On Serializability of Multidatabase Transactions Through Forced Local Conflict

Wright State University CORE Scholar

The Ohio Center of Excellence in Knowledge- Kno.e.sis Publications Enabled Computing (Kno.e.sis)

4-1991

On Serializability of Multidatabase Transactions through Forced Local Conflict

Dimitrios Georgakopoulos

Marek Rusinkiewicz

Amit P. Sheth Wright State University - Main Campus, [email protected]

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Repository Citation Georgakopoulos, D., Rusinkiewicz, M., & Sheth, A. P. (1991). On Serializability of Multidatabase Transactions through Forced Local Conflict. Proceedings of the Seventh International Conference on Data Engineering, 314-323. https://corescholar.libraries.wright.edu/knoesis/845

This Conference Proceeding is brought to you for free and open access by the The Ohio Center of Excellence in Knowledge-Enabled Computing (Kno.e.sis) at CORE Scholar. It has been accepted for inclusion in Kno.e.sis Publications by an authorized administrator of CORE Scholar. For more information, please contact library- [email protected].

ON SERIALIZABILITY OF MULTIDATABASE TRANSACTIONS

THROUGH FORCED LOCAL CONFLICTS

Dimitrios Georgakop oulos and Marek Rusinkiewicz Amit Sheth

Department of Computer Science Bellcore

University of Houston Ho es Lane

Houston TX Picataway NJ

Abstract trievals and to preserve the global consistency in the

presence of multidatabase up dates These ob jectives

The main diculty in enforcing global serializabilityin

are more dicult to achieve in MDBSs than in homo

amultidatabase environment lies in resolving indirect

geneous distributed database systems b ecause in ad

transitive conicts b etween multidatabase transac

dition to the problems caused by data distribution and

tions Indirect conicts intro duced by lo cal transac

replication that all distributed database systems have

tions are dicult to resolve b ecause the the b ehav

to solve transaction management in MDBSs must also

ior or even the existence of lo cal transactions is not

cop e with heterogeneity and autonomy of the partici

known to the multidatabase system Toovercome

pating LDBSs

these problems we prop ose to incorp orate additional

In a multidatabase environment the serializability

data manipulation op erations in the subtransactions of

of lo cal schedules is by itself not sucienttomain

eachmultidatabase transaction We show that if these

tain the multidatabase consistency To assure that

op erations create direct conicts b etween subtransac

global serializability is not violated lo cal schedules

tions at each participating lo cal database system indi

must b e validated bytheMDBSHowever the lo cal

rect conicts can b e resolved even if the multidatabase

serialization orders are neither rep orted by the lo cal

system is not aware of their existence Based on this

database systems nor can they b e determined bycon

approachweintro duce a multidatabase transaction

trolling the submission of the global subtransactions

management metho d that requires the lo cal database

or observing their execution order To determine the

systems to ensure only lo cal serializabilityThepro

serialization order of the global transactions at each

p osed metho d and its renements do not violate the

LDBS the MDBS must deal not only with direct con

autonomy of the lo cal database systems and guaran

icts that may exist b etween the subtransactions of

tee global serializabilityby preventing multidatabase

multidatabase transactions but also with the indirect

transactions from b eing serialized in dierentways at

conicts that may b e caused by the lo cal transactions

the participating database systems

Since the MDBS has no information ab out the exis

tence and b ehavior of the lo cal transactions deter

mining if an execution of global and lo cal transactions

Intro duction

is globally serializable is dicult

Several solutions have b een prop osed in the litera

A Multidatabase System MDBS is a facilitythat

ture to deal with this problem however most of them

supp orts global applications accessing data stored in

are not satisfactory The main problem with the ma

multiple databases It is assumed that the access to

jority of the prop osed solutions is that they do not

these databases is controlled by autonomous and p os

provide a way of assuring that the execution order of

sibly heterogeneous Local Database Systems LDBSs

global transactions whichcanbecontrolled bythe

The MDBS architecture allows local transactions and

MDBS is reected in their lo cal serialization order

global transactions to co exist Lo cal transactions are

pro duced by the LDBSs In this pap er wesolve this

submitted directly to a single LDBS while the multi

problem byintro ducing a technique which disallows

database global transactions are channeled through

schedules in which a global transaction G is executed

the MDBS interface The ob jectives of multidatabase

and committed by some LDBS b efore another transac

transaction management are to avoid inconsistentre

tion G but their lo cal serialization order is reversed

Our solution do es not violate the lo cal autonomyand Current address GTE Lab oratories Waltham MA

