏طاسب‎ ))0( ‏طاسب‎ ))2( موه reud() reud(P) (vrakckat) ‏بوابط:‎ (P+) Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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QOultple Granieriy © low dota tews to be oP vorivus sizes ood dePice 0 hierarchy of dot yrendonies, where the sual groadandies ore vesied wikia hrger vor © Ou be represeuted yruphicdly os 0 tree (but doo't ovoPuse uth treelochiery prviocel) © Okeo ‏ما موی و‎ 9 ade fa the tree expt, it ‏جع اه سا لصا‎ desveadecis to he sane wode. BE ‏ما سارت الط ماخ روم‎ tr doce): © ‏دما ولو‎ trer): high cowry, high beckon overhead © ‏بحصي محص‎ (hiker fa tree): bru beckiey overhead, bu cowry Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 همه‎ 

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‘he levels, starry Brow tee coarsest (jp) evel oe © ‏سس‎ مهو © ord ۰ © record Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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+ Nhaecdoa Look (Dordow Ao ockhicg to G ood Clock wrdes, there ure three uddtizodl look wodes uit: نومب سای ۱ rely wk shared books. © elector excketie (D0): ‏وله‎ expla locker, ot a buver level wil: exch or shared booker ۶ shored vad ‏اج له ۲ :)راومه‎ by trot corde ‏جا‎ ‎bobe ‏تناو‎ ty shored wode od exrlia okie te bet doce oo kav level wth mackie ‏اما‎ BB iotewtiog locks dow o higher fevel cede ty be locked to (8 or XC wode without ‏لت الم اه اه نا مش‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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ح:() جامورا مس بط ولس وا له ی راهم ۱ ۲ + Cowpabliy Detrix wil: GM] x 8 ه66 2 16 ™ | v © | ox] y Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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+ 10 ‏مرف‎ Graxaderiy Lookiay Goberaw ۲ ‏مش‎ 7, oa bok o ade Q, ‏سا رم‎ Pokorny ries! ١ ۳ book ‏لاه سا سم وت موی‎ 21١1ج ‏إن اص‎ he tree wast be booked First, cad coay be locked ‏السب نويج ذا‎ © cede Q oon be backed by Di G or WG corde voy Phe porect of Qi raed ‏روا الما‎ Pia ether DC or WS corde. © @ cere Qo be booked by Tir X, GK, or DC erode only Fhe paral ‏خم‎ © ‏لا رفسمه چا‎ by Ti ether Dor GDC corde. | way vexk (hat 6, te Aeon 1 ‏ای مه‎ a arde Godly Boor oF the chided of Gare curred booked by, ۲ Observe that boke ‏لجمشوجه صنت‎ ta reotied order, whereas hey ore released fr ferro order. Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 woe ©Sbervehnts, Cork ced Cnakershe 

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جوم !0) موم رط() ۲ ‏یی و وا ما ول خن عم لاه ما نی مش(‎ euDy. © Outversiva Tivestrop Ordericr © Ouhversion ‏اما سا کمن‎ ۲ Guth sucpesshl wate resuls ‏مش و‎ oP a cew versiva oP the dats tec ries. © Ose ‏موس‎ to bel versivos. Bl Okew oread(Q) opercion is peued, select oc oppropriie versiva of GQ based ‏مسج بط از سس مت تباب‎ ord peta dhe ‏وج‎ of be ord ee Bread cover hove ty wail os oo appropri versiva ‏باس لا‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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و ۰1۱ دی مر (1) + Bl Cock chia tow GQ ker a senewe of versixe > ‏بي‎ ‎versio Q), coxtctcs tree data Picks! © Content -- the ‏اه لس‎ version GQ). OWA He tee 112 ‏ا ا لس سا‎ a, ام ره ند مس هه رم وا - ( مسج ۰ 0 میس امه مسب 6 0 6۶ مس و و 1 موم و مرن ۴ 0 ۳ ۱ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موم‎ 

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ما سین + ۲ Covetder the ‏تعسو مس شا‎ 4: ‏نی وم عد‎ wre) rie) ‏)سمي‎ ‎BO Gchedhie wih deodock Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wero 

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+ Oore Oradook Prevevirs Ordteqer 0 ‏ما جوا هچ اوه بشما‎ the suhe oP deudock preveciiod doce. جمطامميج ديه سس ورويوارد ولس 19 8 ‏صا صمت لص كسجرر عونا سد‎ release dota ew. Y/ouager froceuniives ever walt Por older voes; hey ore ‏ام ما لا‎ ‎ives bePore arquirtay ceeded date ies‏ سوه بل رون شمان ‎preewpive‏ — اوه لپ ۲ ‎© chlor reaction reuens (Porces rolback) of paneer nxearion keira of ‏مرن‎ Port. Youre tenets way und Por okey care. ‎© oy be Pewer rolbucks thao wate svhewe. ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wero ©Sbervehnts, Cork ced Cnakershe 

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© ok to word ced fo ‏موه الم‎ 0 riled back troceavtiows tr ‏تور‎ wily tte origicd teeestay. Oller traveurizes thus ‏وم سم‎ ver ‏موه لو رو نو‎ ts heace avoided. © Pivert-Bused Orhewer | © ‏مج و‎ uais Por a lock ‏لو و را‎ acount oP tere. Pier tat, the usa tives cut cod the trocsantion & role back. © he deadchs or wl possible © stop to kopkewed,, but starvaiva is possible. (iso dP Picut io detersvice good undue ‏اه موه بل اه‎ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wero ©Sbervehnts, Cork ced Cnakershe 

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+ are cad ‏مایت‎ BAP tuo-phowe ‏و وا‎ | © dete operon ay be perPorned oa Phe ienmunioa deli he hike ‏روصت بنج‎ book oo he ture to be deleted. © 0 tenecnton thot Keer «sew ipl Rio the dotobose ‏ها‎ bet ot Xcode tock oa he tare ۳ ‏موم مد لت موی‎ deer ar ‏لمجم وسو سوا مرو‎ ۱ ‎of‏ امه بنج و بو روم ما جا سا و سس ما موم و ممصم د جابوا نود لصويو ‎prolet spte of oot‏ | ‎© 9P oaiy tuple locks ore used, uzersertatzuble schedules com result the scoa ‏رو وشوو‎ 00 see ‏بو اهر راجت تن‎ be sertaized bePore ‏جما‎ ‏سا بو‎ ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wer ©Sbervehnts, Cork ced Cnakershe 

