Spanner’s key innovation is around time. It includes a novel system using GPS and Atomic Clocks to distribute a globally synchronized “proper time.” The previous dogma in distributed systems was that synchronizing time within and between datacenters is insurmountably hard and uncertain. Ergo, serialization of requests is impossible at global scale. Google’s key innovation is to accept uncertainty, keep it small (via atomic clocks and GPS), quantify the uncertainty and operate around it. In retrospect this is obvious, but it doesn’t make it any less brilliant.