|Developer(s)||Poul-Henning Kamp, Redpill-Linpro, Varnish Software|
|Stable release||3.0.3 / August 20, 2012|
Varnish is an HTTP accelerator designed for content-heavy dynamic web sites. In contrast to other web accelerators, such as Squid, which began life as a client-side cache, or Apache and nginx, which are primarily origin servers, Varnish was designed as an HTTP accelerator. Varnish is focused exclusively on HTTP, unlike other proxy servers that often support FTP, SMTP and other network protocols.
The project was initiated by the online branch of the Norwegian tabloid newspaper Verdens Gang. The architect and lead developer is Danish independent consultant Poul-Henning Kamp (a well-known FreeBSD core developer), with management, infrastructure and additional development originally provided by the Norwegian Linux consulting company Linpro. The support, management and development of Varnish was later spun off into a separate company, Varnish Software.
Varnish stores data in virtual memory and leaves the task of deciding what is stored in memory and what gets paged out to disk to the operating system. This helps avoid the situation where the operating system starts caching data while they are moved to disk by the application.
Furthermore, Varnish is heavily threaded, with each client connection being handled by a separate worker thread. When the configured limit on the number of active worker threads is reached, incoming connections are placed in an overflow queue; when this queue reaches its configured limit incoming connections will be rejected.
The principal configuration mechanism is VCL (Varnish Configuration Language), a domain-specific language (DSL) used to write hooks which are called at critical points in the handling of each request. Most policy decisions are left to VCL code, making Varnish more configurable and adaptable than most other HTTP accelerators. When a VCL script is loaded, it is translated to C, compiled to a shared object by the system compiler, and loaded directly into the accelerator.
A number of run-time parameters control things such as the maximum and minimum number of worker threads, various timeouts etc. 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.
In order to reduce the number of system calls in the fast path to a minimum, log data is stored in shared memory, and the task of filtering, formatting and writing log data to disk is delegated to a separate application.
While Varnish is designed to reduce contention between threads to a minimum, its authors claim that its performance will only be as good as that of the system's pthreads implementation.
Additionally, a poor malloc implementation may add unnecessary contention and thereby limit performance, hence the general recommendation of running Varnish on Linux or Unix based environments.
Load balancing 
Other features 
Varnish Cache also features:
- Plugin support with Varnish Modules, also called VMODs
- Support for Edge Side Includes including stitching together compressed ESI fragments
- Gzip Compression and Decompression
- DNS, Random, Hashing and Client IP based Directors
- Technology preview for HTTP Streaming Pass & Fetch
- Experimental support for Persistent Storage, without LRU eviction
- Saint and Grace mode
See also 
- Web accelerator which discusses host-based HTTP acceleration
- Proxy server which discusses client-side proxies
- Reverse proxy which discusses origin-side proxies
- Comparison of web servers
- Comparison of lightweight web servers
- Internet Cache Protocol