Fail-fast is a property of a system or module with respect to its response to failures. A fail-fast system is designed to immediately report at its interface any failure or condition that is likely to lead to failure. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly flawed process. Such designs often check the system's state at several points in an operation, so any failures can be detected early. A fail-fast module passes the responsibility for handling errors, but not detecting them, to the next-higher system design level.
Fail-fast systems or modules are desirable in several circumstances:
- When building a fault-tolerant system by means of redundant components, the individual components should be fail-fast to give the system enough information to successfully tolerate a failure.
- Fail-fast components are often used in situations where failure in one component might not be visible until it leads to failure in another component.
- Finding the cause of a failure is easier in a fail-fast system, because the system reports the failure with as much information as possible as close to the time of failure as possible. In a fault-tolerant system, the failure might go undetected, whereas in a system that is neither fault-tolerant nor fail-fast the failure might be temporarily hidden until it causes some seemingly unrelated problem later.
- A fail-fast system that is designed to halt as well as report the error on failure is less likely to erroneously perform an irreversible or costly operation.
See also 
- Gray, Jim. WHY DO COMPUTERS STOP AND WHAT CAN BE DONE ABOUT IT?. CiteSeerX: 10.1.1.110.9127, introducing 'Fail Fast'
- "Fail Fast" Article by Jim Shore explaining using 'Fail Fast' concept in software development (from 'columns for IEEE software' edited by Martin Fowler)