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PRESENTED BY S.HAYAT 1

Concurrency Control

ProblemProblem in a multi-user environment, simultaneous

access to data can result in interference and data loss

SolutionSolutionConcurrency Control The process of managing simultaneous operations

against a database so that data integrity is

maintained and the operations do not interfere with

each other in a multi-user environment.

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Concurrency Control ProblemsConcurrency Control Problems

1. Lost Updates Problem

2. Inconsistent Read

LOST UPDATES PROBLEM The most common problem encountered when multiple

users attempt to update a database without adequate

concurrency control is that of lost updates.

The below Fig shows the lost update problem.

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Figure LOST UPDATELOST UPDATE

Simultaneous access causes updates to cancel each other

A similar problem is the inconsistent readinconsistent read problem

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Inconsistent Read Problem

This problem occurs when one user reads data that have

been partially updated by another user.

The read will be incorrect and is sometimes referred to as a

dirty read or an unrepeatable read.

The following fig represent inconsistent read problem.

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Time T3 T4 balx

t1 begin-transaction 100

t2 read(balx) 100

t3

balx =

balx

+100 100

t4 begin_transaction write(balx) 200

t5 read(balx) : 200

t6 balx = balx -10 rollback 100

t7 write(balx) 190

t8 commit 190

Occurs when one transaction can see intermediate results of anothertransaction before it has committed.

T4 updates balx to 200 but it aborts, so balx should be back at originalvalue of 100.

T3 has read new value of balx (200) and uses value as basis of 10reduction, giving a new balance of 190, instead of 90.

Problem avoided by preventing T3 from reading balx until after T4commits or aborts.

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Figure-Updates with locking for concurrency control

This prevents the lost update problem

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Locking Mechanisms

Locking level: Database used during database updates

Table used for bulk updates

Block or page very commonly used

Record only requested row; fairly commonly used

Field requires significant overhead; impractical

Types of locks: Shared lock - Read but no update permitted. Used

when just reading to prevent another user from placingan exclusive lock on the record

Exclusive lock - No access permitted. Used whenpreparing to update

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Deadlock

An impasse that results when two or more transactionshave locked common resources, and each waits for theother to unlock their resources

Figure:

A deadlock situation

UserA and UserB will waitUserA and UserB will wait

forever for each other toforever for each other to

release their locked resources!release their locked resources!

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Managing Deadlock

Deadlock prevention: Lock all records required at the beginning of atransaction

Two-phase locking protocol Growing phase

Shrinking phase

May be difficult to determine all needed resources inadvance

Deadlock Resolution:

Allow deadlocks to occur

Mechanisms for detecting and breaking them Resource usage matrix

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Versioning

Optimistic approach to concurrency control

Instead of locking

Assumption is that simultaneous updates will be

infrequent Each transaction can attempt an update as it wishes

The system will reject an update when it senses a

conflict

Use of rollback and commit for this

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use of versioning

Better performance than locking

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Database Recovery & Its Techniques

Mechanism for restoring a database quickly and accurately afterloss or damage.

OR

The process of restoring the database to the correct state after

loss or damage

Recovery Techniques/facilities:

Backup Facilities

Journalizing Facilities

Checkpoint Facility

Recovery Manager

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Back-up facilities: the DBMS should provide back-up facilitiesthat produce a back-up copy(or save) of the entire database plus

files and journals.

Each DBMS normally provides a COPY utility for this purpose. Including database files , the back-up facility should create a copy

of related database objects including the repository (or system

catalog), database indexes , source libraries and so on.

A back-up copy is produced at least once per day.

The copy should be stored in a secured location where it is

protected from loss or damage.

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Journalizing facilities: A DBMS must provide Journalizingfacilities to produce an audit trial of transactions and database

changes. In the event of a failure, a consistent database state can

be re-established using the information in the journals together.

Transaction log:contains a record of the essential data for eachtransaction include the transaction code or identification, action

or type of transaction, time of the transaction, user ID, input data

values, table and records accessed, records modified, and possibly

the old and new field values.

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Database change log:contains before and after images ofrecords that have been modified by transactions.

Before-image: a copy of a record before it has been modified.

After-image: is a copy of the same record after it has beenmodified

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From the backup and logs,databases can be restored in

case of damage or loss

Database audit trail

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Checkpoint Facility:

DBMS periodically refuses to accept new transactions

system is in a quietstate

Database and transaction logs are synchronized

The DBMS writes a special record( called a checkpoint record) to

the log file, which is like a snapshot of the state of the database.

Checkpoints should be taken frequently when failures do occur, it

is often possible to resume processing from the most recent

checkpoint.

Only a few minutes of processing work must be repeated,

compared with several hours for a complete restart of the days

processing.

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Recovery manager:a module of the DBMS that restores thedatabase to a correct condition when a failure occurs and then

resumes processing user questions

Documents

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