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What's a Web Cache? Why do people use them?

A Web cache sits between Web servers (or origin servers) and a client or many clients, and watches requests for HTML pages, images and files (collectively known as objects) come by, saving a copy for itself. Then, if there is another request for the same object, it will use the copy that it has, instead of asking the origin server for it again.

There are two main reasons that Web caches are used:

• To reduce latency - Because the request is satisfied from the cache (which is closer to the client) instead of the origin server, it takes less time for the client to get the object and display it. This makes Web sites seem more responsive.
• To reduce traffic - Because each object is only gotten from the server once, it reduces the amount of bandwidth used by a client. This saves money if the client is paying by traffic, and keeps their bandwidth requirements lower and more manageable.

Kinds of Web Caches

Browser Caches

If you examine the preferences dialog of any modern browser (like Internet Explorer or Netscape), you'll probably notice a 'cache' setting. This lets you set aside a section of your computer's hard disk to store objects that you've seen, just for you. The browser cache works according to fairly simple rules. It will check to make sure that the objects are fresh, usually once a session (that is, the once in the current invocation of the browser).

This cache is useful when a client hits the 'back' button to go to a page they've already seen. Also, if you use the same navigation images throughout your site, they'll be served from the browser cache 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 ISP's often set them up on their firewalls.

Because proxy caches usually have a large number of users behind them, they are very good at reducing latency and traffic. That's because popular objects are requested only once, and served to a large number of clients.

Most proxy caches are deployed by large companies or ISPs that want to reduce the amount of Internet bandwidth that they use. Because the cache is shared by a large number of users, there are a large number of shared hits (objects that are requested by a number of clients). Hit rates of 50% efficiency or greater are not uncommon. Proxy caches are a type of shared cache.

1. What's a Web Cache? Why do people use them?
2. Kinds of Web Caches
   1. Browser Caches
   2. Proxy Caches
3. Aren't Web Caches bad for me? Why should I help them?
4. How Web Caches Work
5. How (and how not) to Control Caches
   1. HTML Meta Tags vs. HTTP Headers
   2. Pragma HTTP Headers (and why they don't work)
   3. Controlling Freshness with the Expires HTTP Header
   4. Cache-Control HTTP Headers
   5. Validators and Validation
6. Tips for Building a Cache-Aware Site
7. Writing Cache-Aware Scripts
8. Frequently Asked Questions
9. A Note About the HTTP
10. Implementation Notes - Web Servers
11. Implementation Notes - Server-Side Scripting
12. References and Further Information
13. About This Document

Aren't Web Caches bad for me? Why should I help them?