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.
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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.
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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.
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.
