index on Varnish Cache

Ignoring the Vary header for bots

Tue, 07 Sep 2021 00:00:00 +0000

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.

Using extra digits in resp.status

Tue, 07 Sep 2021 00:00:00 +0000

In Varnish the .status variables can hold more than three digits, which is useful to send information to vcl_synth{} about which error message to produce:

sub vcl_recv {
 if ([...]) {
 return(synth(12404));
 }
}

sub vcl_synth {
 if (resp.status == 12404) {
 [...] // this specific 404
 } else if (resp.status % 1000 == 404) {
 [...] // all other 404's
 }
}