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Chapter 15
Transaction Management
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 Example
START TRANSACTION
Display greeting
Get reservation preferences from user
SELECT departure and return flight records
If reservation is acceptable then
UPDATE seats remaining of departure flight record
UPDATE seats remaining of return flight record
INSERT reservation record
Print ticket if requested
End If
On 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
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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
T3
SR = SR -1
T4 If SR > 0 then
SR = SR -1
Write SR (9)
T5
T6 Write SR (9)
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Uncommitted Dependency
Problem
Transaction A
Read SR (10)
SR = SR - 1
Write SR (9)
ROLLBACK
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Time Transaction B
T1
T2
T3
T4 Read SR (9)
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
Read SR1 (10)
SR1 = SR1 - 1
Write SR1 (9)
Read SR2 (5)
SR2 = SR2 - 1
Write SR2 (4)
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Time
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
Transaction B
Read SR1 (9)
Sum = Sum + SR1
Read SR2 (5)
Sum = Sum + SR2
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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
Index
Page
Row
Column
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Deadlock (Mutual Waiting)
Transaction A
XLock SR1
Time Transaction B
T1
T2
XLock SR2 (wait)
T3
T4
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XLock SR2
XLock SR1 (wait)
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Deadlock 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)
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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Locks held
2PL Implementation
BOT
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as
h
p
g
in
w
o
Gr
e
Time
Shrinking
phase
EOT
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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
1
2
3
4
101001
101001
101001
101001
START
UPDATE
UPDATE
INSERT
5
101001
COMMIT 10:33
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10:29
10:30 Acct
10:30 Acct
10:32 Hist
Column Old New
10001 AcctBal 100 200
15147 AcctBal 500 400
25045 *
<1002,
500,
…>
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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
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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
Checkpoint
Failure
Time
T1
T2
T3
T4
T5
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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
T1
T2
T3
T4
T5
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Description
Finished before CP
Started before CP;
finished before failure
Started after CP;
finished before failure
Started before CP; not
yet finished
Started after CP; not
yet finished
Restart Work
None
Redo forward from
checkpoint
Redo forward from
checkpoint
Undo backwards
from most recent log
record
Undo backwards
from most recent log
record
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Deferred Update Recovery
Class Description
T1
Finished before CP
T2
Started before CP;
finished before failure
T3
Started after CP;
finished before failure
T4
Started before CP; not
yet finished
T5
Started after CP; not
yet finished
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Restart Work
None
Redo forward from
first log record
Redo forward from
first log record
None
None
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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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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
Repeatable
read
Long
Long
Short (S),
Long (X)
All except
uncommitted
dependency
Phantom reads
Serializable
Long
Long
Long
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None
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Scholar’s Lost Update
Transaction A
Obtain S lock on SR
Read SR (10)
Release S lock on SR
If SR > 0 then SR = SR -1
Obtain X lock on SR
Write SR (9)
Commit
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Time
Transaction B
T1
T2
T3
T4
T5
T6
Obtain S lock on SR
Read SR (10)
T7
T8
Release S lock on SR
If SR > 0 then SR = SR -1
T9
T10
T11
T12
T13
Obtain X lock on SR
Write SR (9)
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Integrity Constraint Timing
Most constraints checked immediately
 Can defer constraint checking to EOT
 SQL SET CONSTRAINTS statement

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Save Points
Some transactions have tentative actions
 SAVEPOINT statement determines
intermediate points
 ROLLBACK to specified save points

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Workflow Management
Workflow description
 Enabling technologies
 Advanced transaction management

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Workflows
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 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

McGraw-Hill/Irwin
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