Question

An audit of a banking transactions system has found that on an earlier occasion, two joint holders of account A attempted simultaneous transfers of Rs. 10000 each from account A to account B. Both transactions read the same value, Rs. 11000, as the initial balance in A and were allowed to go through. B was credited Rs. 10000 twice. A was debited only once and ended up with a balance of Rs. 1000. Which of the following properties is/are certain to have been violated by the system?

• A. Atomicity
• B. Consistency
• C. Isolation
• D. Durability

Hint:

Remember: Lost update problem arises due to violation of the {Isolation} property.

Answer 1

The Correct Option is C

Step 1: Understanding the scenario.
Two transactions were executed concurrently:
- Each transaction read the same balance Rs. 11000 from account A.
- Each attempted to transfer Rs. 10000 to account B.
- As a result, B received Rs. 20000, but A was debited only Rs. 10000 and ended with Rs. 1000.
This is clearly an incorrect outcome.
Step 2: Understanding ACID properties.
In DBMS:
- Atomicity: Either the entire transaction happens or nothing happens.
- Consistency: Database moves from one valid state to another valid state.
- Isolation: Concurrent transactions should not interfere with each other.
- Durability: Once committed, changes must persist even after failure.
Step 3: Identifying the violation.
Both transactions read the same initial balance before either wrote its update. This is a classic lost update problem, which occurs when isolation is not properly enforced. If isolation had been ensured, the second transaction would have seen the updated balance after the first transaction.
Step 4: Analyzing the options.
(A) Atomicity: Not violated — both transactions completed fully.
(B) Consistency: It appears violated in effect, but the root cause is lack of isolation.
(C) Isolation: Correct — concurrent transactions interfered with each other.
(D) Durability: Not violated — data was stored after transactions.
Step 5: Conclusion.
The property certainly violated is Isolation.