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System which reports likely failures

In systems design, a fail fast system is one that immediately reports at its interface any condition that is likely to indicate a 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 that any failures can be detected early. The responsibility of a fail fast module is detecting errors, and then letting the next highest level of the system handle them.

A fail fast application/system checks that all input/output resources needed for future computations are ready before any computation request arrives. A fail fast application/system checks that all immutable initial configurations are correct at startup. A fail fast function is a function that checks all input to the function in a Precondition before proceeding with any computation or business logic in such function. A fail fast function will normally throw a runtime exception, when some abnormal computation, is found making the system fail if no "catch" has been contemplated by any other, versus returning some error value without making any (optimistic) assumption about the correct management of the raised error. 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. Given an initial state in a state machine, a fail fast system will check such a state and fail fast. Given a state change in a state machine, the fail fast system will halt the machine if the state change is forbidden. It could be the case that the forbidden state change is due to a wrong external input. In that case, the fail fast system will stop processing the request as soon as the wrong input is detected (vs. delegating to the state machine implementation).

The term has been widely employed as a metaphor in business, dating back to at least 2001, meaning that businesses should undertake bold experiments to determine the long term viability of a product or strategy, rather than proceeding cautiously and investing years in a doomed approach. It became adopted as a kind of "mantra" within startup culture, i. e. "Fail fast, fail often."