  This paper proposes a new state transfer method for geographic state machine
replication (SMR) that dynamically allocates the state to be transferred among
replicas according to changes in communication bandwidths. SMR improves fault
tolerance by replicating a service to multiple replicas. When a replica is
newly added or recovered from a failure, the other replicas transfer the
current state of the service to it. However, in geographic SMR, the
communication bandwidths of replicas are different and constantly changing.
Therefore, existing state transfer methods cannot fully utilize the available
bandwidth, and their state transfer time increases. To overcome this problem,
our method divides the state into multiple chunks and assigns them to replicas
based on each replica's bandwidth so that the broader a replica's bandwidth is,
the more chunks it transfers. The proposed method also updates the chunk
assignment of each replica dynamically based on the currently estimated
bandwidth. The performance evaluation on Amazon EC2 shows that the proposed
method reduces the state transfer time by up to 47% compared to the existing
one. In addition, we apply the proposed method to dynamic replacement of
replicas, which can mitigate latency degradation caused by network trouble, and
evaluate how fast the method can relocate a replica.
