Correctness
The DB System must assure that every DB change happens as if the transactions happened 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 such problems do not arise by “locking” the data (only one computer at a time can unlock the data)
“Two changes at once with unsavory results” can happen when
multiple people work on the same document … use locking idea