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In systems design, a fail-fast system is one which 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 any failures can be detected early. A fail-fast module passes the responsibility for handling errors, but not detecting them, to the next-highest level of the system.
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.
Developers also refer to fail-fast code to a code that tries to fail as soon as possible at variable or object initialization. In OOP, a fail-fast designed object initializes the internal state of the object in the constructor, launching an exception if something is wrong (vs allowing non-initialized or partially initialized objects that will fail later due to a wrong "setter"). The object can then be made immutable if no more changes to the internal state are expected. In functions, fail-fast code will check input parameters in the precondition. In client-server architectures, fail-fast will check the client request just upon arrival, before processing or redirecting it to other internal components, returning an error if the request fails (incorrect parameters, ...). Fail-fast designed code decreases the internal software entropy.
- Crash-only software
- Design by contract
- Failing badly vs. failing well
- Fail-silent system
- Gray, Jim. "Why Do Computers Stop And What Can Be Done About It?". CiteSeerX , 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)