1. Databases & Consistency

2. Database
Relational databases : dominant information storage/retrieval system

3. Activity…

4. Consistency
Consistent : no contradictions

5. Consistency
Inconsistent : contradictions

6. How Do we Become Inconsistent?
Only one row can be saved at a time…
Make Rosina and Jingyi friends

7. How Do we Become Inconsistent?
Only one row can be saved at a time…
Make Rosina and Jingyi friends

8. How Do we Become Inconsistent?
Only one row can be saved at a time…
Make Rosina and Jingyi friends
CRASH!!!!

9. Even Worse

10. Concurrency Issues Concurrency : two things happening at the same time
Many things are hard to share:
Printers
Shared memory

11. Concurrency Issues
Race Condition : unpredictable result based on timing of concurrent operations

12. Concurrency Issues X starts as 5, 3 possible answers: Case 1 Case 2
A runs, x = 15
B runs, x =16
B runs, x = 6
A runs, x = 16
A gets x (5)
B gets x (5) A adds 10 (has 15) B adds 1 (has 6) A stores x = 15
B stores x = 6
B gets x (5) A adds 10 (has 15) B adds 1 (has 6)
B stores x = 6 A stores x = 15

13. Locks
Can prevent concurrency problems with locks:

14. Atomicity
Transaction must LOCK all rows it will modify before starting
Want to transfer money from Marie to Pedro

15. Deadlock
Workers A and B both want to use locked resources X and Y:

16. Deadlock Busting Database can resolve: Roll back one transaction
Give up its locks
Make it restart AFTER other is complete

17. To Do List Trick
Make a list of what you intend to do and save it:

18. To Do List Trick
Make a list of what you intend to do and save it:

19. To Do List Trick
Make a list of what you intend to do and save it:

20. To Do List Trick
Delete list when you are done:

21. To Do List Trick
A crash…

22. Idempotent
Idempotent action : same effect no matter how many times you execute it
"Change checking balance to $600"
Non-idempotent
"Subtract $200 from checking"

23. To Do List If every instruction is:
Idempotent
Reversible : Know how to restore original state
Can resume or reverse any partial transaction

24. Atomicity Atomic operation : indivisible / one unit
Never see results of half done transaction

25. Transactional Memory
Current research area : Design regular software with transactional memory interactions

26. Replication Databases must be replicated
Redundancy
Speed
Copies must remain consistent on the fly

27. Prepare Then Commit Trick
Master DB initiates transaction

28. Prepare Then Commit Trick
Master DB initiates transaction
Asks each other DB to do same… waits for OK Prepare

29. Prepare Then Commit Trick
Master DB initiates transaction
Asks each other DB to do same… waits for OK Prepare
Once everyone complete, sends signal to end transaction Commit

30. Prepare Then Commit Trick
Master DB initiates transaction
Asks each other DB to do same… waits for OK ERROR

31. Prepare Then Commit Trick
Master DB initiates transaction
Asks each other DB to do same… waits for OK ERROR
Rollback instead of committing