Download Lecture Note 9

ITEC 3220M
Using and Designing Database Systems
Instructor: Prof. Z. Yang
Course Website:
http://people.yorku.ca/~zyang/itec
3220m.htm
Office: TEL 3049
Chapter 10
Transaction Management and
Concurrent Control
What is a Transaction?
• Any action that reads from and/or writes
to a database may consist of
– Simple SELECT statement to generate a list
of table contents
– A series of related UPDATE statements to
change the values of attributes in various
tables
– A series of INSERT statements to add rows to
one or more tables
– A combination of SELECT, UPDATE, and
INSERT statements
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What is a Transaction? (continued)
• A logical unit of work that must be either entirely
completed or aborted
• Successful transaction changes the database
from one consistent state to another
– One in which all data integrity constraints are
satisfied
• Most real-world database transactions are
formed by two or more database requests
– The equivalent of a single SQL statement in an
application program or transaction
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Example Transaction
• Examine current account balance
SELECT ACC_NUM, ACC_BALANCE
FROM CHECKACC
WHERE ACC_NUM = ‘0908110638’;
• Consistent state after transaction
• No changes made to Database
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Example Transaction
• Register credit sale of 100 units of product X to
customer Y for $500
UPDATE PRODUCT
SET PROD_QOH = PROD_QOH - 100
WHERE PROD_CODE = ‘X’;
UPDATE ACCT_RECEIVABLE
SET ACCT_BALANCE = ACCT_BALANCE + 500
WHERE ACCT_NUM = ‘Y’;
• Consistent state only if both transactions are fully
completed
• DBMS doesn’t guarantee transaction represents
real-world event
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Incomplete Transactions
• Reasons:
– An anomaly arises during execution
(automatically restart)
– System crashes
– An unexpected situation during transaction
execution
• May bring database to inconsistent state
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Transaction Properties
• Atomicity
– All transaction operations must be completed
– Incomplete transactions aborted
• Durability
– Permanence of consistent database state
• Serializability
– Conducts transactions in serial order
– Important in multi-user and distributed databases
• Isolation
– Transaction data cannot be reused until its execution
complete
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Transaction Management with
SQL
• Transaction support
– COMMIT
– ROLLBACK
• User initiated transaction sequence must
continue until:
–
–
–
–
COMMIT statement is reached
ROLLBACK statement is reached
End of a program reached
Program reaches abnormal termination
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Transaction Log
• Tracks all transactions that update database
• May be used by ROLLBACK command
• May be used to recover from system failure
• Log stores
– Record for beginning of transaction
– Each SQL statement
• Operation
• Names of objects
• Before and after values for updated fields
• Pointers to previous and next entries
– Commit Statement
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Transaction Log
Example
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Example
• Suppose that you are a manufacturer of product ABC, which
is composed of parts A, B, C. Each time a new product ABC is
created, it must be added to the product inventory, using the
PROD_QOH in PRODUCT table. And each time the product is
created the parts inventory, using PART_QOH in PART table
must be reduced by one each of parts, A, B, and C.
PART
PRODUCT
PART_CODE
PART_QOH
PROD_CODE
PROD_QOH
A
567
ABC
1205
B
98
C
549
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Example (Cont’d)
Given the information, answer:
• How many database requests can you identify for an
inventory update for both PRODUCT and PART?
• Using SQL, write each database request you have
identified above.
• Write the complete transactions.
• Write the transaction log, using the template in slide 11.
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Concurrency Control
• Coordinates simultaneous transaction
execution in multiprocessing database
– Ensure serializability of transactions in
multiuser database environment
– Potential problems in multiuser
environments
•Lost updates
•Uncommitted data
•Inconsistent retrievals
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Normal Execution of Two
Transactions
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Lost Updates
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More Example
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Correct Execution of Two
Transactions
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An Uncommitted Data Problem
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Retrieval During Update
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Transaction Results:
Data Entry Correction
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Inconsistent Retrievals
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Example
• A department store runs a multiuser DBMS on a local area
network file server which does not enforce concurrency control.
One customer has a balance due of $250 when the following
three transactions related to this customer were processed at
the same time:
–Payment of $250
–Purchase on credit of $100
–Merchandise return of $50.
