Don't Settle for Eventual: Scalable Causal Consistency for Wide-Area Storage With COPS

Geo-replicated, distributed data stores that support complex online applications, such as social networks, must provide an "Always-on" experience where operations always complete with low latency. Today's systems often sacrifice strong consistency to achieve these goals, exposing inconsistencies to their clients and necessitating complex application logic. In this paper, the authors identify and define a consistency model - causal consistency with convergent conflict handling, or causal+ - that is the strongest achieved under these constraints. They present the design and implementation of COPS, a key-value store that delivers this consistency model across the wide-area.