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+ ۳ ‏یفن مت هی‎ (Coa) BO ‏ح ما با وه مش با‎ reader isPoreotica tat todicates uta tuples the ‏وه جوا و صطانت ره مار‎ a tuple updates the score ‏.مادم وت‎ © Dke tPorwaivg should be locked. 1 0 ‏شاد‎ ججاوك؟ تمجابب فججك وصتدوممكما حصا لمجوجمجص صا رمصتاماصم جحلا ‎Ossprtde a dota tew wih‏ © ای ای با رام ‎forks ithe‏ لاه و سوه ما سا وت و ‎on he cd tow‏ اما تاه مه سوت طد و ‎or defer‏ مت مس ‎locke oa the chta tec de cet oobi us locker oa chic tuples.)‏ ] ‎Bl bove prowodl proves ‏ای موجه توا رو‎ ‎19 erder loch protocols provide higher coamurreury whe ‏اما بو وا رومام مور ع رهم وم‎ ‏له وه ون‎ buckets. ‎Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wre ©Sbervehnts, Cork ced Cnakershe 

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Aeadex Lockiag Prowoot © Geer retticg wost hove of leust vor tdex. Oovess too rebaiog aust be wade aly thevugh vor oF the tedices oo the retain. BO txneaioa 7 hot perPorne o bohup cust lock ol he teckex buckets thot it ‏تا و مه‎ ۳ keert a hip | ‏صما‎ reksiog wikout uzdatin of edo wr BO Dinet perPore a bok oo every keen ty Pied ol nde buckets thot cod hove ‏و له میم‎ potter io tiple f, kad existed ready, cad ‏لم سمه سوام‎ Je oc ol hese ‏باقن میاه سوه( ,ماس ی‎ ta X corde or of cee یسب اد با اوه سا نس ام با و ‎Bl Phe rides of‏ عجوت او مساو متام ما مه ۶ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 wero ©Sbervehnts, Cork ced Cnakershe 

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+ Oomnav eda aise ‏ا‎ BO Gaenople oP tadex coowrurreuy prove: § Ose orebbiag fosteud of two-phose lochioy vo the ordes oF the B*-rer, os Polos. Ouray searchlicseriva/detetica: © Crt lock the rvot unde tt shored wode. Rte lockten, ol required chided oP a orde to shored code, reteuse the boty oo the ode. ۱ to exckeive woe. Oke splitec or codesrioy requires chooges too parrot, looks the port ict ‏علس دفص اص‎ و مره و امس هامید وه ی موه امس سا ۲ ‎tree protocol‏ طظ) ع ,سم ‎Geto 10.9 Por var suck‏ Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 ‏سا0 لح 0 لا سواه 1 موه‎ 

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4 Porta Gobedue Oudsr Tw0-Phose boohiog lock-x(A) read(A) lock-S(B) read(B) write(A) unlock(A) lock-x(A) read(A) write(A) unlock(A) lock-S(A) Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 666 1 ‏سا0 لح 0 لا سواه‎ 

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(1ه)5-اءها lock-S (a2) unlock(@,) unlock(@2) lock-S (a1) lock-S (a2) lock-S (a3) lock-S (a4) lock-S (a, ) upgrade (a1) موه Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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read (B) B:=B-50 write (B) read (A) A:=A+50 write (A) display (A + B) موم ۲۵۵۵ )۸( display(A + B) Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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write(Q) read(Q) write(Q) Ceadrwe Gyetre Oncweptr- O* rare, Gey 08, O00 

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Chapter 16 : Concurrency Control Database System Concepts 5th Ed. © Silberschatz, Korth and Sudarshan, 2005 See www.db-book.com for conditions on re-use Chapter 16: Concurrency Control  Lock-Based Protocols  Timestamp-Based Protocols  Validation-Based Protocols  Multiple Granularity  Multiversion Schemes  Deadlock Handling  Insert and Delete Operations  Concurrency in Index Structures Database System Concepts - 5th Edition, Sep 12, 2005 16.2 ©Silberschatz, Korth and Sudarshan Lock-Based Protocols  A lock is a mechanism to control concurrent access to a data item  Data items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as written. X-lock is requested using lock-X instruction. 2. shared (S) mode. Data item can only be read. S-lock is requested using lock-S instruction.  Lock requests are made to concurrency-control manager. Transaction can proceed only after request is granted. Database System Concepts - 5th Edition, Sep 12, 2005 16.3 ©Silberschatz, Korth and Sudarshan Lock-Based Protocols (Cont.)  Lock-compatibility matrix  A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions  Any number of transactions can hold shared locks on an item,   but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item. If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted. Database System Concepts - 5th Edition, Sep 12, 2005 16.4 ©Silberschatz, Korth and Sudarshan Lock-Based Protocols (Cont.)  Example of a transaction performing locking: T2: lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B)  Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong.  A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. Locking protocols restrict the set of possible schedules. Database System Concepts - 5th Edition, Sep 12, 2005 16.5 ©Silberschatz, Korth and Sudarshan Pitfalls of Lock-Based Protocols  Consider the partial schedule  Neither T3 nor T4 can make progress — executing lock-S(B) causes T4 to wait for T3 to release its lock on B, while executing lock-X(A) causes T3 to wait for T4 to release its lock on A.  Such a situation is called a deadlock.  To handle a deadlock one of T3 or T4 must be rolled back and its locks released. Database System Concepts - 5th Edition, Sep 12, 2005 16.6 ©Silberschatz, Korth and Sudarshan Pitfalls of Lock-Based Protocols (Cont.)  