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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.


From the field of software engineering, a Fail Fast Iterator is an iterator that attempts to raise an error if the sequence of elements processed by the iterator is changed during iteration.

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