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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?

$S1$: $r1(X); r2(X); w1(X); r3(X); w2(X)$

$S2$ : $r2(X); r1(X); w2(X); r3(X); w1(X)$

$S3$ : $r3(X); r2(X); r1(X); w2(X); w1(X)$

$S4$ : $r2(X); w2(X); r3(X); r1(X); w1(X)$

  1. $S1$
  2. $S2$
  3. $S3$
  4. $S4$
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Answer (4)

First we need to draw dependency graph for each schedule.

S1: Not possible because cycle exists between transactions 1 and 2

S2: Not possible. Similar reason

S3: Not possible. Similar reason
Answer:

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