Varnish 4 grace

grace.vcl

# conditional grace for healthy/unhealthy (30s/1h)

vcl 4.0;

import std;

sub vcl_hit {
  if (std.healthy(req.backend_hint) && obj.ttl + 30s < 0s) {
      return(fetch);
  }
}

sub vcl_backend_response {
  set beresp.grace = 1h;
}