In this thesis, a hybrid concurrency control scheme for distributed database systems was proposed, which mingles a locking scheme and an optimistic scheme that uses snapshot validation. The proposed scheme determines for each transaction the type of the transaction on the basis of the write/read ratio and the length of the transaction, before the transaction is being executed. There are two types of transactions in the system: locking type and optimistic type. The performance of the proposed scheme was then compared with that of two other schemes: two-phase locking scheme and optimistic scheme proposed by Kung and Robinson. According to the simulation study, the performance of the proposed hybrid concurrency control scheme is shown to be just in between the performance of the two-phase locking scheme and that of the optimistic scheme. And proposed scheme is better than the two other schemes with respect to the mean value of the transaction response times. Since the hybrid scheme has an advantage that takes the benefits of the locking as well as the benefits of the optimistic scheme.
