The trick is to work out what you would do if you had to implement the entire system on paper in a 50s style office with triplicate carbon paper. Then do that, but with computers.
That is not a trick, it is a false analogy. Two humans in eyesight line can be assured that they have exchanged messages successfully. Two computers cannot due to recursing acknowledgements (byzantine generals).
No, it’s not a false analogy. Go look at an old office building and consider whether people really are “in eyesight line” - assuredly they are not.
The unreliable messaging links (internal mail) mean that resilience against missed messages and guards against not making progress must be built into the business process instead of an infrastructure layer.
