Post #1439541

@kindjar Ooh, good question! Here, "externally observable" means "observable to other processes in the system". Making a reservation doesn't count because other processes in the system can't see it happen. (You could imagine a kind of side channel where Alice or Carol could call the restaurant and ask if Bob has made a reservation! But if communication can occur over side channels like that, then you're already out of luck with respect to any guarantee of causally ordered communication.)