Consistency in non-transactional distributed storage systems

Over the years, different meanings have been associated with 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 article, we aim to fill the void in the 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 be useful in future research. Finally, we map the consistency semantics to different practical systems and research prototypes. The scope of this article is restricted to non-transactional semantics, that is, those that apply to single storage object operations. As such, our article complements the existing surveys done in the context of transactional, database consistency semantics.