lo cal transactions and the indirect conicts they may is applicable to all LDBSs that guarantee lo cal serial

cause For example since the MDBS has no informa izabilityWe also discuss the class of schedules and

tion ab out the lo cal transaction T it cannot detect schedulers in which the serialization order of each

the indirect conict b etween G and G at LDBS Al transaction can b e determined bycontrolling its ex

though b oth lo cal schedules are serializable the sched ecution and commitment We show that if the partici

ule is globally nonserializable ie there is no global pating LDBSs use one of the many common schedulers

order involving G G and T that is compatible with that b elong to this class multidatabase transaction

b oth lo cal schedules management is simplied

The pap er is organized as follows In Section we

G G

identify the diculties in maintaining global serializ

ability in MDBSs and review the related work In

Section weintro duce the concept of a subtransac

tion ticket and prop ose the Optimistic Ticket Method

OTM for multidatabase transaction management

To guarantee global serializability OTM requires that

LD B S a LD B S b c

the LDBSs ensure lo cal serializabilityWe also discuss

the Implicit Ticket Method ITM a renementofthe

OTM which reduces the overhead of OTM but works

only for a sub class of the participating LDBSs Section

FinallyinSectionwe summarize our results

LDBS w ar aieG G

G G

Problems in maintain

LDBS w br bw cr c ie G T G

T G G T

ing global serializabilityand

Figure Serial execution of multidatabase transac

related work

tions may violate serializability

Many algorithms that have b een prop osed for trans

In the early work in this area the problems caused

action management in distributed systems are not di

by indirect conicts were not fully recognized In their

rectly applicable in MDBSs b ecause of the p ossibil

early pap er Gligor and Pop escuZeletin stated

ity of indirect conicts caused by the lo cal transac

thataschedule of multidatabase transactions is cor

tions To illustrate this p oint let us consider Figure

rect if multidatabase transactions have the same rel

which depicts twomultidatabase transactions G and

ative serialization order at each LDBS they directly

G and a lo cal transaction T If a transaction G

conict Breitbart and Silb erschatz haveshown

reads a data item awe drawanarcfroma to G An

that the ab ove correctness criterion is insucientto

arc from G to a denotes that G writes a In our ex

i i

guarantee global serializability in the presence of lo cal

ample the global transactions have subtransactions in

transactions They proved that the sucient condition

b oth LDBSs In LDBS G writes a and G reads it

for the global consistency requires the multidatabase

Therefore G and G directly conict in LDBS and

transactions to have the same relative serialization or

the serialization order of the transactions is G G

der in all sites they execute The solutions to the prob

In LDBS G and G access dierent data items ie

lem of concurrency control in MDBSs that havebeen

G reads b and G writes c Hence there is no di

prop osed in the literature can b e divided into several

rect conict b etween G and G in LDBS However

groups

since the lo cal transaction T writes b and and reads

c G and G conict indirectly in LDBS This in

Observing the execution of the global trans

direct conict is caused by the presence of the lo cal

actions at each LDBS The execution order

transaction T In this case the serialization order of

of global transactions do es not determine their rela

the transactions in LDBS b ecomes G T G

tive serialization order at each LDBS For example

In a multidatabase environment the MDBS has con at LDBS in Figure the global transaction G is

trol over the execution of global transactions and the executed b efore G but G precedes G in the lo cal

op erations they issue Therefore the MDBS can de serialization order there To determine lo cal conicts

tect direct conicts involving global transactions such between transactions Logar and Sheth prop osed

as the conict b etween G and G at LDBS in Fig using the commands of the lo cal op erating system and

