Architecture & DesignIntermediate
Fail Open vs Fail Closed
PronunciationFAYL OH-pen versus FAYL KLOHZD
Definition
What a system does when a part breaks. Fail open keeps working without that part. Fail closed blocks everything until it is fixed.
Where you hear it
Resilience, security design, and rate limiting.
Examples
If Redis is down, the rate limiter fails open and lets users in.
Login must fail closed: if the auth service is down, nobody gets in.
Common mistake
Always choosing fail open. For security and payments you usually want fail closed.
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