Ignoring the Vary header for bots

Varnish supports HTTP variants out of the box, but the Vary header is somewhat limited since it operates on complete header values. If you want for example to conduct an A/B testing campaign or perform blue/green deployment you can make clients “remember” their path with a first-party cookie.

When a search engine bot asks for contents however, there’s a high chance that they don’t process cookies and in all likelihood you would prefer to serve a response quickly. In that case you would probably prefer not to even try to attribute a category to the client, but in that case you create a new variant in your cache that is none of A, B, blue, green, or whatever your backend serves.

If the way content is served makes no difference to the bot, because you changed the color of a button or something else orthogonal to the content itself, then you risk a cache miss with the detrimental effects of adding a needless variant to the cache and serving it with extra latency.

If latency is paramount, you can use req.hash_ignore_vary to opt out of the Vary match during the lookup and get the freshest variant.

Ignoring how the cookie is set, and assuming the backend always provides an accurate Cache-Control even when cookies are present, below is an example of an A/B testing setup where bots are served the freshest variant:

import cookie;

include "devicedetect.vcl";

sub vcl_recv {
    call devicedetect;
    if (req.http.X-UA-Device ~ "bot") {
        set req.hash_ignore_vary = true;

sub vcl_req_cookie {
    set req.http.X-AB-Test = cookie.get("ab-test");

sub vcl_deliver {
    unset resp.http.Vary;

It is also assumed that the backend replies with a Vary: X-AB-Test header and varies on no other header.