1. Transaction Management and Concurrent Control

2. 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

3. 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

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 = ‘ ’; 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’; 6

6. Incomplete Transactions
Reasons:
An anomaly arises during execution (automatically restart)
System crashes
An unexpected situation during transaction execution
May bring database to inconsistent state

7. 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 9

8. 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 10

9. 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 12

10. Transaction Log Example

11. 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 A
567 B 98 C
549
PROD_CODE
PROD_QOH
ABC
1205

12. 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.

13. 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 14

14. Normal Execution of Two Transactions

15. Lost Updates

16. More Example

17. Correct Execution of Two Transactions

18. An Uncommitted Data Problem

19. Retrieval During Update

20. Transaction Results: Data Entry Correction

21. Inconsistent Retrievals

22. 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?

23. 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 23

24. Read/Write Conflict Scenarios:

25. 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 25

26. 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

27. 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.

28. A Database-Level Locking Sequence

29. An Example of a Table-Level Lock

30. Example of a Page-Level Lock

31. An Example of a Row-Level Lock

32. 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 31

33. An Example of a Binary Lock

34. 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

35. Shared/Exclusive Locks (Cont’d)_ No
Yes T2 T1

36. 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

37. 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

38. Two-Phase Locking Protocol

39. 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

40. How a Deadlock Condition Is Created

41. 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
Commit B

42. More Example T1 T2 T3 R(C) R(B) W(B) R(A) W(A) W(C) Commit A
Commit A, B & C
Commit B

43. Concurrency Control with Time Stamping Methods
Assigns a global unique time stamp to each transaction
Produces an explicit order in which transactions are submitted to the DBMS
Uniqueness
Ensures that no equal time stamp values can exist
Monotonicity
Ensures that time stamp values always increase

44. Wait/Die and Wound/Wait Schemes
Older transaction waits and the younger is rolled back and rescheduled
Wound/wait
Older transaction rolls back the younger transaction and reschedules it

45. Wait/Die and Wound/Wait Concurrency Control Schemes

46. Example
Concurrency control is implemented based on time stamping method. Consider the following schedule: T1 T2
R(A)
W(A)
W(B)
W(C)
R(C)

47. Concurrency Control with Optimistic Methods
Optimistic approach
Based on the assumption that the majority of database operations do not conflict
Does not require locking or time stamping techniques
Transaction is executed without restrictions until it is committed
Phases are read, validation, and write

48. Better Performance than Locking

49. Example T1 T2
R(A)
W(A)
R(B)
commit

50. Database Recovery Management
Restores database from a given state, usually inconsistent, to a previously consistent state
Based on the atomic transaction property
All portions of the transaction must be treated as a single logical unit of work, in which all operations must be applied and completed to produce a consistent database
If transaction operation cannot be completed, transaction must be aborted, and any changes to the database must be rolled back (undone)

51. Transaction Recovery Deferred write Write-through
Transaction operations do not immediately update the physical database
Only the transaction log is updated
Database is physically updated only after the transaction reaches its commit point using the transaction log information
Write-through
Database is immediately updated by transaction operations during the transaction’s execution, even before the transaction reaches its commit point

52. Example
Describe the restart work if transaction T1 is committed after the checkpoint but prior to the failure. Assume that the recovery manager uses
the deferred update approach
The write though approach
Backup
Checkpoint
Failure T1

53. Review Transaction property Transaction log
Potential problems in multiuser environments
Different locking methods and how they work
Database recovery management