Why is Erlang considered fault-tolerant?

Fault tolerance in Erlang comes from combining process isolation with supervision, not from trying to prevent every possible bug. Because each process has its own heap and crashes independently, a failure in one part of the system doesn't corrupt shared state or take down unrelated processes.

On top of that isolation, supervisors watch for process exits and restart them according to a defined strategy, so the system as a whole recovers automatically from failures that would otherwise require manual intervention. This is often summarized as designing for failure rather than around it — the system expects things to crash and has a built-in recovery path, instead of assuming failures can all be prevented.

This combination is why Erlang systems can report "nine nines" style uptime figures — not because the code never has bugs, but because individual bugs stay contained and self-heal quickly.

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