OCC2T: An Early-Read Dual-Track OCC Algorithm For Mixed Mode Systems
Abdullah Alhajri, Arshad Jhumka, Richard Kirk · 2023
In OLTP systems, serializability is a key property for executions; without this, integrity constraints on data can be violated due to concurrent activities. The serializability property can be guaranteed through the implementation of concurrency control mechanisms such as strict 2-phase locking (S2PL). However, modern OLTP systems are expected to execute millions of transactions concurrently and mechanisms such as S2PL do not scale to handle this level of concurrency. Thus, to get better performance, programmers may choose different isolation levels, rather than the default ones, which are more suitable for their applications. Due to differing requirements, transaction aborts may be high. Thus, this paper investigates the correctness of mixing transactions with different isolation levels and extends the mixing theorem introduced by Adya et al. The extension investigates the correctness of mixing transactions with different isolation levels, especially when optimistic concurrency control (OCC) schemes are used which are not the main focus of the theorem. We thus propose OCC2T, an OCC-based algorithm that is designed to improve the performance of DBMS by decreasing the abort rate of transactions and enhancing mixing transactions with different isolation levels. Our results show that OCC2T is able to reduce the abort rate in highly contended workloads by almost 5x over other algorithms.