ure However the MDBS has no information ab out DBMS to sno op on the LDBS Such approachmay

for a global transaction do es not create a cycle in the b e not always p ossible without violating the autonomy

graph multidatabase consistency is preserved and the of the LDBS

global transaction is allowed to pro ceed Otherwise

Controlling the submission and execution order

inconsistencies are p ossible and the global transaction

of the global transactions Alonso and Garcia

is ab orted

Molina prop osed to use site lo cking in the altruistic

The site graph metho d do es not violate the lo

locking proto col to prevent undesirable conicts

cal autonomy and correctly detects p ossible conicts

between multidatabase transactions Given a pair of

between multidatabase transactions However when

multidatabase transactions G and G the simplest

used for concurrency control it has signicantdraw

altruistic lo cking proto col allows the concurrent exe

backs First the degree of concurrency allowed is

cution of G and G if they access dierent LDBSs

rather low b ecause multidatabase transactions cannot

If there is a LDBS that b oth G and G need to ac

b e executed at the same LDBS concurrently Second

cess G cannot access it b efore G has nished its

and more imp ortantly the MDBS using site graphs

execution there Du and Elmagarmid have shown

has no way of determining when it is safe to remove

that global serializabilitymay b e violated even when

the edges of a committed global transaction Consider

multidatabase transactions are submitted seriallyone

global transactions G and G Supp ose that G is

after the other to their corresp onding LDBS The sce

ab orted b ecause it may p otentially conict with G

nario in Figure illustrates the ab ove problem G

which is currently executing as in Figure If the

is submitted to b oth sites executed completely and

edges corresp onding to G are removed immediately

committed Only then G is submitted for execution

following its commitmentandif G is restarted the

nevertheless the global consistency may b e violated

global serializabilitymay b e violated This is b ecause

Limitingmultidatabase memb ership to LDBSs

a lo cal transaction eg T in Figure whose execu

which use strict schedulers By disallowing lo cal

tion overlaps with the execution of the subtransactions

executions which are serializable but not strict this

of two global transactions may make the serialization

approach places additional restrictions on the execu

order of global transaction dierent than their exe

tion of b oth global and lo cal transactions at each par

cution order The metho d maywork correctly if the

ticipating LDBS A solution in this category called

removal of the edges corresp onding to a committing

the PC Agent Metho d has b een recently prop osed

transaction is delayed However the concurrency will

in The PC Agent Metho d assumes that the

b e sacriced In the scenario in Figure the edge

participating LDBSs use twophase lo cking PL

corresp onding to G can b e removed after the commit

schedulers and pro duce only strict schedules The

ment of the lo cal transaction T However the MDBS

basic idea in this metho d is that strict LDBSs will not

has no way of determining the time of commitmentor

p ermit lo cal executions which violate global serializ

even the existence of the lo cal transaction T

abilityHowever even lo cal strictness is not sucient

Mo difying the lo cal database systems andor

To illustrate this problem consider the LDBSs in Fig

applicationsPu has shown that global serializ

ure and the following lo cal schedules

ability can b e assured if the LDBSs present the lo cal

LDBS w acommit r acommit G G

G G G G

serialization orders to the MDBS Since the traditional

LDBS r br bw bcommit commit

G G G G G

DBMSs usually do not provide their serialization or

G T G

der Pu suggests mo difying the LDBSs to provide it

Both schedules ab ove are strict and are allowed by

Pons and Vilarem suggest mo difying existing ap

PL However global serializability is violated

plications so that all transactions including the lo cal

Assume the p ossibility of conicts among

are channeled through multidatabase interfaces Both

global transactions whenever they execute at

metho ds mentioned here preserve the multidatabase

the same site This idea has b een used by Logar

consistency but at the exp ense of partially violating

and Sheth in the context of distributed deadlo cks

the lo cal autonomy

in MDBSs and by Breitbart et al for concurrency

Rejecting serializability as the correctness cri control in the Amo co Distributed Database System

terion The concepts of sagas hasbeen ADDS Both are based on the notion of the site

