Varnish 的作者Poul-Henning Kamp是FreeBSD的内核开发者之一，他认为现在的计算机比起1975年已经复杂许多。在1975年时，储存媒介只有两种：内存与硬盘。但现在计算 机系统的内存除了主存外，还包括了cpu内的L1、L2，甚至有L3快取。硬盘上也有自己的快取装置，因此squid cache自行处理物件替换的架构不可能得知这些情况而做到最佳化，但操作系统可以得知这些情况，所以这部份的工作应该交给操作系统处理，这就是 Varnish cache设计架构。
The fundamentals of web proxy caching with Varnish — Varnish version 6.0.0 documentation https://varnish-cache.org/docs/6.0/tutorial/introduction.html
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.
Introduction to Varnish — Varnish HTTP Cache http://varnish-cache.org/intro/index.html#intro
The basics
Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture. A high level overview of what Varnish does can be seen in this video.
Performance
Varnish performs really, really well. It is usually bound by the speed of the network, effectively turning performance into a non-issue. We’ve seen Varnish delivering 20 Gbps on regular off-the-shelf hardware.
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. And, you canextend Varnish with modules (VMODs). You can read more about this in our tutorial.
Further reading
There is a good article describing Varnish Cache on Wikipedia.
Varnish Documentation — Varnish version 6.0.0 documentation https://varnish-cache.org/docs/6.0/
Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture.
To get started with Varnish-Cache we recommend that you read the installation guide Varnish Installation. Once you have Varnish up and running we recommend that you go through our tutorial - The Varnish Tutorial, and finally the The Varnish Users Guide.
