Setting the HTTP Cache Expiration Policy

You can also set up your server to check if the cached document is up-to-date by using a last-modified factor or explicit expiration information only.

Explicit expiration information is a header found in some HTTP documents that specifies the date and time when that file will become outdated. Not many HTTP documents use explicit Expires headers, so you should estimate based on the Last-modified header.

If you decide to have your HTTP documents cached based upon the Last-modified header, you need to select a fraction to use in the expiration estimation. This fraction, known as the LM factor, is multiplied by the interval between the last modification and the time that the last up-to-date check was performed on the document. The resulting number is compared with the time since the last up-to-date check. If the number is smaller than the time interval, the document is not expired. Smaller fractions make the proxy check documents more often.

For example, suppose you have a document that was last changed ten days ago. If you set the last-modified factor to 0.1, the proxy interprets the factor to mean that the document is probably going to remain unchanged for one day (10 * 0.1 = 1). The proxy would, in that case, return the document from the cache if the document was checked less than a day ago.

In this same example, if the cache refresh setting for HTTP documents is set to less than one day, the proxy does the up-to-date check more than once a day. The proxy always uses the value, cache refresh or cache expiration, that requires the more frequent update.

You can set the expiration setting for HTTP documents on either the Set Cache Specifics page or the Set Caching Configuration page. The Set Cache Specifics page enables you to configure global caching procedures and the Set Caching Configuration page enables you to control caching procedures, for specific URLs and resources.