GATE CS 2024 Set 2 Q.27: Transactions, Serializability and Concurrency Control

Which of the following statements about the Two Phase Locking (2PL) protocol is/are TRUE?

2PL permits only serializable schedules

With 2PL, a transaction always locks the data item being read or written just before every operation and always releases the lock just after the operation

With 2PL, once a lock is released on any data item inside a transaction, no more locks on any data item can be obtained inside that transaction

A deadlock is possible with 2PL

Official answer and solution

Official answer: A, C and D
From: Official GATE 2024 answer key

Solution

Check the properties of Two-Phase Locking: growing/shrinking phases, guaranteed conflict serializability, and susceptibility to deadlock.

(A) is TRUE: Every schedule permitted by the 2PL protocol is conflict-serializable (and hence serializable), ordered by the transactions' lock points.
(B) is FALSE: Releasing a lock immediately after an operation and then acquiring another lock for a subsequent operation violates the two-phase rule.
(C) is TRUE: By definition of 2PL, a transaction has a growing phase (only acquiring locks) followed by a shrinking phase (only releasing locks); once the first lock is released, no further locks can be acquired.
(D) is TRUE: 2PL does not prevent deadlocks (e.g., $T_1$ holds lock on $X$ and waits for $Y$ , while $T_2$ holds lock on $Y$ and waits for $X$ ).

Final answer

Options (A), (C), (D)

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
Question: 27 of 65
Section: Computer Science and Information Technology

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