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Chapter 18

Concurrency ControlTechniques

Copyright 2004 Pearson Education, Inc.

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Copyright 2004 Ramez Elmasri and Shamant !a"athe

Elmasri/Navathe, Fundamentals of Database ystems, Fourth Edition Chapter 18-3

Chapter 18 Outline

Databases Concurrency Control

# Purpose o$ Concurrency Control

2 %&o'Phase locing

( )imitations o$ CC*s

+ Inde )ocing

- )oc Compatiility *atri/ )oc ranularity

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Copyright 2004 Ramez Elmasri and Shamant !a"athe

Elmasri/Navathe, Fundamentals of Database ystems, Fourth Edition Chapter 18-4

Database Concurrency Control

1 Purpose of Concurrency Control

1 %o en$orce Isolation through mutual eclusion3 amongcon$licting transactions.

1 %o preser"e dataase consistency through consistencypreser"ing eecution o$ transactions.

1 %o resol"e read'&rite and &rite'&rite con$licts.

Eample In concurrent eecution en"ironment i$ %#con$licts &ith %2 o"er a data item 5, then the eistingconcurrency control decides i$ %# or %2 should get the 5and i$ the other transaction is rolled'ac or &aits.

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Copyright 2004 Ramez Elmasri and Shamant !a"athe

Elmasri/Navathe, Fundamentals of Database ystems, Fourth Edition Chapter 18-5

Database Concurrency Control

Two-Phase Locking Techniues

)ocing is an operation &hich secures a3 permission to

Read or 3 permission to 6rite a data item $or a transaction.

Eample )oc 73. 8ata item 7 is loced in ehal$ o$ there9uesting transaction.

:nlocing is an operation &hich remo"es these permissions

$rom the data item. Eample :nloc 73. 8ata item 7 ismade a"ailale to all other transactions.

)oc and :nloc are 5tomic operations.

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Copyright 2004 Ramez Elmasri and Shamant !a"athe

Elmasri/Navathe, Fundamentals of Database ystems, Fourth Edition Chapter 18-!

Database Concurrency Control

Two-Phase Locking Techniues" #ssential co$ponents

%&o locs modes a3 shared read3 and 3 eclusi"e &rite3.

Shared mode shared loc 73. *ore than one transaction can applyshare loc on 7 $or reading its "alue ut no &rite loc can e applied

on 7 y any other transaction.Eclusi"e mode 6rite loc 73. ;nly one &rite loc on 7 can eistat any time and no shared loc can e applied y any other transactionon 7.

Con$lict matri

Read 6riteRead

6rite

!

!!