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