Consistency in Non-Transactional Distributed Storage Systems

TL;DR

This paper provides a comprehensive overview and formalization of over 50 different non-transactional consistency notions in distributed storage systems, clarifying their relationships and practical mappings.

Contribution

It systematically defines, compares, and orders various non-transactional consistency notions, filling gaps in existing literature and aiding future research.

Findings

Defined and clarified over 50 consistency notions

Established a partial order based on semantic strength

Mapped consistency models to practical systems

Abstract

Over the years, different meanings have been associated to the word consistency in the distributed systems community. While in the '80s "consistency" typically meant strong consistency, later defined also as linearizability, in recent years, with the advent of highly available and scalable systems, the notion of "consistency" has been at the same time both weakened and blurred. In this paper we aim to fill the void in literature, by providing a structured and comprehensive overview of different consistency notions that appeared in distributed systems, and in particular storage systems research, in the last four decades. We overview more than 50 different consistency notions, ranging from linearizability to eventual and weak consistency, defining precisely many of these, in particular where the previous definitions were ambiguous. We further provide a partial order among different consistency predicates, ordering them by their semantic "strength", which we believe will reveal useful in future research. Finally, we map the consistency semantics to different practical systems and research prototypes. The scope of this paper is restricted to non-transactional semantics, i.e., those that apply to single storage object operations. As such, our paper complements the existing surveys done in the context of transactional, database consistency semantics.