Consider optimistic concurrency control as applied to the transactions T and U. Suppose that transactions T and U are active at the same time as one another. Describe the outcome in each of the following cases:

Question

i) T's request to commit comes first and backward validation is used;
ii) U's request to commit comes first and backward validation is used;
iii) T's request to commit comes first and forward validation is used;
iv) U's request to commit comes first and forward validation is used.

In each case describe the sequence in which the operations of T and U are performed, remembering that writes are not carried out until after validation

Answer 1

i) T's read(i) is compared with writes of overlapping committed transactions: OK (U has not yet committed).



U - no read operations: OK.

ii) U - no read operations: OK.

T's read(i) is compared with writes of overlapping committed transactions (U’s write(i)): FAILS.

iii)T's write(j) is compared with reads of overlapping active transactions (U): OK.

U's write(i) is compared with reads of overlapping active transactions (none): OK (T is no longer active).

iv)U's write(i) is compared with reads of overlapping active transactions (T’s read(i)): FAILS.

T's write(j) is compared with reads of overlapping active transactions (none): OK.