Transaction Management and Concurrency Control

1. Transaction Management and Concurrency Control MIS 304 Winter 2005

2. Review from last week • The Internet is largely dependent on database technology • Database “Middleware” links HTML and HTTP based systems to traditional Relational Database. • The HTML and HTTP architectures can make it more difficult to implement user friendly interfaces.

3. A leftover from last time. • Demonstration of Altova XML Spy feature to created Relational tables from an XML document.

4. Goals for this class • Understand how transactions are used in the databases and their applications. • Understand the technology of the database transactions. • Understand concurrency and locking technology and how they effect databases.

5. TRANSACTION

6. Sources of Transaction What are the sources of transactions? What generates the data?

7. Consistent State

8. Example of a Transaction Table: Stock X = 40 PartNo=12345 Consistent state UPDATE Stock SET X = X – 10 WHERE PartNo = 12345 Transaction(s) Table: Stock X = 30 PartNo=12345 Consistent state

9. 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

10. 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

11. The Relational Schema for the Ch09_SaleCo Database

12. Double Entry Bookkeeping • The bane of the database programmer. WHY?

13. Accounting Transactions • Represent a large component of “all” Transactions. • “Double Entry” Bookkeeping means that there are at least TWO components per transaction.

14. Evaluating Transaction Results • An accountant wishes to register the credit sale of 100 units of product X to customer Y in the amount of $500.00: • Reducing product X’s Quantity on hand by 100. • Adding $500.00 to customer Y’s accounts receivable. UPDATE PRODUCTSET PROD_QOH = PROD_QOH - 100WHERE PROD_CODE = ‘X’; UPDATE ACCRECSET AR_BALANCE = AR_BALANCE + 500WHERE AR_NUM = ‘Y’; • If the above two transactions are not completely executed, the transaction yields an inconsistent database.

15. 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

16. Tracing the Transaction in the Ch09_SaleCo Database Figure 9.2

17. Transaction Properties • Atomicity • Requires that all operations (SQL requests) of a transaction be completed • Durability • Indicates permanence of database’s consistent state

18. 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

19. 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

20. 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

21. A Transaction Log

22. Concurrency Control • Coordination of simultaneous transaction execution in a multiprocessing database system • Objective is to ensure transaction serializability in a multiuser database environment

23. 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

24. Normal Execution of Two Transactions

25. Lost Updates

26. Correct Execution of Two Transactions

27. An Uncommitted Data Problem

28. Retrieval During Update

29. Transaction Results: Data Entry Correction

30. Inconsistent Retrievals

31. 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

32. 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

33. Read/Write Conflict Scenarios: Conflicting Database Operations Matrix

34. Concurrency Controlwith Locking Methods • Lock • 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

35. Lock Granularity • Indicates the level of lock use • Locking can take place at the following levels: • Database • Table • Page • Row • Field (attribute)

36. Lock Granularity (continued) • Database-level lock • Entire database is locked • Table-level lock • Entire table is locked • Page-level lock • Entire diskpage is locked

37. 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

38. A Database-Level Locking Sequence

39. An Example of a Table-Level Lock

40. Example of a Page-Level Lock

41. An Example of a Row-Level Lock

42. Lock Types • Binary 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

43. An Example of a Binary Lock

44. 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

45. 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

46. Two-Phase Locking Protocol

47. 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

48. How a Deadlock Condition Is Created

49. 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

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