Deterministic scheduling for transactional multithreaded replicas

One way to implement a fault-tolerant service is by replicating it at sites that fail independently. One of the replication techniques is active replication where each request is executed by all the replicas. Thus, the effects of failures can be completely masked, resulting in an increase of service availability. In order to preserve consistency among replicas, replicas must exhibit a deterministic behavior, which has traditionally been achieved by restricting replicas to being single-threaded. However, this approach cannot be applied in some setups like transactional systems, where it is not admissible to process transactions sequentially. The authors present a deterministic scheduling algorithm for multithreaded replicas in a transactional framework. To ensure replica determinism, requests to replicated servers are submitted by means of reliable and totally ordered multicast. Internally, a deterministic scheduler ensures that all threads are scheduled in the same way at all replicas which guarantees replica consistency.