failfast

Fail-fast is a design principle in software engineering whereby a system immediately reports or halts when it detects an invalid state or condition, rather than continuing in a potentially corrupted state. The goal is to detect and address problems early, simplifying debugging and reducing the risk of cascading errors.

Fail-fast is typically implemented through runtime checks, assertions, preconditions, and input validation, with errors raised or exceptions thrown when an invariant is violated. In practice, fail-fast behavior often means that a component will terminate or throw an exception as soon as a fault is detected, rather than proceeding with uncertain data or state.

A classic concrete example occurs in programming languages like Java, where fail-fast iterators throw a ConcurrentModificationException if the underlying collection is modified during iteration. This prevents unpredictable results and helps developers identify concurrent modification bugs. Similar ideas appear in design by contract approaches that enforce preconditions and invariants, triggering immediate failures when violated.

In broader system design, fail-fast can apply to services or components that exit or restart upon critical faults to enable rapid isolation and recovery. It is commonly contrasted with fail-safe or fail-soft approaches, which aim to continue operation in a degraded state to preserve availability.

Trade-offs of the fail-fast approach include improved fault localization and safety at the potential cost of availability and performance. Overly aggressive checks can cause unnecessary interruptions, while false positives can annoy users or disrupt service. Effective use often involves balancing immediate error reporting with resilience and recovery strategies.