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The HTTP histogram shows the breakdown of freshness checks for client HTTP requests. For example:

HTTP histogram:

Count %Total Category

0 0.00 Fresh: request max-stale wildcard

0 0.00 Fresh: request max-stale value

173984 9.76 Fresh: expires time not reached

462757 25.97 Fresh: refresh_pattern last-mod factor percentage

42 0.00 Fresh: refresh_pattern min value

0 0.00 Fresh: refresh_pattern override expires

0 0.00 Fresh: refresh_pattern override lastmod

5521 0.31 Stale: response has must-revalidate

0 0.00 Stale: changed reload into IMS

0 0.00 Stale: request has no-cache directive

470912 26.43 Stale: age exceeds request max-age value

455073 25.54 Stale: expires time reached

65612 3.68 Stale: refresh_pattern max age rule

144706 8.12 Stale: refresh_pattern last-mod factor percentage

3274 0.18 Stale: by default

1781881 100.00 TOTAL

Note, the rules aren't necessarily evaluated in the order in which they appear in the table. Here's what each line means:

Fresh: request max-stale wildcard

Squid considers the cached response fresh because the request includes a max-stale directive without any value. For example:

GET /blah... HTTP/1.1

Cache-control: max-stale

According to RFC 2616: "If no value is assigned to max-stale, then the client is willing to accept a stale response of any age."

Fresh: request max-stale value

Squid considers the cached response fresh because the request includes a max-stale directive with a particular value, which is larger than the amount of time since the object expired.

Fresh: expires time not reached

Squid considers the cached response fresh because its expiration time has not yet been reached.

Fresh: refresh_pattern last-mod factor percentage

Squid considers the cached response fresh because it matches one of the refresh_pattern rules and has a last-modified factor (LM-factor) value that's less than that specified by the rule. See Section 7.7.

Fresh: refresh_pattern min value

Squid considers the cached response fresh because it matches one of the refresh_pattern rules and its age is less than the min value specified by the rule. See Section 7.7.

Fresh: refresh_pattern override expires

Squid considers the cached response fresh because it matched one of the refresh_pattern rules with the override-expire option. This option causes Squid to give precedence to the refresh_pattern minimum value over the object's expiration time. Note: using the override-expire option is a violation of RFC 2616.

Fresh: refresh_pattern override lastmod

Squid considers the cached response fresh because it matched one of the refresh_pattern rules with the override-lastmod option. This option causes Squid to give precedence to the refresh_pattern minimum value over the LM-factor value. Note: using the override-lastmod option is a violation of RFC 2616.

Stale: response has must-revalidate

Squid considers the cached response stale because it contains a Cache-Control: must-revalidate directive.

Stale: changed reload into IMS

Squid considers the cached response stale because it matches one of the refresh_pattern rules with the reload-into-ims option. With this option, Squid turns a request with Cache-Control: no-cache (or similar) into a cache validation. Note: using the reload-into-ims option is a violation of RFC 2616.

Stale: request has no-cache directive

Squid considers the cached response stale because the request contains a Cache-Control: no-cache directive.

Stale: age exceeds request max-age value

Squid considers the cached response stale because the request has a max-age directive, which is less than the response's age.

Stale: expires time reached

Squid considers the cached response stale because its expiration time has been reached.

Stale: refresh_pattern max age rule

Squid considers the cached response stale because it matches one of the refresh_pattern rules, and its age is greater than the max value specified by the rule.

Stale: refresh_pattern last-mod factor percentage

Squid considers the cached response stale