APART+: Boosting APART performance via optimistic pipelining of output events

APART (A Posteriori Active ReplicaTion) is a recently proposed active replication protocol specifically tailored for multi-tier data acquisition systems. It ensures consistency of middle-tier sink replicas by means of an a-posteriori synchronization phase based on reconciliation, which is activated only in case replicas react to an input message from the sensors by generating an output event destined to the back-end tier. This paper enhances APART via a novel non-blocking synchronization scheme which prevents replicas from stalling while waiting for the outcome of an on-going synchronization phase. Contrarily, replicas are allowed to optimistically process data from the sensors, and to immediately propagate any output event towards the back-end tier. The removal of the blocking synchronization phase from the critical path gives rise to striking performance gains via an effective overlapping of event processing and synchronization. On the other hand, system consistency is ensured by enhancing the back-end tier synchronization logic in order to filter out optimistically produced output events that are incompatible with the reconciled state trajectory.