GATE CS 2016 Set 2 — Question 61

Let's analyze the schedule $S$:Recoverability: A schedule is recoverable if for every transaction $T_j$ that reads a value written by $T_i$, $T_i$ commits before $T_j$ commits. In this schedule, there are no "dirty reads" (reading a value written by an uncommitted transaction). $T_2$ reads $X$ and $Y$ before any writes occur. $T_1$ reads $X$ and $Y$ before any writes occur. Since no transaction reads from another, the schedule is trivially recoverable.Cascading Abort: A cascading abort occurs if $T_j$ reads a value written by $T_i$, and then $T_i$ aborts, forcing $T_j$ to abort. Since there are no read dependencies (no transaction reads data written by another uncommitted transaction), there is no cascading abort. The fact that both abort is independent of data dependencies. Thus, $S$ does not have a cascading abort (it is ACA - Avoids Cascading Aborts).Strictness: A schedule is strict if a transaction can neither read nor write a data item $X$ until the last transaction that wrote $X$ has committed or aborted. Here, $T_1$ writes $X$ at step 4 ($w_1(X)$). Then $T_2$ writes $X$ at step 6 ($w_2(X)$). At step 6, $T_1$ has not yet committed or aborted. Thus, $T_2$ overwrites an uncommitted value. This violates the strict property.Therefore, statement (C) is true.