Varnish Cache, also known as a caching HTTP reverse proxy is a web application accelerator It is installed in front of any server speaking HTTP and configured for caching the contents. It is really, really fast. Depending on your architecture, it typically speeds up delivery with a factor of 300 - 1000x.
Varnish is usually bound by the speed of the network and effectively turns performance into a non-issue. Varnish can deliver 20 Gbps on regular off-the-shelf hardware.
The online branch of the Norwegian tabloid newspaper Verdens Gang initiated the project. The architect and lead developer is Poul-Henning Kamp (a Danish independent consultant). Varnish is open source and is available under a two-clause BSD license. Version 1.0 was released in 2006,Varnish 2.0 of Varnish in 2008,Varnish 3.0 in 2011, and Varnish 4.0 of Varnish was released in 2014.
The flexibility of its configuration language, VCL is one of the key features of Varnish Cache, in addition to its performance. VCL allows you to write policies on how incoming requests are to be handled. You can decide what content you want to serve in such a policy, how the request or response should be altered and from where you want to get the content. You can also extend Varnish with modules (VMODs).
A great number of high-profile, high-traffic websites use varnish which includes online newspaper sites such as The Guardian, The New York Times , Corriere della Sera, The Hindu social media and content sites like Facebook, Wikipedia, Twitter, Tumblr, Vimeo and many others. Of the Top 10K sites in the web, use the software.
Varnish Cache also features Gzip Compression and Decompression; HTTP Streaming Pass & Fetch; Plugin support with Varnish Modules, also called VMODs; DNS, Hashing, Random and Client IP based Directors; Experimental support for Persistent Storage; Saint and Grace mode.