transaction

Question

r1(X),W2(X),c2(2 is committed),r1(X)

I know that the precedence graph of this will not contain cycle. But i have not understood why. Its happenning.

if X=100

then T1 read X as 100

then T2 comes and makes it 200

and then T2 is committed

Then T1 reads it it will read it as 200,

but in precedence graph we only have T1->T2, therefore read operation of T1 will be X=100 at the end but its new value is 200, so why we are not taking cycle in this.