The fundamentals of web proxy caching with Varnish

Varnish is a caching HTTP reverse proxy. It receives requests from clients and tries to answer them from the cache. If Varnish cannot answer the request from the cache it will forward the request to the backend, fetch the response, store it in the cache and deliver it to the client.

When Varnish has a cached response ready it is typically delivered in a matter of microseconds, two orders of magnitude faster than your typical backend server, so you want to make sure to have Varnish answer as many of the requests as possible directly from the cache.

Varnish decides whether it can store the content or not based on the response it gets back from the backend. The backend can instruct Varnish to cache the content with the HTTP response header Cache-Control. There are a few conditions where Varnish will not cache, the most common one being the use of cookies. Since cookies indicates a client-specific web object, Varnish will by default not cache it.

This behaviour as most of Varnish functionality can be changed using policies written in the Varnish Configuration Language (VCL). See The Varnish Users Guide for more information on how to do that.

Performance

Varnish has a modern architecture and is written with performance in mind. It is usually bound by the speed of the network, effectively turning performance into a non-issue. You get to focus on how your web applications work and you can allow yourself, to some degree, to care less about performance and scalability.

Flexibility

One of the key features of Varnish Cache, in addition to its performance, is the flexibility of its configuration language, VCL. VCL enables you to write policies on how incoming requests should be handled.

In such a policy you can decide what content you want to serve, from where you want to get the content and how the request or response should be altered.