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ITEC 3220AUsing and Designing Database Systems
Instructor: Prof. Z. YangCourse Website: http://people.yorku.ca/~zyang/itec3220a.htm Office: TEL 3049
Chapter 10
Transaction Management and Concurrent Control
3
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
4
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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• Examine current account balance
• Consistent state after transaction• No changes made to Database
Example Transaction
SELECT ACC_NUM, ACC_BALANCEFROM CHECKACCWHERE ACC_NUM = ‘0908110638’;
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• Register credit sale of 100 units of product X to customer Y for $500
• Consistent state only if both transactions are fully completed
• DBMS doesn’t guarantee transaction represents real-world event
Example Transaction
UPDATE PRODUCTSET PROD_QOH = PROD_QOH - 100WHERE PROD_CODE = ‘X’;UPDATE ACCT_RECEIVABLESET ACCT_BALANCE = ACCT_BALANCE + 500WHERE ACCT_NUM = ‘Y’;
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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
8
• 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
Transaction Properties
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• 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
Transaction Management with SQL
10
• 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
Transaction Log
11
Transaction LogExample
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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.
PROD_CODE PROD_QOH
ABC 1205
PRODUCTPARTPART_CODE PART_QOH
A 567
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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• 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
Concurrency Control
15
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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• 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
The Scheduler
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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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• Two states– Locked (1) – Unlocked (0)
• Locked objects unavailable to other objects– Unlocked objects open to any
transaction– Transaction unlocks object when
complete
Binary Locks
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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)
X S _
X No No Yes
S No Yes Yes
_ Yes Yes Yes
T1T2
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Two-Phase Lockingto 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 Lockingto 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
40
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
T1 T2 T3
R(A)
W(B)
W(A)
Commit A, B
W(B)
Commit B
W(B)
Commit B
Given schedule S1 as follows, and the locks won’t be released until commit. Is there any deadlock in S1 using Shared/Exclusive lock.
43
More Examples
• Let transactions T1, T2, and T3 be defined to perform the following operations:T1: Add one to AT2: Double AT3: 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.
44
More Examples (Cont’d)
T1 T2 T3
T11:Read (A), A ← A+1T12:Update (A)
T21:Read (A), A ← A*2T22:Update (A)
T31:Read (A), A = 1T32:Update (A)
• Suppose the following scheduleT11- T31- T12- T32- T21- T22 obeyed the two-phase locking algorithm. Explain what could be produced by the schedule.