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Apache > HTTP Server > Documentation > Version 2.2 > Modules

Apache Module mod_cache

Description: Content cache keyed to URIs.
Status: Extension
Module Identifier: cache_module
Source File: mod_cache.c

Summary

This module should be used with care and can be used to circumvent Allow and Deny directives. You should not enable caching for any content to which you wish to limit access by client host name, address or environment variable.

mod_cache implements an RFC 2616 compliant HTTP content cache that can be used to cache either local or proxied content. mod_cache requires the services of one or more storage management modules. Two storage management modules are included in the base Apache distribution:

mod_disk_cache
implements a disk based storage manager.
mod_mem_cache
implements a memory based storage manager. mod_mem_cache can be configured to operate in two modes: caching open file descriptors or caching objects in heap storage. mod_mem_cache can be used to cache locally generated content or to cache backend server content for mod_proxy when configured using ProxyPass (aka reverse proxy)

Content is stored in and retrieved from the cache using URI based keys. Content with access protection is not cached.

Further details, discussion, and examples, are provided in the Caching Guide.

Directives

Topics

See also

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Related Modules and Directives

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Sample Configuration

Sample httpd.conf

#
# Sample Cache Configuration
#
LoadModule cache_module modules/mod_cache.so

<IfModule mod_cache.c>
#LoadModule disk_cache_module modules/mod_disk_cache.so
# If you want to use mod_disk_cache instead of mod_mem_cache,
# uncomment the line above and comment out the LoadModule line below.
<IfModule mod_disk_cache.c>
CacheRoot c:/cacheroot
CacheEnable disk /
CacheDirLevels 5
CacheDirLength 3
</IfModule>

LoadModule mem_cache_module modules/mod_mem_cache.so
<IfModule mod_mem_cache.c>
CacheEnable mem /
MCacheSize 4096
MCacheMaxObjectCount 100
MCacheMinObjectSize 1
MCacheMaxObjectSize 2048
</IfModule>

# When acting as a proxy, don't cache the list of security updates
CacheDisable http://security.update.server/update-list/
</IfModule>

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CacheDefaultExpire Directive

Description: The default duration to cache a document when no expiry date is specified.
Syntax: CacheDefaultExpire seconds
Default: CacheDefaultExpire 3600 (one hour)
Context: server config, virtual host
Status: Extension
Module: mod_cache

The CacheDefaultExpire directive specifies a default time, in seconds, to cache a document if neither an expiry date nor last-modified date are provided with the document. The value specified with the CacheMaxExpire directive does not override this setting.

CacheDefaultExpire 86400

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CacheDisable Directive

Description: Disable caching of specified URLs
Syntax: CacheDisable url-string
Context: server config, virtual host
Status: Extension
Module: mod_cache

The CacheDisable directive instructs mod_cache to not cache urls at or below url-string.

Example

CacheDisable /local_files

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CacheEnable Directive

Description: Enable caching of specified URLs using a specified storage manager
Syntax: CacheEnable cache_type url-string
Context: server config, virtual host
Status: Extension
Module: mod_cache

The CacheEnable directive instructs mod_cache to cache urls at or below url-string. The cache storage manager is specified with the cache_type argument. cache_type mem instructs mod_cache to use the memory based storage manager implemented by mod_mem_cache. cache_type disk instructs mod_cache to use the disk based storage manager implemented by mod_disk_cache. cache_type fd instructs mod_cache to use the file descriptor cache implemented by mod_mem_cache.

In the event that the URL space overlaps between different CacheEnable directives (as in the example below), each possible storage manager will be run until the first one that actually processes the request. The order in which the storage managers are run is determined by the order of the CacheEnable directives in the configuration file.

CacheEnable mem /manual
CacheEnable fd /images
CacheEnable disk /

When acting as a forward proxy server, url-string can also be used to specify remote sites and proxy protocols which caching should be enabled for.

# Cache proxied url's
CacheEnable disk /

# Cache FTP-proxied url's
CacheEnable disk ftp://

# Cache content from www.apache.org
CacheEnable disk http://www.apache.org/

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CacheIgnoreCacheControl Directive

Description: Ignore request to not serve cached content to client
Syntax: CacheIgnoreCacheControl On|Off
Default: CacheIgnoreCacheControl Off
Context: server config, virtual host
Status: Extension
Module: mod_cache

Ordinarily, requests containing a Cache-Control: no-cache or Pragma: no-cache header value will not be served from the cache. The CacheIgnoreCacheControl directive allows this behavior to be overridden. CacheIgnoreCacheControl On tells the server to attempt to serve the resource from the cache even if the request contains no-cache header values. Resources requiring authorization will never be cached.

CacheIgnoreCacheControl On

Warning:

This directive will allow serving from the cache even if the client has requested that the document not be served from the cache. This might result in stale content being served.