The potential for deadlock exists in most locking protocols. Deadlocks are a necessary evil.  Starvation is also possible if concurrency control manager is badly designed. For example:   A transaction may be waiting for an X-lock on an item, while a sequence of other transactions request and are granted an S-lock on the same item.  The same transaction is repeatedly rolled back due to deadlocks. Concurrency control manager can be designed to prevent starvation. Database System Concepts - 5th Edition, Sep 12, 2005 16.7 ©Silberschatz, Korth and Sudarshan The Two-Phase Locking Protocol  This is a protocol which ensures conflict-serializable schedules.  Phase 1: Growing Phase    transaction may obtain locks  transaction may not release locks Phase 2: Shrinking Phase  transaction may release locks  transaction may not obtain locks The protocol assures serializability. It can be proved that the transactions can be serialized in the order of their lock points (i.e. the point where a transaction acquired its final lock). Database System Concepts - 5th Edition, Sep 12, 2005 16.8 ©Silberschatz, Korth and Sudarshan The Two-Phase Locking Protocol (Cont.)  Two-phase locking does not ensure freedom from deadlocks  Cascading roll-back is possible under two-phase locking. To avoid this, follow a modified protocol called strict two-phase locking. Here a transaction must hold all its exclusive locks till it commits/aborts.  Rigorous two-phase locking is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit. Database System Concepts - 5th Edition, Sep 12, 2005 16.9 ©Silberschatz, Korth and Sudarshan The Two-Phase Locking Protocol (Cont.)  There can be conflict serializable schedules that cannot be obtained if two-phase locking is used.  However, in the absence of extra information (e.g., ordering of access to data), two-phase locking is needed for conflict serializability in the following sense: Given a transaction Ti that does not follow two-phase locking, we can find a transaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is not conflict serializable. Database System Concepts - 5th Edition, Sep 12, 2005 16.10 ©Silberschatz, Korth and Sudarshan Lock Conversions  Two-phase locking with lock conversions: – First Phase:  can acquire a lock-S on item  can acquire a lock-X on item  can convert a lock-S to a lock-X (upgrade) – Second Phase:   can release a lock-S  can release a lock-X  can convert a lock-X to a lock-S (downgrade) This protocol assures serializability. But still relies on the programmer to insert the various locking instructions. Database System Concepts - 5th Edition, Sep 12, 2005 16.11 ©Silberschatz, Korth and Sudarshan Automatic Acquisition of Locks  A transaction Ti issues the standard read/write instruction, without explicit locking calls.  The operation read(D) is processed as: if Ti has a lock on D then read(D) else begin if necessary wait until no other transaction has a lock-X on D grant Ti a lock-S on D; read(D) end Database System Concepts - 5th Edition, Sep 12, 2005 16.12 ©Silberschatz, Korth and Sudarshan Automatic Acquisition of Locks (Cont.)  write(D) is processed as: if Ti has a lock-X on D then write(D) else begin if necessary wait until no other trans. has any lock on D, if Ti has a lock-S on D then upgrade lock on D to lock-X else grant Ti a lock-X on D end;  write(D) All locks are released after commit or abort Database System Concepts - 5th Edition, Sep 12, 2005 16.13 ©Silberschatz, Korth and Sudarshan Implementation of Locking  A lock manager can be implemented as a separate process to which transactions send lock and unlock requests  The lock manager replies to a lock request by sending a lock grant messages (or a message asking the transaction to roll back, in case of a deadlock)  The requesting transaction waits until its request is answered  The lock manager maintains a data-structure called a lock table to record granted locks and pending requests  The lock table is usually implemented as an in-memory hash table indexed on the name of the data item being locked Database System Concepts - 5th Edition, Sep 12, 2005 16.14 ©Silberschatz, Korth and Sudarshan Lock Table  Black rectangles indicate granted locks, white ones indicate waiting requests  Lock table also records the type of lock granted or requested  New request is added to the end of the queue of requests for the data item, and granted if it is compatible with all earlier locks  Unlock requests result in the request being deleted, and later requests are checked to see if they can now be granted  If transaction aborts, all waiting or granted requests of the transaction are deleted  Database System Concepts - 5th Edition, Sep 12, 2005 16.15 lock manager may keep a list of locks held by each transaction, to implement this efficiently ©Silberschatz, Korth and Sudarshan Graph-Based Protocols  Graph-based protocols are an alternative to two-phase locking  Impose a partial ordering  on the set D = {d1, d2 ,..., dh} of all data items.   If di  dj then any transaction accessing both di and dj must access di before accessing dj.  