An Arbitrary Tree-Structured Replica Control Protocol

Traditional replication protocols that arrange logically the replicas into a tree structure have reasonable availability, low communication costs but induce high system load. We propose in this paper the arbitrary protocol: a tree-based replica control protocol that can be configured based on the frequencies of read and write operations in order to provide lower system load than existing tree replication protocols, yet with comparable cost and availability. Our protocol enables the shifting from one configuration into another by just modifying the structure of the tree. There is no need to implement a new protocol whenever the frequencies of read and write operations change. At the heart of our protocol lies the new idea of logical and physical levels in a tree. In short, read operations are carried out on any physical node of every physical level of the tree whereas the write operation is performed on all physical nodes of a single physical level of the tree. We discuss optimal configurations, proving in particular a new lower bound, of independent interest, for the case of a binary tree.