[kccqzy]

That’s exactly what I’m saying but you simply provided more details. TrueTime guarantees clocks are well synchronized: and of course that means synchronized to a reasonable upper bound. It’s no more possible for clocks to be absolutely synchronized, than for two line segments drawn independently to have absolutely the same length.

[vlovich123]

> you can compare them to determine exactly which commit happened first

This is the part I was referring to. You cannot just compare timestamps and know which happened first. You have to actually handle the case where you don’t know if there’s a happens before relationship between the timestamps. Thats a very important distinction

[kccqzy]

I quote from Spanner docs at https://docs.cloud.google.com/spanner/docs/true-time-externa...

> External consistency states that Spanner executes transactions in a manner that is indistinguishable from a system in which the transactions are executed serially, and furthermore, that the serial order is consistent with the order in which transactions can be observed to commit. Because the timestamps generated for transactions correspond to the serial order, if any client sees a transaction T2 start to commit after another transaction T1 finishes, the system will assign a timestamp to T2 that is higher than T1's timestamp.

Of course there is always the edge case where two commits have the same commit timestamp. Therefore from the perspective of Spanner, they happen simultaneously and there is no way to determine which happens first. But there is no need to. There is no causality relationship between them. If you insist, you can arbitrarily assign a happens-before relationship in your own code and nothing will break.