What is VCL? - Varnish Software
What is VCL?
Varnish Configuration Language (VCL) is a domain-specific language designed for use in Varnish. VCL allows users to configure Varnish exactly how they see fit. It gives total control of content caching policies, HTTP behavior, and routing. When a VCL configuration is loaded, Varnish translates the VCL code to C and compiles it to a shared object, which is then dynamically linked back into Varnish.
The Varnish Book is the bible of VCL. Download it here and learn to master the Varnish Configuration Language (VCL).
What are VMODs?
VMODs are Varnish modules that add new functionality to VCL. A VMOD is a shared library written in C, which is loaded into Varnish and then callable from VCL. VMODs are one of the factors that make Varnish and VCL so effective. VMODs can introduce a wide variety of new features and functionality to Varnish, greatly increasing the power of VCL.