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## Conflict Serializable Schedule - Solved Exercise

Question:
Consider a schedule S with two transactions T1, T2, T3 and T4 as follows;
S: R2(x); W3(x); W1(x); W2(y); R2(y); R4(x); R4(y);
Is the schedule S conflict serializable?

Solution:
 Conflict serializable schedule: A schedule is conflict serializable if it can be transformed into an equivalent serial schedule by swapping pairs of non-conflicting instructions. Two instructions conflict if they involve the same data item and at least one of them is a WRITE.

The schedule S given in questions is as follows;
 Instruction T1 T2 T3 T4 1 2 3 4 5 6 7 W(x) R(x) W(y) R(Z) W(x) R(x) R(y)

We can construct a precedence graph to decide whether the given schedule is conflict serializable or not.
Precedence graph can be constructed by carefully analyzing the conflicting instructions. For every conflicting instruction, an edge can be inserted into the precedence graph. At the end, if the graph has not formed a cycle, then we would say that the schedule S is conflict serializable, otherwise not.

 Conflicting instruction and the edge to be inserted Precedence graph R2(x) and W3(x); T2 → T3 ➔ R2(X) and W1(X); T2 → T1 ➔ W3(X) and W1(X); T3 → T1 ➔ W3(X) and R4(X); T3 → T4 ➔ W1(X) and R4(X); T1 → T4 ➔ W2(Y) and R4(Y); T2 → T4 ➔

From the precedence graph, it is clear that no cycle has been formed through the conflicting instructions. Hence, the schedule is a conflict serializable schedule.

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