Oh yes, that's precisely Spanner's guarantee. If t1.end < t2.start, it is _defined_ as t1 < t2, and t1 definitely happened before t2. t1's effects will always

Oh yes, that's precisely Spanner's guarantee. If t1.end < t2.start, it is _defined_ as t1 < t2, and t1 definitely happened before t2. t1's effects will always be observable before t2's to any observer. The notion of arrival time isn't important really. It is quite possible that the second one arrived at a server before the first one, but the first one's participants responded earlier. At that point, t1's commit time is picked. If t2 is also committed at the same server, then t2's time will be selected to be later than t1 (non-overlapping intervals). If t2 is committed at a different server, and if t1 just happens to be < than t2, then one can be guaranteed that causality is preserved. There is no way t2 could have affected t1. This latter property (external consistency) cannot be guaranteed by pseudotime. In PT it is possible to have t2 affect t1 in an external way (without involving the db, say by making a phone call), and yet get a smaller timestamp. From the db's point of view, t2 happened first, but that's not what really happened in reality, in an externally observable sense.