mod_cache
RFC 2616 compliant HTTP caching filter.
Extension
mod_cache.c
cache_module

This module should be used with care, as when the CacheQuickHandler directive is in its default value of on, the Allow and Deny 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.

mod_cache implements an RFC 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.

As a filter, mod_cache can be placed in front of content originating from any handler, including flat 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.

In the default configuration, mod_cache inserts the caching filter as far forward as possible within the filter stack, utilising the quick handler to bypass all per request processing when returning content to the client. In this mode of operation, mod_cache may be thought of as a caching proxy server bolted to the front of the webserver, while running within the webserver itself.

When the quick handler is switched off using the CacheQuickHandler directive, it becomes possible to insert the CACHE 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 mod_include filter, or optionally compressed by the mod_deflate filter.

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

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 mod_cache when the CacheLock directive is suitably configured. Such responses will contain a Warning 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 mod_cache. Such responses will contain a Warning HTTP header with a 111 response code.

mod_cache requires the services of one or more storage management modules. One storage management module is included in the base Apache distribution:

mod_cache_disk
  Implements a disk based storage manager. Headers and bodies are stored separately on disk, in a directory structure derived from the md5 hash of the cached URL. Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not supported by this module. The htcacheclean tool is provided to list cached URLs, remove cached URLs, or to maintain the size of the disk cache within size and inode limits.

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

Caching Guide

# Related Modules and Directives

mod_cache_disk
CacheRoot
CacheDirLevels
CacheDirLength
CacheMinFileSize
CacheMaxFileSize

# Sample Configuration

## Sample httpd.conf

#
# Sample Cache Configuration
#
LoadModule cache_module modules/mod_cache.so
<IfModule mod_cache.c>
LoadModule cache_disk_module modules/mod_cache_disk.so
<IfModule mod_cache_disk.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 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 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>

# Fine Control with the CACHE Filter

Under the default mode of cache operation, the cache runs as a quick handler, short circuiting the majority of server processing and offering the highest cache performance available.

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 CacheQuickHandler directive can be set to 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.

For example, to cache content before applying compression to the response, place the CACHE filter before the DEFLATE filter as in the example below:

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

Another option is to have content cached before personalisation is applied by mod_include (or another content processing filter). In this example templates containing tags understood by mod_include are cached before being parsed:

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

You may place the CACHE filter anywhere you wish within the filter chain. In this example, content is cached after being parsed by mod_include, but before being processed by mod_deflate:

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

## Warning:

If the location of the CACHE filter in the filter chain is changed for any reason, you may need to flush your cache to ensure that your data served remains consistent. mod_cache is not in a position to enforce this for you.

# Cache Status and Logging

Once mod_cache has made a decision as to whether or not an entity is to be served from cache, the detailed reason for the decision is written to the subprocess environment within the request under the cache-status key. This reason can be logged by the LogFormat directive as follows:

LogFormat "%{cache-status}e ..."

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

cache-hit
  The response was served from cache.
cache-revalidate
  The response was stale and was successfully revalidated, then served from cache.
cache-miss
  The response was served from the upstream server.

This makes it possible to support conditional logging of cached requests as per the following example:

CustomLog cached-requests.log common env=cache-hit
CustomLog uncached-requests.log common env=cache-miss
CustomLog revalidated-requests.log common env=cache-revalidate
CacheEnable
Enable caching of specified URLs using a specified storage manager
CacheEnable cache_type [url-string]
server config
virtual host
directory
.htaccess

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. The CacheEnable directive can alternatively be placed inside either Location or LocationMatch sections to indicate the content is cacheable. cache_type `disk` instructs mod_cache to use the disk based storage manager implemented by mod_cache_disk.

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 directives within Location or LocationMatch sections are processed before globally defined CacheEnable directives.

