GATE CS 2025 Set 2 — Question 27

The scenario described is the classic Lost Update problem, which occurs when two transactions read the same initial value of a data item and then update it based on that value, overwriting each other's updates. The update from $T_1$ is overwritten (lost) by $T_2$. The net effect is that $A$ is decremented by 10000 only once, but $B$ is credited twice (total 20000). This interleaving of operations is a failure to ensure Isolation. The Isolation property requires that concurrent execution of transactions leaves the database in a state that would have been obtained if the transactions were executed serially. In a serial execution, the second transaction would have read the updated balance ($1000$) and likely failed due to insufficient funds (or resulted in a negative balance if allowed, but definitely not $1000$).While the database state becomes inconsistent (violating Consistency), the root cause in the context of ACID properties and concurrency control is the violation of Isolation.