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Chapter 15
Transaction Management
Database Design, Application Development, and
Administration, 5th Edition
Copyright © 2011 by Michael V. Mannino. All rights reserved.
Outline

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Transaction basics
Concurrency control
Recovery management
Transaction design issues
Workflow management
Chapter 15: Transaction Management
Slide 2
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
Chapter 15: Transaction Management
Slide 3
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
Chapter 15: Transaction Management
Slide 4
ATM Transaction Example
START TRANSACTION
Display greeting
Get account number, pin, type, and amount
SELECT account number, type, and balance
If balance is sufficient then
UPDATE account by posting debit
UPDATE account by posting debit
INSERT history record
Display message and dispense cash
Print receipt if requested
End If
On Error: ROLLBACK
COMMIT
Chapter 15: Transaction Management
Slide 5
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
Chapter 15: Transaction Management
Slide 6
Transaction Processing Services
 Concurrency control
 Recovery management
 Service characteristics
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Transparent
Consume significant resources
Significant cost component
Transaction design important
Chapter 15: Transaction Management
Slide 7
Concurrency Control
 Problem definition
 Concurrency control problems
 Concurrency control tools
Chapter 15: Transaction Management
Slide 8
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
Chapter 15: Transaction Management
Slide 9
Conflicting Actions
 Conflicts involve one or more common items
 Same transaction order must be maintained for
each common database item
 No conflicts if just read operations on common
data
 Conflict classification
 W-W conflict: Wi(A)  Wj(A) and Wj(B)  Wi(B)
 R-W conflict: Ri(A)  Wj(A) and Rj(B)  Wi(B)
 W-R conflict: Wj(A)  Ri(A) and Wi(B)  Rj(B)
Chapter 15: Transaction Management
Slide 10
Conflict Examples
 Example 1: R2(A), W2(A), R1(A), R1(B), R2(B), W2(B)
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T1 first: R1(A), R1(B), R2(A), W2(A), R2(B), W2(B)
In T1 first, R1(A) precedes W2(A) and R1(B) precedes W2(B)
T2 first: R2(A), W2(A), R2(B), W2(B), R1(A), R1(B)
In T2 first, W2(A) precedes R1(A) and W2(B) precedes R1(B)
 Example 2: R2(A), W2(A), R1(A), R2(B), W2(B), R1(B)
 Serializable as the ordering of operations on common items is
equivalent to T2 first
 Conflicting actions occur in the same order as T2 first
Chapter 15: Transaction Management
Slide 11
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)
Chapter 15: Transaction Management
Slide 12
Uncommitted Dependency
Problem
Transaction A
Read SR (10)
SR = SR - 1
Write SR (9)
ROLLBACK
Chapter 15: Transaction Management
Time Transaction B
T1
T2
T3
T4 Read SR (9)
T5
Slide 13
Inconsistent Retrieval Problems
 Interference causes inconsistency among
multiple retrievals of a subset of data
 Incorrect summary
 Phantom read
 Non repeatable read
Chapter 15: Transaction Management
Slide 14
Incorrect Summary Problem
Transaction A
Read SR1 (10)
SR1 = SR1 - 1
Write SR1 (9)
Read SR2 (5)
SR2 = SR2 - 1
Write SR2 (4)
Chapter 15: Transaction Management
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
Slide 15
Other Inconsistent Retrieval
Problems
 Non repeatable read
 Transaction A executes a query two times.
 In between the two executions, other transactions
change the data so that the second result is different
than the first result.