Each transaction reads the customer record when the balance was
$250. the updated record was returned to the database in the
order shown above.
• What balance will be for the customer after the last transaction
was completed?
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The Scheduler
• Establishes order of concurrent
transaction execution
• Interleaves execution of database
operations to ensure serializability
• Bases actions on concurrency control
algorithms
– Locking
– Time stamping
• Ensures efficient use of computer’s CPU
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Read/Write Conflict Scenarios:
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Concurrency Control
with Locking Methods
• Lock guarantees current transaction exclusive
use of data item
• Acquires lock prior to access
• Lock released when transaction is completed
• DBMS automatically initiates and enforces
locking procedures
• Managed by lock manager
• Lock granularity indicates level of lock use
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Locking Mechanisms
• Locking level:
–
–
–
–
Database – used during database updates
Table – used for bulk updates
Block or page – very commonly used
Row – only requested row; fairly commonly
used
– Field – requires significant overhead;
impractical
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Locking Granularity
• Granularity refers to the level of the database
item locked.
• A trade-off between overhead and waiting.
• Holding locks at a fine level decreases waiting
among users but increase the system
overhead.
• Holding locks at a coarser level reduces the
number of locks but increases the amount of
waiting.
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A Database-Level Locking
Sequence
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An Example of a Table-Level Lock
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Example of a Page-Level Lock
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An Example of a Row-Level Lock
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Binary Locks
• Two states
– Locked (1)
– Unlocked (0)
• Locked objects unavailable to other
objects
– Unlocked objects open to any transaction
– Transaction unlocks object when complete
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An Example of a Binary Lock
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Shared/Exclusive Locks
• Shared
– Exists when concurrent transactions granted READ
access
– Produces no conflict for read-only transactions
– Issued when transaction wants to read and exclusive
lock not held on item
• Exclusive
– Exists when access reserved for locking transaction
– Used when potential for conflict exists
– Issued when transaction wants to update unlocked
data
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Shared/Exclusive Locks (Cont’d)
T2
X
S
_
X
No
No
Yes
S
No
Yes
Yes
_
Yes
Yes
Yes
T1
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Two-Phase Locking
to Ensure Serializability
• Defines how transactions acquire and
relinquish locks
• Guarantees serializability, but it does
not prevent deadlocks
– Growing phase, in which a transaction
acquires all the required locks without
unlocking any data
– Shrinking phase, in which a transaction
releases all locks and cannot obtain any
new lock
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Two-Phase Locking
to Ensure Serializability (continued)
• Governed by the following rules:
– Two transactions cannot have conflicting
locks
– No unlock operation can precede a lock
operation in the same transaction
– No data are affected until all locks are
obtained—that is, until the transaction is in
its locked point
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Two-Phase Locking Protocol
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Deadlocks
• Condition that occurs when two transactions
wait for each other to unlock data
• Possible only if one of the transactions wants to
obtain an exclusive lock on a data item
– No deadlock condition can exist among shared locks
• Control through
– Prevention
– Detection
– Avoidance
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How a Deadlock Condition Is
Created
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Example on Concurrency Control
Given schedule S1 as follows, and the locks won’t be
released until commit. Is there any deadlock in S1
using Shared/Exclusive lock.
T1
T2
T3
R(A)
W(B)
W(A)
Commit A, B
W(B)
Commit B
W(B)
Commit B
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More Examples
• Let transactions T1, T2, and T3 be defined to
perform the following operations:
T1: Add one to A
T2: Double A
T3: Display A and then set A to one
• Suppose the structure for T1, T2, T3 is
indicated below. If the transactions execute
without any locking, please give an example of
wrong schedules.
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More Examples (Cont’d)
T1
T11:
Read (A), A ←
A+1
T12:
Update (A)
T2
T21:
Read (A), A ←
A*2
T22:
Update (A)
T3
T31:
Read (A), A = 1
T32:
Update (A)
• Suppose the following schedule
T11- T31- T12- T32- T21- T22 obeyed the two-phase
locking algorithm. Explain what could be produced by
the schedule.
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