See also

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CacheIgnoreHeaders Directive

Description: Do not store the given HTTP header(s) in the cache.
Syntax: CacheIgnoreHeaders header-string [header-string] ...
Default: CacheIgnoreHeaders None
Context: server config, virtual host
Status: Extension
Module: mod_cache

According to RFC 2616, hop-by-hop HTTP headers are not stored in the cache. The following HTTP headers are hop-by-hop headers and thus do not get stored in the cache in any case regardless of the setting of CacheIgnoreHeaders:

CacheIgnoreHeaders specifies additional HTTP headers that should not to be stored in the cache. For example, it makes sense in some cases to prevent cookies from being stored in the cache.

CacheIgnoreHeaders takes a space separated list of HTTP headers that should not be stored in the cache. If only hop-by-hop headers not should be stored in the cache (the RFC 2616 compliant behaviour), CacheIgnoreHeaders can be set to None.

Example 1

CacheIgnoreHeaders Set-Cookie

Example 2

CacheIgnoreHeaders None

Warning:

If headers like Expires which are needed for proper cache management are not stored due to a CacheIgnoreHeaders setting, the behaviour of mod_cache is undefined.
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CacheIgnoreNoLastMod Directive

Description: Ignore the fact that a response has no Last Modified header.
Syntax: CacheIgnoreNoLastMod On|Off
Default: CacheIgnoreNoLastMod Off
Context: server config, virtual host
Status: Extension
Module: mod_cache

Ordinarily, documents without a last-modified date are not cached. Under some circumstances the last-modified date is removed (during mod_include processing for example) or not provided at all. The CacheIgnoreNoLastMod directive provides a way to specify that documents without last-modified dates should be considered for caching, even without a last-modified date. If neither a last-modified date nor an expiry date are provided with the document then the value specified by the CacheDefaultExpire directive will be used to generate an expiration date.

CacheIgnoreNoLastMod On

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CacheIgnoreQueryString Directive

Description: Ignore query string when caching
Syntax: CacheIgnoreQueryString On|Off
Default: CacheIgnoreQueryString Off
Context: server config, virtual host
Status: Extension
Module: mod_cache

Ordinarily, requests with query string parameters are cached separately for each unique query string. This is according to RFC 2616/13.9 done only if an expiration time is specified. The CacheIgnoreQueryString directive tells the cache to cache requests even if no expiration time is specified, and to reply with a cached reply even if the query string differs. From a caching point of view the request is treated as if having no query string when this directive is enabled.

CacheIgnoreQueryString On

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CacheLastModifiedFactor Directive

Description: The factor used to compute an expiry date based on the LastModified date.
Syntax: CacheLastModifiedFactor float
Default: CacheLastModifiedFactor 0.1
Context: server config, virtual host
Status: Extension
Module: mod_cache

In the event that a document does not provide an expiry date but does provide a last-modified date, an expiry date can be calculated based on the time since the document was last modified. The CacheLastModifiedFactor directive specifies a factor to be used in the generation of this expiry date according to the following formula: expiry-period = time-since-last-modified-date * factor expiry-date = current-date + expiry-period For example, if the document was last modified 10 hours ago, and factor is 0.1 then the expiry-period will be set to 10*0.1 = 1 hour. If the current time was 3:00pm then the computed expiry-date would be 3:00pm + 1hour = 4:00pm. If the expiry-period would be longer than that set by CacheMaxExpire, then the latter takes precedence.

CacheLastModifiedFactor 0.5

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CacheMaxExpire Directive

Description: The maximum time in seconds to cache a document
Syntax: CacheMaxExpire seconds
Default: CacheMaxExpire 86400 (one day)
Context: server config, virtual host
Status: Extension
Module: mod_cache

The CacheMaxExpire directive specifies the maximum number of seconds for which cachable HTTP documents will be retained without checking the origin server. Thus, documents will be out of date at most this number of seconds. This maximum value is enforced even if an expiry date was supplied with the document.

CacheMaxExpire 604800

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CacheStoreNoStore Directive

Description: Attempt to cache requests or responses that have been marked as no-store.
Syntax: CacheStoreNoStore On|Off
Default: CacheStoreNoStore Off
Context: server config, virtual host
Status: Extension
Module: mod_cache

Ordinarily, requests or responses with Cache-Control: no-store header values will not be stored in the cache. The CacheStoreNoCache directive allows this behavior to be overridden. CacheStoreNoCache On tells the server to attempt to cache the resource even if it contains no-store header values. Resources requiring authorization will never be cached.

CacheStoreNoStore On

Warning:

As described in RFC 2616, the no-store directive is intended to "prevent the inadvertent release or retention of sensitive information (for example, on backup tapes)." Enabling this option could store sensitive information in the cache. You are hereby warned.

See also

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CacheStorePrivate Directive

Description: Attempt to cache responses that the server has marked as private
Syntax: CacheStorePrivate On|Off
Default: CacheStorePrivate Off
Context: server config, virtual host
Status: Extension
Module: mod_cache

Ordinarily, responses with Cache-Control: private header values will not be stored in the cache. The CacheStorePrivate directive allows this behavior to be overridden. CacheStorePrivate On tells the server to attempt to cache the resource even if it contains private header values. Resources requiring authorization will never be cached.

CacheStorePrivate On

Warning:

This directive will allow caching even if the upstream server has requested that the resource not be cached. This directive is only ideal for a 'private' cache.

See also