Simulation Testing For Liveness
Blog post from TigerBeetle
TigerBeetle's simulator, VOPR, has undergone a significant update with the introduction of two operational modes: "safety" and "liveness." Previously, the simulator could only test safety properties by randomly injecting faults, such as crashing replicas or corrupting I/O, to ensure strict serializability amidst various system faults. However, this approach was insufficient for identifying liveness issues, such as scenarios where a cluster remains unavailable despite a healthy operating environment. The new "liveness" mode addresses this gap by ensuring that a quorum of replicas can maintain cluster availability, even if other replicas are down or partitioned. By transitioning from safety to liveness mode, the simulator can now simulate permanent asymmetric partitions, ensuring that the core replicas can still process all transactions. This change led to the discovery of a resonance bug involving the repair algorithm and network load balancing, where the round-robin method used for repair requests inadvertently caused a stalemate in log entry recovery. The liveness mode revealed this bug by maintaining the conditions for indefinite replication failure, thus highlighting the importance of this new testing capability.
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