Spanner implements snapshot isolation across datacenters in this way [59, 60]. It uses the clock’s confidence interval as reported by the TrueTime API, and is based on the following observation: if you have two confidence intervals, each consisting of an earliest and latest possible timestamp (A = [Aearliest, Alatest] and B = [Bearliest, Blatest]), and those two intervals do not overlap (i.e., Aearliest < Alatest < Bearliest < Blatest), then B definitely happened after A — there can be no doubt. Only if the intervals overlap are we unsure in which order A and B happened.
