Question

Consider concurrent execution of two transactions 𝑇1 and 𝑇2 in a DBMS, both of
which access a data object 𝐴. For these two transactions to not conflict on 𝐴, which
one of the following statements must be true?

• A. Both 𝑇1 and 𝑇2 only read 𝐴
• B. 𝑇1 reads 𝐴 and 𝑇2 writes 𝐴
• C. 𝑇1 writes 𝐴 and 𝑇2 reads 𝐴
• D. Both 𝑇1 and 𝑇2 write 𝐴

Hint:

Two operations conflict only if at least one of them is a write on the same data item. A read-read pair never conflicts, while read-write, write-read, and write-write pairs all conflict.

Answer 1

The Correct Option is A

Step 1: In DBMS concurrency control, two operations on the same data object are said to conflict if at least one of them is a write operation. Two read operations on the same object never conflict, because reading does not change the value or interfere with another transaction reading it.

Step 2: Consider each option. If both \(T1\) and \(T2\) only read \(A\), this is a read-read pair, which by definition never conflicts, regardless of execution order.

Step 3: If \(T1\) reads \(A\) and \(T2\) writes \(A\), this is a read-write pair. The value \(T1\) reads depends on whether \(T2\)'s write has happened yet, so the operations conflict (this is a read-write conflict).

Step 4: If \(T1\) writes \(A\) and \(T2\) reads \(A\), this is also a read-write pair (in the opposite order) and conflicts for the same reason.

Step 5: If both \(T1\) and \(T2\) write \(A\), this is a write-write pair. The final value of \(A\) depends on which write executes last, so these operations conflict as well.

Step 6: The only case with no conflict is when both transactions only read \(A\).

\[ \boxed{\text{Option (A): Both T1 and T2 only read A}} \]