If you have a website that you want to be able to cache different versions for country, region or city, and you also do not want to run custom VCL due to the multiple pain points this imposes, but you also want simple purging with acquia purge, consider using custom Vary headers for caching and purging purposes.
Vary header to the rescue
The Vary header is sent by the web server to indicate what makes a HTTP object Vary. This makes a lot of sense with headers like Accept-Encoding. When a server issues a Vary: Accept-Encoding it tells Varnish that its needs to cache a separate version for every different Accept-Encoding that is coming from the client. If a client only accepts gzip encoding, Varnish won't serve the version of the page encoded with the Deflate encoding.
How to use a custom Vary header
If you are using a CDN (e.g. Akamai, Cloudflare or Fastly) then you can get these CDN providers to include an extra HTTP header on all requests back to Acquia. You can also add additional custom Vary headers in the response from Drupal. See drupal_add_http_header on the Drupal API. Here is an example that varies on X-MYCUSTOMHEADER header:
For Drupal 7
/**
* Implements hook_page_build().
*/
function MYMODULE_page_build(&$page) {
drupal_add_http_header('Vary', 'X-MYCUSTOMHEADER', TRUE);
}
For Drupal 8
/**
* Implements hook_page_attachments().
*/
function MY_MODULE_page_attachments(array &$page) {
$page['#attached']['http_header'][] = [
'Vary', 'Accept-Language',
];
}
How to purge all variants in Varnish for a particular URL
Acquia's stock VCL (Varnish Configuration Language) in Acquia Cloud means that if a PURGE request hits (e.g. it matches a variant in cache), or misses (not in cache), purge is still called. This means all variants will be removed from Varnish immediately. You can also use acquia purge
Known Issues
Unfortunately, Drupal's internal page caching mechanism doesn't account for the value of the Vary header when saving rendered pages into its local cache, meaning that Drupal's "cache pages for anonymous users" feature, by default, is not compatible with sites that set custom Vary headers. Happily though, Drupal supports a workaround that will allow you to use custom Vary headers for your Drupal pages and still leverage external page caching mechanisms (like Varnish on your Acquia load balancers, or an external Content Delivery Network). This workaround, which involves setting a "fake" storage backend for Drupal's internal page cache, can be achieved with the following block of code in your site's settings.php file:
if (!class_exists('DrupalFakeCache')) {
$conf['cache_backends'][] = 'includes/cache-install.inc';
}
// Rely on the external cache for page caching.
$conf['cache_class_cache_page'] = 'DrupalFakeCache';
You will also need to be running Acquia's stock VCL with no changes to the vcl_hash() function (e.g. using geo_ip functionality), as any alterations to this function will cause a regular PURGE to not remove all variants.