- In short
- Fail-open and fail-closed describe what a guardrail does when it errors, times out, or becomes unreachable: fail open lets the request through unscreened, fail closed blocks it until the control is healthy. If the direction is not chosen explicitly, the surrounding code usually defaults to letting traffic through, so a screening service that silently passes on error looks healthy while providing none of its protection. High-consequence checkpoints should fail closed even at the cost of availability.
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