Chapter 15Chapter 15Transaction Management

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Outline Outline

Transaction basicsConcurrency controlRecovery managementTransaction design issuesWorkflow management

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Transaction DefinitionTransaction Definition

Supports daily operations of an organization

Collection of database operationsReliably and efficiently processed as one

unit of workNo lost data

– Interference among multiple users– Failures

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Airline Transaction ExampleAirline Transaction Example

START TRANSACTIONDisplay 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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Transaction PropertiesTransaction Properties

Atomic: all or nothingConsistent: database must be consistent

before and after a transactionIsolated: no unwanted interference from

other usersDurable: database changes are permanent

after the transaction completes

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Transaction Processing ServicesTransaction Processing Services

Concurrency controlRecovery managementService characteristics

– Transparent– Consume significant resources– Significant cost component

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Concurrency ControlConcurrency Control

Problem definitionConcurrency control problemsConcurrency control tools

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Concurrency Control ProblemConcurrency Control Problem

Objective: – Maximize work performed– Throughput: number of transactions processed

per unit timeConstraint:

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

data known as hot spots

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Lost Update ProblemLost 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 Uncommitted Dependency ProblemProblem

Transaction A Time Transaction B

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

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Inconsistent Retrieval ProblemsInconsistent Retrieval Problems

Interference causes inconsistency among multiple retrievals of a subset of data

Incorrect summary Phantom readNon repeatable read

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Incorrect Summary ProblemIncorrect Summary ProblemTransaction 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 FundamentalsLocking Fundamentals

Fundamental tool of concurrency controlObtain lock before accessing an itemWait if a conflicting lock is held

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

of locksConcurrency control manager maintains

the lock table

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Locking GranularityLocking GranularityDatabase

Table

Page

Row

Column

Index

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Deadlock (Mutual Waiting)Deadlock (Mutual Waiting)

Transaction A Time Transaction B

XLock SR1 T1

T2 XLock SR2

XLock SR2 (wait) T3

T4 XLock SR1 (wait)

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Deadlock ResolutionDeadlock Resolution

Detection– Can involve significant overhead– Not widely used

Timeout– Waiting limit– Can abort transactions that are not deadlocked– Widely used although timeout interval is

difficult to determine

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Two Phase Locking (2PL)Two Phase Locking (2PL)

Protocol to prevent lost update problemsAll transactions must followConditions

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

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2PL Implementation2PL Implementation

Time

Growing phase

BOT EOT

Shrinkingphase

Lo

cks

he

ld

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Optimistic ApproachesOptimistic Approaches

Assumes conflicts are rareNo locksCheck for conflicts

– After each read and write– At end of transaction

Evaluation– Less overhead– More variability

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Recovery ManagementRecovery Management

Device characteristics and failure typesRecovery toolsRecovery processes

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Storage Device BasicsStorage Device Basics

Volatile: loses state after a shutdownNonvolatile: retains state after a shutdownNonvolatile is more reliable than volatile

but failures can cause loss of dataUse multiple levels and redundant levels

of nonvolatile storage for valuable data

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Failure TypesFailure 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 LogTransaction Log

History of database changesLarge storage overheadOperations

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

Fundamental tool of recovery management

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Transaction Log ExampleTransaction 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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CheckpointsCheckpoints

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

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Other Recovery ToolsOther Recovery Tools

Force writing– Checkpoint time– End of transaction

Database backup– Complete– Incremental

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Recovery from a Media FailureRecovery 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 TimelineRecovery Timeline

Time

T1

T5

T4

T3

T2

Checkpoint Failure

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Recovery ProcessesRecovery Processes

Depend on timing of database writesImmediate 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 RecoveryImmediate Update RecoveryClass 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 RecoveryDeferred Update RecoveryClass 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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Transaction Design IssuesTransaction Design Issues

Transaction boundaryIsolation levelsDeferred constraint checkingSavepoints

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Transaction Boundary DecisionsTransaction Boundary Decisions

Division of work into transactionsObjective: minimize transaction durationConstraint: enforcement of important

integrity constraintsTransaction boundary decision can affect

hot spots

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Registration Form ExampleRegistration Form Example

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Transaction Boundary ChoicesTransaction Boundary Choices

One transaction for the entire formOne transaction for the main form and one

transaction for all subform recordsOne transaction for the main form and

separate transactions for each subform record

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Isolation LevelsIsolation 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 LevelsSQL 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 UpdateScholar’s Lost UpdateTransaction A Time Transaction BObtain S lock on SR T1

Read SR (10) T2

Release S lock on SR T3

If SR > 0 then SR = SR -1 T4

T5Obtain S lock on SR

T6Read SR (10)

T7Release S lock on SR

T8If SR > 0 then SR = SR -1

Obtain X lock on SR T9

Write SR (9) T10

Commit T11

T12Obtain X lock on SR

T13Write SR (9)

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Integrity Constraint TimingIntegrity Constraint Timing

Most constraints checked immediatelyCan defer constraint checking to EOTSQL SET CONSTRAINTS statement

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Save PointsSave Points

Some transactions have tentative actionsSAVEPOINT statement determines

intermediate pointsROLLBACK to specified save points

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Workflow ManagementWorkflow Management

Workflow descriptionEnabling technologiesAdvanced transaction management

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WorkflowsWorkflows

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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Enabling TechnologiesEnabling 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 Advanced Transaction ManagementManagementConversational transactionsTransactions with complex structureTransactions involving legacy systemsCompensating transactionsMore flexible transaction processing

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SummarySummary

Transaction: user-defined collection of workDBMSs support ACID propertiesKnowledge of concurrency control and

recovery important for managing databasesTransaction design issues are importantTransaction processing is an important part

of workflow management