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Transaction management and concurrency control

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Database Systems: Design, Implementation, and Management Eighth Edition

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Transaction management and concurrency control

  1. 1. Database Systems: Design, Implementation, and Management Eighth Edition Chapter 10 Transaction Management and Concurrency Control
  2. 2. Database Systems, 8th Edition 2 Objectives • In this chapter, you will learn: – About database transactions and their properties – What concurrency control is and what role it plays in maintaining the database’s integrity – What locking methods are and how they work
  3. 3. Database Systems, 8th Edition 3 Objectives (continued) • In this chapter, you will learn: (continued) – How stamping methods are used for concurrency control – How optimistic methods are used for concurrency control – How database recovery management is used to maintain database integrity
  4. 4. Database Systems, 8th Edition 4 What is a Transaction? • Logical unit of work that must be either entirely completed or aborted • Successful transaction changes 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 – Equivalent of a single SQL statement in an application program or transaction
  5. 5. Database Systems, 8th Edition 5
  6. 6. Database Systems, 8th Edition 6 Evaluating Transaction Results • Not all transactions update database • SQL code represents a transaction because database was accessed • Improper or incomplete transactions can have devastating effect on database integrity – Some DBMSs provide means by which user can define enforceable constraints – Other integrity rules are enforced automatically by the DBMS
  7. 7. Database Systems, 8th Edition 7 Figure 9.2
  8. 8. Database Systems, 8th Edition 8 Transaction Properties • Atomicity – All operations of a transaction must be completed • Consistency – Permanence of database’s consistent state • Isolation – Data used during transaction cannot be used by second transaction until the first is completed
  9. 9. Database Systems, 8th Edition 9 Transaction Properties (continued) • Durability – Once transactions are committed, they cannot be undone • Serializability – Concurrent execution of several transactions yields consistent results • Multiuser databases subject to multiple concurrent transactions
  10. 10. Database Systems, 8th Edition 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 • Transaction sequence must continue until: – COMMIT statement is reached – ROLLBACK statement is reached – End of program is reached – Program is abnormally terminated
  11. 11. Database Systems, 8th Edition 11 The Transaction Log • Transaction log stores: – A record for the beginning of transaction – For each transaction component: • Type of operation being performed (update, delete, insert) • Names of objects affected by transaction • “Before” and “after” values for updated fields • Pointers to previous and next transaction log entries for the same transaction – Ending (COMMIT) of the transaction
  12. 12. Database Systems, 8th Edition 12
  13. 13. Database Systems, 8th Edition 13 Concurrency Control • Coordination of simultaneous transaction execution in a multiprocessing database • Objective is to ensure serializability of transactions in a multiuser environment
  14. 14. Database Systems, 8th Edition 14 Lost Updates • Lost update problem: – Two concurrent transactions update same data element – One of the updates is lost • Overwritten by the other transaction
  15. 15. Database Systems, 8th Edition 15
  16. 16. Database Systems, 8th Edition 16 Uncommitted Data • Uncommitted data phenomenon: – Two transactions executed concurrently – First transaction rolled back after second already accessed uncommitted data
  17. 17. Database Systems, 8th Edition 17
  18. 18. Database Systems, 8th Edition 18 Inconsistent Retrievals • Inconsistent retrievals: – First transaction accesses data – Second transaction alters the data – First transaction accesses the data again • Transaction might read some data before they are changed and other data after changed • Yields inconsistent results
  19. 19. Database Systems, 8th Edition 19
  20. 20. Database Systems, 8th Edition 20
  21. 21. Database Systems, 8th Edition 21 The Scheduler • Special DBMS program – Purpose is to establish order of operations within which concurrent transactions are executed • Interleaves execution of database operations: – Ensures serializability – Ensures isolation • Serializable schedule – Interleaved execution of transactions yields same results as serial execution
  22. 22. Database Systems, 8th Edition 22 Concurrency Control with 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 transactions
  23. 23. Database Systems, 8th Edition 23 Lock Granularity • Indicates level of lock use • Locking can take place at following levels: – Database – Table – Page – Row – Field (attribute)
  24. 24. Database Systems, 8th Edition 24 Lock Granularity (continued) • Database-level lock – Entire database is locked • Table-level lock – Entire table is locked • Page-level lock – Entire diskpage is locked
  25. 25. Database Systems, 8th Edition 25 Lock Granularity (continued) • Row-level lock – Allows concurrent transactions to access different rows of same table • Even if rows are located on same page • Field-level lock – Allows concurrent transactions to access same row • Requires use of different fields (attributes) within the row
  26. 26. Database Systems, 8th Edition 26
  27. 27. Database Systems, 8th Edition 27
