1. Transaction Management and Concurrency Control
Chapter 9
Transaction Management and Concurrency Control
Database Systems: Design, Implementation, and Management, Sixth 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, 6th Edition, Rob & Coronel

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

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

5. The Relational Schema for the Ch09_SaleCo Database
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

6. Evaluating Transaction Results
Not all transactions update the database
SQL code represents a transaction because database was accessed
Improper or incomplete transactions can have a devastating effect on database integrity
Some DBMSs provide means by which user can define enforceable constraints based on business rules
Other integrity rules are enforced automatically by the DBMS when table structures are properly defined, thereby letting the DBMS validate some transactions
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

7. Tracing the Transaction in the Ch09_SaleCo Database
Figure 9.2
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

8. Transaction Properties
Atomicity
Requires that all operations (SQL requests) of a transaction be completed
Durability
Indicates permanence of database’s consistent state
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

9. Transaction Properties (continued)
Serializability
Ensures that the concurrent execution of several transactions yields consistent results
Isolation
Data used during execution of a transaction cannot be used by second transaction until first one is completed
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

10. Transaction Management with SQL
ANSI has defined standards that govern SQL database transactions
Transaction support is provided by two SQL statements: COMMIT and ROLLBACK
ANSI standards require that, when a transaction sequence is initiated by a user or an application program,
it must continue through all succeeding SQL statements until one of four events occurs
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

11. The Transaction Log Stores A record for the beginning of transaction
For each transaction component (SQL statement)
Type of operation being performed (update, delete, insert)
Names of objects affected by the transaction (the name of the table)
“Before” and “after” values for updated fields
Pointers to previous and next transaction log entries for the same transaction
The ending (COMMIT) of the transaction
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

12. A Transaction Log
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

13. Concurrency Control
Coordination of simultaneous transaction execution in a multiprocessing database system
Objective is to ensure transaction serializability in a multiuser database environment
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

14. Concurrency Control
Important  simultaneous execution of transactions over a shared database can create several data integrity and consistency problems
lost updates
uncommitted data
inconsistent retrievals
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

15. Normal Execution of Two Transactions
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

16. Lost Updates
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

17. Correct Execution of Two Transactions
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

18. An Uncommitted Data Problem
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19. Retrieval During Update
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

20. Transaction Results: Data Entry Correction
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

21. Inconsistent Retrievals
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

22. The Scheduler
Special DBMS program: establishes order of operations within which concurrent transactions are executed
Interleaves the execution of database operations to ensure serializability and isolation of transactions
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

23. The Scheduler (continued)
Bases its actions on concurrency control algorithms
Ensures computer’s central processing unit (CPU) is used efficiently
Facilitates data isolation to ensure that two transactions do not update the same data element at the same time
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

24. Read/Write Conflict Scenarios: Conflicting Database Operations Matrix
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

25. Concurrency Control with Locking Methods
Guarantees exclusive use of a data item to a current transaction
Required to prevent another transaction from reading inconsistent data
Lock manager
Responsible for assigning and policing the locks used by the transactions
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

26. Lock Granularity Indicates the level of lock use
Locking can take place at the following levels:
Database
Table
Page
Row
Field (attribute)
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

27. Lock Granularity (continued)
Database-level lock
Entire database is locked
Table-level lock
Entire table is locked
Page-level lock
Entire diskpage is locked
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

28. Lock Granularity (continued)
Row-level lock
Allows concurrent transactions to access different rows of the same table, even if the rows are located on the same page
Field-level lock
Allows concurrent transactions to access the same row, as long as they require the use of different fields (attributes) within that row
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

29. A Database-Level Locking Sequence
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

30. An Example of a Table-Level Lock
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

31. Example of a Page-Level Lock
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

32. An Example of a Row-Level Lock
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

33. Lock Types Binary lock Exclusive lock Shared lock
Has only two states: locked (1) or unlocked (0)
Exclusive lock
Access is specifically reserved for the transaction that locked the object
Must be used when the potential for conflict exists
Shared lock
Concurrent transactions are granted Read access on the basis of a common lock
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

34. An Example of a Binary Lock
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

35. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

36. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

37. Two-Phase Locking Protocol
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

38. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

39. How a Deadlock Condition Is Created
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

40. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

41. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

42. Wait/Die and Wound/Wait Concurrency Control Schemes
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

43. 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

44. 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)
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

45. Transaction Recovery Makes use of deferred-write and 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
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

46. Transaction Recovery (continued)
Write-through
Database is immediately updated by transaction operations during the transaction’s execution, even before the transaction reaches its commit point
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

47. A Transaction Log for Transaction Recovery Examples
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

48. Summary
Transaction
Sequence of database operations that access the database
Represents real-world events
Must be a logical unit of work
No portion of the transaction can exist by itself
Takes a database from one consistent state to another
One in which all data integrity constraints are satisfied
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

49. Summary (continued)
SQL provides support for transactions through the use of two statements: COMMIT and ROLLBACK
Concurrency control coordinates the simultaneous execution of transactions
Scheduler is responsible for establishing order in which concurrent transaction operations are executed
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

50. Summary (continued)
Lock guarantees unique access to a data item by a transaction
Database recovery restores the database from a given state to a previous consistent state
Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel