Byzantine fault-tolerant (BFT) systems are, in principle, an appealing foundation for transactional applications involving mutually distrustful participants. Yet their adoption has been hampered by two persistent stumbling blocks-performance and developer convenience-which are often in tension with one another. Recent systems show promising progress on both fronts by shifting to a client-centric architecture; clients execute transactions locally and concurrently, while the system resolves any data conflicts to maintain database serializability. We argue that, in its current form, this approach introduces a critical vulnerability: it leaves the integrity of the database exposed to Byzantine clients, which may issue malicious or incorrect transactions. We address this threat with Sintr, a framework that prevents Byzantine clients from compromising database integrity by executing rogue transactions. Sintr combines redundant execution to validate transaction outcomes with a flexible, heterogeneous policy framework for expressing an application's data-integrity requirements. We apply Sintr to harden several existing BFT database systems and find that it imposes only modest overheads-3%-16% in throughput and 3%-21% in latency.

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