7.7 refresh_pattern

The refresh_pattern directive controls the disk cache only indirectly. It helps Squid decide whether or not a given request can be a cache hit or must be treated as a miss . Liberal settings increase your cache hit ratio but also increase the chance that users receive a stale response. Conservative settings, on the other hand, decrease hit ratios and stale responses.

The refresh_pattern rules apply only to responses without an explicit expiration time. Origin servers can specify an expiration time with either the Expires header, or the Cache-Control : max-age directive.

You can put any number of refresh_pattern lines in the configuration file. Squid searches them in order for a regular expression match. When Squid finds a match, it uses the corresponding values to determine whether a cached response is fresh or stale. The refresh_pattern syntax is as follows :

 refresh_pattern [-i]   regexp     min     percent     max   [options] 

For example:

 refresh_pattern -i \.jpg$ 30 50% 4320 reload-into-ims refresh_pattern -i \.png$ 30 50% 4320 reload-into-ims refresh_pattern -i \.htm$ 0 20% 1440 refresh_pattern -i \.html$ 0 20% 1440 refresh_pattern -i . 5 25% 2880 

The regexp parameter is a regular expression that is normally case-sensitive. You can make them case-insensitive with the -i option. Squid checks the refresh_pattern lines in order; it stops searching when one of the regular expression patterns matches the URI.

The min parameter is some number of minutes. It is, essentially , a lower bound on stale responses. A response can't be stale unless its time in the cache exceeds the minimum value. Similarly, max is an upper limit on fresh responses. A response can't be fresh unless its time in the cache is less than the maximum time.

Responses that fall between the minimum and maximum are subject to Squid's last-modified factor (LM-factor) algorithm. For such responses, Squid calculates the response age and the LM-factor and compares it to the percent value. The response age is simply the amount of time passed since the origin server generated, or last validated , the response. The resource age is the difference between the Last-Modified and Date headers. The LM-factor is the ratio of the response age to the resource age.

Figure 7-2 demonstrates the LM-factor algorithm. Squid caches an object that is 3 hours old (based on the Date and Last-Modified headers). With an LM-factor value of 50%, the response will be fresh for the next 1.5 hours, after which the object expires and is considered stale. If a user requests the cached object during the fresh period, Squid returns an unvalidated cache hit. For a request that occurs during the stale period, Squid forwards a validation request to the origin server.

Figure 7-2. Calculating expiration times based on LM-factor

It's important to understand the order that Squid checks the various values. Here is a simplified description of Squid's refresh_pattern algorithm:

• The response is stale if the response age is greater than the refresh_pattern max value.

• The response is fresh if the LM-factor is less than the refresh_pattern percent value.

• The response is fresh if the response age is less than the refresh_pattern min value.

• Otherwise, the response is stale.

The refresh_pattern directive also has a handful of options that cause Squid to disobey the HTTP protocol specification. They are as follows:

override-expire

When set, this option causes Squid to check the min value before checking the Expires header. Thus, a non-zero min time makes Squid return an unvalidated cache hit even if the response is preexpired.

override-lastmod

When set, this option causes Squid to check the min value before the LM-factor percentage.

reload-into-ims

When set, this option makes Squid transform a request with a no-cache directive into a validation ( If-Modified-Since ) request. In other words, Squid adds an If-Modified-Since header to the request before forwarding it on. Note that this only works for objects that have a Last-Modified timestamp. The outbound request retains the no-cache directive, so that it reaches the origin server.

ignore-reload

When set, this option causes Squid to ignore the no-cache directive, if any, in the request.