Consider the following four schedules due to three transactions (indicated by the subscript) using read and write on a data item $X$, denoted by $r(X)$ and $w(X)$ respectively. Which one of them is conflict serializable?
$S1$: $r1(X); r2(X); w1(X); r3(X); w2(X)$
$S2$ : $r2(X); r1(X); w2(X); r3(X); w1(X)$
$S3$ : $r3(X); r2(X); r1(X); w2(X); w1(X)$
$S4$ : $r2(X); w2(X); r3(X); r1(X); w1(X)$
- $S1$
- $S2$
- $S3$
- $S4$