GATE CS 2025 Set 2 Q.27: Transactions, Serializability and Concurrency Control

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?

Atomicity

Consistency

Isolation

Durability

Official answer and solution

Official answer: B and C
From: Official GATE 2025 answer key

Solution

Analyze the lost-update anomaly in terms of ACID properties: concurrent unisolated execution (Isolation) caused money creation/invariant violation (Consistency).

Isolation (C) is violated: The two concurrent transactions read the same initial balance of $A$ ( $11000$ ) and overwrote $A$ 's balance ( $11000 - 10000 = 1000$ ) in an interleaved schedule not equivalent to any serial execution (a classic lost update anomaly).
Consistency (B) is violated: The sum of balances $A + B$ was not preserved ( $A$ decreased by Rs. $10000$ while $B$ increased by Rs. $20000$ ), and the second Rs. $10000$ transfer was allowed despite $A$ having only Rs. $11000$ initially.
Both transactions completed and their final writes persisted, so Atomicity (A) and Durability (D) were not violated.

Final answer

Options (B), (C)

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2025 Computer Science and Information Technology - Session 2 (CS2)
Question: 27 of 65
Section: Computer Science and Information Technology

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