Implies that the set D may now be viewed as a directed acyclic graph, called a database graph. The tree-protocol is a simple kind of graph protocol. Database System Concepts - 5th Edition, Sep 12, 2005 16.16 ©Silberschatz, Korth and Sudarshan Tree Protocol  Only exclusive locks are allowed.  The first lock by Ti may be on any data item. Subsequently, a data Q can be locked by Ti only if the parent of Q is currently locked by Ti.  Data items may be unlocked at any time. Database System Concepts - 5th Edition, Sep 12, 2005 16.17 ©Silberschatz, Korth and Sudarshan Graph-Based Protocols (Cont.)  The tree protocol ensures conflict serializability as well as freedom from deadlock.  Unlocking may occur earlier in the tree-locking protocol than in the two-phase locking protocol.   shorter waiting times, and increase in concurrency  protocol is deadlock-free, no rollbacks are required Drawbacks  Protocol does not guarantee recoverability or cascade freedom    Need to introduce commit dependencies to ensure recoverability Transactions may have to lock data items that they do not access.  increased locking overhead, and additional waiting time  potential decrease in concurrency Schedules not possible under two-phase locking are possible under tree protocol, and vice versa. Database System Concepts - 5th Edition, Sep 12, 2005 16.18 ©Silberschatz, Korth and Sudarshan Timestamp-Based Protocols  Each transaction is issued a timestamp when it enters the system. If an old transaction Ti has time-stamp TS(Ti), a new transaction Tj is assigned time-stamp TS(Tj) such that TS(Ti) <TS(Tj).  The protocol manages concurrent execution such that the time-stamps determine the serializability order.  In order to assure such behavior, the protocol maintains for each data Q two timestamp values:  W-timestamp(Q) is the largest time-stamp of any transaction that executed write(Q) successfully.  R-timestamp(Q) is the largest time-stamp of any transaction that executed read(Q) successfully. Database System Concepts - 5th Edition, Sep 12, 2005 16.19 ©Silberschatz, Korth and Sudarshan Timestamp-Based Protocols (Cont.)  The timestamp ordering protocol ensures that any conflicting read and write operations are executed in timestamp order.  Suppose a transaction Ti issues a read(Q) 1. If TS(Ti)  W-timestamp(Q), then Ti needs to read a value of Q that was already overwritten.  2. Hence, the read operation is rejected, and Ti is rolled back. If TS(Ti) W-timestamp(Q), then the read operation is executed, and Rtimestamp(Q) is set to the maximum of R-timestamp( Q) and TS(Ti). Database System Concepts - 5th Edition, Sep 12, 2005 16.20 ©Silberschatz, Korth and Sudarshan Timestamp-Based Protocols (Cont.)  Suppose that transaction Ti issues write(Q). 1. If TS(Ti) < R-timestamp(Q), then the value of Q that Ti is producing was needed previously, and the system assumed that that value would never be produced.  2. If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q.  3. Hence, the write operation is rejected, and Ti is rolled back. Hence, this write operation is rejected, and Ti is rolled back. Otherwise, the write operation is executed, and W-timestamp(Q) is set to TS(Ti). Database System Concepts - 5th Edition, Sep 12, 2005 16.21 ©Silberschatz, Korth and Sudarshan Example Use of the Protocol A partial schedule for several data items for transactions with timestamps 1, 2, 3, 4, 5 T1 T2 read(Y) read(Y) read(X) T3 write(Y) write(Z) read(X) abort Database System Concepts - 5th Edition, Sep 12, 2005 write(Z) abort 16.22 T4 T5 read(X) read(Z) write(Y) write(Z) ©Silberschatz, Korth and Sudarshan Correctness of Timestamp-Ordering Protocol  The timestamp-ordering protocol guarantees serializability since all the arcs in the precedence graph are of the form: transaction with smaller timestamp transaction with larger timestamp Thus, there will be no cycles in the precedence graph  Timestamp protocol ensures freedom from deadlock as no transaction ever waits.  But the schedule may not be cascade-free, and may not even be recoverable. Database System Concepts - 5th Edition, Sep 12, 2005 16.23 ©Silberschatz, Korth and Sudarshan Recoverability and Cascade Freedom   Problem with timestamp-ordering protocol:  Suppose Ti aborts, but Tj has read a data item written by Ti  Then Tj must abort; if Tj had been allowed to commit earlier, the schedule is not recoverable.  Further, any transaction that has read a data item written by Tj must abort  This can lead to cascading rollback --- that is, a chain of rollbacks Solution 1:  A transaction is structured such that its writes are all performed at the end of its processing  All writes of a transaction form an atomic action; no transaction may execute while a transaction is being written  A transaction that aborts is restarted with a new timestamp  Solution 2: Limited form of locking: wait for data to be committed before reading it  Solution 3: Use commit dependencies to ensure recoverability Database System Concepts - 5th Edition, Sep 12, 2005 16.24 ©Silberschatz, Korth and Sudarshan Thomas’ Write Rule  Modified version of the timestamp-ordering protocol in which obsolete write operations may be ignored under certain circumstances.  When Ti attempts to write data item Q, if TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of {Q}.  Rather than rolling back Ti as the timestamp ordering protocol would have done, this {write} operation can be ignored.  