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GATE CSE 2025 Session 2 Q27: ACID properties violated by a lost-update bank transfer

Question

An audit of a banking transactions system has found that on an earlier occasion, two joint holders of account AA attempted simultaneous transfers of Rs. 10000 each from account AA to account BB. Both transactions read the same value, Rs. 11000, as the initial balance in AA and were allowed to go through. BB was credited Rs. 10000 twice. AA 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?

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Answer: B, C (official GATE 2025 final answer key)

Concept used: A lost update comes from concurrent transactions interfering (isolation) and leaves the data violating an invariant (consistency).

First step: Check the invariant: total money in AA and BB rose by Rs. 10000, and both transactions read the same stale balance.

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Source: official GATE 2025 Session 2 CS question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key

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