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control protocol ? I n thi s paper we assumed that the GTMcannot take an advantage of knowl edge about mi xed types of l ocal DBMSs. For exampl e, i f one of the DBMSs i s ri gorous and another one i s strongl y seri al i zabl e, then the GTMassumes that each l ocal DBMS i s strongl y seri al i zabl e; the knowl edge that one of the DBMS i s more restri cti ve (and, theref ore, the GTMcoul d be mor permi ssi ve) i s not used. Avai l abi l i ty of such knowl edge coul d possi bl y i ncrease the c l evel of gl obal transacti ons and i mprove transacti on throughput. Ful l data consi stency and seri al i zabi l i ty can onl y be achi eved i n a i mposi ng restri cti ons that many consi der severe. Thus, there i s a n of consi stency and ways of restri cti ng \standard" noti o can be stated rather than i mpossi bi l i ty resul t other opti ons f or correctness i ncl ud

1. parti ti oned noti ons of con consi stency.

2. temporal consi promi ses (

3. degrees

4

there wi l l be a cycl e i n the approxi mate wai t- f or- graph. Cl earl y, the converse i s not true; a cycl e i n the approxi mate wai t- f or- graph that i s not a real deadl ock i s cal l ed a f al se deadl ock. To r the l i kel i hood of f al se deadl ocks, the arc Ti ! Tj may be added to the approxi mate wai onl y af ter Ti has been bl ocked f or some threshol d amount of ti me. These i dea deadl ock detecti on schemes of [ 8] and [ 61] . Very l i ttl e work has been done to determi ne the perf ormance of deadl schemes. I n parti cul ar, i t wi l l be i mportant to eval uate the nu ken, and to compare detecti on schemes to si mpl e ti meou that some of the opti ons we have revi ewed are dea and strategi es where there i s no gl obal not eecti ve, then the deadl ock

8 Conclusions Mul ti databases are one Foundati on (USA) mul ti database as o has sponsore that mul

commi tted. Note that aborti ng a transacti on may i mpl y that a compensati ng transacti on needs to be schedul ed at the si tes on whi ch compensatabl e subtransacti ons have successf ul l y commi tted. On recei pt of the commi t acknowl edgment f romthe p- server, the GTMsubmi ts a commi t to the remai ni ng servers. I f i n case a subtransacti on i s aborted af ter the pi vot has commi tted (not the subtransacti on must be ei ther a retri abl e subtransacti on or a redoabl e one), i t or a redo transacti on i s executed f or i t dependi ng upon i ts type. The above protocol combi nes each of the scheme that we have di sc of gl obal transacti ons. Obvi ousl y, we assume that each redo s i s appropri atel y restri cted and m- seri al i zabi l i ty o assume that no other subtransacti on of i tems read by each retri abl e subtr abl e subtransacti ons. Recal there be no data dep ti on) gl obal or w

condi ti ons of the mul ti l evel transacti on model can be repl aced by the i sol ati on of recovery condi ti on as we have di scussed i n our transacti on model .

6.5 Combinati o n o f t he Di e re nt Appr o a che s

W e have so f ar descri bed the vari ous approaches that have been studi ed i n the l i terature atomi ci ty of gl obal transacti ons i n a mul ti database system. Each of the app meri ts and demeri ts. For exampl e, whi l e the redo techni que seems at depend upon the semanti cs of the transacti ons, i ts appl i cabi l that need to be i mposed upon the data i tems accessed though the retry and the compensate approaches the semanti cs of the appl i cati ons. Furth thei r appl i cabi l i ty i s al so One i nteresti ng cha and can thus b sol uti

system. The l ong transacti on i s broken up i nto subtransacti ons that commi t and rel ease thei r resources when compl eted. Long durati on transacti ons are used f or many sci enti c and engi neeri ng appl i cati ons [ 36] . I t i s al so shown that the l og and state i nf ormati on needed f or compensat be stored wi thi n the same appl i cati on database. The noti on of sagas i s extended i n sagas, where a subtransacti on may be f urther decomposed i nto steps that are c i deas f or usi ng semanti c atomi ci ty f or copi ng wi th l ong l i ved acti vi ti One i ssue that we have not addressed i n thi s secti on transacti on. Note that some subtransacti on may no say a subtransacti on deposi ts f unds i n an acco may have been wi thdrawn by another transa customer a penal ty or sendi ng a message not be compensatabl e, e. g. , ri n di scussed i n the l i teratu Compensati on f or mul t

two phase commit (O2PC) protocol i s i ntroduced to guarantee semanti c atomi ci ty. The protocol works as f ol l ows. When a transacti on compl etes, the GTMsends \prepare" messages to the servers at each si te, as i t i s done i n the 2PC protocol . However, unl i ke the 2PC protocol , upon recei vi ng the \prepare" message, the servers opti mi sti cal l y try to commi t thei r subtransacti ons at that poi The resul t i s reported to the GTM. I f al l subtransacti ons commi tted, then the transacti on commi tted. I f not, the transacti on i s decl ared aborted, and compensati ng trans al l the subtransacti ons that di d commi t. I n the common case where subtr the O2PC l ets si tes commi t sooner than i n the 2PC protocol , l e The O2PCprotocol was al so devel oped i ndependentl y i n [ 45] wi thout an atomi c gl obal commi t protocol i s al so s commi t protocol s do not requi re each l ocal DB mul ti - si te transacti ons and are thus attr W e have so f ar i gnored the f act that others may vi ol ate database con ti ons T11 and T12 execut I t i s possi bl e con

