Download Chapter 15 of Database Design, Application Development and

Chapter 15
Transaction Management
McGraw-Hill/Irwin
Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.
Outline





Transaction basics
Concurrency control
Recovery management
Transaction design issues
Workflow management
15-2
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
15-3
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
15-4
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
15-5
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
15-6
Transaction Processing
Services
 Concurrency control
 Recovery management
 Service characteristics




Transparent
Consume significant resources
Significant cost component
Transaction design important
15-7
Concurrency Control
 Problem definition
 Concurrency control problems
 Concurrency control tools
15-8
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
15-9
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)
15-10
Uncommitted Dependency
Problem
Transaction A
Read SR (10)
SR = SR - 1
Write SR (9)
ROLLBACK
Time Transaction B
T1
T2
T3
T4 Read SR (9)
T5
15-11
Inconsistent Retrieval Problems
 Interference causes inconsistency among
multiple retrievals of a subset of data
 Incorrect summary
 Phantom read
 Non repeatable read
15-12
Incorrect Summary Problem
Transaction A
Read SR1 (10)
SR1 = SR1 - 1
Write SR1 (9)
Read SR2 (5)
SR2 = SR2 - 1
Write SR2 (4)
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
15-13
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
15-14
Locking Granularity
Database
Table
Index
Page
Row
Column
15-15
Deadlock (Mutual Waiting)
Transaction A
XLock SR1
Time Transaction B
T1
T2
XLock SR2 (wait)
XLock SR2
T3
T4
XLock SR1 (wait)
15-16
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
15-17
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
15-18
Locks held
2PL Implementation
BOT
as
h
p
g
in
w
o
Gr
e
Time
Shrinking
phase
EOT
15-19
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
15-20
Recovery Management
 Device characteristics and failure types
 Recovery tools
 Recovery processes
15-21
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
15-22
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
15-23
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
15-24
Transaction Log Example
LSN TransNo Action
Time Table Row
1
2
3
4
101001
101001
101001
101001
START
UPDATE
UPDATE
INSERT
10:29
10:30 Acct
10:30 Acct
10:32 Hist
5
101001
COMMIT 10:33
Column Old New
10001 AcctBal 100 200
15147 AcctBal 500 400
25045 *
<1002,
500,
…>
15-25
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
15-26
Other Recovery Tools
 Force writing
 Checkpoint time
 End of transaction
 Database backup
 Complete
 Incremental
15-27
Recovery from a Media Failure
 Restore database from the most recent
backup
 Redo all committed transactions since the
most recent backup
 Restart active transactions
15-28
Recovery Timeline
Checkpoint
Failure
Time
T1
T2
T3
T4
T5
15-29
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
15-30
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
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
15-31
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
Restart Work
None
Redo forward from
first log record
Redo forward from
first log record
None
None
15-32
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
15-33
Transaction Design Issues

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Transaction boundary
Isolation levels
Deferred constraint checking
Savepoints
15-34
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
15-35
Registration Form Example
15-36
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
15-37
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
15-38
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
15-39
SQL Isolation Levels
Level
XLocks SLocks
PLocks
Interference
Read
None
uncommitted
None
None
Uncommitted
dependency
Read
committed
Long
Short
None
All except
uncommitted
dependency
Repeatable
read
Long
Long
Short (S), Phantom reads
Long (X)
Serializable
Long
Long
Long
None
15-40
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
Transaction B
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)
15-41
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
15-42
Save Points
 Some transactions have tentative actions
 SAVEPOINT statement determines
intermediate points
 ROLLBACK to specified save points
Safe
operations
Save point
Tentative
operations
Rollback to
save point
Commit
15-43
Workflow Management
 Workflow description
 Enabling technologies
 Advanced transaction management
15-44
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
15-45
Workflow Classification
Human-oriented
Computer-oriented
Communication
support
Transaction
support
Task complexity
Complex
Property sale
Insurance
claim
Utility service
Bid proposal
Travel
request
Simple
Meeting
notification
Low
Task structure
High
15-46
Enabling Technologies
 Distributed object management
 Many kinds of non traditional data
 Data often dispersed in location
 Workflow modeling
 Specification
 Simulation
 Optimization
15-47
Advanced Transaction Management

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Conversational transactions
Transactions with complex structure
Transactions involving legacy systems
Compensating transactions
More flexible transaction processing
15-48
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
15-49