Chapter 15: Transactions

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پوس ی و © 0 rewwton & 9 vail prog execuivg hot uccesses od possibly updates various data tec. © 0 ‏لول موه و و ام‎ © Ort ‏مس‎ exevuiiog he dotcbuse wey be tewporady tacveststiect. مح لیب وا سپس اوه بو بج مسجو لجل سنا ‎bene,‏ ۲ Pero rensuivs cows, the chooges it has wade iy he dotabuse persist, euro there ore spstew ‏.وس اننا‎ ۱ ‏طسو و و وی اون لو(‎ © Pw wa eves ty deol with ۶ ‏اه یلو‎ vatour bods, suck os horde Polures ond syoteu ‏اه‎ © Cowered exevutiod oP ‏ول‎ tronartiods Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. wo ©Sbervehnts, Cork ced Cnakershe 

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2010 Properties وی ‎ond possibly updates‏ یه اب ماج مس و لس د دا وماصحصز 09 ‎of dota the database sysiew oust rou!‏ فيج جا رم وه مد © @wrty. Ciker dl ppercicgs of the trowsuctiva ore propery reAeried ia the database ‏و وه مه‎ ۲ ‏ماه و قح مین‎ fa ‏مت‎ preserves the coca of ‏بو‎ _ Beckton. Dlhougs cults tracsurtiois way exevute coorurrediy, euck ‏مه‎ ‏واه ۴اه نوی سا عم‎ coazunreuly executes ‏لصا شم‎ trocsuntica results o7ust be kiddeo Proow ober ocacurredly executed trocrarivc. © ‏بط‎ ts, Por every par oP ‏مس‎ Ton 7, toppeas io Phot ether T, Priched executod bere “P started, or “T) started exertion Pier “?) Praisked. ۲ ‏اون و و ۳( ی(‎ survesshuly, the choayes it has wade to the database persist, eved P there ore systew Pobures. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. we ©Sbervehnts, Cork ced Cnakershe 

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+ (Backopls OP and Tractor ‎rexePer $60 Proc wernt Dy aorant @:‏ ام الا ‎vee)‏ .0 0-0 :0 ,6 ‎wrte()‏ .9 (0 )سر .6 ۵+۵۵ :0 ,9 (© )سم .© ,6 وه ما لمی 6 هط مسا بط مور روموت 19 ‎ore opt rePered tothe daubase, ebro‏ لین با فا مه لاه مره بل 0 ‎© Onwstewy requrewed — he sav of @ ocd © is varhooged by ‏شحو‎ ‎OP the troceruntion. ‎Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. wo ©Sbervehnts, Cork ced Cnakershe 

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+ Oxo oP Pund TraePer (Ova) Beckton requrewed — Fbetwera eps O ond O, caker raat i sho epee hee pach ‏پم‎ Heder idk ene on ‏مسجم عجوم‎ her (he sco + Pail be bros trom t shoal be). © ecktioc mac be peured irundy by nenicny inneucioas serklly, teat is oe Hee ert © Ahowever, exeruien, wulipie ‏تنیز سا راو موه‎ ] biter. ۲ ‏سا مج نم ات ما ماج و مور رای‎ ‏وین‎ (ie., he rceber of te SOO hor theo pice), fhe updos to the ‏ای‎ by the traxmurtioa cant pero ‏سج واي موس‎ Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. wo ©Sbervehnts, Cork ced Cnakershe 

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Traswird Onde ove — the Rad ste) fee ‏وممصم‎ stave fa ier ste whe fer exert arty courted - «Pier ihe Phd stata har bee exerted. Oded — Per he devovery ha word exer ica mn oo baer proceed. bored — Pier he weneurtins hos bert red back oud he dace rested ‏اماد جا جا‎ prior 7 he stort oP ike trceuton. Tuan optocs Per i har bee تم و اما لیوا مه ۳ ‎cou be dow ool‏ راما ما مس © صمحم ‎(hil the‏ Bl Opwtted — Per sucess mxopleton. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. wor ©Sbervehnts, Cork ced Cnakershe 

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Treenivd Orie (Ovc.) Pe “ye Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ~ i 

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+ ‘koplewweciaiod oP Biowiiy wad Ourablliy a ۲ ‏لول وه موه‎ syetew ‏اوه‎ ‎the support Por ‏لجی روت‎ durcbiliy. ل لا و و سره سا مت و رای نما مج ۶ ۰ موه مه اه سا خر ور ‎odled db_potcier‏ ور ور سیب بر © dhe oe wae oo a shack copy oF he dacbose, ord ‏لاه ای سا ما مور و عمج اسر بل‎ copy ody Pier her ‏سس‎ ‏ایا با‎ och, ارو ‎copy potted ty by‏ موه همم بت ‎pent be sed, ood the shabu copy oor be dotted.‏ Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. wo ©Sbervehnts, Cork ced Cnakershe 

