Redirect to another backend based on response status


In this example, if a certain request comes in that causes a backend to return both a status code of 409 and if that same backend sends a header X-Fastly-Instructions with the contents "Use Other Backend", we restart the request, and point to a new backend.

sub vcl_recv {
#FASTLY recv

  # This header disables intra-POP clustering, allowing a request to process 
  # through the same Varnish machine, rather than a cluster. We don't reccomend
  # setting this unless Fastly support advises 
  set req.http.Fastly-No-Shield = "1";

  # If the header `Redirect-Header` is present and equal to 1, redirect to
  # a new backend, and set original-host equal to the previous host
  if (req.http.Redirect-Header == "1")  {
    set req.http.original-host =;
    set req.backend = F_my_new_backend_com;


sub vcl_fetch {
  # If the backend returns a status of 409 and the header
  # X-Fastly-Instructions:"Use Other Backend" is returned, set Redirect-Header:1
  # in the request and restart. This will send the request back to vcl_recv and
  # trigger the backend redirection logic.
  if (beresp.status == 409 && beresp.http.X-Fastly-Instructions == "Use Other Backend") {
    set req.http.Redirect-Header = "1";

sub vcl_hash {
  # This adds Original-URL and Original-Host to the cache key. We absolutely 
  # recommend _not_  changing the cache key unless so advised by Fastly support. 
  set req.hash += req.http.original-url;
  set req.hash += req.http.original-host;
#FASTLY hash

Inconsistent cache lookup