Correctness
The DB System must assure that every change to the database happens as if the transactions took place one-at-a-time
The one-at-a-time protocol solves the “problem” with T1 and T2:
- T1 applied first, then T2: $500 --> $600 --> $500
- T2 applied first, then T1: $500 --> $400 --> $500
Transaction processing systems make sure that such problems do not arise by “locking” the data (only one computer at a time can unlock the data) and using a special protocol to perform the locking