A pair of (partial) database operations, p and q, are said to forward-commute if, in every database state in which both p and q are defined, the sequences p, q and q, p are both defined and, in both sequences, p and q return the same values and the final database state is the same.
a. Describe howforward commutativity can be used in the design of a deferred-update
pessimistic concurrency control.
b. Give a conflict table for such a control for the account object described in
Figure 20.18.
Note that the only conflict that occurs is for two withdrawOK() transactions
because there might not be enough money in the account to satisfy both withdrawals.
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