prop osed do deal with longlived transactions byre graph In the ADDS metho d when a global trans

leasing transaction atomicity and isolation Quasi action issues a subtransaction to a LDBS a no de cor

serializability assumes that no value dep endencies resp onding to it is included to the site graph Fur

exist among databases so indirect conicts can b e ig thermore undirected edges are added to connect the

nored Stransactions and exible transactions no des of the LDBSs that participate in the execution

use transaction semantics to allow nonserializable ex of each global transaction If the addition of the edges

ecutions of global transactions These solutions do not

G G

violate the autonomy of the LDBSs and can b e used

I MB

whenever the correctness guarantees they oer are ap

plicable In this pap er we will assume that the global

schedules must b e serializable

b c LD B S t a t LD B S

The Optimistic Ticket

Metho d OTM

In this section we describ e a metho d for multidatabase

transaction management called OTM whichdoesnot

violate the LDBS autonomy and guarantees global se

rializability if the participating LDBSs assure lo cal se

LDBS r t w t w ar t w t

G G G G G

rializability The prop osed metho d addresses two com

r a ie G G

plementary issues

LDBS w br t w t r br t

T G G G G

how the MDBS can obtain the information ab out

w t w cr c ie

G G T

the relative serialization order of subtransactions

G T G

of global transactions at each LDBS and

Figure The eects of the TakeATicket approach

how the MDBS can guarantee that the subtrans

actions of eachmultidatabase transaction have

the same relative serialization order in all par

Figure illustrates the eects of the TakeATicket

ticipating LDBSs

pro cess on the example in Figure The ticket data

In the following discussion we do not consider site

items at LDBS and LDBS are denoted by t and

failures commitment and recovery of multidatabase

t resp ectivelyInLDBS the t values obtained by

transactions are discussed among others in

the subtransactions of G and G reect their rela

tive serialization order This schedule will b e p ermit

Determining the lo cal serialization

ted by the lo cal concurrency controller at LDBS In

LDBS the lo cal transaction T causes an indirect con

order

ict such that G T G However by requiring

OTM uses tickets to determine the relative serializa

the subtransactions to take tickets we force an addi

tion order of the subtransactions of global transactions

tional conict G G This additional ticket con

at each LDBS A ticket is a logical timestamp whose

ict causes the execution at LDBS to b ecome lo cally

value is stored as a regular data item in eachLDBS

nonserializable Therefore the lo cal schedule

Each subtransaction of a global transaction is required

w br t w t r b r t t

T G G G G G

to issue the TakeATicket op eration which consist of

w cr c

G T

reading the value of the ticket ie r tick et and in

will b e not allowed by the lo cal concurrency control

crementing it ie w tick et through regular data

ie the subtransaction of G or the subtransaction

manipulation op erations The value of a ticket and all

of G or T will b e blo cked or ab orted On the other

op erations on tickets issued at each LDBS are sub ject

hand if the lo cal schedule in LDBS were for example

to the lo cal concurrency control and other database

r t w t r br t w t w b

G G G G G T

constraints Only a single ticket value is needed p er

w cr c

G T

LDBS The TakeATicket op eration do es not violate

the tickets obtained by G and G would reect their

lo cal autonomy b ecause no mo dication of the lo cal

relative serialization order there and the lo cal schedule

systems is required Only the subtransactions of global

would b e p ermitted by the lo cal concurrency control

transactions havetotaketickets lo cal transactions

at LDBS Theorem formally proves that the tick

are not aected

ets obtained by the subtransactions at eachLDBSare

This may create a hot sp ot in the LDBSs However since

guaranteed to reect their relative serialization order

only subtransactionsofmultidatabase transactions and not lo

cal LDBS transactionshave to comp ete for tickets wedonot

consider this to b e a ma jor problem aecting the p erformance

