ASAP: An Aggressive SpeculAtive Protocol for Actively Replicated Transactional Systems

Recent advances in the field of replicated, fault tolerant transactional systems make systematic use of Optimistic Atomic Broadcast (OAB) group communication primitives in order to coordinate the replicas. According to this scheme, the replicas gain information on the existence of transactional requests before a final and global agreement is reached on the transaction serialization order. Hence, speculative processing schemes can be exploited in order to maximize the overlap between local computation and distributed coordination activities. In this article we present ASAP, an innovative Aggressive SpeculAtive Protocol, which exhibits the following two peculiarities: (A) it allows speculating along different transaction serialization orders, thus increasing the likelihood of successful overlap between local processing and coordination in case of mismatches between the optimistic and the final delivery sequence of incoming requests, (B) it speculates along chains of conflicting transactions, tracking data dependencies among transactions via an innovative concurrency control mechanism, which allows determining in a timely fashion the alternative serialization orders to be speculatively explored. Via a simulation study in the context of Software Transactional Memory systems we show ASAP can achieve robust performance independently of the likelihood of reorder between optimistic and final deliveries, providing remarkable performance improvements (enhancing the maximum sustainable throughput up to a 2x factor) with respect to state of the art speculative replication protocols.