Transactions introduction
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Transactions address the problems of concurrent database access and resilience to system failures. A transaction is defined as a sequence of SQL statements treated as a single unit. Transactions allow statements to appear isolated from each other to avoid inconsistencies during concurrent access, and ensure all-or-nothing execution of statements if a failure occurs. The combination of transactions with the ACID properties provides a solution for both concurrency control and recovery from failures.
Transactions introduction
- 2. Motivated by two independent requirements Transactions Concurrent database access Resilience to system failures Jennifer Widom
- 3. Transactions Concurrent Database Access Even more software More software DBMS Select… Update… Create Table… Drop Index… Help… Delete… Data Jennifer Widom
- 4. Concurrent Access: Attribute-level Inconsistency Transactions Update College Set enrollment = enrollment + 1000 Where cName = ‘Stanford’ concurrent with … Update College Set enrollment = enrollment + 1500 Where cName = ‘Stanford’ Jennifer Widom
- 5. Concurrent Access: Tuple-level Inconsistency Transactions Update Apply Set major = ‘CS’ Where sID = 123 concurrent with … Update Apply Set decision = ‘Y’ Where sID = 123 Jennifer Widom
- 6. Concurrent Access: Table-level Inconsistency Transactions Update Apply Set decision = ‘Y’ Where sID In (Select sID From Student Where GPA > 3.9) concurrent with … Update Student Set GPA = (1.1) GPA Where sizeHS > 2500 Jennifer Widom
- 7. Concurrent Access: Multi-statement inconsistency Transactions Insert Into Archive Select * From Apply Where decision = ‘N’; Delete From Apply Where decision = ‘N’; concurrent with … Select Count(*) From Apply; Select Count(*) From Archive; Jennifer Widom
- 8. Concurrency Goal Transactions Execute sequence of SQL statements so they appear to be running in isolation Simple solution: execute them in isolation But want to enable concurrency whenever safe to do so Jennifer Widom
- 9. Resilience to System Failures Transactions Bulk Load DBMS Data Jennifer Widom
- 10. Resilience to System Failures Transactions Insert Into Archive Select * From Apply Where decision = ‘N’; Delete From Apply Where decision = ‘N’; DBMS Data Jennifer Widom
- 11. Resilience to System Failures Transactions Lots of updates buffered in memory DBMS Data Jennifer Widom
- 12. System-Failure Goal Guarantee all-or-nothing execution, regardless of failures Transactions DBMS Data Jennifer Widom
- 13. Solution for both concurrency and failures Transactions Transactions A transaction is a sequence of one or more SQL operations treated as a unit Transactions appear to run in isolation If the system fails, each transaction’s changes are reflected either entirely or not at all Jennifer Widom
- 14. Solution for both concurrency and failures Transactions Transactions A transaction is a sequence of one or more SQL operations treated as a unit. SQL standard: Transaction begins automatically on first SQL statement On “commit” transaction ends and new one begins Current transaction ends on session termination “Autocommit” turns each statement into transaction Jennifer Widom
- 15. Solution for both concurrency and failures Transactions Transactions A transaction is a sequence of one or more SQL operations treated as a unit Jennifer Widom