Stalewhilerevalidate

Stale-While-Revalidate is a Cache-Control directive used in HTTP responses to improve performance by allowing caches to serve stale content while refreshing it in the background. When a cached response is no longer fresh, a cache that supports this directive may continue to serve that stale copy for a specified period, even as it asynchronously revalidates the resource with the origin server.

The directive works by adding a time delta in seconds to the cache policy. For example, a header such as Cache-Control: max-age=60, stale-while-revalidate=30 indicates that after the resource becomes stale, the cache can serve the stale response for up to 30 additional seconds while it fetches a fresh copy in the background. If the background revalidation succeeds, the cache updates its entry; subsequent requests may then receive the refreshed content. If the revalidation fails, the stale content may still be served until the window closes.

Scope and compatibility vary. The directive is primarily used by shared caches, such as content delivery networks and reverse proxies, to reduce latency for end users. Browser support exists for some implementations but is not universal, and not all caches honor the directive. Consequently, behavior can differ across networks and configurations.

Usage considerations include the nature of the content and the acceptable staleness. Stale-While-Revalidate is well-suited for resources that can tolerate brief periods of staleness, such as images or static assets, while still benefiting from faster responses and background updating. It is one of several Cache-Control extensions designed to optimize web performance.