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Transaction concurrency control

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Transaction concurrency control

  1. 1. Transaction Management and Concurrency Control
  2. 2. What is a Transaction?• Logical unit of work• Must be either entirely completed or aborted• No intermediate states are acceptable Figure 9.1 2
  3. 3. Example Transaction • Examine current account balanceSELECT ACC_NUM, ACC_BALANCEFROM CHECKACCWHERE ACC_NUM = ‘0908110638’; • Consistent state after transaction • No changes made to Database 3
  4. 4. Example Transaction• Register credit sale of 100 units of product X to customer Y for $500 UPDATE PRODUCT SET PROD_QOH = PROD_QOH - 100 WHERE PROD_CODE = ‘X’; UPDATE ACCT_RECEIVABLE SET ACCT_BALANCE = ACCT_BALANCE + 500 WHERE ACCT_NUM = ‘Y’;• Consistent state only if both transactions are fully completed• DBMS doesn’t guarantee transaction represents real-world event 4
  5. 5. Transaction Properties (ACID)• Atomicity – All transaction operations must be completed – Incomplete transactions aborted• Durability – Permanence of consistent database state• Consistency / Serializability – Conducts transactions in serial order – Important in multi-user and distributed databases• Isolation – Transaction data cannot be reused until its execution complete 5
  6. 6. Example of Fund Transfer• Transaction to transfer $50 from account A to account B: 1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B) 6
  7. 7. Example of Fund Transfer• Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.• Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result. 7
  8. 8. Example of Fund Transfer• Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures. 8
  9. 9. Example of Fund Transfer• Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be). Can be ensured by running transactions serially, that is one after the other. 9
  10. 10. Transaction state• Active – the initial state; the transaction stays in this state while it is executing• Partially committed – after the final statement has been executed.• Failed – after the discovery that normal execution can no longer proceed. 10
  11. 11. Transaction state• Aborted – after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted: • restart the transaction – only if no internal logical error • kill the transaction• Committed, after successful completion. 11
  12. 12. DBMS Transaction Subsystem Transaction Scheduler/ Database Lock Manager Manager Manager Buffer Recovery Manager ManagerAccess FileManager ManagerSystems DatabaseManager and system catalog 12
  13. 13. DBMS Transaction Subsystem• Trans. Mgr. – coordinates transactions on behalf of application program. It communicates with scheduler.• Scheduler – implements a strategy for concurrency control.• Recovery manager – If any failure occurs, recovery manager handles it.• Buffer manager – in charge of transferring data between disk storage and main memory. 13
  14. 14. DBMS Transaction Subsystem• File manager – manipulates the underlying storage files and manages the allocation of storage space on disk.• Access manager – File manager does not directly manage the physical input and output of data, rather it passes the requests on to the access manager.• System manager – Appropriate access method is used to either read or write data into the system manager. 14
  15. 15. 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 15
  16. 16. Transaction Log• Tracks all transactions that update database• May be used by ROLLBACK command• May be used to recover from system failure• Log stores – Record for beginning of transaction – Each SQL statement • Operation • Names of objects • Before and after values for updated fields • Pointers to previous and next entries – Commit Statement 16
  17. 17. Transaction Log Example Table 9.1 17
  18. 18. Concurrency Control• Coordinates simultaneous transaction execution in multiprocessing database – Ensure serializability of transactions in multiuser database environment – Potential problems in multiuser environments • Lost updates • Uncommitted data • Inconsistent retrievals 18
  19. 19. Lost Updates Table 9.2 Table 9.3 19
  20. 20. Uncommitted Data Table 9.4 Table 9.5 20
  21. 21. Inconsistent Retrievals Table 9.6 Table 9.7 21
  22. 22. Inconsistent Retrievals (con’t.) Table 9.8 22
  23. 23. The Scheduler• Establishes order of concurrent transaction execution• Interleaves execution of database operations to ensure serializability• Bases actions on concurrency control algorithms – Locking – Time stamping• Ensures efficient use of computer’s CPU 23
  24. 24. Read/Write Conflict Scenarios:Conflicting Database Operations Matrix Table 9.9 24
  25. 25. Concurrency Control with Locking Methods• Lock guarantees current transaction exclusive use of data item• Acquires lock prior to access• Lock released when transaction is completed• DBMS automatically initiates and enforces locking procedures• Managed by lock manager• Lock granularity indicates level of lock use 25
  26. 26. Database-Level Locking Sequence Figure 9.2 26
  27. 27. Table-Level Lock ExampleFigure 9.3 27
  28. 28. Page-Level Lock ExampleFigure 9.4 28
  29. 29. Row-Level Lock ExampleFigure 9.5 29
  30. 30. Field Level Lock• Access same row as long as they require different fields (attributes) within that row 30
  31. 31. Lock Types• Binary Locks• Shared/Exclusive Locks 31
  32. 32. Binary Locks• Two states – Locked (1) – Unlocked (0)• Locked objects unavailable to other objects – Unlocked objects open to any transaction – Transaction unlocks object when complete 32
  33. 33. Example of Binary Lock Table Table 9.10 33
  34. 34. Shared/Exclusive Locks• Shared – Exists when concurrent transactions granted READ access – Produces no conflict for read-only transactions – Issued when transaction wants to read and exclusive lock not held on item• Exclusive – Exists when access reserved for locking transaction – Used when potential for conflict exists – Issued when transaction wants to update unlocked data 34
  35. 35. Problems with Locking• Transaction schedule may not be serializable – Managed through two-phase locking• Schedule may create deadlocks – Managed by using deadlock detection and prevention techniques 35
  36. 36. Two-Phase Locking• 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 36
  37. 37. Two-Phase Locking ProtocolFigure 9.6 37
  38. 38. Deadlocks• Occurs when two transactions wait for each other to unlock data• Called deadly embrace• Control techniques – Deadlock prevention – Deadlock detection – Deadlock avoidance 38
  39. 39. How Deadlock Conditions Created Table 9.11 39
  40. 40. Concurrency Control with Time Stamping Methods• Assigns global unique time stamp to each transaction• Produces order for transaction submission• Properties – Uniqueness – Monotonicity• DBMS executes conflicting operations in time stamp order• Each value requires two additional time stamps fields – Last time field read – Last update 40
  41. 41. Concurrency Control with Optimistic Methods• Assumes most database operations do not conflict• Transaction executed without restrictions until committed• Phases: – Read Phase – Validation Phase – Write Phase 41
  42. 42. Database Recovery Management• Restores a database to previously consistent state• Based on the atomic transaction property• Level of backup – Full backup – Differential – Transaction log 42
  43. 43. Causes of Database Failure • Software • Hardware • Programming Exemption • Transaction • External 43
  44. 44. Transaction Recovery• Deferred-write and Deferred-update – Changes are written to the transaction log – Database updated after transaction reaches commit point• Write-through – Immediately updated by during execution – Before the transaction reaches its commit point – Transaction log also updated – Transaction fails, database uses log information to ROLLBACK 44