Parameterized and Runtime-Tunable Snapshot Isolation in Distributed Transactional Key-Value Stores

Abstract

Several relaxed variants of Snapshot Isolation (SI) have been proposed for improved performance in distributed transactional key-value stores. These relaxed variants, however, provide no specification or control of the severity of the anomalies with respect to SI. They have also been designed to be used statically throughout the whole system life cycle. To overcome these drawbacks, we propose the idea of parameterized and runtime-tunable snapshot isolation. We first define a new transactional consistency model called Relaxed Version Snapshot Isolation (RVSI), which can formally and quantitatively specify the anomalies it may produce with respect to SI. To this end, we decompose SI into three "view properties", for each of which we introduce a parameter to quantify one of three kinds of possible anomalies: k1-BV (k1-version bounded backward view), k2-FV (k2-version bounded forward view), and k3-SV (k3-version bounded snapshot view). We then implement a prototype partitioned replicated distributed transactional key-value store called Chameleon across multiple data centers. While achieving RVSI, Chameleon allows each transaction to dynamically tune its consistency level at runtime. The experiments show that RVSI helps to reduce the transaction abort rates when applications are willing to tolerate certain anomalies. We also evaluate the individual impacts of k1-BV, k2-FV, and k3-SV on reducing the transaction abort rates in various scenarios. We find that it depends on the issue delays between clients and replicas which of k1 and k2 plays a major role in reducing transaction abort rates.