GATE CS 2010 Q20 — Transactions and Concurrency Control

To determine which protocol ensures both conflict serializability and freedom from deadlock, let us analyze each protocol:

1. I. $2$-Phase Locking (2PL):
   • Conflict Serializability: Yes. The basic 2PL protocol guarantees that any schedule it generates is conflict serializable.
   • Freedom from Deadlock: No. Under 2PL, transactions acquire locks dynamically. If Transaction $T_1$ holds a lock on resource $A$ and waits for resource $B$, while Transaction $T_2$ holds a lock on resource $B$ and waits for resource $A$, a deadlock occurs. Thus, 2PL does not prevent deadlocks.
2. II. Time-stamp Ordering (TO):
   • Conflict Serializability: Yes. The protocol uses transaction timestamps to decide the serialization order. Any conflicting operations are executed in the order of their timestamps.
   • Freedom from Deadlock: Yes. In the timestamp ordering protocol, transactions do not wait for each other by holding locks. If an operation violates the timestamp order, the transaction is immediately aborted and restarted with a new timestamp. Since there is no waiting, circular wait is impossible, ensuring absolute freedom from deadlock.

Therefore, only II (Time-stamp ordering) ensures both properties.

Correct Option: B (II only)