IoT Fire and forget - Atlanta, GA

IoT encompasses many areas of tech that pose difficulties, but chief among them is the problem of scale. How does one process a few-billion messages a day? an hour? a second?
The answer to this burning question is microservices. But that in itself poses more questions.
How do you do this at scale?
How do you account for reliability?
Can you do edge computing?
What about state? Application state? User state?
What happens to state during a failover?