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GATE 2024 CS (CS2) – Question 27

Databases · Transactions and Concurrency Control · 1 mark · Multiple select

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

  1. 2PL permits only serializable schedules
  2. 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
  3. 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
  4. A deadlock is possible with 2PL

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Show answer and explanation

Correct answer: (A) 2PL permits only serializable schedules; (C) 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; (D) A deadlock is possible with 2PL

Explanation

2PL guarantees conflict serializability, has a growing then shrinking phase (no new locks after first release), and can deadlock. Releasing right after each operation would violate 2PL.