In the context of concurrency control, a given pair of operations in a schedule is called conflict schedule if

(A) At least one of the operations is write operation

(B) Both the operations are performed on the same data item

(C) Both the operations are performed by different transactions

(D) Both the operations are performed on different data items

Choose the correct answer from the options given below:

Question

Question

This question was previously asked in

UGC NET Computer Science (Paper 2) 2020 Official Paper

Answer 1

The correct answer is option 2.

Key Points

Two functions are said to be incompatible if any of the following conditions are met:

If you note the issues mentioned below, it's because different transactions have performed at least one write operation on the same data object.

a) Item X has an incorrect value because its update by T1is lost(overwritten).

T1

T2

read_item(X);
X=X-N;

write_item(X);
read_item(Y);

Y=Y+N;

write_item(Y);

read_item(X);
X=X+m;

write_item(X);

b) Transaction T1 fails and must change the value of X back to its old value; meanwhile, T2 has read the temporary incorrect value of x.

T1

T2

read_item(X);
X=X-N;

write_item(X);

read_item(Y);

read_item(X);
X=X+m;

write_item(X);

c)T3 reads X after N is subtracted and reads Y before N is added; a wrong summary is a result(off by N).

T1

T3

read_item(X);
x=x-N;
write_item(X);

read_item(Y);
Y=Y+N;
write_item(Y);

sum=0
read_item(A);
sum=sum+A;

.

read_item(X);
sum=sum+X;
read_item(Y);
sum=sum+Y;

∴ Hence the correct answer is (A), (B) and (C) only