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+13 votes

Consider two transactions $T_1$ and $T_2$, and four schedules $S_1, S_2, S_3, S_4$, of $T_1$ and $T_2$ as given below:

$T_1: R_1[x]W_1[x]W_1[y]$

$T_2: R_2[x]R_2[y]W_2[y]$

$S_1: R_1[x]R_2[x]R_2[y] W_1[x] W_1[y] W_2[y]$

$S_2: R_1[x]R_2[x]R_2[y] W_1[x] W_2[y] W_1[y]$

$S_3: R_1[x]W_1[x]R_2[x] W_1[y] R_2[y] W_2[y]$

$S_4: R_2[x]R_2[y]R_1[x] W_1[x] W_1[y] W_2[y]$

Which of the above schedules are conflict-serializable?

- $S_1 \text{ and } S_2$
- $S_2 \text{ and } S_3$
- $S_3$ only
- $S_4$ only

+19 votes

Best answer

The answer is B.

- S1 has a cycle from $T1\to T2$ and $T2 \to T1.$
- S2-- It is uni-directional and has only $T2\to T1.$
- S3-- It is uni-directional and has only $T1\to T2.$
- S4-- same as S1.

A schedule is conflict serializable if there is no cycle in the directed graph made by the schedules.

In the schedules we check for RW, WR, WW conflicts between the schedules and only these conflicts contribute in the edges of the graph.

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