mod_cache Content cache keyed to URIs. Extension mod_cache.c cache_module 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. One storage management module is included in the base Apache distribution:

mod_disk_cache
implements a disk based storage manager.

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.

 Caching Guide
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
<IfModule mod_disk_cache.c>
CacheRoot c:/cacheroot
CacheEnable disk /
CacheDirLevels 5
CacheDirLength 3
 </IfModule>

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

When a cached entry becomes stale, mod_cache will submit a conditional request to the backend, which is expected to confirm whether the cached entry is still fresh, and send an updated entity if not.

A small but finite amount of time exists between the time the cached entity becomes stale, and the time the stale entity is fully refreshed. On a busy server, a significant number of requests might arrive during this time, and cause a thundering herd of requests to strike the backend suddenly and unpredicably.

To keep the thundering herd at bay, the CacheLock directive can be used to define a directory in which locks are created for URLs in flight. The lock is used as a hint by other requests to either suppress an attempt to cache (someone else has gone to fetch the entity), or to indicate that a stale entry is being refreshed (stale content will be returned in the mean time).

Initial caching of an entry

When an entity is cached for the first time, a lock will be created for the entity until the response has been fully cached. During the lifetime of the lock, the cache will suppress the second and subsequent attempt to cache the same entity. While this doesn't hold back the thundering herd, it does stop the cache attempting to cache the same entity multiple times simultaneously.

Refreshment of a stale entry

When an entity reaches its freshness lifetime and becomes stale, a lock will be created for the entity until the response has either been confirmed as still fresh, or replaced by the backend. During the lifetime of the lock, the second and subsequent incoming request will cause stale data to be returned, and the thundering herd is kept at bay.

Locks and Cache-Control: no-cache

Locks are used as a hint only to enable the cache to be more gentle on backend servers, however the lock can be overridden if necessary. If the client sends a request with a Cache-Control header forcing a reload, any lock that may be present will be ignored, and the client's request will be honoured immediately and the cached entry refreshed.

As a further safety mechanism, locks have a configurable maximum age. Once this age has been reached, the lock is removed, and a new request is given the opportunity to create a new lock. This maximum age can be set using the CacheLockMaxAge directive, and defaults to 5 seconds.

Example configuration Enabling the cache lock #
# Enable the cache lock
#
<IfModule mod_cache.c>
CacheLock on
CacheLockPath /tmp/mod_cache-lock
CacheLockMaxAge 5
 </IfModule>
CacheEnable Enable caching of specified URLs using a specified storage manager CacheEnable cache_type url-string server configvirtual host

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 disk instructs mod_cache to use the disk based storage manager implemented by mod_disk_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.

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.

 Environment Variables in Apache CacheDisable Disable caching of specified URLs CacheDisable url-string server configvirtual host

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.

 Environment Variables in Apache CacheMaxExpire The maximum time in seconds to cache a document CacheMaxExpire seconds CacheMaxExpire 86400 (one day) server configvirtual host

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 CacheMinExpire The minimum time in seconds to cache a document CacheMinExpire seconds CacheMinExpire 0 server configvirtual host

The CacheMinExpire directive specifies the minimum number of seconds for which cachable HTTP documents will be retained without checking the origin server. This is only used if no valid expire time was supplied with the document.

CacheMinExpire 3600 CacheDefaultExpire The default duration to cache a document when no expiry date is specified. CacheDefaultExpire seconds CacheDefaultExpire 3600 (one hour) server configvirtual host

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 CacheIgnoreNoLastMod Ignore the fact that a response has no Last Modified header. CacheIgnoreNoLastMod On|Off CacheIgnoreNoLastMod Off server configvirtual host

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 CacheIgnoreCacheControl Ignore request to not serve cached content to client CacheIgnoreCacheControl On|Off CacheIgnoreCacheControl Off server configvirtual host

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. CacheStorePrivate CacheStoreNoStore CacheIgnoreQueryString Ignore query string when caching CacheIgnoreQueryString On|Off CacheIgnoreQueryString Off server configvirtual host

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 CacheLastModifiedFactor The factor used to compute an expiry date based on the LastModified date. CacheLastModifiedFactor float CacheLastModifiedFactor 0.1 server configvirtual host

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 CacheIgnoreHeaders Do not store the given HTTP header(s) in the cache. CacheIgnoreHeaders header-string [header-string] ... CacheIgnoreHeaders None server configvirtual host

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. CacheIgnoreURLSessionIdentifiers Ignore defined session identifiers encoded in the URL when caching CacheIgnoreURLSessionIdentifiers identifier [identifier] ... CacheIgnoreURLSessionIdentifiers None server configvirtual host

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 CacheStorePrivate Attempt to cache responses that the server has marked as private CacheStorePrivate On|Off CacheStorePrivate Off server configvirtual host

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. CacheIgnoreCacheControl CacheStoreNoStore CacheStoreNoStore Attempt to cache requests or responses that have been marked as no-store. CacheStoreNoStore On|Off CacheStoreNoStore Off server configvirtual host

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. CacheIgnoreCacheControl CacheStorePrivate CacheLock Enable the thundering herd lock. CacheLock on|off CacheLock off server configvirtual host

The CacheLock directive enables the thundering herd lock for the given URL space.

In a minimal configuration the following directive is all that is needed to enable the thundering herd lock in the default system temp directory.

# Enable cache lock
CacheLock on

 CacheLockPath Set the lock path directory. CacheLockPath directory CacheLockPath /tmp/mod_cache-lock server configvirtual host

The CacheLockPath directive allows you to specify the directory in which the locks are created. By default, the system's temporary folder is used. Locks consist of empty files that only exist for stale URLs in flight, so is significantly less resource intensive than the traditional disk cache.

 CacheLockMaxAge Set the maximum possible age of a cache lock. CacheLockMaxAge integer CacheLockMaxAge 5 server configvirtual host

The CacheLockMaxAge directive specifies the maximum age of any cache lock.

A lock older than this value in seconds will be ignored, and the next incoming request will be given the opportunity to re-establish the lock. This mechanism prevents a slow client taking an excessively long time to refresh an entity.