Question 10781 – Transactions

Consider the following two transactions: T₁ and T₂.

T1: read(A);
               read(B);
               if A=0 then B ← B+1;
               write(B);
T2: read(B);
               read(A);
               if B≠0 then A ← A-1;
               write(A); 
 
Which of the following schemes, using shared and exclusive locks, satisfy the requirements for strict two phase locking for the above transactions? 
 
 
Correct Answer: C
 
Question 9 Explanation: 
 
 For strict 2PL the requirements are,

i) Exclusive locks should be released after the commit.

ii) No locking can be done after the first unlock.

(A) Wrong because to write x you need Exclusive Lock.

(B) Wrong because violating the 1st requirement.

(C) Correct.

(D) Wrong because violating the 1st requirement.
      
A
 
   
     
B
 
   
     
C
 
   
     
D