Theorem The tickets obtained by the subtransac

of our metho d In fact if the volume of global transactions is

tions of multidatabase transactions determine their

high it is likely that the value of the ticket can b e read from the

relative serialization order database buer with minimal IO overhead

rect conict caused by lo cal transactions suchthat

g T T T g n Figure c

g g

i n j

i j

the resulting schedule is serializable and b oth g and

g are allowed to commit In this case g is serial

j i

ized b efore g and this is reected by the order of

their tickets However if there is an indirect conict

g g

i j

g T T T g n Figure d

j n i

the ticket conict g g creates a cycle in the lo

i j

cal serialization graph Hence this execution b ecomes

nonserializable and is not allowed by the LDBS con

T T T

currency control Therefore indirect conicts can b e

resolved through the use of tickets by the lo cal con

currency control even if the MDBS cannot detect their

existence

Case b in Figure is explained separately in Sec

g g

i j

tion

Enforcing global serializability

T T T

To maintain global consistency OTM must ensure

that the subtransactions of each global transactions

have the same relative serialization order in their cor

resp onding LDBSs Since the relative serialization

g g

i j

order of the subtransactions at each LDBS is reected

in the values of their tickets the basic idea in OTM

is to allow the subtransactions of each global transac

ticket conicts

tion to pro ceed but commit them only if their ticket

transaction conicts

values have the same relative order in all participat

ing LDBSs This requires that the lo cal database sys

Figure The eects of ticket conicts in OTM

tems supp ort a visible preparedtocommit state for all

subtransactions of global transactions Wesay that a

Pro of Let g and g b e the subtransactions of global

i j

transaction enters its prepared to commit state when

transactions G and G resp ectively at some LDBS

i j

it completes the execution of its op erations and leaves

Without loss of generalitywe can assume that g takes

this state when it is committed or ab orted During

its ticket b efore g ier tick et precedes r tick et

j g g

this time all up dates reside in its private workspace

i j

in the lo cal execution order Since a subtransaction

and are installed in the database when the transaction

takes its ticket rst and then increments the ticket

is committed The prepared to commit state is visi

value only the following execution orders are p ossible

ble if the application program can decide whether the

E r tick etr tick etw tick et w tick et

g g g g

transaction should commit or ab ort Many database

i j i j

E r tick etr tick etw tick et w tick et

management systems designed using the clientserver

g g g g

i j j i

E r tick etw tick et r tick etw tick et

architecture eg SYBASE provide a visible prepared

g g g g

i i j j

to commit state and can directly participate in a multi

However among these executions only E is serial

database commitment However even if the prepared izable and can b e allowed by the LDBS concurrency

state is not explicitly supp orted by the lo cal systems control Therefore g increments the ticket value b e

it can b e simulated by forcing a handshake after each fore g reads it and g obtains a smaller ticket than

j i

read and write op eration Under this assumption

a transaction enters its simulated prepared to com

To shownow that g can only b e serialized b efore

mit state when the completion of its last op eration is

g it is sucienttopoint out that the op erations to

acknowledged

take and increment the ticket issued rst by g and

OTM pro cesses a multidatabase transaction G as then by g create a direct conict g g This di

j i j

follows Initially it sets a timeout for G and submits rect conict forces g and g to b e serialized according

i j

its subtransactions to their corresp onding LDBSs All to the order in which they take their tickets More

subtransactions are allowed to interleave under the sp ecically if there is another direct conict b etween

control of the LDBSs until they enter their prepared g and g suchthatg g Figure a or indi

i j i j

to commit state If they all enter their prepared to Each multidatabase transaction has at most one