Otherwise this protocol is the same as the timestamp ordering protocol.  Thomas' Write Rule allows greater potential concurrency.  Allows some view-serializable schedules that are not conflict-serializable. Database System Concepts - 5th Edition, Sep 12, 2005 16.25 ©Silberschatz, Korth and Sudarshan Validation-Based Protocol  Execution of transaction Ti is done in three phases. 1. Read and execution phase: Transaction Ti writes only to temporary local variables 2. Validation phase: Transaction Ti performs a ``validation test'' to determine if local variables can be written without violating serializability. 3. Write phase: If Ti is validated, the updates are applied to the database; otherwise, Ti is rolled back.  The three phases of concurrently executing transactions can be each transaction must go through the three phases in that order.  Assume for simplicity that the validation and write phase occur together, atomically and serially   interleaved, but I.e., only one transaction executes validation/write at a time. Also called as optimistic concurrency control since transaction executes fully in the hope that all will go well during validation Database System Concepts - 5th Edition, Sep 12, 2005 16.26 ©Silberschatz, Korth and Sudarshan Validation-Based Protocol (Cont.)   Each transaction Ti has 3 timestamps  Start(Ti) : the time when Ti started its execution  Validation(Ti): the time when Ti entered its validation phase  Finish(Ti) : the time when Ti finished its write phase Serializability order is determined by timestamp given at validation time, to increase concurrency.   Thus TS(Ti) is given the value of Validation(Ti). This protocol is useful and gives greater degree of concurrency if probability of conflicts is low.  because the serializability order is not pre-decided, and  relatively few transactions will have to be rolled back. Database System Concepts - 5th Edition, Sep 12, 2005 16.27 ©Silberschatz, Korth and Sudarshan Validation Test for Transaction Tj  If for all Ti with TS (Ti) < TS (Tj) either one of the following condition holds:  finish(Ti) < start(Tj)  start(Tj) < finish(Ti) < validation(Tj) and the set of data items written by Ti does not intersect with the set of data items read by Tj. then validation succeeds and Tj can be committed. Otherwise, validation fails and Tj is aborted.  Justification: Either the first condition is satisfied, and there is no overlapped execution, or the second condition is satisfied and  the writes of Tj do not affect reads of Ti since they occur after Ti has finished its reads.  the writes of Ti do not affect reads of Tj since Tj does not read any item written by Ti. Database System Concepts - 5th Edition, Sep 12, 2005 16.28 ©Silberschatz, Korth and Sudarshan Schedule Produced by Validation  Example of schedule produced using validation T14 read(B) read(A) (validate) display (A+B) Database System Concepts - 5th Edition, Sep 12, 2005 T15 read(B) B:= B-50 read(A) A:= A+50 (validate) write (B) write (A) 16.29 ©Silberschatz, Korth and Sudarshan Multiple Granularity  Allow data items to be of various sizes and define a hierarchy of data granularities, where the small granularities are nested within larger ones  Can be represented graphically as a tree (but don't confuse with tree-locking protocol)  When a transaction locks a node in the tree explicitly, it implicitly locks all the node's descendents in the same mode.  Granularity of locking (level in tree where locking is done):  fine granularity (lower in tree): high concurrency, high locking overhead  coarse granularity (higher in tree): low locking overhead, low concurrency Database System Concepts - 5th Edition, Sep 12, 2005 16.30 ©Silberschatz, Korth and Sudarshan Example of Granularity Hierarchy The levels, starting from the coarsest (top) level are     database area file record Database System Concepts - 5th Edition, Sep 12, 2005 16.31 ©Silberschatz, Korth and Sudarshan Intention Lock Modes   In addition to S and X lock modes, there are three additional lock modes with multiple granularity:  intention-shared (IS): indicates explicit locking at a lower level of the tree but only with shared locks.  intention-exclusive (IX): indicates explicit locking at a lower level with exclusive or shared locks  shared and intention-exclusive (SIX): the subtree rooted by that node is locked explicitly in shared mode and explicit locking is being done at a lower level with exclusive-mode locks. intention locks allow a higher level node to be locked in S or X mode without having to check all descendent nodes. Database System Concepts - 5th Edition, Sep 12, 2005 16.32 ©Silberschatz, Korth and Sudarshan Compatibility Matrix with Intention Lock Modes  The compatibility matrix for all lock modes is: IS IX S S IX IS      IX      S      S IX      X      Database System Concepts - 5th Edition, Sep 12, 2005 16.33 X ©Silberschatz, Korth and Sudarshan Multiple Granularity Locking Scheme   Transaction Ti can lock a node Q, using the following rules: 1. The lock compatibility matrix must be observed. 