Apache Module mod_cache
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.
Related Modules and Directives
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>
CacheDefaultExpire Directive
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.
CacheDisable Directive
The CacheDisable
directive instructs mod_cache
to not cache urls at or below url-string.
Example
CacheDisable /local_files
The no-cache
environment variable can be set to disable caching on a finer grained set of resources in versions 2.2.12 and later.
See also
CacheEnable Directive
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/
The no-cache
environment variable can be set to disable caching on a finer grained set of resources in versions 2.2.12 and later.
See also
CacheIgnoreCacheControl Directive
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
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
:
Connection
Keep-Alive
Proxy-Authenticate
Proxy-Authorization
TE
Trailers
Transfer-Encoding
Upgrade
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. CacheIgnoreNoLastMod Directive
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.
CacheIgnoreQueryString Directive
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
CacheIgnoreURLSessionIdentifiers Directive
Description: | Ignore defined session identifiers encoded in the URL when caching |
---|
Syntax: | CacheIgnoreURLSessionIdentifiers identifier [identifier] ... |
---|
Default: | CacheIgnoreURLSessionIdentifiers None |
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Context: | server config, virtual host |
---|
Status: | Extension |
---|
Module: | mod_cache |
---|
Sometimes applications encode the session identifier into the URL like in the following Examples:
/someapplication/image.gif;jsessionid=123456789
/someapplication/image.gif?PHPSESSIONID=12345678
This causes cachable resources to be stored separately for each session, which is often not desired. CacheIgnoreURLSessionIdentifiers
lets define a list of identifiers that are removed from the key that is used to identify an entity in the cache, such that cachable resources are not stored separately for each session.
CacheIgnoreURLSessionIdentifiers None
clears the list of ignored identifiers. Otherwise, each identifier is added to the list.
Example 1
CacheIgnoreURLSessionIdentifiers jsessionid
Example 2
CacheIgnoreURLSessionIdentifiers None
CacheLastModifiedFactor Directive
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
CacheMaxExpire Directive
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.
CacheStoreNoStore Directive
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.
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
CacheStorePrivate Directive
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.
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