I tried to read the document multiple times but failed to understand it. Can someone explain it in layman's terms?
TrueTime is an API available at Google that directly exposes clock uncertainty.
Comparing to standard datetime libraries, instead of a particular timestamp, TrueTime's now() function returns an interval of time [earliest, latest].
It also provides two functions:
- after(t) returns true if t has definitely passed. E.g. t < now().earliest.
- before(t) returns true if t has definitely not arrived, or t > now().latest.
What's impressive, is that the implementation of now() returns intervals with generally small uncertainty.
Spanner uses TrueTime API to assign a commit timestamps to transactions in the way that it satisfies external consistency: if a transaction T1 commits before another transaction T2 starts, then T1’s commit timestamp is smaller than T2’s.
More in the Spanner paper http://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf
来源：https://stackoverflow.com/questions/40274040/what-is-the-truetime-api-in-googles-spanner