1. Transaction Management and Concurrency Control
Chapter 7
Transaction Management and Concurrency Control

2. Study Objectives
Understand what a database transaction is and what its properties are
Identify how database transactions are managed
Learn what concurrency control is and what role it plays in maintaining the database’s integrity
Learn what locking methods are and how they work
Understand how database recovery management is used to maintain database integrity

3. Fundamental capability of a DBMS
Users don’t need to know how data is stored or manipulated
Users should be able to add, change, and delete records during updates
Users should be able to view and manipulate data during retrieval

4. What is a Transaction?
A set of steps completed by a DBMS to accomplish a single user task.
Must be either entirely completed or aborted
No intermediate states are acceptable 4

5. Transaction Properties
Atomicity
All transaction operations must be completed
Incomplete transactions must be aborted
Durability
Once a transaction is completed, its changes are made is permanent.
Serializability
Conducts transactions in serial order
Important in multi-user and distributed databases
Isolation
Transaction data cannot be used until its execution is completed 9

6. 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

7. Concurrency Control
Coordinates simultaneous transaction execution in multiprocessing database
Ensure serializability of transactions in multiuser database environment
Solve potential problems in multiuser environments
Lost updates
Uncommitted data
Inconsistent retrievals 14

8. Read from this slide to slide #11
Best scenario of updating data
Read from this slide to slide #11
Before update
DBMS reads data from database into RAM for Ryan
Ryan changes data in RAM
DBMS updates database with Ryan’s change

9. Best scenario of updating data
After Ryan’s update and before Elena’s
DBMS reads database data into RAM for Elena
Elena changes data in RAM
DBMS updates database with Elena’s change

10. Concurrency Control problem
Database before updates
DBMS reads database data into RAM for Ryan
DBMS reads database data into RAM for Elena
Ryan changes data in RAM

11. Concurrency Control problem
Slide #11
Elena updates data in RAM
DBMS updates database with Ryan’s change
DBMS updates database with Elena’s change; Ryan’s update is lost!

12. How to avoid previous slide problem??
Simply use of batch processing concept
Update all data once a day
Considered as one phase locking
What is a problem with this method?

13. Locking Schemes Two-Phase Locking Deadlock Locking on PC-Based DBMSs
Locks are held until required updates completed
Deadlock
Occurs when two users hold more than one lock at a time
DBMS chooses method to break deadlock
One user becomes ‘victim’
Locking on PC-Based DBMSs
Table or row locked, not both
Usually more limited than locking facilities on mainframe DBMSs

14. Read from this slide to slide #16
Two-Phase Locking
Read from this slide to slide #16
Database before updates
DBMS reads database data into RAM for Ryan and locks record
Elena requests same record and request fails
Ryan changes data in RAM; Elena’s request for same record again fails

15. Two-Phase Locking (con’t.)
DBMS updates database with Ryan’s change; Elena’s request for same record again fails
DBMS unlocks record; DBMS reads database data into RAM for Elena and locks record
Elena changes data in RAM

16. Two-Phase Locking (con’t.)
Slide #16
DBMS updates database with Elena’s change
DBMS unlocks record

17. Two-Phase Locking Protocol
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

18. Two-Phase Locking Protocol

19. Database-Level Locking Sequence

20. Table-Level Lock Example

21. Page-Level Lock Example

22. Row-Level Lock Example
Lock row 2 request

23. Deadlocks
Occurs when two transactions wait for each other to unlock data
Called deadly embrace
Control techniques
Abort entire transactions..
Abort most recent transaction
Abort transactions that require least changes

24. Deadlock

25. How Deadlock Conditions Created

26. Time Stamping Methods Helps detect and resolve deadlocks
DBMS assigns a unique time when the update started
Last time field read
Last update time
DBMS executes conflicting operations in time stamp order
Eliminates processing time needed to apply and release locks 42

27. Recovery Mechanism for recovering damaged database
The return of database to correct state is called recovery
Simplest recovery involves using backups
Other recovery methods
Journaling
Forward recovery
Backward recovery
PC-based

28. Forward Recovery
What would be the answer of the question on page 239 if the catastrophe occurred after 9:00pm?
Different than backward

29. Forward Recovery Useful when a catastrophe destroys the database.
Since the Db is no longer current, the DBA executes a DBMS recovery program that applies the after committed transactions from the log to bring the database up to date.
Figure in next slide

30. Backward Recovery

31. Backward recovery Useful when the DB has not actually been destroyed.
The DB is still valid (unlike forward recovery case)
Only transactions are either incorrect or in the midstream
Figure in next slide
Still possible to use “roll back – see SQL note for more details” as a recovery mechanism.
Roll back: go back to the previous state

32. Data Replication
Manage multiple copies of same data in multiple locations
Maintained for performance or other reasons
Ease of access and portability

33. Replicas From Master Database

34. DBMS Synchronizes Databases