Minimal Byzantine Storage

Byzantine fault-tolerant storage systems can provide high availability in hazardous environments, but the redundant servers they require increase software development and hardware costs. In order to minimize the number of servers required to implement fault-tolerant storage services, we develop a new algorithm that uses a “Listeners” pattern of network communication to detect and resolve ordering ambiguities created by concurrent accesses to the system. Our protocol requires 3f +1 servers to tolerate up to f Byzantine faults—f fewer than the 4f + 1 required by existing protocols for non-self-verifying data. In addition, SBQ-L provides atomic consistency semantics, which is stronger than the regular or pseudo-atomic semantics provided by these existing protocols. We show that this protocol is optimal in the number of servers—any protocol that provides safe semantics or stronger requires at least 3f + 1 servers to tolerate f Byzantine faults in an asynchronous system. We also examine protocols that store self-verifying data (i.e. data that cannot be undetectably altered). Existing protocols can use self-verifying data to reduce the number of servers required to tolerate faults but because SBQ-L already uses the minimum possible number of servers for its semantics, self-verifying data provides no advantage. Finally, we examine a non-confirmable writes variation of the SBQ-L protocol where a client cannot determine when its writes complete. We show that SBQ-L with non–confirmable writes provides regular semantics with 2f + 1 servers and that this number of servers is minimal.