Sintr: Safe Interactive Transactions in the Presence of Byzantine Clients

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.