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

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

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

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

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

Transaction_1 Transaction_2 Shared resources Transaction_1Transaction_2 Transaction_1Transaction_2 Two concurrent transactions Give the same result as: Or: Isolation of transactions (Serializability)

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

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

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

Application serverResource managerTransaction manager request(tp_ID,….) join(tp_ID) More requests…. Prepare (tp_ID) commit_or_abort(tp_ID) Lock Commit or rollback Summary OK or Already_locked Unlock