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Byzantine fault tolerance (BFT) enhances the reliability and availability of replicated systems subject to software bugs, malicious attacks, or other unexpected events. This paper presents Byzantium, a BFT database replication middleware that provides snapshot isolation semantics. It is the first BFT database system that allows for concurrent transaction execution without relying on a centralized component, which is essential for having both performance and robustness. Byzantium builds on an existing BFT library but extends it with a set of techniques for increasing concurrency in the execution of operations, for optimistically executing operations in a single replica, and for striping and load-balancing read operations across replicas. Experimental results show that our replication protocols introduce only a modest performance overhead for read-write dominated workloads and perform better than a non-replicated …