The Transaction

1. The Transaction Durable starting state Durable, consistent, ending state Collection of resource actions Successful completion Abort Rollback

2. Withdraw $100 Read current balance new balance = current - 100 Write new balance Dispense cash Transfer $100 Read savings (sub 100) Read checking (add 100) Write savings Write checking Transaction Examples

3. Transaction architecture Transaction manager Join Application logic Prepare, commit, abort Resource managers

4. Transaction manager Phase 1 commit() Transaction manager Phase 2 prepare() yes_or_no Transaction manager abort() Commit or abort One or more “no’s” Rollback All “yes”

5. Withdraw $100 Read checking balance new balance = current - 100 Write new balance Dispense cash Transfer $100 Read savings (sub 100) Read checking (add 100) Write savings Write checking Transaction Interference time

6. Isolation of transactions (Serializability) Two concurrent transactions Transaction_1 Shared resources Give the same result as: Transaction_2 Transaction_1 Transaction_2 Or: Transaction_2 Transaction_1

7. Withdraw $100 Read checking balance new balance = current - 100 Write new balance Dispense cash Transfer $100 Read savings (sub 100) Read checking (add 100) Write savings Write checking Bad (Not Serializable) time

8. Withdraw $100 Read checking balance new balance = current - 100 Write new balance Dispense cash Transfer $100 Read savings (sub 100) Read checking (add 100)Write savings Write checking OK (Same as Withdraw first) time

9. Transaction 1: Lock A Lock B … Release all Transaction 2: Lock B Lock A … Release all Danger from Waiting: Deadlock

10. Summary Application server Resource manager Transaction manager request(tp_ID,….) join(tp_ID) Lock OK or Already_locked More requests…. Prepare (tp_ID) commit_or_abort(tp_ID) Commit or rollback Unlock