commit states they wait for the OTM to validate G subtransaction at each LDBS

The validation can b e p erformed using a Global Seri

Each subtransaction has a visible preparetocom

alization Graph GSG test The no des in GSG corre

mit state

sp ond to recently committed global transactions In

its simplest form the set of recently committed global

Pro of Wehave already shown that the order in which

transactions in OTM do es not contain transactions

the subtransactions take their tickets reects their rel

committed b efore the oldest of the currently active

ative serialization order Theorem After the tick

global transactions started its execution For anypair

ets are obtained by a global transaction at all sites it

c c

of recently committed global transactions G and G

i j

executes OTM p erforms the global serialization test

c c

GSG contains a directed edge G G if at least one

i j

describ ed in earlier in this section Global transac

subtransaction of G was serialized b efore obtained

tions pass validation and are allowed to commit only

a smaller ticket than the subtransaction of G in the

if their relative serialization order is the same at all

same LDBS Similarly if the subtransaction of G in

participating LDBSs which guarantees global serializ

some LDBS was serialized b efore the subtransaction

ability

c c c

of G a directed edge G G connects their no des

i i j

in GSG

Eect of the Ticketing Time on the

Initially GSG contains no cycles During the vali

Performance of OTM

dation of G OTM rst creates a no de for G in GSG

Then it attempts to insert edges b etween Gs no de

OTM can pro cess anynumber of multidatabase trans

and no des corresp onding to every recently commit

actions concurrentlyeven if they conict at multiple

ted multidatabase transaction G More sp ecically

LDBS However since OTM forces the subtransac

if the ticket obtained by a subtransaction of G at some

tions of multidatabase transactions to directly conict

LDBS is smaller larger than the the ticket of the sub

on the ticket it may cause some subtransactions to get

c c c

transaction of G there an edge G G G G is

ab orted or blo cked b ecause of ticket conicts Figure

added to GSG If all such edges can b e added without

b Since subtransactions maytake their tickets

creating a cycle in GSG G is validated Otherwise

at any time during their lifetime without aecting the

G do es not pass validation its no de together with all

correctness of OTM optimization based on the char

incident edges is removed from the graph and G is

acteristics of each subtransaction eg numb er time

restarted This validation test is enclosed in a single

and typ e of the data manipulation op erations issued or

critical section

their semantics is p ossible For example if all global

G is also restarted if at least one LDBS forces a

transactions conict directly at some LDBS there is

subtransaction of G to ab ort for lo cal concurrency

no need for them to take tickets To determine their

control reasons eg lo cal deadlo ck or its timeout

relative serialization order there it is sucienttoob

expires eg global deadlo ck AlternativelyOTM

serve the order in which they issue their conicting

may set a new timeout and restart only the subtrans

op erations

actions that did not rep ort prepared to commit in

The appropriate choice of the p oint in time to take

time If more than one of the participating LDBSs

the ticket during the lifetime of a subtransaction can

uses a blo cking mechanism for concurrency control

minimize the synchronization conicts among sub

the timeouts ab ove are necessary to resolve global

transactions For instance if a LDBS uses PL it is

deadlo cks An alternative approachistomaintain a

more appropriate to take the ticket immediately b e

waitfor graph WFG having LDBS as no des Then

fore a subtransaction enters its prepared to commit

if a cycle is found in the WFG and the cycle involves

state To show the eect of this convention consider

LDBS that use a blo cking technique to synchronize

a LDBS that uses PL for lo cal concurrency control

conicting transactions a deadlo ck is p ossible Deal

Figure a PL requires that each subtransaction

ing with deadlo cks in MDBSs constitutes a problem

sets a write lo ck on the ticket b efore it increments its

for further research

value Given four concurrent subtransactions g g

g and g g do es not interfere with g which can take

Theorem OTM guarantees global serializability if

its ticket and commit b efore g takes its ticket Sim

the fol lowing conditions are satisedbytheLDBSs

ilarly g do es not interfere with g sog can take

The concurrency control mechanisms of the

its ticket and commit b efore g takes its ticket How

LDBSs assurelocal serializability

ever when g attempts to takeitsticket after g has

taken its ticket but b efore g commits and releases its

Other validation tests such as the certication scheme pro

ticket lo ck it gets blo cked until g is committed The p osed in can b e also used to validate global transactions

a Preferred ticketing in a LDBS using PL

lo ck t

b Preferred ticketing in a LDBS using TO

let tsg tsg

abort g g

wt rt

abort g

wt rt

c Preferred ticketing in a LDBS using an optimistic proto col

serial g abort g

parallel g Q

abort g wt

Figure Preferred ticketing in LDBSs

ticket values always reect the serialization order of transactions

the subtransactions of multidatabase transactions but

To simplify transaction pro cessing and recovery the

the ticket conicts are minimized if the time when g

transaction managementmechanisms in most DBMSs

takes its ticket is as close as p ossible to its commitment

pro duce schedules that are not only serializable but

time

also cascadeless or strict Under a strict scheduler

If a LDBS uses timestamp ordering TO Fig no transaction can read or write a data item until all

