Caching Tutorial
By Mark Nottingham2007-12-22
Kinds of Web Caches
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Browser Caches
If you examine the preferences dialog of any modern Web browser (like Internet Explorer, Safari or Mozilla), you’ll probably notice a “cache” setting. This lets you set aside a section of your computer’s hard disk to store representations that you’ve seen, just for you. The browser cache works according to fairly simple rules. It will check to make sure that the representations are fresh, usually once a session (that is, the once in the current invocation of the browser).
This cache is especially useful when users hit the “back” button or click a link to see a page they’ve just looked at. Also, if you use the same navigation images throughout your site, they’ll be served from browsers’ caches almost instantaneously.
Proxy Caches
Web proxy caches work on the same principle, but a much larger scale. Proxies serve hundreds or thousands of users in the same way; large corporations and ISPs often set them up on their firewalls, or as standalone devices (also known as intermediaries).
Because proxy caches aren’t part of the client or the origin server, but instead are out on the network, requests have to be routed to them somehow. One way to do this is to use your browser’s proxy setting to manually tell it what proxy to use; another is using interception. Interception proxies have Web requests redirected to them by the underlying network itself, so that clients don’t need to be configured for them, or even know about them.
Proxy caches are a type of shared cache; rather than just having one person using them, they usually have a large number of users, and because of this they are very good at reducing latency and network traffic. That’s because popular representations are reused a number of times.
Gateway Caches
Also known as “reverse proxy caches” or “surrogate caches,” gateway caches are also intermediaries, but instead of being deployed by network administrators to save bandwidth, they’re typically deployed by Webmasters themselves, to make their sites more scalable, reliable and better performing.
Requests can be routed to gateway caches by a number of methods, but typically some form of load balancer is used to make one or more of them look like the origin server to clients.
Content delivery networks (CDNs) distribute gateway caches throughout the Internet (or a part of it) and sell caching to interested Web sites. Speedera and Akamai are examples of CDNs.
This tutorial focuses mostly on browser and proxy caches, although some of the information is suitable for those interested in gateway caches as well.
Tutorial Pages:
» What’s a Web Cache? Why do people use them?
» Kinds of Web Caches
» Aren’t Web Caches bad for me? Why should I help them?
» How Web Caches Work
» How (and how not) to Control Caches
» Tips for Building a Cache-Aware Site
» Writing Cache-Aware Scripts
» Frequently Asked Questions
» Implementation Notes — Web Servers
» Implementation Notes — Server-Side Scripting
» References and Further Information
