Fail Open / Fail Closed

Two opposing failure modes for a system check or gate. Fail open allows requests through when the check itself is unavailable, prioritizing availability over correctness — common in rate limiters and feature flags where a brief outage is less harmful than blocking all traffic. Fail closed blocks all requests when the check fails, prioritizing security or data integrity — common in authentication middleware and fraud detection. Choosing the right mode depends on the cost of false positives versus false negatives.