Varnish Cache – caching HTTP reverse proxy

Varnish is a web accelerator written with performance and flexibility in mind. Its modern architecture gives it a significantly better performance. Varnish stores web pages in memory so the web servers do not have to create the same web page repeatedly. The web server only recreates a page when it is changed. When content is served from memory this happens a lot faster then anything.

Additionally Varnish can serve web pages much faster then any application server is capable of – giving the website a significant speed enhancement.

It uses the advanced features in Linux 2.6, FreeBSD 6/7 and Solaris 10 to achieve its high performance. For a cost-effective configuration, Varnish Cache uses between 1-16GB and a SSD disk.

Features include:

  • Modern design.
  • VCL – a very flexible configuration language. The VCL configuration is translated to C, compiled, loaded and executed giving flexibility and speed.
  • Load balancing using both a round-robin and a random director, both with a per-backend weighting.
  • DNS, Random, Hashing and Client IP based Directors.
  • Load balance between multiple backends.
  • Support for Edge Side Includes including stitching together compressed ESI fragments.
  • Heavily threaded.
  • URL rewriting.
  • Cache multiple vhosts with a single Varnish.
  • Log data is stored in shared memory.
  • Basic health-checking of backends.
  • Graceful handling of “dead” backends.
  • Administered by a command line interface.
  • Use In-line C to extend Varnish.
  • Can be used on the same system as Apache.
  • Run multiple Varnish on the same system.
  • Support for HAProxy’s PROXY protocol. This is a protocol adds a small header on each incoming TCP connection that describes who the real client is, added by (for example) an SSL terminating process.
  • Warm and cold VCL states.
  • Plugin support with Varnish Modules, called VMODs.
  • Backends defined through VMODs.
  • Gzip Compression and Decompression.
  • HTTP Streaming Pass & Fetch.
  • Saint and Grace mode. Saint Mode allows for unhealthy backends to be blacklisted for a period of time, preventing them from serving traffic when using Varnish as a load balancer. Grace mode allows Varnish to serve an expired version of a page or other asset in cases where Varnish is unable to retrieve a healthy response from the backend.
  • Experimental support for Persistent Storage, without LRU eviction.

Support: Documentation
Developer: Varnish Software
License: BSD-2-Clause

Varnish is written in C. Learn C with our recommended free books and free tutorials.

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