Have you used varnishtest? Maybe not, but we’ve just made it a lot easier for you to adopt and put into practice!
Varnish has an ecosystem for third-party modules – Varnish Cache Modules (VMODs). A VMOD is a shared library that can be imported in your VCL code. VMODs allow you to use standard C libraries, meaning that you can extend the functionality of your Varnish servers.