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.
It’s finally here. The latest edition of the Varnish Book for Varnish 4 and Varnish Plus. This edition is a major revision that updates the content from Varnish 3 to Varnish 4 and adds new content for Varnish Plus including Varnish Administration Console (VAC), Varnish Custom Statistics (VCS), Massive Storage Engine (MSE), SSL/TLS support and more. It also clearly illustrates differences between the open source Varnish Cache and the enhanced commercial edition Varnish Cache Plus.