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+ Choplercition oP Brwty wed Durdbity (Ova.) ‘db-pointer سس old copy of database (to be deleted) new copy of database Dke shadou-dotobase schewe: db-pointer 53 old copy of database © Qssanves disks do wt Pal B OsePu Por tent echiors, but © exrewel ke Pitot Por haw chtcbares (uty?) مس © Oves wi kane cared ۲ Oil study beter schewes ta Chopter 17. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 

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+ Oowured Creviow Bl Dukpe rocsunioce ore dlewed i rus poscurreaiy fa he syetew. ‏م0‎ ore: © erewed provessor oad deh uiizaia, leudoy to better trocsurtion troup oe ieeurion rat be usiog the CPO utile rather ie ready Pro or write ty the choke © rekord were resp koe Por irexewizes: shen rerio ceed wot ura behind br oer. © Opwwrewy cours! svhewes — wechodisws tp achieve tsvhiiod; thot ts, 17 ‏او ها و و سس موه سا روت موه با ایو‎ thew Prow desiroying the ‏صلاخان ری‎ © Olean is Chapter (0, Per stain seta of porreviiess oF شوه سور سا0 لح 0 لا سواه 1 موه ۱ 

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ی + ۲ ‏امن با رتاو ناوخا سم و - سل‎ order ict hick ‏اجه جوا موی تن وا‎ © ached Por o set of ‏اس‎ ust oposite of oll etruniizes oP those ‏سس‎ © wet preserve the order in which the ‏اف ای و و‎ ‏مسر‎ BO trenton hd success Ruy cowpbtes tis ‏اوه سم رت مج‎ ‏جما جد جملا اص‎ ket stalewedt (wll be occted a te obvi) BO tnrewoioa ot Paks ty surcessPMy omopket ts exert wll have or bert ‏جما جب جات اص‎ ket stalewedt (wll be occited Ft te obvi) Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 10.00 ©Sbervehnts, Cork ced Cnakershe 

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+ Ookerhis 1 B Let Ty ‏ام‎ 160 Prow Pio B, ‏امه‎ Ty trcePer (D% of he beknee Prine 9 0 B® veri ‏ات‎ ia which T, ts Polowed by To: read(A) AzA—50 write (A) read(B) B:=B+50 write(B) read(A) temp := A *0.1 A:=A- temp write(A) read(B) B:=B+temp write(B) Ez Oeadrwr Gyetrer Ovcwptr- O* Brame, Gey ©, 2008. 

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© ط یی + + sernd schekie where Ti Polowed ‏برط‎ 10 read(A) temp :=A*0.1 A:=A-— temp write(A) read(B) B:=B+ temp write(B) read(A) A:=A-50 write(A) read(B) B:=B+50 write(B. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 16.00 

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+ ‏یی‎ O 19 Let Ty ocd 7, be be inxeantions dePed previa. Phe Policy sohede toot a serid sokedle, but itis eguivaialte Ockedve ۰ read(A) A=A-50 write(A) read(A) temp :- ‏ل‎ * 1 A A- temp write(A) read(B) B:=B+50 write(B) read(B) B:=B+temp write(B) 410 Gokeddes (, O und 8, he une + @ preserved. Ez Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 10.00 ©Sbervehnts, Cork ced Cnakershe 

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@ یی + 18 Dh Polowiey ooourred schedde does ot preserve the vue ‏خام‎ ))© +O). read(A) temp :=A*0.1 A:=A — temp write(A) read(B) write (A) read(B) B:=B+50 write (B) B:=B+ temp write(B) ka Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 16.00 ©Sbervehnts, Cork ced Cnakershe 

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Gorichabity © ‏یدوجو موه‎ rie “Dhue gerd executog of a get of ‏ره سل مرو موم‎ او و ها افو بت ها ره مه ‎(poeebly‏ ۲ يخم مجهي جما صر صم سق سترفوت مایت تن ‎sukecle. OP Perea Poros‏ ون ی ‎٩‏ © Oe kewre ‏اجه ,و نمب كحت لمج حكا عجكات ممصت‎ we osu thot ‏وه تمه موم رو وس‎ oo dota ‏لیا و‎ buPPers ke ‏ای دوه لاه )ات له و ما‎ oP ody read ord ‏ری ات رن‎ ۱ 10.0? ©Sbervehnts, Cork ced Cnakershe 

