The problem

Marc sells second hand bikes on a marketplace app. A buyer comments on his listing at 21:04. Marc wants a push on his phone, a badge in the app, and no email, because he turned email off in his settings.

At 21:05 his phone buzzes three times for the same comment. The comment service had retried its event twice after a slow reply. Marc turns off all notifications, and the next buyer gets no answer.

You design the service that decides who gets told what, on which channel, exactly once.