Consider the following schedules,
S1: R1(A)W2(B)R2(A)W1(B)W3(B)W1(C)R3(B)W1(A)
S2: W2(C)R1(A)W3(B)R1(C)R3(B)R3(A)W1(B)
S3: R3(B)R2(A)R1(A)R2(C)W3(A)W2(B)W1(C)
S4: W1(A)W2(B)W1(B)W2(A)
Which of the following schedule are conflict serializable?
(A) Only S1 and S3
(B) Only S1 and S2
(C) Only S2 and S4
(D) Only S2 and S3
Answer: (B)
Explanation: Using Precedence graphs for given schedules,
S1: R1(A)W2(B)R2(A)W1(B)W3(B)W1(C)R3(B)W1(A)
Serializable as T2 → T1 → T3.
S2: W2(C)R1(A)W3(B)R1(C)R3(B)R3(A)W1(B)
Serializable as either T2 → T3 → T1 or T3 → T2 → T1.
S3: R3(B)R2(A)R1(A)R2(C)W3(A)W2(B)W1(C)
Cycle exist, so can not be conflict serializable.
S4: W1(A)W2(B)W1(B)W2(A)
Cycle exist, so can not be conflict serializable.
Therefore, option (B) is correct.
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