Consider the following ordering schedule —
S’ of transactions:

T1 : R(A);  
T1 : A := A + 5;  
T1 : commit;  

T2 : R(B);  
T2 : B := B + 5;  

T3 : R(C);  
T3 : C := C + 5;  
T3 : C := C + 5;  

T4 : R(A);  
T4 : A := A + 5;  
T4 : R(D);  
T4 : D := D + 5;  
T4 : commit;  

T2 : commit;  
T3 : commit;

Let the initial value of A = B = C = D = 0.
The system follows log-based recovery process of immediate database modification.

The assumption is the concurrency control system uses strict 2PL, and all transactions share a common disk buffer and single log.

Explain what happens during the recovery process if a failure occurs at T2 : commit statement.