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هس سره | بو زو( و را له سای هل 7 مس ‎[oP‏ ام | ما ۳ ‎these‏ و اه لو | لو سا روا له 0 ما مه سوه سا 0 vead(Q), | =read(@). ‏اد ام‎ rew(Q), |= urte(Q). Phey coh. wrte(Q), | =rea(Q). ‏اه بو‎ wrte(Q), | = wrte(Q). ‏ممت و۳‎ 18 ‏لحب | موا سناجت د ,باعش‎ [Porces « (locied) tewopord order beter tow. 4B fod fore coweenuive ‏اه و و‎ ond they do oot ‏رت‎ rede ‎key hud bevu rizrckuuued inhe scbedvie.‏ من سوت بط من لت ‎ ‎ ‎Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 16.00 ©Sbervehnts, Cork ced Cnakershe 

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راب6 بزیین + ۲ ‏اه‎ G voc be trensPorwed toto 0 scheduie 6۵ by a sertes oP swaps ‏مومس خر‎ Retrunions, we say thot G ont Gore ooh equided. © We sw that a schedule Gis vodPlet sertatzable Pitts ‏ی‎ equivdedt 7 seridl shed Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 16.00 ©Sbervehnts, Cork ced Cnakershe 

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+ OvdMtt Osridtrbity (Ova) Bl Ocherki 0 van be inxePorned tay Oohedde O, 9 sertd sohechle where 1, Polaws Py, by eres oP swape oF ueoosP ry 032000 © DherePory Ooheckle © ts out sertakzable. read(A) write(A) write(A) read(B) write(B) read(B) write(B) Gckedue D Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 

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+ OvdMtt Osridtrbity (Ova) اه ‎fri oot cob‏ او و و ۴ 14 write(Q) Be ae unbe sup ketrtoas ta he chove sched to obtaia ther the seid ocherhie <T,, Tig >, o the gertd ‏جامد‎ < Ty, TD >. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 16.00 ©Sbervehnts, Cork ced Cnakershe 

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رسب سل + Bl Let Gord 6’ be tu aches wih he save ‏واه بو‎ Gord GO” ae Vew equivdedt Phe ‏مت سبط مر(‎ ore wet (. Por pack dota tw Q, P iroceurton 7, reads the rtd ude of Gis ‏مس ما ,۵ طساو‎ 1 met, tr sched © ‏قحم سا ام و‎ ‏سس‎ of Q. ©. Por cock data tow GP rxearton 7) executes read(Q) it ochechie G, cen hot vk wor prockerd by inneurion 7) (Poy), hea inxeurtion 4 ‏اس‎ roche (G ‏سای سا ام‎ of G trot war produced by ‏ی‎ 9. Por ‏اس‎ chia tew Q, the trout (Pony) that ‏اس وت‎ ‏جا موه( رن‎ sched G cust perPorer the Pred wrte(@) ‏اه ما من‎ & Os coo be sero, view equivdeare is dev based purely oo reads ‏ات اج‎ dow. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 موی‎ 

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+ ‏و‎ (Ovw.) © O sthedle Gis vew vertdbrabl itis view equivdedt ty 0 serial schedule. ۱ ‏ری رای الا تور‎ ts uv view sericizabe. © ‏یاه بای بت ها طورش ع امان طللاه وج تلد(‎ qe write(Q) write(Q) a write(Q) © vey view seridizable schedule thot is ot cooPict seriakzoble has bled writes. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 20 ©Sbervehnts, Cork ced Cnakershe 

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+ Ober Ovtow oP Oeridtzdbity Bl Dhe ochedde below prodces suwe pure ar the serid schedle < Dy Po, >, ves coh oobi ‏و مرهج تسه مهوت‎ read(A) A:=A-50 write(A) read(B) B:=B-10 write(B) read(B) B:=B+50 write(B) read(A) A:=A+10 write(A) B Ort oraiey suck equideure requires univets oP pperdicas ‏اه‎ ‎read ord rite. Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 ممه‎ 

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+ Testy Por Ovrickrbily © Oousider sowe schedule of a set oP ‏اس‎ 1 7, ‎Pores‏ با ‎Bl Prevebwe graph ‏و‎ drent wok where he vertoes ore the wearer (sxe). ‎B We draw wor Pow Tp 7B ke ‏اجه رات مس مس‎ 7, ‏تخس‎ fhe da few va heb the ou ‏لماعب ميمت‎ ‎۴ We coay kabel he ory by the te thot woes orvessed. 0 ‎<a ‎5 SS) ‎v ‎Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 موص‎ 

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+ @xavple Ootedue (Gvbedue ®) + Prevedewe Brak Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 وهم‎ 

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0 .تاسوه حا عامسو ‎prevedeure‏ 11 ‏ف لسن حجان جأسجانن اعقب جص اجا باس سا0‎ koe, where ois he oneber of vertices ia he soph. © (eter cherie take order 0+ e where oi the nncber of eckes.) BOP precede wroph te cts, the sertokzcbliy order oa be ‏جما خاب متحت اسح اجوز و بالات‎ ‏اي‎ ‏مس ما و و‎ wih he porta order of the raph. © Por ‏طسو و ی‎ order Por Goheddle B wokd be Ratatat ۱ @re tere vers? Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏جوصه‎ 

