chapter 15

McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.

Chapter 15

Transaction Management

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Outline

Transaction basics Concurrency control Recovery management Transaction design issues Workflow management

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Transaction Definition Supports daily operations of an

organization Collection of database operations Reliably and efficiently processed as one

unit of work No lost data

Interference among multiple users Failures

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Airline Transaction ExampleSTART TRANSACTION

Display greetingGet reservation preferences from userSELECT departure and return flight recordsIf reservation is acceptable then

UPDATE seats remaining of departure flight recordUPDATE seats remaining of return flight recordINSERT reservation recordPrint ticket if requested

End IfOn Error: ROLLBACK

COMMIT

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ATM Transaction ExampleSTART TRANSACTION

Display greetingGet account number, pin, type, and amountSELECT account number, type, and balanceIf balance is sufficient then

UPDATE account by posting debitUPDATE account by posting debitINSERT history recordDisplay message and dispense cashPrint receipt if requested

End IfOn Error: ROLLBACK

COMMIT

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Transaction Properties Atomic: all or nothing Consistent: database must be consistent

before and after a transaction Isolated: no unwanted interference from

other users Durable: database changes are

permanent after the transaction completes

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Transaction Processing Services Concurrency control Recovery management Service characteristics

Transparent Consume significant resources Significant cost component Transaction design important

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Concurrency Control Problem definition Concurrency control problems Concurrency control tools

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Concurrency Control Problem Objective:

Maximize work performed Throughput: number of transactions

processed per unit time

Constraint: No interference: serial effect Interference occurs on commonly manipulated

data known as hot spots

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Lost Update Problem

Transaction A Time Transaction B

Read SR (10) T1 T2 Read SR (10) If SR > 0 then SR = SR -1

T3

T4 If SR > 0 then SR = SR -1

Write SR (9) T5 T6 Write SR (9)

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Uncommitted Dependency Problem


Read SR (10) T1 SR = SR - 1 T2 Write SR (9) T3 T4 Read SR (9) ROLLBACK T5

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Inconsistent Retrieval Problems Interference causes inconsistency among

multiple retrievals of a subset of data Incorrect summary Phantom read Non repeatable read

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Incorrect Summary Problem

Transaction A Time Transaction B Read SR1 (10) T1 SR1 = SR1 - 1 T2 Write SR1 (9) T3 T4 Read SR1 (9) T5 Sum = Sum + SR1 T6 Read SR2 (5) T7 Sum = Sum + SR2 Read SR2 (5) T8 SR2 = SR2 - 1 T9 Write SR2 (4) T10

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Locking Fundamentals Fundamental tool of concurrency control Obtain lock before accessing an item Wait if a conflicting lock is held

Shared lock: conflicts with exclusive locks Exclusive lock: conflicts with all other kinds of

locks

Concurrency control manager maintains the lock table

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Locking Granularity

Database

Table

Page

Row

Column

Index

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Deadlock (Mutual Waiting)


XLock SR1 T1

T2 XLock SR2

XLock SR2 (wait) T3

T4 XLock SR1 (wait)

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Deadlock Resolution Detection

Overhead is reasonable for deadlocks among 2 or 3 transactions

Used by enterprise DBMSs

Timeout Waiting limit Can abort transactions that are not

deadlocked Timeout interval is difficult to determine

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Two Phase Locking (2PL) Protocol to prevent lost update problems All transactions must follow Conditions

Obtain lock before accessing item Wait if a conflicting lock is held Cannot obtain new locks after releasing locks

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2PL Implementation

Time

Growing phase

BOT EOT

Shrinkingphase

Lo

cks

he

ld

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Optimistic Approaches Assumes conflicts are rare No locks Check for conflicts

After each read and write At end of transaction

Evaluation Less overhead More variability

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Recovery Management Device characteristics and failure types Recovery tools Recovery processes

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Storage Device Basics Volatile: loses state after a shutdown Nonvolatile: retains state after a shutdown Nonvolatile is more reliable than volatile

but failures can cause loss of data Use multiple levels and redundant levels

of nonvolatile storage for valuable data

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Failure Types

Local Detected and abnormal termination Limited to a single transaction

Operating System Affects all active transactions Less common than local failures

Device Affects all active and past transactions Least common

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Transaction Log History of database changes Large storage overhead Operations

Undo: revert to previous state Redo: reestablish a new state

Fundamental tool of recovery management

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Transaction Log Example

LSN TransNo Action Time Table Row Column Old New

1 101001 START 10:29 2 101001 UPDATE 10:30 Acct 10001 AcctBal 100 200 3 101001 UPDATE 10:30 Acct 15147 AcctBal 500 400 4 101001 INSERT 10:32 Hist 25045 * <1002,

