Varnish is a reverse caching proxy[2] used as HTTP accelerator for content-heavy dynamic web sites as well as APIs.

Varnish is focused exclusively on HTTP, unlike other proxy servers that often support FTP, SMTP, and other network protocols.

This helps avoid the situation where the operating system starts caching data while it is moved to disk by the application.

Most policy decisions are left to VCL code, making Varnish more configurable and adaptable than most other HTTP accelerators.

A command-line management interface allows these parameters to be modified, and new VCL scripts to be compiled, loaded and activated, without restarting the accelerator.

It provides access to in-memory statistics such as cache hits and misses, resource consumption, threads created, and more.