1. Databases & Consistency

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

3. Database Data stored in tables Each row is a record
Columns define attributes

4. Consistency
Consistent : no contradictions

5. Consistency
Inconsistent : contradictions

6. How Do we Become Inconsistent?
Only one row can be saved at a time…

7. How Do we Become Inconsistent?
Only one row can be saved at a time…

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

9. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Program 2: Make Matt & Jingi friends

10. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Make Rosina friend Jingyi
Program 2: Make Matt & Jingi friends

11. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Make Rosina friend Jingyi
Program 2: Make Matt & Jingi friends
Make Matt friend Jingyi

12. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Make Rosina friend Jingyi
Check who Jingyi is friends with none
Program 2: Make Matt & Jingi friends

13. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Make Rosina friend Jingyi
Check who Jingyi is friends with none
Set Jingyi friend to none + Rosina
Program 2: Make Matt & Jingi friends

14. How Do we Become Inconsistent?
Two programs interact
Program 1: Make Rosina & Jingyi friends
Make Rosina friend Jingyi
Check who Jingyi is friends with none
Set Jingyi friend to none + Rosina
Program 2: Make Matt & Jingi friends
Set Jingyi friend to none + Matt

15. Even Worse
Zadie transfers $200 from checking to savings

16. Even Worse Zadie transfers $200 from checking to savings
Look at checking… decide it should be $600
Look at savings… decide it should be $500
Write change to checking
BOOM

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. OK smart guy… Zadie sends $200 to Pedro Pedro withdraws $100
Begin transaction
Change Zadie checking from $600 to $400
Change Pedro checking from $150 to $350
End transaction
Pedro withdraws $100
Change Pedro checking from $150 to $50
Spit out money

25. OK smart guy… Zadie sends $200 to Pedro Pedro withdraws $100
Begin transaction
Change Zadie checking from $600 to $400
Change Pedro checking from $150 to $350
End transaction
Pedro withdraws $100
Change Pedro checking from $150 to $50
Spit out money
$400
$350

26. OK smart guy… Zadie sends $200 to Pedro Pedro withdraws $100
Begin transaction
Change Zadie checking from $600 to $400
Change Pedro checking from $150 to $350
End transaction
Pedro withdraws $100
Change Pedro checking from $150 to $50
Spit out money
$400
$50
Pedro got cheated!!!

27. OK smart guy… Zadie sends $200 to Pedro Pedro withdraws $100
Begin transaction
Change Zadie checking from $600 to $400
Change Pedro checking from $150 to $350
End transaction
Pedro withdraws $100
Change Pedro checking from $150 to $50
Spit out money
$50

28. OK smart guy… Zadie sends $200 to Pedro Pedro withdraws $100
Begin transaction
Change Zadie checking from $600 to $400
Change Pedro checking from $150 to $350
End transaction
Pedro withdraws $100
Change Pedro checking from $150 to $50
Spit out money
$400
$350
Pedro got $100 free!!!

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

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

31. Deadlock Two Programs both trying to transfer money
Process A locks Marie
Process B Pedro

32. Deadlock Process A locks Marie… now needs Pedro
Process B Pedro… now needs Marie

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

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

35. Prepare Then Commit Trick
Master DB initiates transaction

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

37. 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

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

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

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