GATE2025_CS1_Q15

A schedule of three database transactions T₁, T₂, and T₃ is shown. R_i(A) and W_i(A) denote read and write of data item A by transaction T_i, i = 1,2,3. The transaction T₁ aborts at the end. Which other transaction(s) will be required to be rolled back?

R₁(X)   W₁(Y)   R₂(X)   R₂(Y)   R₃(Y)   ABORT(T₁)

✅ Final Answer:

The correct answer is option (C) Both T₂ and T₃.

🔹 Short Answer:

This situation requires a cascading rollback. Transaction T₁ writes to data item Y (W1(Y)). Subsequently, both T₂ (with R2(Y)) and T₃ (with R3(Y)) read the value of Y written by T₁. Since T₁ has not committed, this is a "dirty read." When T₁ aborts, the value it wrote to Y is invalidated. Therefore, any transaction that read this dirty value must also be rolled back to maintain database consistency. Both T₂ and T₃ are dependent on T₁ and must be rolled back.

🔸 Long Answer:

This question tests the concept of recoverability in database schedules, specifically focusing on cascading rollbacks which occur due to dirty reads.

Step 1: Understand Dirty Read and Cascading Rollback

• Dirty Read: A transaction is said to perform a "dirty read" when it reads a data item that has been modified (written) by another transaction that has not yet committed. If the writing transaction later aborts, the data read by the first transaction becomes invalid.
• Cascading Rollback: If a transaction T₁ aborts, and another transaction T₂ had performed a dirty read on data from T₁, then T₂ must also be rolled back. This can create a chain reaction, or "cascade," where the rollback of one transaction forces others to roll back as well. [1, 4]

Step 2: Analyze the Transaction Schedule and Dependencies

Let's trace the operations and identify the dependencies between the transactions.

1. R1(X): T₁ reads X.
2. W1(Y): T₁ writes a new value to Y. At this point, T₁ is uncommitted.
3. R2(X): T₂ reads X. This is not a dirty read, as X was not written by an uncommitted transaction in this schedule.
4. R2(Y): T₂ reads Y. Since T₁ wrote to Y and has not yet committed, T₂ has performed a dirty read. T₂ is now dependent on T₁.
5. R3(Y): T₃ also reads Y. It reads the same value written by the uncommitted transaction T₁. Therefore, T₃ has also performed a dirty read and is dependent on T₁.
6. ABORT(T1): Transaction T₁ fails and is rolled back.

Step 3: Determine which transactions must be rolled back

Because T₁ aborts, all its operations are undone. The value it wrote to Y is now considered invalid. Any transaction that used this invalid data must also be rolled back to ensure the database remains in a consistent state.

• Transaction T₂: It read the dirty value of Y from T₁. Therefore, T₂ must be rolled back.
• Transaction T₃: It also read the dirty value of Y from T₁. Therefore, T₃ must be rolled back.

Both T₂ and T₃ are affected by the failure of T₁ and must be part of the cascading rollback.

Bibliography:
[1] Tutorialspoint. (2022). "Different Types of Recoverability of Schedules in DBMS."
[2] GeeksforGeeks. (2025). "Types of Schedules Based on Recoverability in DBMS."
[3] "Transaction Management Schedules," Course Notes.
[4] Gate Vidyalay. "Cascading Rollback | Cascadeless Schedule."
[5] "ACR schedules: Avoiding Cascading Rollback," Course Notes.

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