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Chapter 9

Chapter 9. Transaction Management and Concurrency Control Database Systems: Design, Implementation, and Management, Sixth Edition, Rob and Coronel. In this chapter, you will learn:. What a database transaction is and what its properties are How database transactions are managed

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Chapter 9

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

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

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

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

  43. Concurrency Controlwith 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 • Database recovery • 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 • Deferred write • 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

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