alt Hacker NewsSpanner depends on having a time source with bounded error to maintain consistency. Google accomplishes this by having GPS and atomic clocks in several datacenters.
https://static.googleusercontent.com/media/research.google.c...
https://static.googleusercontent.com/media/research.google.c...
And more importantly, the tighter the time bound, the higher the performance, so more accurate clocks easily pay for themselves in other saved infrastructure costs to service the same number of users.