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Software should refuse an operation when it cannot verify that the action is permitted, the system is in a valid state, and the inputs are trustworthy—and continuing could cause harm or an unacceptable failure. The right response is not always a full shutdown: it may be safer to block one command, continue with reduced functionality, correct the fault in time, or ask a qualified person to take control.
What should trigger a refusal?
Base the decision on the system’s state and the consequences of acting, not on a blanket preference for stopping. In safety-critical software, NASA requirements identify concrete checks: whether a command is allowed in the current mode, whether its prerequisites are satisfied, whether commands arrive in a valid sequence, and whether input and output integrity checks pass. If one of these conditions fails, the software should not initiate an unsafe action. NASA Software Engineering Handbook: Software Safety Requirements
For other applications, applying the same checks is a risk-based design inference, not a universal regulatory rule. A failed check matters most when continuing could cause substantial harm, loss, or an unrecoverable result. A harmless, reversible action may need a warning or confirmation rather than a hard stop.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems- Authority and mode: Is this action allowed for the current user, operating mode, and system state?
- Prerequisites and sequence: Are required conditions met, and would an out-of-order command create risk?
- Data and state integrity: Are the inputs, outputs, and relevant system state reliable enough to predict the action’s effects?
- Time to mitigate: Can the system correct the problem before the hazard or other serious consequence occurs?
How should software choose a safe response?
First consider what happens if the software continues, stops, or waits. Compare the severity and timing of possible consequences with the time needed to correct the fault. Also consider confidence in the system state and inputs, whether reduced functionality remains safe and useful, whether the action can be reversed, and whether a qualified operator can intervene safely. These comparison factors synthesize NASA guidance on safe states, timely mitigation, recovery, and human control.
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- Block or cancel the operation if a required condition fails and the action cannot safely proceed.
- Correct the fault in time if the system can do so reliably before the hazard occurs.
- Degrade to a safe mode when a reduced set of functions can remain safe and useful. A safe state need not mean that every feature or the entire system is switched off. NASA guidance describes safe states that may retain reduced functionality and support later recovery. NASA Software Safety Guidebook
- Stop or shut down when no safe corrective action or reduced mode is available. The transition itself should leave the system in a known safe state.
- Request human control when automation reaches a defined safety limit and an operator can safely take over. NASA crew-interface guidance calls for protective action or a request for safe operator control when an operational safety threshold is exceeded. It also requires human initiation of autonomous robotic operations, including restart after an emergency or protective stop. NASA Human Factors Requirements
These choices are not interchangeable defaults. A system’s hazard analysis and applicable domain requirements should define the thresholds, fallback behavior, and who is authorized to restart it. NASA and FAA guidance cited here applies to its stated safety-critical and human-spaceflight contexts; it is not a blanket legal requirement for every application.
Make the refusal clear and recoverable
A refusal should tell the user or operator what was blocked, why it was blocked, what the system did, and what can happen next. For example: “Transfer paused because the destination account could not be verified. No funds were sent. Verify the account details or contact support.” This is an illustrative example, not a tested message.
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For safety-critical interfaces, distinguish critical errors from noncritical status messages. Show whether a command was received, initiated, or is still processing; do not leave an operator guessing whether an action took effect. FAA software safety guidance calls for unambiguous safety-critical error messages and a single-action route out of current processing to a known stable state. FAA Advisory Circular AC 20-115D
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Set system-specific thresholds, not a universal number
There is no general-purpose numeric threshold that determines when all software should refuse to continue. The trigger depends on the system, the possible hazard, how quickly consequences could occur, and how reliably a mitigation works. Establish thresholds through the system’s hazard analysis and validation, and check the standards and regulations for the relevant domain and jurisdiction. NASA guidance describes system-specific requirements rather than a universal stopping rule. NASA Software Engineering Handbook: Software Safety Requirements NASA Software Engineering Handbook: Software Safety Requirements
The practical rule is: do not proceed on an assumption the system cannot trust when the cost of being wrong is serious. Refuse the risky action, choose the least hazardous safe state available, and make the reason and recovery path clear.
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