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LectureLecture- - 2323

Concurrency ControlConcurrency Control

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ContentsContents

Introduction

Concurrency Control vs. Serializability Tests

ea eve s o ons stency

Transaction Definition in SQL

Locking Schemes

Applications

Scope of Research

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IntroductionIntroduction

A database must provide a mechanism that will ensure that all possibleschedules are

,

are recoverable and preferably cascadeless A policy in which only one transaction can execute at a time generates

,

Are serial schedules recoverable/cascadeless?

Testing a schedule for serializability after it has executed is a little tooa e

Goal – to develop concurrency control protocols that will assureserializability.

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Concurrency Control vs. Serializability TestsConcurrency Control vs. Serializability Tests

Concurrency-control protocols allow concurrent schedules, but ensure,

and cascadeless .Concurrency control protocols generally do not examine the

recedence ra h as it is bein created

Instead a protocol imposes a discipline that avoids nonseralizableschedules.

.

Different concurrency control protocols provide different tradeoffsbetween the amount of concurrency they allow and the amount ofoverhead that the incur.

Tests for serializability help us understand why a concurrency controlprotocol is correct.

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Weak Levels of ConsistencyWeak Levels of Consistency

Some applications are willing to live with weak levels of consistency,allowing schedules that are not serializable

-. .

balance of all accountsE.g. database statistics computed for query optimization can bea roximate wh ?

Such transactions need not be serializable with respect to othertransactions

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Transaction Definition in SQLTransaction Definition in SQL

Data manipulation language must include a construct forspecifying the set of actions that comprise a transaction.

, .

A transaction in SQL ends by:Commit work commits current transaction and begins a new

.

Rollback work causes current transaction to abort.

In almost all database systems, by default, every SQL statementa so comm s mp c y execu es success u y

Implicit commit can be turned off by a database directive

E.g. in JDBC, connection.setAutoCommit(false);

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Implementation of IsolationImplementation of Isolation

Schedules must be conflict or view serializable, and recoverable,for the sake of database consistency, and preferably cascadeless.

generates serial schedules, but provides a poor degree ofconcurrency.

Concurrenc -control schemes tradeoff between the amount of concurrency they allow and the amount of overhead that theyincur.

Some schemes allow only conflict-serializable schedules to begenerated, while others allow view-serializable schedules that arenot conflict-serializable.

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Exclusive LockExclusive Lock

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of this lock is to provide exclusive use of the data item to onetransaction. If a transaction T locks a data item Q in an exclusivemode no other transaction can access Q until the lock is released bytransaction T.

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Shared LockShared Lock

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locking is to ensure that the data item does not undergo anymodifications while it is locked in this mode. Any number of transactions can concurrently lack and access a data item in theshared mode, but none of these transactions can modify the dataitem. A data item locked in a shared mode can not be locked in the

exclusive mode until the shared lock is released by all transactions.

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Applications Applications

Concurrency control in data bases is used to avoid the problems of dead locks as this is one of the serious problems in data basemanagement systems

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ScopeScope of Researchof Research

so a on requ remen s n concurrency con ro

Thread safety for concurrent clients

Concurrenc control in distributed database s stems

Realization of Transaction Concurrency Control in Grid

Database