Stale If Error

stale-if-error is defined in RFC 5861 along stale-while-revalidate as HTTP Cache-Control extensions.

The latter is supported out of the box since Varnish Cache 4.1.0; if stale-while-revalidate is present beresp.grace will be set to the specified number of seconds.

The stale-if-error directive, however, does not map very well to Varnish but is still possible to implement it entirely in VCL.