Performance evaluation of an efficient multiple copy update algorithm

A well-known algorithm for updating multiple copies is the Thomas majority consensus algorithm. This algorithm, before performing an update, needs to obtain permission from a majority of the nodes in the system. We study the response-time behavior of a symmetric (each node seeks permission from the same number of other nodes and each node receives requests for update permission from the same number of other nodes) distributed update-synchronization algorithm where nodes need to obtain permission from only O(√N) (N being the number of database copies) other nodes before performing an update. The algorithm we use is an adaptation of Maekawa´s O(√N) distributed mutual exclusion algorithm to multiple-copy update-synchronization. This increase in the efficiency of the update-synchronization algorithm enhances performance in two ways. First, the reduction in transaction service time reduces the response time. Second, for a given arrival rate of transactions, the decrease in response time reduces the number of waiting transactions in the system. This reduces the probability of conflict between transactions. To capture the interaction between the probability of conflict and the transaction response time, we define a new measure called the conflict response-time product. Based on the solution of a queueing model we show that optimizing this measure yields a different and more appropriate choice of system parameters than simply minimizing the mean transaction response time