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 content
<Location /foo>
CacheEnable disk
</Location>
# Cache regex
<LocationMatch foo$>
CacheEnable disk
</LocationMatch>
# Cache proxied url's
CacheEnable disk /
# Cache FTP-proxied url's
CacheEnable disk ftp://
# Cache content from www.example.org
CacheEnable disk http://www.example.org/

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

# 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/

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 | on
server config
virtual host
directory
.htaccess

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

# Example

CacheDisable /local_files

If used in a Location 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.

# Example

<Location /foo>
CacheDisable on
</Location>

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 config
virtual host
directory
.htaccess

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 config
virtual host
directory
.htaccess

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 config
virtual host
directory
.htaccess

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 config
virtual host
directory
.htaccess

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 config
virtual 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 config
virtual 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 config
virtual host
directory
.htaccess

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 config
virtual 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 config
virtual 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
CacheStoreExpired
Attempt to cache responses that the server reports as expired
CacheStoreExpired On|Off
CacheStoreExpired Off
server config
virtual host
directory
.htaccess

Since httpd 2.2.4, responses which have already expired are not stored in the cache. The CacheStoreExpired directive allows this behavior to be overridden. CacheStoreExpired On tells the server to attempt to cache the resource if it is stale. Subsequent requests would trigger an If-Modified-Since request of the origin server, and the response may be fulfilled from cache if the backend resource has not changed.

CacheStoreExpired On
CacheStorePrivate
Attempt to cache responses that the server has marked as private
CacheStorePrivate On|Off
CacheStorePrivate Off
server config
virtual host
directory
.htaccess

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 config
virtual host
directory
.htaccess

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 config
virtual host
Available in Apache 2.2.15 and later

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 config
virtual 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 config
virtual 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.

CacheQuickHandler
Run the cache from the quick handler.
CacheQuickHandler on|off
CacheQuickHandler on
server config
virtual host

The CacheQuickHandler directive controls the phase in which the cache is handled.

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.

# Run cache as a normal handler
CacheQuickHandler off

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.

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

If the CACHE filter is specified more than once, the last instance will apply.

CacheHeader
Add an X-Cache header to the response.
CacheHeader on|off
CacheHeader off
server config
virtual host
directory
.htaccess
Available in Apache 2.3.9 and later

When the CacheHeader directive is switched on, an X-Cache header will be added to the response with the cache status of this response. If the normal handler is used, this directive may appear within a <Directory> or <Location> directive. If the quick handler is used, this directive must appear within a server or virtual host context, otherwise the setting will be ignored.

HIT
  The entity was fresh, and was served from cache.
REVALIDATE
  The entity was stale, was successfully revalidated and was served from cache.
MISS
  The entity was fetched from the upstream server and was not served from cache.

# Enable the X-Cache header
CacheHeader on
X-Cache: HIT from localhost
CacheDetailHeader
Add an X-Cache-Detail header to the response.
CacheDetailHeader on|off
CacheDetailHeader off
server config
virtual host
directory
.htaccess
Available in Apache 2.3.9 and later

When the CacheDetailHeader directive is switched on, an X-Cache-Detail header will be added to the response containing the detailed reason for a particular caching decision.

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.

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

# Enable the X-Cache-Detail header
CacheDetailHeader on
X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost
CacheKeyBaseURL
Override the base URL of reverse proxied cache keys.
CacheKeyBaseURL URL
CacheKeyBaseURL http://example.com
server config
virtual host
Available in Apache 2.3.9 and later

When the CacheKeyBaseURL 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.

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

Take care when setting this directive. If two separate virtual hosts are accidentally given the same base URL, entries from one virtual host will be served to the other.

CacheStaleOnError
Serve stale content in place of 5xx responses.
CacheStaleOnError on|off
CacheStaleOnError on
server config
virtual host
directory
.htaccess
Available in Apache 2.3.9 and later

When the CacheStaleOnError directive is switched on, and when stale data is available in the cache, the cache will respond to 5xx responses from the backend by returning the stale data instead of the 5xx response. While the Cache-Control headers sent by clients will be respected, and the raw 5xx responses returned to the client on request, the 5xx response so returned to the client will not invalidate the content in the cache.

# Serve stale data on error.
CacheStaleOnError on