squid中的refresh_pattern详解

refresh_pattern 大概是 squid 最有意思但最不好懂的配置参数了，经过看书，大概明白如何使用，写出来贡献。

记住refresh_pattern 只对后端没设置Expires过期时间的页面起作用，比如论坛页面；而对类似apache mod_expires 设置过的页面不起作用。

说明之前，先将个概念LM，LM就是页面Header里时间(Date)和Last-Modified时间的差。Date一般是Squid从后面取页面的时间，Last-Modified 一般是页面生成时间。

refresh_pattern 的语法是refresh_pattern [-i] regexp min percent max [options]regexp 就不讲了，大家都明白的;)

min, max的单位是分钟，percent就是百分比。

refresh_pattern 的算法如下：(当前时间定义为CURRENT_DATE)
1) If ((CURRENT_DATE-DATE(就是LM里定义的时间)) < min)，cache是新鲜的
2) else if ((CURRENT_DATE-DATE) < (min + (max-min)*percent)，cache是新鲜的
3) else cache是过期的
cache过期就需要从后面server取新鲜内容。

如果希望页面一进入cache就不删除，直到被主动purge掉为止，可以加上ignore-reload选项
一般情况可以使用 reload-into-ims。
举例：

refresh_pattern -i \.gif$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.jpg$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.png$ 1440 50% 2880 ignore-reload

refresh_pattern -i \.mp3$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.wmv$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.rm$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.swf$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.mpeg$ 1440 50% 2880 ignore-reload
refresh_pattern -i \.wma$ 1440 50% 2880 ignore-reload

refresh_pattern -i \.css$ 10 50% 60 reload-into-ims
refresh_pattern -i \.js$ 10 50% 60 reload-into-ims
refresh_pattern -i \.xml$ 10 50% 30 reload-into-ims

英文版:
Remember 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.



refresh_pattern syntax is as follows:

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

definitions:

resource age LM = abs(Last-Modified Time - Date) in headers.
LM factor = (Current_Date-Date)/LM
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.
if percent = 50%, see below image for better understanding:


If you want to cache object until it got purged by explicitly command, use ignore-reload option.

you can always cache something for some time using reload-into-ims option.

Examples:

refresh_pattern -i \.gif$ 10 100% 20 ignore-reload
refresh_pattern -i \.jpg$ 10 100% 20 ignore-reload

refresh_pattern -i \.css$ 10 50% 60 reload-into-ims
refresh_pattern -i \.js$ 10 50% 60 reload