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Chapter 15

Chapter 15. Transaction Management. Outline. Transaction basics Concurrency control Recovery management Transaction design issues Workflow management. Transaction Definition. Supports daily operations of an organization Collection of database operations

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Chapter 15

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  1. Chapter 15 Transaction Management

  2. Outline • Transaction basics • Concurrency control • Recovery management • Transaction design issues • Workflow management

  3. Transaction Definition • Supports daily operations of an organization • Collection of database operations • Reliably and efficiently processed as one unit of work • No lost data • Interference among multiple users • Failures

  4. Airline Transaction Example START TRANSACTION Display greeting Get reservation preferences from user SELECT departure and return flight records If reservation is acceptable then UPDATE seats remaining of departure flight record UPDATE seats remaining of return flight record INSERT reservation record Print ticket if requested End If On Error: ROLLBACK COMMIT

  5. Transaction Properties • Atomic: all or nothing • Consistent: database must be consistent before and after a transaction • Isolated: no unwanted interference from other users • Durable: database changes are permanent after the transaction completes

  6. Transaction Processing Services • Concurrency control • Recovery management • Service characteristics • Transparent • Consume significant resources • Significant cost component

  7. Concurrency Control • Problem definition • Concurrency control problems • Concurrency control tools

  8. Concurrency Control Problem • Objective: • Maximize work performed • Throughput: number of transactions processed per unit time • Constraint: • No interference: serial effect • Interference occurs on commonly manipulated data known as hot spots

  9. Lost Update Problem

  10. Uncommitted Dependency Problem

  11. Inconsistent Retrieval Problems • Interference causes inconsistency among multiple retrievals of a subset of data • Incorrect summary • Phantom read • Non repeatable read

  12. Incorrect Summary Problem

  13. Locking Fundamentals • Fundamental tool of concurrency control • Obtain lock before accessing an item • Wait if a conflicting lock is held • Shared lock: conflicts with exclusive locks • Exclusive lock: conflicts with all other kinds of locks • Concurrency control manager maintains the lock table

  14. Locking Granularity

  15. Deadlock (Mutual Waiting)

  16. Deadlock Resolution • Detection • Can involve significant overhead • Not widely used • Timeout • Waiting limit • Can abort transactions that are not deadlocked • Widely used although timeout interval is difficult to determine

  17. Two Phase Locking (2PL) • Protocol to prevent lost update problems • All transactions must follow • Conditions • Obtain lock before accessing item • Wait if a conflicting lock is held • Cannot obtain new locks after releasing locks

  18. 2PL Implementation

  19. Optimistic Approaches • Assumes conflicts are rare • No locks • Check for conflicts • After each read and write • At end of transaction • Evaluation • Less overhead • More variability

  20. Recovery Management • Device characteristics and failure types • Recovery tools • Recovery processes

  21. Storage Device Basics • Volatile: loses state after a shutdown • Nonvolatile: retains state after a shutdown • Nonvolatile is more reliable than volatile but failures can cause loss of data • Use multiple levels and redundant levels of nonvolatile storage for valuable data

  22. Failure Types • Local • Detected and abnormal termination • Limited to a single transaction • Operating System • Affects all active transactions • Less common than local failures • Device • Affects all active and past transactions • Least common

  23. Transaction Log • History of database changes • Large storage overhead • Operations • Undo: revert to previous state • Redo: reestablish a new state • Fundamental tool of recovery management

  24. Transaction Log Example

  25. Checkpoints • Reduces restart work but adds overhead • Checkpoint log record • Write log buffers and database buffers • Checkpoint interval: time between checkpoints • Types of checkpoints • Cache consistent • Fuzzy

  26. Other Recovery Tools • Force writing • Checkpoint time • End of transaction • Database backup • Complete • Incremental

  27. Recovery from a Media Failure • Restore database from the most recent backup • Redo all committed transactions since the most recent backup • Restart active transactions

  28. Recovery Timeline

  29. Recovery Processes • Depend on timing of database writes • Immediate update approach: • Before commit • Log records written first (write-ahead log protocol) • Deferred update approach • After commit • Undo operations not needed

  30. Immediate Update Recovery

  31. Deferred Update Recovery

  32. Transaction Design Issues • Transaction boundary • Isolation levels • Deferred constraint checking • Savepoints

  33. Transaction Boundary Decisions • Division of work into transactions • Objective: minimize transaction duration • Constraint: enforcement of important integrity constraints • Transaction boundary decision can affect hot spots

  34. Registration Form Example

  35. Transaction Boundary Choices • One transaction for the entire form • One transaction for the main form and one transaction for all subform records • One transaction for the main form and separate transactions for each subform record

  36. Isolation Levels • Degree to which a transaction is separated from the actions of other transactions • Balance concurrency control overhead with interference problems • Some transactions can tolerate uncommitted dependency and inconsistent retrieval problems • Specify using the SET TRANSACTION statement

  37. SQL Isolation Levels

  38. Scholar’s Lost Update

  39. Integrity Constraint Timing • Most constraints checked immediately • Can defer constraint checking to EOT • SQL SET CONSTRAINTS statement

  40. Save Points • Some transactions have tentative actions • SAVEPOINT statement determines intermediate points • ROLLBACK to specified save points

  41. Workflow Management • Workflow description • Enabling technologies • Advanced transaction management

  42. Workflows • Set of tasks to accomplish a business process • Human-oriented vs. computer-oriented • Amount of judgment • Amount of automation • Task structure vs. task complexity • Relationships among tasks • Difficulty of performing individual tasks

  43. Enabling Technologies • Distributed object management • Many kinds of non traditional data • Data often dispersed in location • Workflow modeling • Specification • Simulation • Optimization

  44. Advanced Transaction Management • Conversational transactions • Transactions with complex structure • Transactions involving legacy systems • Compensating transactions • More flexible transaction processing

  45. Summary • Transaction: user-defined collection of work • DBMSs support ACID properties • Knowledge of concurrency control and recovery important for managing databases • Transaction design issues are important • Transaction processing is an important part of workflow management

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