 Phantom read
 Transaction A: SELECT COUNT(*) … WHERE Salary
> 50000
 Transaction B: insert a new row with salary = 55000
 Transaction A: reexecute query again with Salary >
50000
Chapter 15: Transaction Management
Slide 16
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
Chapter 15: Transaction Management
Slide 17
Basic Compatibility Matrix
User 2 Requests
User 1 Holds
S Lock
X Lock
S Lock
Lock granted
User 2 waits
X Lock
User 2 waits
User 2 waits
Chapter 15: Transaction Management
Slide 18
Locking Granularity
Database
Table
Index
Page
Row
Column
Chapter 15: Transaction Management
Slide 19
Intention Locks
 Allow transactions to quickly determine if rows
(or pages) in a given table have been locked by
other transactions
 Obtain intention lock on larger grained object
before smaller grained object
 Types
 Intention shared (IS): obtain IS lock on a larger
grained object if needing shared locks on some finer
grained objects
 Intention exclusive (IX): obtain IX lock on a larger
grained object if needing exclusive locks on some
finer grained objects
Chapter 15: Transaction Management
Slide 20
Extended Compatibility Matrix
User 2 Requests
User 1 Holds IS lock
IX lock
SIX lock
S lock
X lock
IS
Grant
Grant
Grant
Grant
Wait
IX
Grant
Grant
Wait
Wait
Wait
SIX
Grant
Wait
Wait
Wait
Wait
S
Grant
Wait
Wait
Grant
Wait
X
Wait
Wait
Wait
Wait
Wait
Chapter 15: Transaction Management
Slide 21
Deadlock (Mutual Waiting)
Transaction A
XLock SR1
Time Transaction B
T1
T2
XLock SR2 (wait)
T3
T4
Chapter 15: Transaction Management
XLock SR2
XLock SR1 (wait)
Slide 22
Deadlock Resolution
 Detection
 Overhead is reasonable for deadlocks among
a small number of transactions
 Used by enterprise DBMSs
 Timeout
 Waiting limit
 Can abort transactions that are not
deadlocked
 Timeout interval is difficult to determine
Chapter 15: Transaction Management
Slide 23
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
Chapter 15: Transaction Management
Slide 24
Locks held
2PL Implementation
e
as
h
p
g
in
w
o
Gr
BOT
Chapter 15: Transaction Management
Time
Shrinking
phase
EOT
Slide 25
Extensions for Locking Granularity
 Small extension of 2PL
 Lock from root (coarsest level) to finer
levels
 Release locks in opposite order (finest to
coarsest)
 Locking details
 Obtain S or IS lock: must hold IS or IX lock on
parent
 Obtain X, IX, or SIX: must hold IX or SIX lock
on parent
Chapter 15: Transaction Management
Slide 26
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
Chapter 15: Transaction Management
Slide 27
Recovery Management
 Device characteristics and failure types
 Recovery tools
 Recovery processes
Chapter 15: Transaction Management
Slide 28
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
Chapter 15: Transaction Management
Slide 29
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
Chapter 15: Transaction Management
Slide 30
Transaction Log
 History of database changes
 Large storage overhead
 Operations
 Undo: revert to previous state (write old value)
 Redo: use new state (write new value)
 Fundamental tool of recovery
management
Chapter 15: Transaction Management
Slide 31
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
Chapter 15: Transaction Management
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,
…>
Slide 32
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
Chapter 15: Transaction Management
Slide 33
Cache Consistent Checkpoint
 Traditional checkpoint approach
 Longest checkpoint time but smallest
recovery time
 Actions
 No new transactions can start and existing
transactions must stop.
 Write all log buffers to disk.
 Write all dirty database pages to disk.
 Write checkpoint record to the log file.
Chapter 15: Transaction Management
Slide 34
Fuzzy Checkpoint
 Reduced checkpoint time but increased
recovery time
 Actions
 No new transactions can start and existing
transactions must stop.
 Write all log buffers to disk.
 Write all dirty database pages older than the previous
checkpoint to disk: fewer disk pages than cache
consistent checkpoint.
 Write checkpoint record to the log file.
 Record the set of log pages that are dirty since the
previous checkpoint
Chapter 15: Transaction Management
Slide 35
Incremental Checkpoint
 Smallest checkpoint time but largest recovery
time
 Oracle approach
 Actions
 No new transactions can start and existing
transactions must stop.
 Write all log buffers to disk.
 Write log position of the oldest dirty database page:
provides a starting point for recovery
 Control amount of restart work by the frequency of
writing dirty database pages
Chapter 15: Transaction Management
Slide 36
Comparison of Checkpoint
Types
Log Pages
All Dirty
DB Pages
Cache Consistent
Log Pages
Older Dirty
DB Pages
Fuzzy
Log Pages
Log
Position
Incremental
Chapter 15: Transaction Management
Slide 37
Other Recovery Tools
 Force writing
 Checkpoint time
 End of transaction
 Database backup
 Complete
 Incremental
Chapter 15: Transaction Management
Slide 38
Recovery from a Media Failure
 Restore database from the most recent
backup
 Redo all committed transactions since the
most recent backup
 Restart active transactions
Chapter 15: Transaction Management
Slide 39
Recovery Timeline
Checkpoint
Failure
Time
T1
T2
T3
T4
T5
Chapter 15: Transaction Management
Slide 40
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
Chapter 15: Transaction Management
Slide 41
Immediate Update Recovery
Class
T1
T2
T3
T4
T5
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
Chapter 15: Transaction Management
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
Slide 42
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
Chapter 15: Transaction Management
Restart Work
None
Redo forward from
first log record
Redo forward from
first log record
None
None
Slide 43
Recovery Examples
 Chapter 15 Recovery Example document
 Transaction log and failure time
 Read log backwards starting with the last
log record
 Immediate update method
 Deferred update method
Chapter 15: Transaction Management
Slide 44
Oracle Recovery Features
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Incremental checkpoints
Immediate update approach
Mean Time to Recover (MTTR) parameter
MTTR advisor
Dynamic dictionary views to monitor
recovery state
 Flashback recovery
Chapter 15: Transaction Management
Slide 45
Oracle Recovery Parameters
 Tradeoff: checkpoint overhead versus recovery time
 Parameter choice
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Log checkpoint timeout: number of seconds
Log checkpoint interval: number of redo blocks
MTTR target: expected mean time to recover in seconds
Set either the MTTR target parameter or the checkpoint timeout
and interval parameters
 MTTR advisor executes transaction loads with different
parameter MTTR values.