reservati on si nce the i ght i s al ready f ul l . Had T11 not executed T2 woul d have been abl e to procure the reservati on. Thus, the state that resul ts af ter the executi on of CT11 di ers f romthe state t woul d have resul ted had T11 not executed at al l . Thi s, as i n the current i ght reservati i s neverthel ess qui te acceptabl e. W e stress that compensati ng transacti on f or a commi tted gl obal subtra regul ar transacti on and, thus, i t must preserve database consi sten onl y consi st of an i nverse f uncti on of the ori gi nal subtr other acti ons. I n our exampl e, f or i nstance, t coul d have tri ggered another transa (re ecti ng that the i ght i s states that i f the the co

subtransacti on, si nce those reads are now i nval i d. I n other words, there are no data dependenci es between Ti2 and any other subtransacti on of Ti . Techni ques such as [ 41, 35] can be used f or checkpoi nti ng transacti on programs and tracki ng data dependenci es among subtransacti ons. Further, i t must be the case that subtransacti on Ti2 i s retriable [ 51] ; that i s, i f Ti suci ent number of ti mes (f romany database state) i t wi l l eventual l y commi si nce bef ore the subtransacti on i s retri ed the state of the l ocal DBMS executi on of other l ocal transacti ons. Thi s shoul d not resul t i n cannot be commi tted. I t must be noted that not every t f or exampl e, a subtransacti on that i s to debi retri ed, dependi ng upon the bal ance i n hand, i f a subtransacti on i s to i f i t i s retri ed a suci ent The techni que atomi ci ty o d

Another opti on of ensuri ng gl obal seri al i zabi l i ty i s to use some mechani smf or preventi ng cycl es i n the gl obal seri al i zati on graph through i ndi rect con i cts between gl obal transacti ons. Note as di scussed i n Secti on 4, executi ng gl obal transacti ons seri al l y, or usi ng one of al trui sti c l ocki ng or the commi t graph approach can be used f or thi s purpos i n [ 10, 11] uses the commi t graph approach to prevent cycl es throu i t i s assumed there that l ocal DBMSs f ol l ow the stri ct 2PL schedul es) and ri gorousness of GS i s ensured (by mai 2PL l ocki ng scheme on gl obal l ocks) to ensur W ol ski and Vei j al ai nen al so pro and i t assumes that the pa however, requi res t equi val en

of f ai l ures. I f we were to ensure the 2LSR correctness cri teri on of gl obal schedul es, then besi des ensuri ng m- seri al i zabi l i ty, we must f urther ensure that the proj ecti on of the gl obal schedul operati ons bel ongi ng to gl obal transacti ons (whi ch we ref er to as GS ) i s al so ser that to ensure m- seri al i zabi l i ty i tsel f , the GTMneeds to ensure ri gorous the rst more weaker restri cti on on the i nteracti ons between gl every ri gorous schedul e i s al so seri al i zabl e, 2LSR i were to adopt the more restri cti ve second r the gl obal schedul es are 2LSR, be that the schedul e GS i s dependi ng upon t possi bl e

transacti on T3 i s executed to redo the wri te operati ons perf ormed by the gl obal l y commi tted but l ocal l y aborted transacti on T1. I n that exampl e, si nce the l ocal DBMS consi dered T3 as a di erent transacti on than T1, the resul ti ng l ocal schedul e was not seri al i zabl e f romthe GTMvi ew Note that each of our correctness cri teri a di scussed i n Secti on 4 and 5 (that i s, gl o LSR, or 2LSR) requi res that the schedul es at the l ocal DBMSs be seri poi nt of vi ew. W e ref er to the l ocal schedul e as bei ng m-serializable [ the MDBS poi nt of vi ew. M- seri al i zabi l i ty can be de ned as De ni t i o n 6 . 1 : Let Sj be a l ocal schedul e c acti ons and redo transacti ons. Let m( S al so over the read operati ons per DBMS but are commi tted by by Ti and the wri te operat of Ti i s consi dered seri al i z Fo

I f the g

si tes, regardi ng i ssues such as error handl i ng and who control s the gl obal commi t. I f there were a si ngl e standard 2PC protocol , these probl ems woul d be avoi ded, but i t i s unl i kel y that thi s w occur. Al ready there are several competi ng \standards" (e. g. , LU6. 2 [ 15] , OSI TP [ 63] ). probl emof coordi nati ng heterogeneous commi t protocol s wi l l persi st. Some i ni coordi nati on i s reported i n [ 37] . As we argued i n Secti on 3, there may be cases where the prepareprovi ded by al l si tes. Thi s may be due to the f ol l owi ng:

1. Si tes onl y oer a Servi ce Request i nterf ace, gi vi ng control over servi ce commi tment;

2. Si tes wi sh to retai n thei r executi on or c

3. Perf ormance of 2PCi n a di stri buted s to remai n i n the prepared stat ti me and throughput m I n the rest of thi s sec bei ng used.

