concurrency control technique

8/9/2019 Concurrency Control Technique

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Module - 5Module - 5Transaction Management,Concurrency Control and

Distributed System


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What is Concurrency?What is Concurrency?

Allowing multiple users to accessthe data contained within adatabase at the same time


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These collections o operations that orm asingle logical unit o wor! are calledtransactions"


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The AC%D properties oThe AC%D properties oTransactionTransactionAtomicity$ Transactions are atomic 7all

or nothing8Consistency$ 1&ecution o a transaction

preser2es the consistency o thedatabase%solation $ Transactions do not

interact6interere with one another9

transactions act as i they areindependentDurability$ 1:ects o a completed

transaction are persistent


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To gain a betterTo gain a betterunderstanding o AC%Dunderstanding o AC%Dpropertiesproperties)et Ti be a transaction that

transers ;5 rom account A toaccount *" this transaction can be

de A-59

write7A89

read7*89*=> *59

write 7*89


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AtomicityAtomicity

Transaction should either ta!eplace completely or nothingshould happen in case o ailure

This is handled by a componentcalled transaction managementcomponent and ensured by the

database itsel"


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ConsistencyConsistency

Sum o A and * be unchanged bythe e&ecution o the transaction"4esponsibility o the programmer

who codes the transaction toensure consistency


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DurabilityDurability

Ater the e&ecution o transactionis completed successully thenthere should not be any loss o

data corresponding to thetranser o unds e2en i a systemailure occurs"


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Durability is ensured byDurability is ensured by

eithereitherThe updates carried out by thetransaction ha2e been written to dis!beore the transaction completes%normation about the updates carried

out by the transaction and written todis! is su@cient to enable the databaseto reconstruct the updates when thedatabase system is restarted ater the

ailure"1nsuring durability is the responsibility o

a component o database system calledthe reco2ery management component"


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%solation%solationDb is temporarily inconsistent while the

transaction to transer unds rom A to * ise&ecuting, with the deduced total writtento A and the increased total yet to be

written to *" % a second concurrentlyrunning transaction reads A and * at thisintermediate point and computes A*,which gi2es an inconsistent 2alue

To a2oid this problem 1&ecute transactions serially6 other solutions

1nsuring isolation is the responsibility o acomponent called Concurrency-Control

Component


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Transaction StateTransaction State

Aborted= a transaction which does notcomplete its e&ecution successully4olled *ac!= signals unsuccessul end-

o-transaction" %t tells the transaction

manager that something has gonewrong, the database might be in aninconsistent state and all theoperations that had been done so arhas been rolled bac! or undone

Committed= a transaction thatcompletes its e&ecution successullyTerminated= a transaction that has

either committed or aborted


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State Diagram o aState Diagram o a

TransactionTransactionActive

Partiallycommitt

ed

Committed

FailedAborte

d


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Acti2e= the initial state9 thetransaction stays in this state whileit is e&ecutingartially Committed= ater the


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Transactions andTransactions and

SchedulesSchedulesTransaction is a series or list oactionsActions that can be e&ecuted by a

transaction include reads and writeso database obects1ach transaction must speciy as its


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System logSystem log

the system needs to essentiallymaintain a trac! record o alltransaction operations that are

ta!ing place and that are li!ely toa:ect the status o the database"

This inormation is called a

System logE and may becomeuseul when the system is tryingto reco2er rom ailures"


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)et T reer to a uniFuetransaction id, generatedautomatically whene2er a new

transaction is encountered andthis can be used to uniFuelyidentiy the transaction" Then the

ollowing entries are made withrespect to the transaction T"


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ScheduleSchedule

A list o actions 7reading, writing, aborting,or committing8 rom a set o concurrenttransactions and the order6arrangement inwhich two actions o a transaction Tappear in a schedule must be the same as

the order in which they appear in T

47A8

W7A8 47*8 W7*8

47C8

W7C8

A schedule in2ol2ing two transactions

T( T


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Complete Schedule $ a schedule thatcontain an abort or a commit or each

transaction whose actions are listed in it"

Serial Schedule $ i the actions o di:erenttransactions are not interlea2ed $ i"e"transactions are e&ecuted rom start to


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A Complete Schedule= A schedule S on transactions T(, T,KK"" Tn is said to be

a Complete ScheduleE i the ollowingconditions are satis


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Concurrent 1&ecutions oConcurrent 1&ecutions o

TransactionsTransactionsTransaction processing systems

allow multiple transactions to runconcurrently

Cause se2eral complications withconsistency o the dataood reasons or allowing

concurrencyTransaction consists o multiple steps"

Some in2ol2e %6# acti2ity and somein2ol2e C acti2ity


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There may be a mi& o transactionsrunning on a system i"e" some short andsome long"

Serial Transaction= a short transaction mayha2e to wait or a preceding longtransaction to complete which leads tounpredictable delays in running atransaction"

% the transactions are operating ondi:erent parts o the database, then it isbetter to let them run concurrentlysharing the C cycles and dis! accesses

among them


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Ad2antages o ConcurrentAd2antages o Concurrent1&ecution1&ecution4educes the unpredictable delays

in running transactions"

4educes the a2erage responsetime, the a2erage time or atransaction to be completed ater

it has been submitted"

concurrency control technique

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