1. Accessing data Transactions

2. Agenda Questions from last class? Transactions concurrency Locking rollback

3. Transactions 1+ SQL statements that are performed completely, or not at all ACID properties –Atomic –Consistent –Isolation –Durability Why develop these?

4. Banking CustomerAccount has

5. Transactions T1 T2 T3 Checking Savings Loan T1 – CustomerA checks her balance and withdraws money from checking account T2 – An automatic withdrawal from CustomerA’s checking account goes towards a mortgage loan that she holds with the same bank T3 – An automatic transfer moves money from CustomerA’s checking account into her savings account

6. Airline Reservations CustomerReservation makes Ticket purchases converts to

7. Concurrency Problems: Lost Update T1 - read R1 T2 - read R1 T1 - write new R1 T2 - write new R1

8. Concurrency Problems: Temporary Update or Dirty Read T1 - read R T1 - update R T2 - read R T1 - rollback to original value of R

9. Concurrency Problems: Incorrect Summary T1 - read T2 - update R1 R2 R3 R1 R2 R3

10. Concurrency Problems: Phantom Record T1 - query DB, retrieve set of records T2 - insert new record that satisfies T1’s query

11. Dealing with concurrency (OR: how to handle the pressure!) Timestamp ordering Locking Multiversion protocols Optimistic protocols (validation protocol)

12. Locks Granularity – lock on table, record, page, ets. Escalation – raising granularity in the middle of a transaction Index locking – primarily for adding/deleting records, but also changes to index field

13. Locking: Binary Lock 2 states: locked or unlocked Rules: 1.Transaction must issue lock on item before doing any reads or writes of it. 2.Transaction must issue unlock of item after completing all reads and writes of it.

14. Locking: Multiple-mode Lock 2 types of locks: Shared, S, or Read lock. Any number of S locks are allowed on an item. Exclusive, X, or Write lock. If an item is X- locked, there can be no other locks on it. Transaction must have appropriate lock on item before performing the action.

15. Locking: Multiple-mode Lock (1) Rules: 1.Transaction must issue S or X lock before reading an item. 2.Transaction must issue an X lock before writing an item. 3.Transaction must release locks after all reads and writes are completed.

16. Locking: Multiple-mode Lock (2) 4.Transaction may upgrade S lock to X lock if no one else has S lock. 5.Transaction may downgrade X lock to S lock when all writes are completed.

17. Intention locking Acts on next higher level – to write to a record, acquire intention lock on table and then acquire lock on record 3 types of locks –Intention read –Intention write –Read intention write Must check for locks on at least two levels

18. Concurrency with Locking: Lost Update T1 - request S lock on R T1 - read R - S lock T2 - request S lock on R T2 - read R - S lock T1 - request X lock - wait T2 - request X lock - wait

19. Concurrency with Locking: Temporary Update T1 - request S lock on R T1 - read R - S lock T1 - request X lock on R T1 - update R - X lock T2 - request S lock on R - wait T1 - release X lock on R T2 - read R - S lock

20. Concurrency with Locking: Incorrect Summary (1) T1 - request S lock on R1 T1 - read R1 - S lock T1 - request S lock on R2 T1 - read R2 - S lock T2 - request S lock on R3 T2 - read R3 - S lock

21. Concurrency with Locking: Incorrect Summary (2) T2 - request X lock on R3 T2 - update R3 - X lock T2 - request S lock on R1 T2 - read R1 - S lock T2 - request X lock on R1 - wait T1 - request S lock on R3 - wait

22. Concurrency with Locking: Deadlock Simplest example: T1 - write R1 - X lock T2 - write R2 - X lock T1 - request X lock on R2 - wait T2 - request X lock on R1 - wait