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Comment by exikyut | Hacker News Reader

Very curious, how is state persisted where non-volatile storage is needed, and how is that persistence structured?

It is never persisted outside of RAM. The flight computers are never powered off during a mission. You rely on two flight computers to always be functioning, that’s why the system is one-fault redundant.

You can think as each string being “functional“: given the same set of inputs you expect the same set of outputs.

What happens if some anomaly happens where the shuttle loses power for a second?

Are you asking about what happens if all the power goes out or are you asking how they get state back? If everything reboots it’s fine. On restart the computers will look at all the sensors add project the state of now. There is no reason to keep the state of the past is there? Why does it matter where the craft was 5 minutes before? What matters is what is what is in front of them and the future right? If you tell the computer I want to fly home it looks at where you are and tells you how to get there. It does not need to remember the past parts of the trip does it?

I'm reminded of the quote from Linus:

"Real men don’t use backups, they post their stuff on a public ftp server and let the rest of the world make copies."

Real programmers don't use persistent storage, they just rely on physical reality in general.

It'd have to know that it's destination was earth, or does it just sit idle after a power on and wait for the command? What happens if the astronauts are asleep? I feel like it needs some persisted data.

State is, as far as the system is concerned, is what values the multitude of sensors are reporting at any given moment.

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