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مطاح دهن 5) ببد2) عروذا زوع ص تصحف 1 ‎b‏ بقام احا جد نمست عوذا دجا عوسي صاصم 21157 1 یاه تن سوا ]۱ Por view sertakrobliy hes oot expooruitd ia the size oP the prevedeuce yruph. BL Dhe problew of chevhiog Po sokedle view seridkable Pols to the ches of OP-axmpet: probes. ای سوه ها منوا مه مد خا مه عط © رش ۲ لین موی وروی اه اس ما واه امش و 18 ‎sertakzdbliy coc sill be weed.‏ Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 وهم‎ 

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QRevovervbls Ookeduss ره مه یط مج اه ای با سای وا لو( تس مت ۳ هجو مه مهم 1 ماه 11 خأ مس و عم بسح (10 )اه پا ۱ ۲ ‎senha cae bert‏ To read(A) write(A) read(A) read(B) BP Ty shod obo, 7, would we edd (ued possibly shows ty the veer) oo inoousistedt dutcbosr state. Weare, dotcbosr wut eusure thot schedules ore Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 موه‎ 

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+ Owen Rolohs Bl Ocenudos rolbuck — a sine haxeuriva Baki buds to seven of fexeurion rolbacks. Onvekder fe Polpuien schedde where ace of he resect bee pet ‏و میور بط ایا ی اس‎ read(A) read(B) write(A) read(A) write(A) B Oca ‏مه وتو و اه موی سا نا لا‎ oP words سا0 لح 0 لا سواه 1 همه ۱ 

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Ovscadeless Ovlhedules Bl Ocevadeles sheddes — cucu rolbucks came pozur; Por euch pur of ‏مسر‎ Tod Puck tet Prensa cata few previnwly writen by Tbe maxima operates oP P oppeurs before ber read operive of P. ۴ Cie ewnleben sched & dou ‏ترا رم‎ ۳ ‏تا ما تا‎ w resin he ‏ی مر سم با ورن‎ ewrnbboe ۱ 16.00 ©Sbervehnts, Cork ced Cnakershe 

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Cowurrewwy ‏اهبوهن)‎ و دوه وم و ما عمجت نب ام و ‎database wet provide‏ 0 © اه تاه رت و ی ی ۴ عولط موی لام لو مرس و ۶ او ‎yeueriies‏ دوجو و موس امن مایت و و ۵ ها مه ۴اه صرح عجوم د دسلتصصم الجا روعت ل صداوه © Ore seri schedles revovercblelrosradeless? ۲ Desay a schedule Por ‏و تلو‎ it hos executed ts o hie tov kt! © od - ‏بت‎ tevebp cowerewy ood protools thet wil assure ‏,اوه‎ Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 00 ©Sbervehnts, Cork ced Cnakershe 

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+ ‏راوس و ریق‎ eer ما یحو نها امه لوج ممجممه وله مر او تروص ول ۲ ‎ued are recoverable oad vases.‏ اوه تیالو و امد ‎raph‏ طلسم ها عم بت ول اي وم امه جوز ‎we tis been created‏ ‎© Vestecd 0 protocol koposes 0 derighor thot avoids omeercizable svhethies. © We shady suck protons to Chupter 10. ‎© OPPercot ‏ام روصم‎ protools provide dPPered iradevPPs betwerd the nt oP ‏له سطاه وروی‎ ۷ nn oP overkead thot they tamu. ‎Vests Por sertatzabtiy help ue voderstond why ‏ج اسصو اوه تمحصجوه و‎ ‏ات ‎۱ ‎ 

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+ Oak eee tt ‏ون‎ © Gowe uppicuives ore wiles to hve wih weds levels oP ‏شاه ,موی‎ schedules thot are ant sertatzable © Gx). ‏بوحصم م‎ romsoriva thot wets fo get oo opprontcrate total bokrare oP ‏أن‎ corns © Cy, chtcbwse stutoins copied Por query opikoizaioa oe be upproiocte (ary?) ‏مهو اس‎ ved ont be sertdkzuble wik respent ip ‏سوه مان‎ ‏ل ا‎ Por perPorwone Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. woe ©Sbervehnts, Cork ced Cnakershe 

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00 ۰ رین و مرا + ميك — ‎Bl Gericke‏ اه وم او ام ‎records io be‏ و را ‎Repeokbl: read‏ ۲ 0 ام ‎bat ot‏ ماه رو او لاجو جور و لت رو ۱ باصا مم ات ‎recs‏ مود بط ام ‎can be‏ و لو را لس 0 ‎(but cowed) vokes.‏ سل ما رو لو خر ام ع9 رو ور امس ‎Red woowwited — even‏ © مرو رو خی موی اه و ‎Bo bower‏ ‎about the database‏ ماو Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 66م‎ 

