Consider the following interleaving of transactions T and U:

Question

The outcome of optimistic concurrency control with backward validation is that T will be aborted because its read operation conflicts with U's write operation on ai, although the interleavings are serially equivalent. Suggest a modification to the algorithm that deals with such cases.

Answer 1

Keep ordered read sets for active transactions. When a transaction commits after passing its validation, note the fact in the read sets of all active transactions. For example, when U commits, note the fact in T’s read set.



Thus, T’s read set = {U commit, i}



Then the new validate procedure becomes: