AB testing with Varnish (and Django)

abtest.vcl

# import the abtesting vmod
import abtest;

sub vcl_deliver {

  # only continue, if the config file with all the rules can be loaded
  if (abtest.load_config("/etc/varnish/abtest-rules.cfg") == 0) {
  
    # if there isn't a named cookie 'ab' in the request, we can add one
    if (req.http.Cookie !~ "ab=") {
    
      # if there is a experiment defined in the rules files for this url
      if(abtest.get_rand(req.url)) {
      
        # this is where the magic happens, a cookie is set for this url (page), and 
        # for the lifetime of the cookie, the user will be in the given random testgroup
        set resp.http.Set-Cookie = "ab=" + abtest.get_rand(req.url) + "; path=" + req.url + "; expires=" + abtest.get_expire(req.url);
        
      }
    }
  }
}

experiments.js

if(location.pathname.indexOf("foo") != -1) {
  if(readCookie('ab') == '1') {
    // set button color to red
  }
  if(readCookie('ab') == '2') {
    // set button color to blue
  }
  if(readCookie('ab') == '3') {
    // set button color to green
  }
}
if(location.pathname.indexOf("bar") != -1) {
  if(readCookie('ab') == '1') {
    // set button text to 'now'
  }
  if(readCookie('ab') == '2') {
    // set button text to 'later'
  }
  if(readCookie('ab') == '3') {
    // set button text to 'ponies!'
  }
}

rules.cfg

# location : groups : ttl ;
/bar/:A:33;B:33;C:33;300;
/foo/:A:20;B:20;C:20;D:20;E:20;300;
/unicorns/:A:66;B:34;300;