In globally distributed systems, shared state is never perfect. When communication is neither fast nor reliable, it is not possible to achieve strong consistency, low latency, and availability at the same time. Unfortunately, abandoning strong consistency has wide ramifications. Eventual consistency, though attractive from a performance viewpoint, is challenging to understand and reason about for both system architects and programmers. To provide robust abstractions, we need not just systems but also principles; we need the ability to articulate what a consistency protocol is supposed to...