GATE CSE 2025 | Set 2 | Question: 17

Question

​​​​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?

• A. Atomicity
• B. Consistency
• C. Isolation
• D. Durability

Comments

https://cs.stackexchange.com/questions/173518/violation-of-only-the-acid-consistency-property-for-these-transactions/

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Atomicity means , once started will completely execute or not execute at all,  and Durablity means that , Database will preserve changes.
Here neither of them is being violated, as the amount is debited as well as credited .
Isolation and Consistency has been breached, as amount credited twice, and  resources of account  A is accessed by two users simultaneously , which led to violation of isolation.

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1. Isolation (Violated)

Isolation ensures that concurrent transactions are executed as if they were the only ones running on the system. If the transactions were properly isolated, Transaction 2 (T2) would not have been able to read the initial balance of Rs. 11,000 while Transaction 1 (T1) was still processing it.

• The Failure: The system allowed "dirty reads" or "unrepeatable reads" by letting both transactions see the same starting state.

• Serialized Result: If isolated, T1 would have finished (leaving Rs. 1,000), and then T2 would have either failed (insufficient funds) or left a balance of Rs. -9,000.

2. Consistency (Violated)

Consistency ensures that a transaction takes the database from one valid state to another, maintaining all predefined rules (like "Money cannot be created or destroyed").

• The Failure: Before the transactions, the total sum was Rs. 11,000. After the transactions, B received Rs. 20,000 while A only lost Rs. 10,000. The system "created" Rs. 10,000 out of thin air, breaking the integrity constraints of a banking system.

Answer 1

Given:

• Account $A$ has $\text{Rs. 11,000.}$

• Two simultaneous transfers of $\text{Rs. 10,000}$ each are attempted.

• Account $B$ is credited twice (total $\text{Rs. 20,000}$).

• Account $A$ is debited only once (final balance $\text{Rs. 1,000}$).

Analysis of ACID Properties:

1. Atomicity:
   Ensures all parts of a transaction are completed or none are.
   Each transfer individually was complete (money was debited from $A$ and credited to $B$), so atomicity is likely not violated.
   $✘$ Not necessarily violated.

2. Consistency:
   Ensures that the database moves from one valid state to another.
   If $\text{Rs. 20,000}$ was credited to $B$, $\text{Rs. 20,000}$ should be debited from $A$.
   But only $\text{Rs. 10,000}$ was debited. This breaks balance rules.
   $✓$ Violated.

3. Isolation:
   Ensures concurrent transactions do not interfere with each other.
   Both transactions read the same initial balance ( $\text{Rs. 11,000}$) and proceeded independently, leading to an incorrect final state.
   This is a classic lost update.
   $✓$Violated.

4. Durability:
   Ensures that once a transaction is committed, it remains so even in case of failure.
   No crash or rollback problem here.
   $✘$ Not violated.

Only $\color{red}{\text{Consistency}}$ and $\color{red}{\text{Isolation}}$ are violated. 

Hence,  Correct Answer :-  ${B,C}$

Comments

Both transactions read the same initial balance ( Rs. 11,000) and proceeded independently, leading to an incorrect final state.
This is a classic lost update or write skew issue.

@Umesh Shelke can you please explain how this statement is referring to "write skew issue"?

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@Shubham Sharma 2  Thanks for point-ing out 

it is just "classic lost update" not "write skew issue" I had written it mistakenly.

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Corrected now.

Answer 2

Two transactions, ( $T_1$ ) and ( $T_2 $), each attempt to transfer ₹10,000 from account ( A ) (initial balance ₹11,000) to account ( B ). Due to inadequate concurrency control, the following interleaving occurs:

$$
\begin{array}{|c|c|}
\hline
T_1 & T_2 \\
\hline
\text{Read}(A) \rightarrow 11000 & \\
 & \text{Read}(A) \rightarrow 11000 \\
\text{Write}(A) \leftarrow 11000 - 10000 = 1000 & \\
 & \text{Write}(A) \leftarrow 11000 - 10000 = 1000 \\
\text{Write}(B) \leftarrow B + 10000 & \\
 & \text{Write}(B) \leftarrow B + 10000 \\
\hline
\end{array}
$$
 

Both transactions read the same initial value of ( A = 11000 ). Each independently computes the new balance as ( 1000 ) and writes it back. The second write overwrites the first but does not incorporate the effect of both transfers. Consequently:

• Only one debit of ₹10,000 is reflected in ( A ) (final balance = ₹1,000),

• Both credits are applied to ( B ) (final balance = ₹20,000).

This is a classic lost update anomaly: the update by one transaction on ( A ) is effectively lost due to the concurrent write by the other transaction.

ACID Property Violations

Consistency:
The system invariant “total money conserved” is violated:
$$[
\text{Initial total} = 11000, \quad \text{Final total} = 1000 + 20000 = 21000
]$$
$\color{red} \boxed{ \text{Violated.}}$

Isolation:
The lost update arises because transactions are not isolated; they interfere via uncoordinated reads and writes.
$\color{red} \boxed{ \text{Violated.}}$

Atomicity and Durability:
Both transactions complete all steps and persist their results; no partial execution or data loss is indicated.
$\color{lime} \boxed{ \text{Not Violated.}}$

$$
\boxed{\text{B. Consistency and C. Isolation}}
$$

Answer 3

Correct Option: B,C (Consistency, Isolation) or Only C (Isolation)

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Isolation is certain to be violated because the Isolation property states that when multiple transactions run concurrently, their effect should be equivalent to some serial schedule of those transactions. This is not the case here, since the balance of A was not deducted twice.

Atomicity and Durability are not likely to be violated.

Now, according to some Standard Books (including Korth, Navathe, etc) Consistency (of transaction) is not violated and according to other books (including ACID paper), Consistency is violated.

I asked on cs.StackExchange - https://cs.stackexchange.com/questions/173518/violation-of-only-the-acid-consistency-property-for-these-transactions/

Answer 4

The answer is B.Consistency because in a transaction the total amount before performing the transaction should be exactly equal to amount after performing the transaction ,not less not more. but here before performing the transaction let us assume the amount in account B = 0, amount in the so called joint account (A)=11000,the total amount in both the accounts sums up to 11000 which is 11000+0. but soon after transaction the total amount is 20000(credited in acount B) + 1000(left out balance of account  A)=21000,which is not at all equal to total amount before transaction i.e 11000 which implies DB is not consistent.

Therefore the answer is "B.Consistency"

Comments

• Conflict-serializable ⇒ view-serializable ⇒ serializable (equivalence to some serial run).

• Contrapositive: not view-serializable ⇒ not serializable.

View-serializability is the broadest standard notion of serializability (it requires the same reads-from relations and final writers as some serial schedule). If a schedule is not view-serializable, there is no serial schedule producing the same read/write behavior — so it is not serializable, and therefore the isolation guarantee is not preserved.

@Shubham Sharma 2 sir please verify this.

Feel free to correct me.

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@akhil1 that's correct. If the schedule is not serializable then we can encounter many issues like dirty-read, dirty-write, phantom and others.

So, yes serializability is the highest level of isolation.

Table reference : SQL Isolation Page no. 7

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Thank you @Shubham Sharma 2 sir.