GATE CS 1999 Q2.6 — Transactions and Concurrency Control

To determine the properties of the given schedule, let us analyze the conflicts between transactions $T_1$ (left column) and $T_2$ (right column):

1. Conflict on $A$: $T_1$ writes $A$ at step 3, and $T_2$ reads $A$ at step 4. This creates a conflict dependency: $T_1 \to T_2$.
2. Conflict on $B$: $T_2$ writes $B$ at step 6, and $T_1$ reads $B$ at step 7. This creates a conflict dependency: $T_2 \to T_1$.

The precedence graph contains a cycle: $T_1 \to T_2 \to T_1$. Because of this cycle, the schedule is not conflict serializable. Since there are only two transactions, conflict serializability is equivalent to view serializability, meaning the schedule is not serializable.

Furthermore, any schedule allowed by the Two-Phase Locking (2PL) protocol is guaranteed to be conflict serializable. Since this schedule is not conflict serializable, it cannot occur under any 2PL scheme.

Thus, the schedule is not serializable and cannot occur in a scheme using 2PL protocol.