2. The root of the tree must be locked first, and may be locked in any mode. 3. A node Q can be locked by Ti in S or IS mode only if the parent of Q is currently locked by Ti in either IX or IS mode. 4. A node Q can be locked by Ti in X, SIX, or IX mode only if the parent of Q is currently locked by Ti in either IX or SIX mode. 5. Ti can lock a node only if it has not previously unlocked any node (that is, Ti is two-phase). 6. Ti can unlock a node Q only if none of the children of Q are currently locked by Ti. Observe that locks are acquired in root-to-leaf order, whereas they are released in leaf-to-root order. Database System Concepts - 5th Edition, Sep 12, 2005 16.34 ©Silberschatz, Korth and Sudarshan Multiversion Schemes  Multiversion schemes keep old versions of data item to increase concurrency.  Multiversion Timestamp Ordering  Multiversion Two-Phase Locking  Each successful write results in the creation of a new version of the data item written.  Use timestamps to label versions.  When a read(Q) operation is issued, select an appropriate version of Q based on the timestamp of the transaction, and return the value of the selected version.  reads never have to wait as an appropriate version is returned immediately. Database System Concepts - 5th Edition, Sep 12, 2005 16.35 ©Silberschatz, Korth and Sudarshan Multiversion Timestamp Ordering  Each data item Q has a sequence of versions <Q1, Q2,...., Qm>. Each version Qk contains three data fields:  Content -- the value of version Qk.  W-timestamp(Qk) -- timestamp of the transaction that created (wrote) version Qk  R-timestamp(Qk) -- largest timestamp of a transaction that successfully read version Qk  when a transaction Ti creates a new version Qk of Q, Qk's W-timestamp and R-timestamp are initialized to TS(Ti).  R-timestamp of Qk is updated whenever a transaction Tj reads Qk, and TS(Tj) > R-timestamp(Qk). Database System Concepts - 5th Edition, Sep 12, 2005 16.36 ©Silberschatz, Korth and Sudarshan Multiversion Timestamp Ordering (Cont)    Suppose that transaction Ti issues a read(Q) or write(Q) operation. Let Qk denote the version of Q whose write timestamp is the largest write timestamp less than or equal to TS(Ti). 1. If transaction Ti issues a read(Q), then the value returned is the version Qk. 2. If transaction Ti issues a write(Q) content of 1. if TS(Ti) < R-timestamp(Qk), then transaction Ti is rolled back. 2. if TS(Ti) = W-timestamp(Qk), the contents of Qk are overwritten 3. else a new version of Q is created. Observe that  Reads always succeed  A write by Ti is rejected if some other transaction Tj that (in the serialization order defined by the timestamp values) should read Ti's write, has already read a version created by a transaction older than Ti. Protocol guarantees serializability Database System Concepts - 5th Edition, Sep 12, 2005 16.37 ©Silberschatz, Korth and Sudarshan Multiversion Two-Phase Locking  Differentiates between read-only transactions and update transactions  Update transactions acquire read and write locks, and hold all locks up to the end of the transaction. That is, update transactions follow rigorous two-phase locking.   Each successful write results in the creation of a new version of the data item written.  each version of a data item has a single timestamp whose value is obtained from a counter ts-counter that is incremented during commit processing. Read-only transactions are assigned a timestamp by reading the current value of ts-counter before they start execution; they follow the multiversion timestampordering protocol for performing reads. Database System Concepts - 5th Edition, Sep 12, 2005 16.38 ©Silberschatz, Korth and Sudarshan Multiversion Two-Phase Locking (Cont.)  When an update transaction wants to read a data item:   When it wants to write an item   it obtains a shared lock on it, and reads the latest version. it obtains X lock on; it then creates a new version of the item and sets this version's timestamp to . When update transaction Ti completes, commit processing occurs:  Ti sets timestamp on the versions it has created to ts-counter + 1  Ti increments ts-counter by 1  Read-only transactions that start after Ti increments ts-counter will see the values updated by Ti.  Read-only transactions that start before Ti increments the ts-counter will see the value before the updates by Ti.  Only serializable schedules are produced. Database System Concepts - 5th Edition, Sep 12, 2005 16.39 ©Silberschatz, Korth and Sudarshan Deadlock Handling  Consider the following two transactions: T1: write (X) write(Y)  T2: write(Y) write(X) Schedule with deadlock T1 lock-X on X write (X) T2 lock-X on Y write (X) wait for lock-X on X wait for lock-X on Y Database System Concepts - 5th Edition, Sep 12, 2005 16.40 ©Silberschatz, Korth and Sudarshan Deadlock Handling  System is deadlocked if there is a set of transactions such that every transaction in the set is waiting for another transaction in the set.  Deadlock prevention protocols ensure that the system will never enter into a deadlock state. Some prevention strategies :  Require that each transaction locks all its data items before it begins execution (predeclaration).  Impose partial ordering of all data items and require that a transaction can lock data items only in the order specified by the partial order (graph-based protocol). Database System Concepts - 5th Edition, Sep 12, 2005 16.41 ©Silberschatz, Korth and Sudarshan More Deadlock Prevention Strategies  Following schemes use transaction timestamps for the sake of deadlock prevention alone.  