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اوقت مج + خم اه سا ره 3 اس ه ‎ust fochide‏ مها مومت میا ۲ ‎ircesariioc.‏ و موی ۳ موم ان ما شوه د , ب[ ©) 115 0 ‎treceutivs is QL cade by:‏ 0 19 وه سوه و لكك ‎trocsutica‏ ی اروت ‎Cnet work‏ © اه 0 متس مه موه اون اوق ۶ :اه روا لام رم ‎beveb of‏ © لو ت۱۹ لس ادوس ۶ لام لد © مص لد« ‎Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 وهم‎ 

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Gad oP Okaper 

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read(A) write(A) read(B) write(B) read(A) write(A) read(B) write(B) Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 موم‎ 

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read(A) write(A) read(A) read(B) write(A) write(B) read(B) write(B) Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 666 1 ‏سا0 لح 0 لا سواه‎ 

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write(Q) read(Q) write(Q) Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. 

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+ (a) Gokedule ( acd (b) Goleedule O Ceadrwe Gyetrer Ovcwptr- O* Brame, Gey ©, O08. ‏سا0 لح 0 لا سواه 1 موم‎ 

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۱ 

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Chapter 15: Transactions Database System Concepts, 5th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 15: Transactions  Transaction Concept  Transaction State  Concurrent Executions  Serializability  Recoverability  Implementation of Isolation  Transaction Definition in SQL  Testing for Serializability. Database System Concepts - 5th Edition, Sep 10, 2005. 15.2 ©Silberschatz, Korth and Sudarshan Transaction Concept  A transaction is a unit of program execution that accesses and possibly updates various data items.  A transaction must see a consistent database.  During transaction execution the database may be temporarily inconsistent.  When the transaction completes successfully (is committed), the database must be consistent.  After a transaction commits, the changes it has made to the database persist, even if there are system failures.  Multiple transactions can execute in parallel.  Two main issues to deal with:  Failures of various kinds, such as hardware failures and system crashes  Concurrent execution of multiple transactions Database System Concepts - 5th Edition, Sep 10, 2005. 15.3 ©Silberschatz, Korth and Sudarshan ACID Properties A transaction is a unit of program execution that accesses and possibly updates various data items.To preserve the integrity of data the database system must ensure:  Atomicity. Either all operations of the transaction are properly reflected in the database or none are.  Consistency. Execution of a transaction in isolation preserves the consistency of the database.  Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions.   That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished. Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures. Database System Concepts - 5th Edition, Sep 10, 2005. 15.4 ©Silberschatz, Korth and Sudarshan Example of Fund Transfer  Transaction to transfer $50 from account A to account B: 1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B)  Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.  Consistency requirement – the sum of A and B is unchanged by the execution of the transaction. Database System Concepts - 5th Edition, Sep 10, 2005. 15.5 ©Silberschatz, Korth and Sudarshan Example of Fund Transfer (Cont.)   Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).  Isolation can be ensured trivially by running transactions serially, that is one after the other.  However, executing multiple transactions concurrently has significant benefits, as we will see later. Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures. Database System Concepts - 5th Edition, Sep 10, 2005. 15.6 ©Silberschatz, Korth and Sudarshan Transaction State  Active – the initial state; the transaction stays in this state while it is executing  Partially committed – after the final statement has been executed.  Failed -- after the discovery that normal execution can no longer proceed.  Aborted – after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted:   restart the transaction; can be done only if no internal logical error  kill the transaction Committed – after successful completion. Database System Concepts - 5th Edition, Sep 10, 2005. 15.7 ©Silberschatz, Korth and Sudarshan Transaction State (Cont.) Database System Concepts - 5th Edition, Sep 10, 2005. 15.8 ©Silberschatz, Korth and Sudarshan Implementation of Atomicity and Durability  The recovery-management component of a database system implements the support for atomicity and durability.  The shadow-database scheme:  assume that only one transaction is active at a time.  a pointer called db_pointer always points to the current consistent copy of the database.  all updates are made on a shadow copy of the database, and db_pointer is made to point to the updated shadow copy only after the transaction reaches partial commit and all updated pages have been flushed to disk.  in case transaction fails, old consistent copy pointed to by db_pointer can be used, and the shadow copy can be deleted. Database System Concepts - 5th Edition, Sep 10, 2005. 