Amazon CloudFront announces support for stale-while-revalidate and stale-if-error cache control directives, which can improve performance and availability. The stale-while-revalidate directive instructs CloudFront to immediately deliver stale responses to users while it revalidates caches in the background. The stale-if-error directive defines how long CloudFront should reuse stale responses if there’s an error during revalidation, which provides a better user experience.
Source:: Amazon AWS