A Family of Speculative Concurrency Control Algorithms for Real-Time Databases

Speculative Concurrency Control (SCC) was proposed in [Best92b] as a new concurrency control approach especially suited for real-time database applications. SCC 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. Using SCC, several shadow transactions execute on behalf of a given uncommitted transaction so as to protect against the hazards of blockages and restarts, which are characteristics of Pessimistic and Optimistic Concurrency Control algorithms, respectively. We present SCC-nS, a generic algorithm that characterizes a family of SCCbased algorithms. Under the SCC-nS algorithm, shadows executing on behalf of a transaction are either optimistic or speculative. Optimistic shadows execute under an assumed serialization order, which requires them to wait for no other con icting transactions. They execute unhindered until they are either aborted or committed. Speculative shadows are more conservative. They execute under an assumed serialization order, which requires them to wait for some con icting transactions to commit. In this paper, we provide a description of the SCC-nS algorithm, establish its correctness by showing that it only admits serializable histories, and demonstrate its superiority for RTDBMS through numerous examples. Three members of the SCC-nS family (namely SCC-1S, SCC-2S and SCC-MS) are singled out and contrasted. SCC-1S and SCC-MS represent two extremes in a spectrum of choices regarding the total amount of spared redundancy in the system. SCC-1S is notable for its minimal use of redundancy, whereas SCC-MS is notable for its liberal use thereof.