GATE | GATE CS 1999 | Question 31

For the schedule given below, which of the following is Correct?

1   Read A
2                               Read B
3   Write A
4                               Read A
5                               Write A
6                               Write B
7   Read B
8   Write B 

(A) This schedule is serialisable and can occur in a scheme using 2PL protocol.
(B) This schedule is serialisable but cannot occur in a scheme using 2PL protocol.
(C) This schedule is not serialisable but can occur in a scheme using 2PL protocol.
(D) This schedule is not serialisable and cannot occur in a scheme using 2PL protocol.


Answer: (D)

Explanation: Initial read of data item A is done by T1 whereas final write of data item B os done by T1. therefore W1( A) , R2( A) and W2(B) , R1(B ) are conflicting pairs. The schedule is neither T1 –> T2 nor T2 –> T1 serializable. Since schedule is not serializable , it can not occur in scheme using 2PL protocol .

Option (D) is correct.

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