6. 2

Re do A

Consi der th I n thi s

3. Co mpe ns a t e . At each si te where a subtransacti on of a gl obal transacti on di d commi t, a compensati ng subtransacti on i s run to semanti cal l y undo the eects of the commi tted subtransacti on.

W e di scuss these approaches i n Secti ons 6. 2 through 6. 4. Whi l e redo and retry techni que the standard atomi ci ty of transacti ons, i n the case of compensati on a weaker noti o used, si nce i t i s possi bl e that the eects of the aborted gl obal transacti transacti ons. Thi s i mpacts the preservati on of consi stency i n i ssue i n Secti on 6. 4. Fi nal l y, each of the above tech to combi ne themi nto a si ngl e uni f ormsol

6. 1

Two Pha s e Co mmi t

I f the l ocal DBMSs support a pre consi stency i n a f ai l ure control mechani sms tocol [ 7] (or one o the executi each

A di erent noti on of correctness i s used i n [ 27] . Here transacti ons are grouped i nto di sj oi nt types. An appl i cati on admi ni strator then determi nes that transacti ons of certai n types can i nterl eaved arbi trari l y wi thout causi ng constrai nts to be vi ol ated. For exampl e, i n a be saf e f or deposi t transacti on to i nterl eave wi th other deposi ts and wi th t The concurrency control mechani smproposed i n [ 27] uses l ocal l ocks gl obal l ocks to avoi d undesi rabl e i nterl eavi ngs. The concept of compati bi l i ty i s re ned i n [ 44] and s acti ons are de ned. These l evel s are structure i ncl ude those at l ower l evel s. Furt acti ons whi ch represent p the use of comp poi nt

I n Exampl e 5. 3, we coul d say that there i s a second type of correctness cri teri a, i n addi ti on to strong correctness. I n thi s case we do not want the transf er transacti on to be i nvol ved i n a seri al i zati on cycl e. One \arti ci al " way of deal i ng wi th thi s probl emi s to decl are i temt ot al and de ne an i ntegri ty constrai nt t ot al =a + b . I f thi s constrai schedul e of Exampl e 5. 3 woul d not be strongl y correct and woul d be However, one coul d argue that de ni ng addi ti onal constrai nt be no real i ntegri ty constrai nt between accounts a and val ue of a and b not equal to t ot al , that may are speci al . I f we decl are the con audi ts. Second, i f we constrai nts bet gener

seri al i zabl e. Adi erent i dea f or enf orci ng gl obal constrai nts i s presented i n [ 5] . The cl ai mi s that gl obal constrai nts tend to be very si mpl e i n practi ce and that the GTMcan enf orce themdi rec wi thout concerni ng i tsel f wi th seri al i zabi l i ty. Asecond cl ai mi s that gl obal constr \approxi mate, " gi vi ng the GTMeven more exi bi l i ty i n enf orci ng them. To i l l ustrate, consi der a copy constrai nt between i temg 1 at si te s cati ons, especi al l y i f they run on i ndependent si tes, can tol constrai nt may be jg 1 0 g 2 j   , where  i s some every update to g 1 needs to be reproduced can keep track of a wi ndowof al l owab of the new val ues are not prop apparent when f ai l ures I n summary, i n [ sati s ed. E by th

1. I f gl obal transacti ons are not al l owed to access l ocal data, then we can drop the GDPrequi rement. (Actual l y, i f gl obal transacti ons cannot read l ocal data, then they are necessari l y GD So the requi rement i s not dropped; i t i s repl aced by a more restri cti ve one). I f assume that l ocal transacti ons cannot read gl obal data, and that the gl oba not wri te l ocal data, then the l ocal and gl obal data i s total l y deco al ways be seri al i zabl e, wi thout any requi rements on the tr

2. I f there are no gl obal /l ocal constrai nts, then th proof of thi s i s l engthy [ 48] , but the i ntui t the l ocal schedul es at each si te a (regardl ess of whethe by gl obal tran onto

De ni t i o n 5 . 3 [ 4 9] : A gl obal schedul e S i s two level serializable (2LSR) i f i t i s LSR and i ts

proj ecti on to a set of gl obal transacti ons i s seri al i zabl e 4. Gl obal l y seri al i zabl e schedul es are al ways 2LSR, but the converse i s not true. Thi s i s i l by the f ol l owi ng exampl e, whi ch al so shows that 2LSR schedul es may vi ol ate con contai n \unusual " transacti ons: Exa mpl e 5 . 2 [ 4 9 ] : Consi der an MDBS where there i s a si ngl e l o gl obal i tems, b and c at s 1 and d at s 2 . There i s one gl obal

a >0 ! b >0 d > 0 ! ( b > 0 or

Consi der the f ol l owi ng two gl obal and one l ocal tra

T1 :

i f ( a