15.9 ©Silberschatz, Korth and Sudarshan Implementation of Atomicity and Durability (Cont.) The shadow-database scheme:  Assumes disks do not fail  Useful for text editors, but extremely inefficient for large databases (why?)  Does not handle concurrent transactions Will study better schemes in Chapter 17.   Database System Concepts - 5th Edition, Sep 10, 2005. 15.10 ©Silberschatz, Korth and Sudarshan Concurrent Executions   Multiple transactions are allowed to run concurrently in the system. Advantages are:  increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the disk  reduced average response time for transactions: short transactions need not wait behind long ones. Concurrency control schemes – mechanisms to achieve isolation; that is, to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database  Will study in Chapter 16, after studying notion of correctness of concurrent executions. Database System Concepts - 5th Edition, Sep 10, 2005. 15.11 ©Silberschatz, Korth and Sudarshan Schedules  Schedule – a sequences of instructions that specify the chronological order in which instructions of concurrent transactions are executed  a schedule for a set of transactions must consist of all instructions of those transactions  must preserve the order in which the instructions appear in each individual transaction.  A transaction that successfully completes its execution will have a commit instructions as the last statement (will be omitted if it is obvious)  A transaction that fails to successfully complete its execution will have an abort instructions as the last statement (will be omitted if it is obvious) Database System Concepts - 5th Edition, Sep 10, 2005. 15.12 ©Silberschatz, Korth and Sudarshan Schedule 1  Let T transfer $50 from A to B, and T transfer 10% of the 1 2 balance from A to B.  A serial schedule in which T is followed by T : 1 2 Database System Concepts - 5th Edition, Sep 10, 2005. 15.13 ©Silberschatz, Korth and Sudarshan Schedule 2 • A serial schedule where T2 is followed by T1 Database System Concepts - 5th Edition, Sep 10, 2005. 15.14 ©Silberschatz, Korth and Sudarshan Schedule 3  Let T1 and T2 be the transactions defined previously. The following schedule is not a serial schedule, but it is equivalent to Schedule 1. In Schedules 1, 2 and 3, the sum A + B is preserved. Database System Concepts - 5th Edition, Sep 10, 2005. 15.15 ©Silberschatz, Korth and Sudarshan Schedule 4  The following concurrent schedule does not preserve the value of ( A + B). Database System Concepts - 5th Edition, Sep 10, 2005. 15.16 ©Silberschatz, Korth and Sudarshan Serializability  Basic Assumption – Each transaction preserves database consistency.  Thus serial execution of a set of transactions preserves database consistency.  A (possibly concurrent) schedule is serializable if it is equivalent to a serial schedule. Different forms of schedule equivalence give rise to the notions of: 1. conflict serializability 2. view serializability  We ignore operations other than read and write instructions, and we assume that transactions may perform arbitrary computations on data in local buffers in between reads and writes. Our simplified schedules consist of only read and write instructions. Database System Concepts - 5th Edition, Sep 10, 2005. 15.17 ©Silberschatz, Korth and Sudarshan Conflicting Instructions  Instructions li and lj of transactions Ti and Tj respectively, conflict if and only if there exists some item Q accessed by both li and lj, and at least one of these instructions wrote Q. 1. li = read(Q), lj = read(Q). li and lj don’t conflict. 2. li = read(Q), lj = write(Q). They conflict. 3. li = write(Q), lj = read(Q). They conflict 4. li = write(Q), lj = write(Q). They conflict  Intuitively, a conflict between li and lj forces a (logical) temporal order between them.  If li and lj are consecutive in a schedule and they do not conflict, their results would remain the same even if they had been interchanged in the schedule. Database System Concepts - 5th Edition, Sep 10, 2005. 15.18 ©Silberschatz, Korth and Sudarshan Conflict Serializability  If a schedule S can be transformed into a schedule S´ by a series of swaps of non-conflicting instructions, we say that S and S´ are conflict equivalent.  We say that a schedule S is conflict serializable if it is conflict equivalent to a serial schedule Database System Concepts - 5th Edition, Sep 10, 2005. 15.19 ©Silberschatz, Korth and Sudarshan Conflict Serializability (Cont.)  Schedule 3 can be transformed into Schedule 6, a serial schedule where T2 follows T1, by series of swaps of non-conflicting instructions.  Therefore Schedule 3 is conflict serializable. Schedule 6 Schedule 3 Database System Concepts - 5th Edition, Sep 10, 2005. 15.20 ©Silberschatz, Korth and Sudarshan Conflict Serializability (Cont.)  Example of a schedule that is not conflict serializable:  We are unable to swap instructions in the above schedule to obtain either the serial schedule < T3, T4 >, or the serial schedule < T4, T3 >. Database System Concepts - 5th Edition, Sep 10, 2005. 