ure b it is b etter to obtain the ticket when the transactions that previously wrote it commit or ab ort

subtransaction b egins its execution More sp ecically

In wehaveintro duced the concept of rigorous trans

TO assigns a a timestamp tsg to a subtransaction

action pro cessing mechanism In addition of guaran

teeing strictness a rigorous scheduler do es not allow g when it b egins its execution Let g b e another

transactions to write a data item until the transac subtransaction suchthattsg tsg If the ticket

tions that previously read it either commit or ab ort obtained by g has a larger value than the ticket of g

then g is ab orted Clearlyif g increments the ticket

That is the notion of rigorousness eectively elimi

value b efore g then since g is younger than g either

nates conicts b etween uncommitted transactions In

r tick etorw tick et conicts with the w tick et

wehave also shown that the class of rigorous trans

g g g

action managementmechanisms includes several com and g is ab orted Hence only g is allowed to incre

mon transaction management mechanisms suchas ment the ticket value b efore g Similarlyif g reads

conservativeTO the optimistic proto col with se the ticket b efore g increments it then when g issues

w tick et it conicts with the r tick et op eration

rial validation and a variant of strict twophase

g g

issued b efore and g is ab orted Therefore given that

lo cking PL under which a transaction must hold its

read and write lo cks until it terminates tsg tsT either g takes its ticket b efore g or it

is ab orted Therefore its is b etter for subtransactions

The p oint out the imp ortance of rigorousness in a

to take their tickets as close as p ossible to the p oint

multidatabase environment consider a MDBS in which

they are assigned their timestamps under TO ie at

the transaction managementmechanisms of all LDBSs

the b eginning of their execution

are rigorous In suchamultidatabase environment

Finally if a LDBS uses an optimistic proto

the MDBS can determine the serialization order of

col which uses transaction readsets and writesets to

global transactions bycontrolling the submission and

validate transactions there is no b est time for the

execution order of their subtransaction at the par

subtransactions to obtain their tickets Figure c

ticipating LDBSs In particular wehaveshown

Each subtransaction g reads the ticket value b efore it

that rigorous schedulers guarantee that for anypair

starts its serial or parallel validation but increments

of transactions T and T such that T is committed

i j i

b efore T T also precedes T in the serialization order it at the end of its write phase If another transaction

j i j

g is able to increment the ticket in the meantime g

corresp onding to the execution schedule It should b e

is restarted

observed that strictness of the lo cal transaction man

agement mechanisms is not sucient to assure this

The basic advantage of OTM is that it requires the

prop erty lo cal systems to ensure only lo cal serializability It

main disadvantage is that it intro duces additional con

To takeadvantage of rigorous LDBSs weintro

icts b etween global transactions whichmay not con

duce a renement of OTM called the Implicit Ticket

ict otherwise If additional assumptions can b e made

Method ITM ITM takes advantage of the fact that

concerning the schedules that are pro duced by the lo

if all LDBSs pro duce rigorous schedules then ticket

cal systems the OTM can b e simplied and the ticket

conicts can b e eliminated To guarantee global seri

conicts can b e eliminated

alizability in the presence of lo cal transactions ITM

requires the following condition to b e satised in ad

dition to Conditions and which are stated in the

Implicit tickets a renement for

Theorem presented in Section Rigorous schedules

rigorous LDBSs

are serializable therefore Condition can replace

Condition

As wehave discussed the basic problem in multi

database concurrency control is that the lo cal seri

All local database systems use rigorous transac

alization orders do not necessarily reect the order

tion management mechanisms

in which global transactions are submitted executed

Like OTM ITM ensures global serializabilityby and committed in the LDBSs In this section weshow

preventing the subtransactions of eachmultidatabase that if the LDBSs do not pro duce schedules with such

