Apache Module mod_cache

Sample Cache Configuration

LoadModule cache_module modules/mod_cache.so LoadModule cache_disk_module modules/mod_cache_disk.so CacheRoot c:/cacheroot CacheE

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

Enable the cache lock

CacheLock on CacheLockPath /tmp/mod_cache-lock CacheLockMaxAge 5

Cache content before optional compression CacheQuickHandler off AddOutputFilterByType CACHE;DEFLATE text/plain

Cache content before mod_include and mod_deflate CacheQuickHandler off AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html

Cache content between mod_include and mod_deflate CacheQuickHandler off AddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html

Enable the X-Cache-Detail header CacheDetailHeader on

Cache content (normal handler only) CacheQuickHandler off CacheEnable disk

Cache regex (normal handler only) CacheQuickHandler off CacheEnable disk

Cache all but forward proxy url's (normal or quick handler) CacheEnable disk /

Cache FTP-proxied url's (normal or quick handler) CacheEnable disk ftp://

Cache forward proxy content from www.example.org (normal or quick handler) CacheEnable disk http://www.example.org/

Match www.example.org, and fooexample.org CacheEnable disk http://*example.org/

Match www.example.org, but not fooexample.org CacheEnable disk http://.example.org/

Enable the X-Cache header CacheHeader on

Override the base URL of the cache key. CacheKeyBaseURL http://www.example.com/

Enable cache lock CacheLock on

Run cache as a normal handler CacheQuickHandler off

Serve stale data on error. CacheStaleOnError on

directives will be circumvented. You should not enable quick handler caching for any content to which you wish to limit access by client host name, address or environment variable.

implements an mod_cacheRFC 2616 compliant HTTP content caching filter, with support for the caching of content negotiated responses containing the Vary header.

RFC 2616 compliant caching provides a mechanism to verify whether stale or expired content is still fresh, and can represent a significant performance boost when the origin server supports conditional requests by honouring the If-None-Match HTTP request header. Content is only regenerated from scratch when the content has changed, and not when the cached entry expires.

can be placed in front of content originating from any handler, including mod_cacheflat files (served from a slow disk cached on a fast disk), the output of a CGI script or dynamic content generator, or content proxied from another server.

inserts the caching filter as far forward as possible within the filter stack, utilising the mod_cachequick handler to bypass all per request processing when returning content to the client. In this mode of operation,

may be thought of as a caching proxy server bolted to the front of the webserver, while running within the webserver itself.mod_cache

directive, it becomes possible to insert the CacheQuickHandlerCACHE filter at a point in the filter stack chosen by the administrator. This provides the opportunity to cache content before that content is personalised by the

filter, or optionally compressed by the mod_include

will respond to and can be controlled by the mod_cacheCache-Control and Pragma headers sent from a client in a request, or from a server within a response. Under exceptional circumstances,

can be configured to override these headers and force site specific behaviour, however such behaviour will be limited to this cache only, and will not affect the operation of other caches that may exist between the client and server, and as a result is not recommended unless strictly necessary.mod_cache

RFC 2616 allows for the cache to return stale data while the existing stale entry is refreshed from the origin server, and this is supported by

directive is suitably configured. Such responses will contain a CacheLockWarning HTTP header with a 110 response code. RFC 2616 also allows a cache to return stale data when the attempt made to refresh the stale data returns an error 500 or above, and this behaviour is supported by default by

requires the services of one or more storage management modules. The following storage management modules are included in the base Apache distribution:mod_cache

tool is provided to list cached URLs, remove cached URLs, or to maintain the size of the disk cache within size and inode limits.modcachesocache

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

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 unpredictably.

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).

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.

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 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 honored 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

In this mode, the cache bolts onto the front of the server, acting as if a free standing RFC 2616 caching proxy had been placed in front of the server.

While this mode offers the best performance, the administrator may find that under certain circumstances they may want to perform further processing on the request after the request is cached, such as to inject personalisation into the cached page, or to apply authorisation restrictions to the content. Under these circumstances, an administrator is often forced to place independent reverse proxy servers either behind or in front of the caching server to achieve this.

To solve this problem the

directive can be set to CacheQuickHandler off, and the server will process all phases normally handled by a non-cached request, including the authentication and authorisation phases.

In addition, the administrator may optionally specify the precise point within the filter chain where caching is to take place by adding the CACHE filter to the output filter chain.

Another option is to have content cached before personalisation is applied by

(or another content processing filter). In this example templates containing tags understood by mod_include

You may place the CACHE filter anywhere you wish within the filter chain. In this example, content is cached after being parsed by

is not in a position to enforce this for you.Once

Based on the caching decision made, the reason is also written to the subprocess environment under one the following four keys, as appropriate:

CustomLog cached-requests.log common env=cache-hit CustomLog uncached-requests.log common env=cache-miss CustomLog revalidated-requests.log common env=cache-revalidate CustomLog invalidated-requests.log common env=cache-invalidate

For module authors, a hook called cache_status is available, allowing modules to respond to the caching outcomes above in customised ways.

CacheDefaultExpire 3600 (one hour)

It can be useful during development of cached RESTful services to have additional information about the caching decision written to the response headers, so as to confirm whether Cache-Control

and other headers have been correctly used by the service and client.

directive. If the quick handler is used, this directive must appear within a server or virtual host context, otherwise the setting will be ignored.

directive, the path needs to be specified below the Location, or if the word "on" is used, caching for the whole location will be disabled.

environment variable can be set to disable caching on a finer grained set of resources in versions 2.2.12 and later.

to cache urls at or below modcacheurl-string. The cache storage manager is specified with the cachetype argument. The CacheEnable

to use the shared object cache based storage manager implemented by mod_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

sections are processed before globally defined CacheEnable

When acting as a forward proxy server, url-string must minimally begin with a protocol for which caching should be enabled.

A hostname starting with a "*" matches all hostnames with that suffix. A hostname starting with "." matches all hostnames containing the domain components that follow.

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.

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.

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

which are needed for proper cache management are not stored due to a CacheIgnoreHeaders

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.

/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.

directive is specified, the URL provided will be used as the base URL to calculate the URL of the cache keys in the reverse proxy configuration. When not specified, the scheme, hostname and port of the current virtual host is used to construct the cache key. When a cluster of machines is present, and all cached entries should be cached beneath the same cache key, a new base URL can be specified with this directive.CacheKeyBaseURL

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 *

For example, if the document was last modified 10 hours ago, and factor expiry-date = current-date + expiry-periodfactor 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.

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

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.

CacheMaxExpire 86400 (one day)

In the default enabled configuration, the cache operates within the quick handler phase. This phase short circuits the majority of server processing, and represents the most performant mode of operation for a typical server. The cache bolts onto the front of the server, and the majority of server processing is avoided.

When disabled, the cache operates as a normal handler, and is subject to the full set of phases when handling a server request. While this mode is slower than the default, it allows the cache to be used in cases where full processing is required, such as when content is subject to authorisation.

It is also possible, when the quick handler is disabled, for the administrator to choose the precise location within the filter chain where caching is to be performed, by adding the CACHE filter to the chain.

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Apache HTTP Server Version 2.4