More optimism about real-time distributed commit processing

In (Gupta et al., 1996), we proposed a new commit protocol, OPT, specially designed for use in distributed firm-deadline real-time database systems. OPT allows transactions to optimistically borrow uncommitted prepared data in a controlled manner. This controlled borrowing reduces the data inaccessibility and the priority inversion that is inherent in real-time commit processing. Experimental evaluations showed the new OPT protocol to be highly successful, as compared to the classical distributed commit protocols, in minimizing the number of missed transaction deadlines. In this paper, we extend and improve upon this prior work in the following ways. First, we consider parallel distributed transactions whereas the previous study was restricted to sequential transactions. Second, we evaluate the extent to which OPT's real-time performance is adversely affected by those cases where its optimism turns out to be misplaced. This is achieved by comparing OPT's performance with that of Shadow-OPT, a protocol that augments OPT with the shadow transaction approach and ensures that the right decision about access to uncommitted data is always eventually made. In all of our experiments, which considered a wide range of workloads and system configurations, the difference between OPT and Shadow-OPT never exceeded ten percent. Moreover, the difference was reduced to less than two percent when OPT was enhanced with a simple healthy lenders heuristic. Finally, we compare the performance of OPT to that of an alternative priority, inheritance-based approach to addressing priority inversion during commit processing.