What are Caches and How do They Work?
from the March 2001 Actrix Newsletter

by Rob Zorn

For Internet users, there are two types of caches that are important. One is on your hard drive and the other is at your Internet Service Provider (ISP). Putting it really simply for this article, a cache (usually pronounced (kaysh") is a storehouse of web files that a computer can use instead of having to download the same ones from somewhere further away on the Internet.

ISP Caches

ISPs each have a cache and when you enter a web address into your browser (e.g. Internet Explorer or Netscape Navigator) your ISP directs your browser to the page you have requested. Your ISP's servers then do a quick check of that page, and then check their own cache which is stored locally. If the page you want is on the local cache, you will receive the page you wanted from your ISP's cache instead of from the web site you have requested. The reason for this, of course to save time for you and server traffic for your ISP.

When the server checks the page you have requested, it does a quick content comparison. If the page you have requested is different or has been updated, or if it has never seen that page before, it downloads the new version of the page to itself at the same time as it downloads it for you. This way it makes sure that it always has the latest content to give to the next person who requests the exact same page.

Pragma No Cache

If you're not sure you trust the ISP's cache to be giving you the latest version of the page, you can add a command to your page request that tells your ISP's servers that you are not interested in their cache, but that you want your content straight from the source. To do this, once you have downloaded the page requested, hold the control key down on your keyboard and click the Refresh or Reload button in your browser's toolbar with your mouse. This adds the words "pragma no cache" to your web request, and your ISP's servers then ignore their own cache and download for you straight from the horse's mouth.

Your Browser's Cache

Your browser also works with a cache of its own. In Internet Explorer this cache is called your Temporary Internet Files. It is designed to work the same way as an ISP's cache. It checks the page it is downloading and if there is content from the same page with the same name in there, it will give you that content instead of from the source, but only if it is exactly the same, and again, for the purposes of saving time. Your browser also has a RAM cache. This means that it saves a portion of its memory to store Internet content in so that it can display it for you immediately. You'll notice this when you click the Back button through a series of previous pages and notice that they are there with no waiting at all.

You can play with your hard drive cache settings in Internet Explorer by clicking Tools/Options, and then the Settings button on the Internet Options box. There (in later versions of IE) you can set the size of your cache (the bigger you have it set, the more it can remember, but the more hard disk space it will absorb) view what's in it, and delete its content if you wish.

In Netscape Navigator 4.7 you can play with your cache settings by clicking Edit/Preferences and then Advanced/Cache. There you will find various settings as well as the directory pathway on your hard drive to where Navigator keeps its temporary Internet files.

In Opera 4, you can find your cache settings under File/Preferences, and then by clicking Cache in the left hand column.