I don't think there's a catchy one-liner which can explain why Spanner "needs" real time instead of Lamport clock. Spanner is a system with some very concrete c

I don't think there's a catchy one-liner which can explain why Spanner "needs" real time instead of Lamport clock. Spanner is a system with some very concrete characteristics (Paxos consensus, strongly consistent pessimistic writes, non-blocking snapshot reads etc.). There are ways to use Lamport clock instead of TrueTime, but then you'll also have to lessen some of the correctness/performance guarantees [and thus it would no longer be "Spanner"]. To fully understand the trade-offs you kinda have to dive into it and understand the system as a whole. These lecture notes are a bit brief, but they go through why TrueTime is needed by explaining the correctness issues if we don't have it. http://www.cs.utah.edu/~stutsman/cs6963/lecture/15/ http://www.cs.utah.edu/~stutsman/cs6963/lecture/15/