GATE CS 2025 Set 2 — Question 27
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Databases → Transactions & Concurrency → ACID Properties
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Question
An audit of a banking transactions system has found that on an earlier occasion, two joint holders of account attempted simultaneous transfers of Rs. 10000 each from account to account . Both transactions read the same value, Rs. 11000, as the initial balance in and were allowed to go through. was credited Rs. 10000 twice. 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?
Correct answer
(B) Consistency; (C) Isolation
Solution
Both transactions read , each computed and wrote it back, so one debit was lost (a lost update), while was credited twice.
- Isolation — violated. No serial order of the two transfers can produce this result: run one after the other, the second transfer would see a balance of Rs. 1000, so could not end at Rs. 1000 with credited Rs. 20000. The interleaving was not serializable.
- Consistency — violated. A transfer must conserve money ( unchanged). Here lost Rs. 10000 but gained Rs. 20000, so the system went from a consistent state to an inconsistent one.
- Atomicity — not certain. Each transaction performed both its debit and its credit; nothing shows a partially applied transaction.
- Durability — not certain. The committed writes persisted (the audit sees them); no committed effect was lost because of a failure.
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