15.21 ©Silberschatz, Korth and Sudarshan View Serializability  Let S and S´ be two schedules with the same set of transactions. S and S´ are view equivalent if the following three conditions are met: 1. For each data item Q, if transaction Ti reads the initial value of Q in schedule S, then transaction Ti must, in schedule S´, also read the initial value of Q. 2. For each data item Q if transaction Ti executes read(Q) in schedule S, and that value was produced by transaction Tj (if any), then transaction Ti must in schedule S´ also read the value of Q that was produced by transaction Tj . 3. For each data item Q, the transaction (if any) that performs the final write(Q) operation in schedule S must perform the final write(Q) operation in schedule S´. As can be seen, view equivalence is also based purely on reads and writes alone. Database System Concepts - 5th Edition, Sep 10, 2005. 15.22 ©Silberschatz, Korth and Sudarshan View Serializability (Cont.)  A schedule S is view serializable it is view equivalent to a serial schedule.  Every conflict serializable schedule is also view serializable.  Below is a schedule which is view-serializable but not conflict serializable.  What serial schedule is above equivalent to?  Every view serializable schedule that is not conflict serializable has blind writes. Database System Concepts - 5th Edition, Sep 10, 2005. 15.23 ©Silberschatz, Korth and Sudarshan Other Notions of Serializability  The schedule below produces same outcome as the serial schedule < T1, T5 >, yet is not conflict equivalent or view equivalent to it.  Determining such equivalence requires analysis of operations other than read and write. Database System Concepts - 5th Edition, Sep 10, 2005. 15.24 ©Silberschatz, Korth and Sudarshan Testing for Serializability  Consider some schedule of a set of transactions T1, T2, ..., Tn  Precedence graph — a direct graph where the vertices are the transactions (names).  We draw an arc from Ti to Tj if the two transaction conflict, and Ti accessed the data item on which the conflict arose earlier.  We may label the arc by the item that was accessed.  Example 1 x y Database System Concepts - 5th Edition, Sep 10, 2005. 15.25 ©Silberschatz, Korth and Sudarshan Example Schedule (Schedule A) + Precedence Graph T1 T2 read(X) read(Y) read(Z) read(V) read(W) read(W) read(Y) write(Y) write(Z) read(U) read(Y) write(Y) read(Z) write(Z) T3 T4 T5 T 1 2 T 3 read(U) write(U) Database System Concepts - 5th Edition, Sep 10, 2005. T 15.26 T 4 ©Silberschatz, Korth and Sudarshan Test for Conflict Serializability  A schedule is conflict serializable if and only if its precedence graph is acyclic.  Cycle-detection algorithms exist which take order n2 time, where n is the number of vertices in the graph.   (Better algorithms take order n + e where e is the number of edges.) If precedence graph is acyclic, the serializability order can be obtained by a topological sorting of the graph.  This is a linear order consistent with the partial order of the graph.  For example, a serializability order for Schedule A would be T5  T1  T3  T2  T4  Are there others? Database System Concepts - 5th Edition, Sep 10, 2005. 15.27 ©Silberschatz, Korth and Sudarshan Test for View Serializability  The precedence graph test for conflict serializability cannot be used directly to test for view serializability.   The problem of checking if a schedule is view serializable falls in the class of NP-complete problems.   Extension to test for view serializability has cost exponential in the size of the precedence graph. Thus existence of an efficient algorithm is extremely unlikely. However practical algorithms that just check some sufficient conditions for view serializability can still be used. Database System Concepts - 5th Edition, Sep 10, 2005. 15.28 ©Silberschatz, Korth and Sudarshan Recoverable Schedules Need to address the effect of transaction failures on concurrently running transactions.  Recoverable schedule — if a transaction Tj reads a data item previously written by a transaction Ti , then the commit operation of Ti appears before the commit operation of Tj.  The following schedule (Schedule 11) is not recoverable if T9 commits immediately after the read  If T8 should abort, T9 would have read (and possibly shown to the user) an inconsistent database state. Hence, database must ensure that schedules are recoverable. Database System Concepts - 5th Edition, Sep 10, 2005. 15.29 ©Silberschatz, Korth and Sudarshan Cascading Rollbacks  Cascading rollback – a single transaction failure leads to a series of transaction rollbacks. Consider the following schedule where none of the transactions has yet committed (so the schedule is recoverable) If T10 fails, T11 and T12 must also be rolled back.  Can lead to the undoing of a significant amount of work Database System Concepts - 5th Edition, Sep 10, 2005. 15.30 ©Silberschatz, Korth and Sudarshan Cascadeless Schedules  Cascadeless schedules — cascading rollbacks cannot occur; for each pair of transactions Ti and Tj such that Tj reads a data item previously written by Ti, the commit operation of Ti appears before the read operation of Tj.  