> Establishing a single externally consistent order of events for all possible observers is physically impossible No, it is not. It does however require commun

> Establishing a single externally consistent order of events for all possible observers is physically impossible No, it is not. It does however require communication. Additionally, what's true in an absolute physical sense, is somewhat removed from what's practical engineered product. For example, we cannot literally make a physical part that is perfectly 1 meter in length. This has not stopped us from making everything from microchips to spacecraft. > Abel puts in money in Spain, Beth removes it in Australia -- do you charge an overdraft fee/interest? (N.B., Abel and Beth may be names of HFT algorithms.) Spanner handles this by using two phase commit over sets of independent consensus replication groups. By using TrueTime's ability to provide absolute intervals, it can wait out uncertainty windows to enforce the total order. > P.S. Not sure what you mean by "hidden communication channels." Schemes such as lamport and vector clocks require all messages exchanged to include clock data. If two clients establish their own communication channel using some other protocol, they may not agree on the same event order, because the system has no way to know it needs to advance the logical clocks based on this hidden communication. Spanner in combination with TrueTime avoids this limitation. External observers will see the same commit order in real time, regardless of how they communicate. Please read and understand the papers to get what you're missing.