Consistency without borders

Distributed consistency is a perennial research topic; in recent years it has become an urgent practical matter as well. The research literature has focused on enforcing various flavors of consistency at the I/O layer, such as linearizability of read/write registers. For practitioners, strong I/O consistency is often impractical at scale, while looser forms of I/O consistency are difficult to map to application-level concerns. Instead, it is common for developers to take matters of distributed consistency into their own hands, leading to application-specific solutions that are tricky to write, test and maintain. In this paper, we agitate for the technical community to shift its attention to approaches that lie between the extremes of I/O-level and application-level consistency. We ground our discussion in early work in the area, including our own experiences building programmer tools and languages that help developers guarantee distributed consistency at the application level. Much remains to be done, and we highlight some of the challenges that we feel deserve more attention.