1. Transaction Management and Concurrency Control
Chapter 9
Transaction Management and Concurrency Control
Database Systems: Design, Implementation, and Management, Fifth Edition, Rob and Coronel

2. In this chapter, you will learn:
What a database transaction is and what its properties are
How database transactions are managed
What concurrency control is and what role it plays in maintaining the database’s integrity
What locking methods are and how they work
How database recovery management is used to maintain database integrity
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

3. What is a Transaction? Logical unit of work
Must be either entirely completed or aborted
No intermediate states are acceptable
Figure 9.1
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 4

4. Example Transaction
Examine current account balance
Consistent state after transaction
No changes made to Database
SELECT ACC_NUM, ACC_BALANCE FROM CHECKACC WHERE ACC_NUM = ‘ ’;
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 6

5. Example Transaction
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
UPDATE PRODUCT SET PROD_QOH = PROD_QOH WHERE PROD_CODE = ‘X’;
UPDATE ACCT_RECEIVABLE
SET ACCT_BALANCE = ACCT_BALANCE WHERE ACCT_NUM = ‘Y’;
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 6

6. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 9

7. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 10

8. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 12

9. Transaction Log Example
Table 9.1
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 12

10. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 14

11. Lost Updates Table 9.2 Table 9.3
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

12. Uncommitted Data Table 9.4 Table 9.5
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

13. Inconsistent Retrievals
Table 9.6
Table 9.7
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

14. Inconsistent Retrievals (con’t.)
Table 9.8
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

15. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 23

16. Read/Write Conflict Scenarios: Conflicting Database Operations Matrix
Table 9.9
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

17. 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 25

18. Database-Level Locking Sequence
Figure 9.2
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

19. Table-Level Lock Example
Figure 9.3
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

20. Page-Level Lock Example
Figure 9.4
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

21. Row-Level Lock Example
Figure 9.5
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

22. Binary Locks Two states Locked objects unavailable to other objects
Unlocked (0)
Locked objects unavailable to other objects
Unlocked objects open to any transaction
Transaction unlocks object when complete
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 31

23. Example of Binary Lock Table
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 31

24. Shared/Exclusive Locks
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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

25. Problems with Locking Transaction schedule may not be serializable
Managed through two-phase locking
Schedule may create deadlocks
Managed by using deadlock detection and prevention techniques
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

26. Two-Phase Locking Growing phase Shrinking phase Governing 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
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

27. Two-Phase Locking Protocol
Figure 9.6
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

28. Deadlocks
Occurs when two transactions wait for each other to unlock data
Called deadly embrace
Control techniques
Deadlock prevention
Deadlock detection
Deadlock avoidance
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

29. How Deadlock Conditions Created
Table 9.11
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel

30. Concurrency Control with Time Stamping Methods
Assigns global unique time stamp to each transaction
Produces order for transaction submission
Properties
Uniqueness
Monotonicity
DBMS executes conflicting operations in time stamp order
Each value requires two additional time stamps fields
Last time field read
Last update
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 42

31. Concurrency Control with Optimistic Methods
Assumes most database operations do not conflict
Transaction executed without restrictions until committed
Phases:
Read Phase
Validation Phase
Write Phase
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 43

32. Database Recovery Management
Restores a database to previously consistent state
Based on the atomic transaction property
Level of backup
Full backup
Differential
Transaction log
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 44

33. Causes of Database Failure
Software
Hardware
Programming Exemption
Transaction
External
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 44

34. Transaction Recovery Deferred-write and Deferred-update Write-through
Changes are written to the transaction log
Database updated after transaction reaches commit point
Write-through
Immediately updated by during execution
Before the transaction reaches its commit point
Transaction log also updated
Transaction fails, database uses log information
to ROLLBACK
Database Systems: Design, Implementation, & Management, 5th Edition, Rob & Coronel 47