Selection Guidelines for Geographical SMR Protocols: A Communication Pattern-based Latency Modeling Approach

State machine replication (SMR) is a replication technique that ensures fault tolerance by duplicating a service. Geographical SMR can enhance its robustness against disasters by distributing replicas in separate geographical locations. Several geographical SMR protocols have been proposed in the literature, each of which tailored to specific requirements; for example, protocols designed to meet the requirement of latency reduction by either sacrificing a part of their fault tolerance or limiting the content of responses to clients. However, this diversity complicates the decision-making process for selecting the best protocol for a particular service. In this study, we introduce a latency estimation model for these SMR protocols based on the communication patterns of the protocols and perform simulations for various cases. Based on the simulation results and an experimental evaluation, we present five selection guidelines for geographical SMR protocols based on their log management policy, distances between replicas, number of replicas, frequency of slow paths, and client distribution. These selection guidelines enable determining the best geographical SMR protocol for each situation.