Every cascadeless schedule is also recoverable  It is desirable to restrict the schedules to those that are cascadeless Database System Concepts - 5th Edition, Sep 10, 2005. 15.31 ©Silberschatz, Korth and Sudarshan Concurrency Control   A database must provide a mechanism that will ensure that all possible schedules are  either conflict or view serializable, and  are recoverable and preferably cascadeless A policy in which only one transaction can execute at a time generates serial schedules, but provides a poor degree of concurrency  Are serial schedules recoverable/cascadeless?  Testing a schedule for serializability after it has executed is a little too late!  Goal – to develop concurrency control protocols that will assure serializability. Database System Concepts - 5th Edition, Sep 10, 2005. 15.32 ©Silberschatz, Korth and Sudarshan Concurrency Control vs. Serializability Tests  Concurrency-control protocols allow concurrent schedules, but ensure that the schedules are conflict/view serializable, and are recoverable and cascadeless.  Concurrency control protocols generally do not examine the precedence graph as it is being created  Instead a protocol imposes a discipline that avoids nonseralizable schedules.  We study such protocols in Chapter 16.  Different concurrency control protocols provide different tradeoffs between the amount of concurrency they allow and the amount of overhead that they incur.  Tests for serializability help us understand why a concurrency control protocol is correct. Database System Concepts - 5th Edition, Sep 10, 2005. 15.33 ©Silberschatz, Korth and Sudarshan Weak Levels of Consistency   Some applications are willing to live with weak levels of consistency, allowing schedules that are not serializable  E.g. a read-only transaction that wants to get an approximate total balance of all accounts  E.g. database statistics computed for query optimization can be approximate (why?)  Such transactions need not be serializable with respect to other transactions Tradeoff accuracy for performance Database System Concepts - 5th Edition, Sep 10, 2005. 15.34 ©Silberschatz, Korth and Sudarshan Levels of Consistency in SQL-92   Serializable — default  Repeatable read — only committed records to be read, repeated reads of same record must return same value. However, a transaction may not be serializable – it may find some records inserted by a transaction but not find others.  Read committed — only committed records can be read, but successive reads of record may return different (but committed) values.  Read uncommitted — even uncommitted records may be read. Lower degrees of consistency useful for gathering approximate information about the database Database System Concepts - 5th Edition, Sep 10, 2005. 15.35 ©Silberschatz, Korth and Sudarshan Transaction Definition in SQL  Data manipulation language must include a construct for specifying the set of actions that comprise a transaction.  In SQL, a transaction begins implicitly.  A transaction in SQL ends by:   Commit work commits current transaction and begins a new one.  Rollback work causes current transaction to abort. Levels of consistency specified by SQL-92:  Serializable — default  Repeatable read  Read committed  Read uncommitted Database System Concepts - 5th Edition, Sep 10, 2005. 15.36 ©Silberschatz, Korth and Sudarshan End of Chapter Database System Concepts, 5th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Database System Concepts - 5th Edition, Sep 10, 2005. 15.38 ©Silberschatz, Korth and Sudarshan Database System Concepts - 5th Edition, Sep 10, 2005. 15.39 ©Silberschatz, Korth and Sudarshan Schedule 7 Database System Concepts - 5th Edition, Sep 10, 2005. 15.40 ©Silberschatz, Korth and Sudarshan Precedence Graph for (a) Schedule 1 and (b) Schedule 2 Database System Concepts - 5th Edition, Sep 10, 2005. 15.41 ©Silberschatz, Korth and Sudarshan Illustration of Topological Sorting Database System Concepts - 5th Edition, Sep 10, 2005. 15.42 ©Silberschatz, Korth and Sudarshan Precedence Graph Database System Concepts - 5th Edition, Sep 10, 2005. 15.43 ©Silberschatz, Korth and Sudarshan fig. 15.21 Database System Concepts - 5th Edition, Sep 10, 2005. 15.44 ©Silberschatz, Korth and Sudarshan Implementation of Isolation  Schedules must be conflict or view serializable, and recoverable, for the sake of database consistency, and preferably cascadeless.  A policy in which only one transaction can execute at a time generates serial schedules, but provides a poor degree of concurrency.  Concurrency-control schemes tradeoff between the amount of concurrency they allow and the amount of overhead that they incur.  Some schemes allow only conflict-serializable schedules to be generated, while others allow view-serializable schedules that are not conflict-serializable. Database System Concepts - 5th Edition, Sep 10, 2005. 15.45 ©Silberschatz, Korth and Sudarshan Figure 15.6 Database System Concepts - 5th Edition, Sep 10, 2005. 15.46 ©Silberschatz, Korth and Sudarshan Figure 15.12 Database System Concepts - 5th Edition, Sep 10, 2005. 15.47 ©Silberschatz, Korth and Sudarshan