We investigate the performance implications of providing transaction atomicity for firm-deadline real-time applications, operating on distributed data. Using a detailed simulation model, the real-time performance of a representative set of classical transaction commit protocols is evaluated. The experimental results show that data distribution has a significant influence on real-time performance and that the choice of commit protocol clearly affects the magnitude of this influence. We also propose and evaluate a new commit protocol, PROMPT (Permits Reading Of Modified Prepared-data for Timeliness), that is specifically designed for the real-time domain. PROMPT allows transactions to "optimistically" borrow, in a controlled manner, the updated data of transactions currently in their commit phase. This controlled borrowing reduces the data inaccessibility and the priority inversion that is inherent in distributed real-time commit processing. A simulation-based evaluation shows PROMPT to be highly successful, as compared to the classical commit protocols, in minimizing the number of missed transaction deadlines. In fact, its performance is close to the best on-line performance that could be achieved using the optimistic lending approach. Further, it is easy to implement and incorporate in current database system software. Finally, PROMPT is compared against an alternative priority inheritance-based approach to addressing priority inversion during commit processing. The results indicate that priority inheritance does not provide tangible performance benefits.

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.

A significant body of literature is available on distributed transaction commit protocols. Surprisingly, however, the relative merits of these protocols have not been studied with respect to their quantitative impact on transaction processing performance. In this paper, using a detailed simulation model of a distributed database system, we profile the transaction throughput performance of a representative set of commit protocols. A new commit protocol, OPT, that allows transactions to “optimistically” borrow uncommitted data in a controlled manner is also proposed and evaluated. The new protocol is easy to implement and incorporate in current systems, and can coexist with most other optimizations proposed earlier. For example, OPT can be combined with current industry standard protocols such as Presumed Commit and Presumed Abort.

We investigate the performance implications of supporting transaction atomicity in a distributed real-time database system. Using a detailed simulation model of a firm-deadline distributed real-time database system, we profile the real-time performance of a representative set of commit protocols. A new commit protocol that is designed for the real-time domain and allows transactions to "optimistically" read uncommitted data is also proposed and evaluated. The experimental results show that data distribution has a significant influence on the real-time performance and that the choice of commit protocol clearly affects the magnitude of this influence. Among the protocols evaluated, the new optimistic commit protocol provides the best performance for a variety of workloads and system configurations.