How to return banned objects length in varnish purge?

i am Using Varnish 6.0 LTS in debian

iam using magento2 vcl 6 from here:

https://github.com/magento/magento2/blob/2.4-develop/app/code/Magento/PageCache/etc/varnish6.vcl

when i issue a curl request with a tag pattern i would like the response to be:

"Found and purged X object hits"

the section from the vcl is here :

   if (req.http.X-Magento-Tags-Pattern) {
          ban("obj.http.X-Magento-Tags ~ " + req.http.X-Magento-Tags-Pattern);
        }
        if (req.http.X-Pool) {
          ban("obj.http.X-Pool ~ " + req.http.X-Pool);
        }
 #Here i would like to return the Ban Purged Hits counter
        
return (synth(200, "You have Purged X items"));

vmod_xkey

I suggest you use the vmod_xkey for that. It's an open source Varnish module that performs tag-based invalidation. You can download the source code from https://github.com/varnish/varnish-modules.

However, you have to compile this module from source.

One you have successfully installed the VMOD, you can find the API here: https://github.com/varnish/varnish-modules/blob/master/src/vmod_xkey.vcc.

There are some restrictions though: currently you're tagging responses with the X-Magento-Tags keyword. This will have to change to the xkey keyword instead. Hopefully you can make this change in Magento.

Once you've done that, you can call set req.http.n-gone = xkey.purge(req.http.X-Magento-Tags-Pattern);. The req.http.n-gone header contains the number of purged items.

vmod_ykey

If vmod_xkey is too restrictive, you can also use vmod_ykey, which is the Varnish Enterprise alternative. It offers more flexibility.

See https://docs.varnish-software.com/varnish-cache-plus/vmods/ykey/ for the documentation.

As mentioned in the documentation, you can tag content as follows:

sub vcl_backend_response {
    ykey.add_header(beresp.http.X-Magento-Tags);
    ykey.add_header(beresp.http.X-Pool);
}

Invalidation can be done through set req.http.n-gone = ykey.purge_header(req.http.X-Magento-Tags-Pattern);.

DISCLAIMER: Varnish Enterprise is commercial software that requires a license. However, you can use official machine images in the Cloud. This allows you to pay the license on a pay-as-you-go basis. There are Developer Edition images for AWS where lower fees are charged.