GATE CS 2025 Set 2 Q17 — Transactions and Concurrency Control

To determine which ACID properties are violated, let us analyze the execution of the two concurrent transactions, $T_1$ and $T_2$:

• Initial State: Balance of $A = \text{Rs. } 11000$.
• Both $T_1$ and $T_2$ read the balance of $A$ as $\text{Rs. } 11000$ simultaneously.
• $T_1$ calculates the new balance of $A$ as $11000 - 10000 = 1000$ and writes it. $T_1$ also credits $\text{Rs. } 10000$ to $B$.
• $T_2$ calculates the new balance of $A$ as $11000 - 10000 = 1000$ and writes it. $T_2$ also credits $\text{Rs. } 10000$ to $B$.
• Final State: Balance of $A = \text{Rs. } 1000$, and $B$ is credited with $\text{Rs. } 20000$ in total.

This scenario is a classic example of the Lost Update concurrency anomaly. Let us evaluate each ACID property:

1. Isolation (Violated): Isolation requires that concurrent execution of transactions results in a state equivalent to some serial execution.
   • If $T_1$ and $T_2$ executed serially, the first transaction would debit $A$ to $\text{Rs. } 1000$. The second transaction would then read $\text{Rs. } 1000$ and either fail (due to insufficient funds) or debit $A$ to a negative balance ($\text{Rs. } -9000$).
   • Since the concurrent execution allowed both to proceed using the stale balance of $\text{Rs. } 11000$, the schedule is not serializable. Thus, Isolation is violated.
2. Consistency (Violated): Consistency ensures that a transaction transforms the database from one valid state to another, preserving all integrity constraints (such as the conservation of money during transfers).
   • Here, a total of $\text{Rs. } 20000$ was credited to account $B$, but only $\text{Rs. } 10000$ was debited from account $A$. This mismatch violates the fundamental consistency invariant of a banking system. Thus, Consistency is violated.
3. Atomicity (Not Violated): Atomicity requires that either all operations of a transaction are completed, or none are. In this case, both $T_1$ and $T_2$ executed all of their respective write operations (debiting $A$ and crediting $B$) completely. There was no partial transaction execution.
4. Durability (Not Violated): Durability guarantees that once a transaction commits, its updates survive any subsequent system failures. Since the incorrect final state ($A = 1000$, $B$ credited twice) persisted successfully, durability was maintained.

Therefore, the properties certain to have been violated are Consistency and Isolation.