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mod_cache - Apache HTTP Server Version 2.4









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Apache HTTP Server Version 2.4



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Apache Module mod_cache

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Description:RFC 2616 compliant HTTP caching filter.
Status:Extension
ModuleIdentifier:cache_module
SourceFile:mod_cache.c
Summary

    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. The following storage management modules are 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.
    mod_cache_socache
    Implements a shared object cache based storage manager. Headers and
    bodies are stored together beneath a single key based on the URL of the
    response being cached. Multiple content negotiated responses can
    be stored concurrently, however the caching of partial content is not
    supported by this module.
    

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

Topics

 Related Modules and Directives
 Sample Configuration
 Avoiding the Thundering Herd
 Fine Control with the CACHE Filter
 Cache Status and Logging
Directives

 CacheDefaultExpire
 CacheDetailHeader
 CacheDisable
 CacheEnable
 CacheHeader
 CacheIgnoreCacheControl
 CacheIgnoreHeaders
 CacheIgnoreNoLastMod
 CacheIgnoreQueryString
 CacheIgnoreURLSessionIdentifiers
 CacheKeyBaseURL
 CacheLastModifiedFactor
 CacheLock
 CacheLockMaxAge
 CacheLockPath
 CacheMaxExpire
 CacheMinExpire
 CacheQuickHandler
 CacheStaleOnError
 CacheStoreExpired
 CacheStoreNoStore
 CacheStorePrivate

Bugfix checklisthttpd changelogKnown issuesReport a bugSee also

Caching Guide
Comments


Related Modules and Directives
    Related ModulesRelated Directivesmod_cache_diskmod_cache_socacheCacheRootCacheDirLevelsCacheDirLengthCacheMinFileSizeCacheMaxFileSizeCacheSocacheCacheSocacheMaxTimeCacheSocacheMinTimeCacheSocacheMaxSizeCacheSocacheReadSizeCacheSocacheReadTime


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 unpredictably.
  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 four keys, as appropriate:

  
    cache-hitThe response was served from cache.
    cache-revalidateThe response was stale and was successfully
      revalidated, then served from cache.
    cache-missThe response was served from the upstream server.
    cache-invalidateThe cached entity was invalidated by a request
      method other than GET or HEAD.
  

  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
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 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, directory, .htaccess
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




CacheDetailHeader Directive

Description:Add an X-Cache-Detail header to the response.
Syntax:CacheDetailHeader on|off
Default:CacheDetailHeader off
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_cache
Compatibility: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
  




CacheDisable Directive

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

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

    ExampleCacheDisable "/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.


See also

Environment Variables in Apache



CacheEnable Directive

Description:Enable caching of specified URLs using a specified storage
manager
Syntax:CacheEnable cache_type [url-string]
Context:server config, virtual host, directory
Status:Extension
Module:mod_cache
Compatibility:A url-string of '/' applied to forward proxy content in 2.2 and
 earlier.

    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. cache_type
    socache instructs mod_cache to use the
    shared object cache based storage manager implemented by
    mod_cache_socache.
    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 must
    minimally begin with a protocol for which caching should be enabled.

    # Cache content (normal handler only)
CacheQuickHandler off
<Location "/foo">
    CacheEnable disk
</Location>

# Cache regex (normal handler only)
CacheQuickHandler off
<LocationMatch "foo$">
    CacheEnable disk
</LocationMatch>

# 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/


    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.


See also

Environment Variables in Apache



CacheHeader Directive

Description:Add an X-Cache header to the response.
Syntax:CacheHeader on|off
Default:CacheHeader off
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_cache
Compatibility: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.

  
    HITThe entity was fresh, and was served from
    cache.
    REVALIDATEThe entity was stale, was successfully
    revalidated and was served from cache.
    MISSThe 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





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

CacheStorePrivate
CacheStoreNoStore



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:

    
      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 1CacheIgnoreHeaders Set-Cookie


    Example 2CacheIgnoreHeaders 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

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




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





CacheIgnoreURLSessionIdentifiers Directive

Description:Ignore defined session identifiers encoded in the URL when caching

Syntax:CacheIgnoreURLSessionIdentifiers identifier [identifier] ...
Default:CacheIgnoreURLSessionIdentifiers None
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 cacheable 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 cacheable 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 1CacheIgnoreURLSessionIdentifiers jsessionid


    Example 2CacheIgnoreURLSessionIdentifiers None





CacheKeyBaseURL Directive

Description:Override the base URL of reverse proxied cache keys.
Syntax:CacheKeyBaseURL URL
Default:CacheKeyBaseURL http://example.com
Context:server config, virtual host
Status:Extension
Module:mod_cache
Compatibility: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.




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, directory, .htaccess
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




CacheLock Directive

Description:Enable the thundering herd lock.
Syntax:CacheLock on|off
Default:CacheLock off
Context:server config, virtual host
Status:Extension
Module:mod_cache
Compatibility: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





CacheLockMaxAge Directive

Description:Set the maximum possible age of a cache lock.
Syntax:CacheLockMaxAge integer
Default:CacheLockMaxAge 5
Context:server config, virtual host
Status:Extension
Module:mod_cache

  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.




CacheLockPath Directive

Description:Set the lock path directory.
Syntax:CacheLockPath directory
Default:CacheLockPath /tmp/mod_cache-lock
Context:server config, virtual host
Status:Extension
Module:mod_cache

  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.




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, directory, .htaccess
Status:Extension
Module:mod_cache

    The CacheMaxExpire directive specifies the maximum number of
    seconds for which cacheable 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 Directive

Description:The minimum time in seconds to cache a document
Syntax:CacheMinExpire seconds
Default:CacheMinExpire 0
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_cache

    The CacheMinExpire directive specifies the minimum number of
    seconds for which cacheable 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




CacheQuickHandler Directive

Description:Run the cache from the quick handler.
Syntax:CacheQuickHandler on|off
Default:CacheQuickHandler on
Context:server config, virtual host
Status:Extension
Module:mod_cache
Compatibility:Apache HTTP Server 2.3.3 and later

    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.

  


CacheStaleOnError Directive

Description:Serve stale content in place of 5xx responses.
Syntax:CacheStaleOnError on|off
Default:CacheStaleOnError on
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_cache
Compatibility: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





CacheStoreExpired Directive

Description:Attempt to cache responses that the server reports as expired
Syntax:CacheStoreExpired On|Off
Default:CacheStoreExpired Off
Context:server config, virtual host, directory, .htaccess
Status:Extension
Module:mod_cache

    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




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, directory, .htaccess
Status:Extension
Module:mod_cache

    Ordinarily, requests or responses with Cache-Control: no-store header
       values will not be stored in the cache.  The
       CacheStoreNoStore directive allows this
       behavior to be overridden.  CacheStoreNoStore 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

CacheIgnoreCacheControl
CacheStorePrivate



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, directory, .htaccess
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

CacheIgnoreCacheControl
CacheStoreNoStore




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