An error-reflective consistency model for distributed data stores

Abstract

Consistency models for distributed data stores offer insights and paths to reasoning about what a user of such a system can expect. However, often consistency models are defined or implemented in coarse-grained manners, making it difficult to achieve precisely the consistency required. Further, many domains are already written to handle anomalies in distributed systems, yet they have little opportunity for expressing or taking advantage of their leniency. We propose reflective consistency-an active solution which adapts an underlying data store to changing loads and resource availability to meet a given consistency level. We implement reflective consistency in Cassandra, an existing distributed data store supporting per-read and per-write consistency. Our implementation allows users to express their anomaly leniency directly and the system will react to the presence of anomalies, changing Cassandra's consistency only when needed. Users of Reflective Cassandra can expect minimal overhead (anywhere from 1% to 14% depending on configuration) and a 50% decrease in the amount of costly strong reads.