500, …>

5 101001 COMMIT 10:33

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Checkpoints Reduces restart work but adds overhead

Checkpoint log record Write log buffers and database buffers

Checkpoint interval: time between checkpoints

Types of checkpoints Cache consistent Fuzzy Incremental

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Other Recovery Tools Force writing

Checkpoint time End of transaction

Database backup Complete Incremental

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Recovery from a Media Failure Restore database from the most recent

backup Redo all committed transactions since the

most recent backup Restart active transactions

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Recovery Timeline

Time

T1

T5

T4

T3

T2

Checkpoint Failure

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Recovery Processes Depend on timing of database writes Immediate update approach:

Before commit Log records written first (write-ahead log

protocol)

Deferred update approach After commit Undo operations not needed

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Immediate Update Recovery

Class Description Restart Work

T1 Finished before CP None

T2 Started before CP; finished before failure

Redo forward from checkpoint

T3 Started after CP; finished before failure

Redo forward from checkpoint

T4 Started before CP; not yet finished

Undo backwards from most recent log record

T5 Started after CP; not yet finished

Undo backwards from most recent log record

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Deferred Update Recovery

Class Description Restart Work

T1 Finished before CP None

T2 Started before CP; finished before failure

Redo forward from first log record

T3 Started after CP; finished before failure

Redo forward from first log record

T4 Started before CP; not yet finished

None

T5 Started after CP; not yet finished

None

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Oracle Recovery Features Incremental checkpoints Immediate update approach Mean Time to Recover (MTTR) parameter MTTR advisor Dynamic dictionary views to monitor

recovery state

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Transaction Design Issues Transaction boundary Isolation levels Deferred constraint checking Savepoints

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Transaction Boundary Decisions Division of work into transactions Objective: minimize transaction duration Constraint: enforcement of important

integrity constraints Transaction boundary decision can affect

hot spots

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Registration Form Example

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Transaction Boundary Choices One transaction for the entire form One transaction for the main form and one

transaction for all subform records One transaction for the main form and

separate transactions for each subform record

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Avoiding User Interaction Time Avoid to increase throughput Possible side effects: user confusion due

to database changes Balance increase in throughput with

occurrences of side effects Most situations increase in throughput

more important than possible user confuusion

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Isolation Levels Degree to which a transaction is separated

from the actions of other transactions Balance concurrency control overhead

with interference problems Some transactions can tolerate

uncommitted dependency and inconsistent retrieval problems

Specify using the SET TRANSACTION statement

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SQL Isolation Levels

Level XLocks SLocks PLocks Interference

Read uncommitted

None None None Uncommitted dependency

Read committed

Long Short None All except uncommitted dependency

Repeatable read

Long Long Short (S), Long (X)

Phantom reads

Serializable Long Long Long None

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Scholar’s Lost Update


Obtain S lock on SR T1

Read SR (10) T2

Release S lock on SR T3

If SR > 0 then SR = SR -1 T4

T5 Obtain S lock on SR

T6 Read SR (10)

T7 Release S lock on SR

T8 If SR > 0 then SR = SR -1

Obtain X lock on SR T9

Write SR (9) T10

Commit T11

T12 Obtain X lock on SR

T13 Write SR (9)

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Integrity Constraint Timing Most constraints checked immediately Can defer constraint checking to EOT SQL

Constraint timing clause for constraints in a CREATE TABLE statement

SET CONSTRAINTS statement

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Save Points Some transactions have tentative actions SAVEPOINT statement determines

intermediate points ROLLBACK to specified save points

CommitSafe

operationsSave point

Rollback tosave point

Tentativeoperations

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Workflow Management Workflow description Enabling technologies Advanced transaction management

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Workflow Basics Set of tasks to accomplish a business

process Human-oriented vs. computer-oriented

Amount of judgment Amount of automation

Task structure vs. task complexity Relationships among tasks Difficulty of performing individual tasks

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Workflow Classification

Human-oriented Computer-oriented

Communicationsupport

Transactionsupport

Task structure

Simple

Complex

Low High

Meetingnotification

Travelrequest

Bid proposal

Insuranceclaim

Utility service

Property sale

Task

com

plex

ity

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Enabling Technologies Distributed object management

Many kinds of non traditional data Data often dispersed in location

Workflow modeling Specification Simulation Optimization

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Advanced Transaction Management

Conversational transactions Transactions with complex structure Transactions involving legacy systems Compensating transactions More flexible transaction processing

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Summary Transaction: user-defined collection of work DBMSs support ACID properties Knowledge of concurrency control and recovery

important for managing databases Transaction design issues are important Transaction processing is an important part of

workflow management

chapter 15

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