Performance Evaluation of Two-Shadow Speculative Concurrency Control

Speculative Concurrency Control (SCC) [Best92a] is a new concurrency control approach especially suited for real-time database applications. It relies on the use of redundancy to ensure that serializable schedules are discovered and adopted as early as possible, thus increasing the likelihood of the timely commitment of transactions with strict timing constraints. In [Best92b], SCC-nS, a generic algorithm that characterizes a family of SCC-based algorithms was described, and its correctness established by showing that it only admits serializable histories. In this paper, we evaluate the performance of the Two-Shadow SCC algorithm (SCC-2S), a member of the SCC-nS family, which is notable for its minimal use of redundancy. In particular, we show that SCC-2S (as a representative of SCC-based algorithms) provides significant performance gains over the widely used Optimistic Concurrency Control with Broadcast Commit (OCC-BC), under a variety of operating conditions and workloads.

Multi-version Speculative Concurrency Control with Delayed Commit

This paper presents an algorithm which extends the relatively new notion of speculative concurrency control by delaying the commitment of transactions, thus allowing other conflicting transactions to continue execution and commit rather than restart. This algorithm propagates uncommitted data to other outstanding transactions thus allowing more speculative schedules to be considered. The algorithm is shown always to find a serializable schedule, and to avoid cascading aborts. Like speculative concurrency control, it considers strictly more schedules than traditional concurrency control algorithms. Further work is needed to determine which of these speculative methods performs better on actual transaction loads.

Speculative Concurrency Control for Real-Time Databases

In this paper, we propose a new class of Concurrency Control Algorithms that is especially suited for real-time database applications. Our approach relies on the use of (potentially) redundant computations to ensure that serializable schedules are found and executed as early as possible, thus, increasing the chances of a timely commitment of transactions with strict timing constraints. Due to its nature, we term our concurrency control algorithms Speculative. The aforementioned description encompasses many algorithms that we call collectively Speculative Concurrency Control (SCC) algorithms. SCC algorithms combine the advantages of both Pessimistic and Optimistic Concurrency Control (PCC and OCC) algorithms, while avoiding their disadvantages. On the one hand, SCC resembles PCC in that conflicts are detected as early as possible, thus making alternative schedules available in a timely fashion in case they are needed. On the other hand, SCC resembles OCC in that it allows conflicting transactions to proceed concurrently, thus avoiding unnecessary delays that may jeopardize their timely commitment.

Value-cognizant Speculative Concurrency Control

A problem with Speculative Concurrency Control algorithms and other common concurrency control schemes using forward validation is that committing a transaction as soon as it finishes validating, may result in a value loss to the system. Haritsa showed that by making a lower priority transaction wait after it is validated, the number of transactions meeting their deadlines is increased, which may result in a higher value-added to the system. SCC-based protocols can benefit from the introduction of such delays by giving optimistic shadows with high value-added to the system more time to execute and commit instead of being aborted in favor of other validating transactions, whose value-added to the system is lower. In this paper we present and evaluate an extension to SCC algorithms that allows for commit deferments.