wait-die scheme — non-preemptive   older transaction may wait for younger one to release data item. Younger transactions never wait for older ones; they are rolled back instead.  a transaction may die several times before acquiring needed data item wound-wait scheme — preemptive  older transaction wounds (forces rollback) of younger transaction instead of waiting for it. Younger transactions may wait for older ones.  may be fewer rollbacks than wait-die scheme. Database System Concepts - 5th Edition, Sep 12, 2005 16.42 ©Silberschatz, Korth and Sudarshan Deadlock prevention (Cont.)  Both in wait-die and in wound-wait schemes, a rolled back transactions is restarted with its original timestamp. Older transactions thus have precedence over newer ones, and starvation is hence avoided.  Timeout-Based Schemes :  a transaction waits for a lock only for a specified amount of time. After that, the wait times out and the transaction is rolled back.  thus deadlocks are not possible  simple to implement; but starvation is possible. Also difficult to determine good value of the timeout interval. Database System Concepts - 5th Edition, Sep 12, 2005 16.43 ©Silberschatz, Korth and Sudarshan Deadlock Detection  Deadlocks can be described as a wait-for graph, which consists of a pair G = (V,E),  V is a set of vertices (all the transactions in the system)  E is a set of edges; each element is an ordered pair Ti Tj.  If Ti  Tj is in E, then there is a directed edge from Ti to Tj, implying that Ti is waiting for Tj to release a data item.  When Ti requests a data item currently being held by Tj, then the edge Ti Tj is inserted in the wait-for graph. This edge is removed only when Tj is no longer holding a data item needed by Ti.  The system is in a deadlock state if and only if the wait-for graph has a cycle. Must invoke a deadlock-detection algorithm periodically to look for cycles. Database System Concepts - 5th Edition, Sep 12, 2005 16.44 ©Silberschatz, Korth and Sudarshan Deadlock Detection (Cont.) Wait-for graph with a cycle Wait-for graph without a cycle Database System Concepts - 5th Edition, Sep 12, 2005 16.45 ©Silberschatz, Korth and Sudarshan Deadlock Recovery  When deadlock is detected :  Some transaction will have to rolled back (made a victim) to break deadlock. Select that transaction as victim that will incur minimum cost.  Rollback -- determine how far to roll back transaction   Total rollback: Abort the transaction and then restart it.  More effective to roll back transaction only as far as necessary to break deadlock. Starvation happens if same transaction is always chosen as victim. Include the number of rollbacks in the cost factor to avoid starvation Database System Concepts - 5th Edition, Sep 12, 2005 16.46 ©Silberschatz, Korth and Sudarshan Insert and Delete Operations   If two-phase locking is used :  A delete operation may be performed only if the transaction deleting the tuple has an exclusive lock on the tuple to be deleted.  A transaction that inserts a new tuple into the database is given an X-mode lock on the tuple Insertions and deletions can lead to the phantom phenomenon.  A transaction that scans a relation (e.g., find all accounts in Perryridge) and a transaction that inserts a tuple in the relation (e.g., insert a new account at Perryridge) may conflict in spite of not accessing any tuple in common.  If only tuple locks are used, non-serializable schedules can result: the scan transaction may not see the new account, yet may be serialized before the insert transaction. Database System Concepts - 5th Edition, Sep 12, 2005 16.47 ©Silberschatz, Korth and Sudarshan Insert and Delete Operations (Cont.)  The transaction scanning the relation is reading information that indicates what tuples the relation contains, while a transaction inserting a tuple updates the same information.   The information should be locked. One solution:  Associate a data item with the relation, to represent the information about what tuples the relation contains.  Transactions scanning the relation acquire a shared lock in the data item,  Transactions inserting or deleting a tuple acquire an exclusive lock on the data item. (Note: locks on the data item do not conflict with locks on individual tuples.)  Above protocol provides very low concurrency for insertions/deletions.  Index locking protocols provide higher concurrency while preventing the phantom phenomenon, by requiring locks on certain index buckets. Database System Concepts - 5th Edition, Sep 12, 2005 16.48 ©Silberschatz, Korth and Sudarshan Index Locking Protocol  Every relation must have at least one index. Access to a relation must be made only through one of the indices on the relation.  A transaction Ti that performs a lookup must lock all the index buckets that it accesses, in S-mode.  A transaction Ti may not insert a tuple ti into a relation r without updating all indices to r.  Ti must perform a lookup on every index to find all index buckets that could have possibly contained a pointer to tuple ti, had it existed already, and obtain locks in Xmode on all these index buckets. Ti must also obtain locks in X-mode on all index buckets that it modifies.  