  28. 28. Database Systems, 8th Edition 28
  29. 29. Database Systems, 8th Edition 29
  30. 30. Database Systems, 8th Edition 30 Lock Types • Binary lock – Two states: locked (1) or unlocked (0) • Exclusive lock – Access is specifically reserved for transaction that locked object – Must be used when potential for conflict exists • Shared lock – Concurrent transactions are granted read access on basis of a common lock
  31. 31. Database Systems, 8th Edition 31
  32. 32. Database Systems, 8th Edition 32 Two-Phase Locking to Ensure Serializability • Defines how transactions acquire and relinquish locks • Guarantees serializability, but does not prevent deadlocks – Growing phase • Transaction acquires all required locks without unlocking any data – Shrinking phase • Transaction releases all locks and cannot obtain any new lock
  33. 33. Database Systems, 8th Edition 33 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
  34. 34. Database Systems, 8th Edition 34
  35. 35. Database Systems, 8th Edition 35 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
  36. 36. Database Systems, 8th Edition 36 Deadlocks (continued) • Three techniques to control deadlock: – Prevention – Detection – Avoidance • Choice of deadlock control method depends on database environment – Low probability of deadlock, detection recommended – High probability, prevention recommended
  37. 37. Database Systems, 8th Edition 37
  38. 38. Database Systems, 8th Edition 38 Concurrency Control with Time Stamping Methods • Assigns global unique time stamp to each transaction • Produces explicit order in which transactions are submitted to DBMS • Uniqueness – Ensures that no equal time stamp values can exist • Monotonicity – Ensures that time stamp values always increase
  39. 39. Database Systems, 8th Edition 39 Wait/Die and Wound/Wait Schemes • Wait/die – Older transaction waits and younger is rolled back and rescheduled • Wound/wait – Older transaction rolls back younger transaction and reschedules it
  40. 40. Database Systems, 8th Edition 40
  41. 41. Database Systems, 8th Edition 41 Concurrency Control with Optimistic Methods • Optimistic approach – Based on assumption that 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: read, validation, and write
  42. 42. Database Systems, 8th Edition 42 Database Recovery Management • Restores database to previous consistent state • Based on atomic transaction property – All portions of transaction treated as single logical unit of work – All operations applied and completed to produce consistent database • If transaction operation cannot be completed – Transaction aborted – Changes to database rolled back
  43. 43. Database Systems, 8th Edition 43 Transaction Recovery • Write-ahead-log protocol: ensures transaction logs are written before data is updated • Redundant transaction logs: ensure physical disk failure will not impair ability to recover • Buffers: temporary storage areas in primary memory • Checkpoints: operations in which DBMS writes all its updated buffers to disk
  44. 44. Database Systems, 8th Edition 44 Transaction Recovery (continued) • Deferred-write technique – Only transaction log is updated • Recovery process: – Identify last checkpoint – If transaction committed before checkpoint • Do nothing – If transaction committed after checkpoint • Use transaction log to redo the transaction – If transaction had ROLLBACK operation • Do nothing
  45. 45. Database Systems, 8th Edition 45 Transaction Recovery (continued) • Write-through technique – Database is immediately updated by transaction operations during transaction’s execution • Recovery process – Identify last checkpoint – If transaction was committed before checkpoint • Do nothing – If transaction committed after last checkpoint • DBMS redoes the transaction using “after” values – If transaction had ROLLBACK or was left active • Do nothing because no updates were made
  46. 46. Database Systems, 8th Edition 46
  47. 47. Database Systems, 8th Edition 47 Summary • Transaction: sequence of database operations that access database – Logical unit of work • No portion of transaction can exist by itself – Five main properties: atomicity, consistency, isolation, durability, and serializability • COMMIT saves changes to disk • ROLLBACK restores previous database state • SQL transactions are formed by several SQL statements or database requests
  48. 48. Database Systems, 8th Edition 48 Summary (continued) • Transaction log keeps track of all transactions that modify database • Concurrency control coordinates simultaneous execution of transactions • Scheduler establishes order in which concurrent transaction operations are executed • Lock guarantees unique access to a data item by transaction • Two types of locks: binary locks and shared/exclusive locks
  49. 49. Database Systems, 8th Edition 49 Summary (continued) • Serializability of schedules is guaranteed through the use of two-phase locking • Deadlock: when two or more transactions wait indefinitely for each other to release lock • Three deadlock control techniques: prevention, detection, and avoidance • Time stamping methods assign unique time stamp to each transaction – Schedules execution of conflicting transactions in time stamp order
  50. 50. Database Systems, 8th Edition 50 Summary (continued) • Optimistic methods assume the majority of database transactions do not conflict – Transactions are executed concurrently, using private copies of the data • Database recovery restores database from given state to previous consistent state

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