fallback

Fallback refers to an alternative option used when a primary option cannot be used. It is a form of contingency designed to preserve functionality, accessibility, or communication. Fallbacks are common in technology, writing systems, and service design, and are chosen to be less costly than repeated failures.

In software engineering, a fallback is a mechanism that provides an alternate resource or behavior when the preferred one is unavailable or invalid. Examples include default configuration values, alternative data sources, and error-handling paths. Common patterns include try-catch blocks, default parameters, and graceful degradation when a subsystem fails.

In web design and networking, fallbacks ensure content renders across environments. If an image cannot load, browsers may display alternative content or alt text. Font fallbacks supply a chain of typefaces to use if the selected font is missing. Failover systems switch to a backup server to maintain service availability.

In linguistics and typography, a fallback term or glyph may be used when the preferred option is unavailable, or a script substitutes a character from another set. In data formats, fallback elements provide backward compatibility by supplying older schemas when newer ones are not understood.

Good fallback design balances reliability with simplicity. Fallbacks should be predictable, testable, and well-documented, and they should degrade gracefully rather than failing hard. When appropriate, users should be informed of limitations.