The rules of the two-phase locking protocol must be observed  Guarantees that phantom phenomenon won’t occur Database System Concepts - 5th Edition, Sep 12, 2005 16.49 ©Silberschatz, Korth and Sudarshan Weak Levels of Consistency   Degree-two consistency: differs from two-phase locking in that S-locks may be released at any time, and locks may be acquired at any time  X-locks must be held till end of transaction  Serializability is not guaranteed, programmer must ensure that no erroneous database state will occur] Cursor stability:  For reads, each tuple is locked, read, and lock is immediately released  X-locks are held till end of transaction  Special case of degree-two consistency Database System Concepts - 5th Edition, Sep 12, 2005 16.50 ©Silberschatz, Korth and Sudarshan Weak Levels of Consistency in SQL  SQL allows non-serializable executions  Serializable: is the default  Repeatable read: allows only committed records to be read, and repeating a read should return the same value (so read locks should be retained)  However, the phantom phenomenon need not be prevented – T1 may see some records inserted by T2, but may not see others inserted by T2  Read committed: same as degree two consistency, but most systems implement it as cursor-stability  Read uncommitted: allows even uncommitted data to be read Database System Concepts - 5th Edition, Sep 12, 2005 16.51 ©Silberschatz, Korth and Sudarshan Concurrency in Index Structures  Indices are unlike other database items in that their only job is to help in accessing data.  Index-structures are typically accessed very often, much more than other database items.  Treating index-structures like other database items leads to low concurrency. Two-phase locking on an index may result in transactions executing practically oneat-a-time.  It is acceptable to have nonserializable concurrent access to an index as long as the accuracy of the index is maintained.  In particular, the exact values read in an internal node of a B+-tree are irrelevant so long as we land up in the correct leaf node.  There are index concurrency protocols where locks on internal nodes are released early, and not in a two-phase fashion. Database System Concepts - 5th Edition, Sep 12, 2005 16.52 ©Silberschatz, Korth and Sudarshan Concurrency in Index Structures (Cont.)  Example of index concurrency protocol:  Use crabbing instead of two-phase locking on the nodes of the B +-tree, as follows. During search/insertion/deletion:   First lock the root node in shared mode.  After locking all required children of a node in shared mode, release the lock on the node.  During insertion/deletion, upgrade leaf node locks to exclusive mode.  When splitting or coalescing requires changes to a parent, lock the parent in exclusive mode. Above protocol can cause excessive deadlocks. Better protocols are available; see Section 16.9 for one such protocol, the B-link tree protocol Database System Concepts - 5th Edition, Sep 12, 2005 16.53 ©Silberschatz, Korth and Sudarshan End of Chapter Database System Concepts 5th Ed. © Silberschatz, Korth and Sudarshan, 2005 See www.db-book.com for conditions on re-use Partial Schedule Under Two-Phase Locking Database System Concepts - 5th Edition, Sep 12, 2005 16.55 ©Silberschatz, Korth and Sudarshan Incomplete Schedule With a Lock Conversion Database System Concepts - 5th Edition, Sep 12, 2005 16.56 ©Silberschatz, Korth and Sudarshan Lock Table Database System Concepts - 5th Edition, Sep 12, 2005 16.57 ©Silberschatz, Korth and Sudarshan Tree-Structured Database Graph Database System Concepts - 5th Edition, Sep 12, 2005 16.58 ©Silberschatz, Korth and Sudarshan Serializable Schedule Under the Tree Protocol Database System Concepts - 5th Edition, Sep 12, 2005 16.59 ©Silberschatz, Korth and Sudarshan Schedule 3 Database System Concepts - 5th Edition, Sep 12, 2005 16.60 ©Silberschatz, Korth and Sudarshan Schedule 4 Database System Concepts - 5th Edition, Sep 12, 2005 16.61 ©Silberschatz, Korth and Sudarshan Schedule 5, A Schedule Produced by Using Validation Database System Concepts - 5th Edition, Sep 12, 2005 16.62 ©Silberschatz, Korth and Sudarshan Granularity Hierarchy Database System Concepts - 5th Edition, Sep 12, 2005 16.63 ©Silberschatz, Korth and Sudarshan Compatibility Matrix Database System Concepts - 5th Edition, Sep 12, 2005 16.64 ©Silberschatz, Korth and Sudarshan Wait-for Graph With No Cycle Database System Concepts - 5th Edition, Sep 12, 2005 16.65 ©Silberschatz, Korth and Sudarshan Wait-for-graph With A Cycle Database System Concepts - 5th Edition, Sep 12, 2005 16.66 ©Silberschatz, Korth and Sudarshan Nonserializable Schedule with Degree-Two Consistency Database System Concepts - 5th Edition, Sep 12, 2005 16.67 ©Silberschatz, Korth and Sudarshan B+-Tree For account File with n = 3. Database System Concepts - 5th Edition, Sep 12, 2005 16.68 ©Silberschatz, Korth and Sudarshan Insertion of “Clearview” Into the B+-Tree of Figure 16.21 Database System Concepts - 5th Edition, Sep 12, 2005 16.69 ©Silberschatz, Korth and Sudarshan Lock-Compatibility Matrix Database System Concepts - 5th Edition, Sep 12, 2005 16.70 ©Silberschatz, Korth and Sudarshan