varnish is a reverse-proxy build exclusively for web acceleration. It speed up websites by caching their contents in memory and serve the requested objects (HTML pages, images, css, js, etc…) directly instead of the HTTP server.
It not only reduce user-perceived page load time, but also increase platform load capacity. Varnish is licensed under the two-clause BSD license, but commercial support is also available from Varnish-software.
Architecture / Performance
The big plus of Varnish compared to using a classic proxy, like Squid, in reverse mode, is it ability to store data in virtual memory and lets the kernel decide what to do with it (place the object in RAM or swap or anything else). This behavior make Varnish very memory efficient.
Varnish is also massively multi-threaded, queuing incoming connection only when it reaches its limit or when multiple users ask for the same non-cacheable resource. Also in order to reduce system calls, varnish logs requests in shared memory. In a nutshell: varnish is clearly build for performance.
Configuration
Configuration is done via a specific ‘pseudo-language’ called VCL. VCL is trans-coded into C code and compiled as a shared object when reloading the configuration. If you know what you do, you can add C code into a VCL setting file to increase varnish functionalities.
Speaking of functionalities, varnish supports round-robin load balancing and most ESI standard tags, which simplify a lot the inclusion of dynamic content into ‘static’ cachable pages.