Chapter 15: Transaction Management
Slide 46
Dynamic Recovery View
(V$Instance_Recovery)
 Monitors the mechanisms available to
users to limit recovery I/O
 Columns
 Estimated number of data blocks to be
processed during recovery
 Estimated and current number of redo blocks
required for recovery
 Target and estimated MTTR
 Optimal log file size
Chapter 15: Transaction Management
Slide 47
Flashback Recovery
 Flash database to store old versions
 Historical queries
 Specify past table states using timestamps
 Insert data into current table using timestamp
query
 Alternative recovery method
 Fast recovery of individual tables to a
timestamp
 Useful for short-term restoration: 2 to 7 days
 Separate flashback logs
Chapter 15: Transaction Management
Slide 48
Transaction Design Issues
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Transaction boundary
Isolation levels
Deferred constraint checking
Savepoints
Chapter 15: Transaction Management
Slide 49
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
Chapter 15: Transaction Management
Slide 50
Registration Form Example
Chapter 15: Transaction Management
Slide 51
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
Chapter 15: Transaction Management
Slide 52
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
confusion
Chapter 15: Transaction Management
Slide 53
Revised Airline Reservation Example
Get reservation preferences
SELECT departure and return flight rows
if reservation is acceptable then
START TRANSACTION
UPDATE seats remaining of departure flight row
UPDATE seats remaining of return flight row
INSERT reservation row
End If
On Error: ROLLBACK
COMMIT
Send receipt to customer
Chapter 15: Transaction Management
Slide 54
Transaction Design Example
 Tennis reservations
 Reserve court 7 days in advance
 Choose date, start time, duration, court, and
partner(s)
 Subject to partner availability
 Predictable hot spots
 Reservation table: date, start time, duration,
court, player1, player2, player3, player4
 Wait time for partner selection after date, time,
court, and duration selected
Chapter 15: Transaction Management
Slide 55
Tennis Reservation Calendar
Chapter 15: Transaction Management
Slide 56
Tennis Reservation Selection
Chapter 15: Transaction Management
Slide 57
Choose Partner(s)
Chapter 15: Transaction Management
Slide 58
Solutions for Transaction Design
 Minimal duration
 Transaction begins after final selection (date, time,
court, duration, and partners)
 No wait time for transaction duration
 Reservation may not succeed after selection
 Longer duration with wait time
 Transaction begins if selection of date, time, court,
and duration are not already selected
 Locks must be held until wait time limit
 Reservation will succeed after partner selection
Chapter 15: Transaction Management
Slide 59
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
Chapter 15: Transaction Management
Slide 60
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
Chapter 15: Transaction Management
Slide 61
Scholar’s Lost Update
Transaction A
Time
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
Chapter 15: Transaction Management
Transaction B
T12
Obtain X lock on SR
T13
Write SR (9)
Slide 62
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
Chapter 15: Transaction Management
Slide 63
Save Points
 Some transactions have tentative actions
 SAVEPOINT statement determines
intermediate points
 ROLLBACK to specified save points
Safe
operations
Save point
Chapter 15: Transaction Management
Tentative
operations
Rollback to
save point
Commit
Slide 64
Workflow Management
 Workflow description
 Enabling technologies
 Advanced transaction management
Chapter 15: Transaction Management
Slide 65
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
Chapter 15: Transaction Management
Slide 66
Workflow Classification
Human-oriented
Communication
support
Computer-oriented
Transaction
support
Task complexity
Complex
Property sale
Insurance
claim
Utility service
Bid proposal
Travel
request
Simple
Meeting
notification
Low
Chapter 15: Transaction Management
Task structure
High
Slide 67
Enabling Technologies
 Distributed object management
 Many kinds of non traditional data
 Data often dispersed in location
 Workflow modeling
 Specification
 Simulation
 Optimization
Chapter 15: Transaction Management
Slide 68
Advanced Transaction
Management
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Conversational transactions
Transactions with complex structure
Transactions involving legacy systems
Compensating transactions
More flexible transaction processing
Chapter 15: Transaction Management
Slide 69
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: Transaction Management
Slide 70