Isolation is a sufficient but not necessary condition to achieve correctness. Consider an application that reserves seats for a concert. Each reservation transaction (1) reads the list of seats that have not yet been reserved, (2) presents them to a potential customer who selects one of them, and (3) marks the selected seat as reserved. An integrity constraint asserts that the same seat cannot be reserved by two different customers. Describe a situation in which two such transactions that reserve two different seats execute in a nonisolated fashion but are nevertheless correct.

What will be an ideal response?

Assume there are two unreserved seats, Sa and Sb. T1 reads that both are unreserved and while its customer is thinking which seat she wants to reserve, T2 also reads that Sa and Sb are unreserved. Then the customer at T1 reserves Sa and the customer at T2 reserves Sb . That is a perfectly correct execution, but T1 and T2 cannot be serialized in either order because neither sawthe results of the other’s execution. (The system must guarantee that if both customers had tried to reserve the same seat, only one would have been allowed to.)