transaction from b eing serialized in dierentways at anomalies the MDBS can determine the lo cal seri

their corresp onding LDBSs Unlike OTM ITM do es alization order bycontrolling the execution of global

not need to maintain tickets and the subtransactions tributed databases systems The additional diculties

of global transactions do not need to explicitly take in this environment are caused by the autonomyand

and incrementtickets In a rigorous LDBS the im the heterogeneity of the participating LDBSs

plicit ticket of each subtransaction executed there is To enforce global serializability we intro duced

determined by its commitment order That is the or OTM an optimistic multidatabase transaction man

der in whichwe commit subtransactions at eachLDBS agement mechanism that p ermits the commitmentof

determines the relativevalues of their implicit tickets multidatabase transactions only if their relativeseri

Toachieve global serializability ITM controls the com alization order is the same in all participating LDBSs

mitment execution order and thus the serialization OTM requires the LDBSs to guarantee only lo cal seri

order of multidatabase subtransactions as follows Let alizability The basic idea in OTM is to create direct

G and G be twomultidatabase transactions As conicts b etween multidatabase transactions at each

i j

suming rigorous LDBSs ITM guarantees that in all LDBS that allow us to determine the relative serial

participating LDBSs either the subtransactions of G ization order of their subtransactions ITM is a re

are committed b efore the subtransactions of G or the nement of OTM that uses implicit tickets and elim

subtransactions of G are committed prior to the sub inates ticket conicts but works only when the par

transactions of G ticipating LDBSs use rigorous transaction scheduling

ITM achieves this ob jective as follows Initiallythe mechanisms ITM uses the lo cal commitmentorder

MDBS sets timeouts for G and G and submits their of each subtransaction to determine its implicit ticket

i j

subtransactions to the corresp onding LDBSs All sub value It achieves global serializabilityby controlling

transactions are allowed to interleave under the control the commitment execution order and thus the seri

of the LDBSs until they enter their prepared to com alization order of multidatabase transactions Com

mit state If all subtransactions of G rep ort prepared pared to the the ADDS approach and Altruistic Lo ck

to commit to the ITM b efore the subtransactions of ing ITM can pro cess anynumber of multidatabase

G do the ITM commits each subtransaction of G be transactions concurrentlyeven if they have concur

j i

fore any subtransaction of G If the subtransactions rent and conicting subtransactions at multiple sites

of G are prepared to commit rst each subtransac Both OTM and ITM do not violate the autonomyof

tion of G is committed b efore any subtransaction of the LDBSs and can b e combined in a single compre

G If neither of these happ ens the MDBSs ab orts hensive mechanism

and restarts anymultidatabase transaction that has Rigorousness is a very useful prop erty in a MDBS

subtransactions which did not rep ort their prepared For example it can b e shown that ADDS scheme

to commit state b efore the timeout expired Altruistic Lo cking and PC Agent Metho d pro

duce globally serializable schedules if the participat

Theorem ITM ensures global serializability if al l

ing LDBSs are rigorous Similarly quasiserializable

LDBSs satisfy conditions and

schedules b ecome serializable if all the LDBSs are

Pro of Given twomultidatabase transactions G and

rigorous On the other hand if the lo cal systems are

G ITM commits each subtransaction of G b efore

j i

not rigorous some of the ab ove metho ds may lead to

the corresp onding subtransaction of G or vice versa

schedules that are not globally serializable

In the b eginning of this section we explained that in

rigorous LDBSs the commitment order of eachsub

Acknowledgments

transaction implicit ticket order determines its rela

The idea to use tickets in multidatabase transaction

tive serialization order Therefore all subtransactions

management had emerged during a discussion with

of eachmultidatabase transaction are serialized the

Gomer Thomas We thank Yuri Breitbart for p oint

same way in their corresp onding LDBSs

ing out an error in one of our denitions in an earlier

Although ITM works only for rigorous LDBSs it can

version of this pap er

b e combined with OTM into a single comprehensive

mechanism where OTM is rst used to synchronize the

subtransactions in all nonrigorous LDBSs and then

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