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|
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<title>mod_cache - Apache HTTP Server</title>
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<div id="page-content">
<div id="preamble"><h1>Apache Module mod_cache</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_cache.html" title="English"> en </a> |
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>RFC 2616 compliant HTTP caching filter.</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>cache_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_cache.c</td></tr></table>
<h3>Summary</h3>
<div class="warning">This module should be used with care, as when the
<code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive is
in its default value of <strong>on</strong>, the <code class="directive"><a href="../mod/mod_authz_host.html#allow">Allow</a></code> and <code class="directive"><a href="../mod/mod_authz_host.html#deny">Deny</a></code> 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.</div>
<p><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> implements an <a href="http://www.ietf.org/rfc/rfc2616.txt">RFC 2616</a> compliant
<strong>HTTP content caching filter</strong>, with support for the caching
of content negotiated responses containing the Vary header.</p>
<p>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 <strong>conditional
requests</strong> by honouring the
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26">If-None-Match</a>
HTTP request header. Content is only regenerated from scratch when the content
has changed, and not when the cached entry expires.</p>
<p>As a filter, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> can be placed in front of
content originating from any handler, including <strong>flat
files</strong> (served from a slow disk cached on a fast disk), the output
of a <strong>CGI script</strong> or <strong>dynamic content
generator</strong>, or content <strong>proxied from another
server</strong>.</p>
<p>In the default configuration, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> inserts the
caching filter as far forward as possible within the filter stack,
utilising the <strong>quick handler</strong> to bypass all per request
processing when returning content to the client. In this mode of
operation, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> may be thought of as a caching
proxy server bolted to the front of the webserver, while running within
the webserver itself.</p>
<p>When the quick handler is switched off using the
<code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive,
it becomes possible to insert the <strong>CACHE</strong> 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
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> filter, or optionally compressed by the
<code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code> filter.</p>
<p>Under normal operation, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> will respond to
and can be controlled by the
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9">Cache-Control</a>
and
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32">Pragma</a>
headers sent from a client in a request, or from a
server within a response. Under exceptional circumstances,
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> 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.</p>
<p>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 <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> when the
<code class="directive"><a href="#cachelock">CacheLock</a></code> directive is suitably
configured. Such responses will contain a
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
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
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>. Such responses will contain a
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
HTTP header with a 111 response code.</p>
<p><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> requires the services of one or more
storage management modules. One storage management module is included in
the base Apache distribution:</p>
<dl>
<dt><code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code></dt>
<dd>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 <code class="program"><a href="../programs/htcacheclean.html">htcacheclean</a></code> tool is
provided to list cached URLs, remove cached URLs, or to maintain the size
of the disk cache within size and inode limits.</dd>
</dl>
<p>Further details, discussion, and examples, are provided in the
<a href="../caching.html">Caching Guide</a>.</p>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#cachedefaultexpire">CacheDefaultExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachedetailheader">CacheDetailHeader</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachedisable">CacheDisable</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheenable">CacheEnable</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheheader">CacheHeader</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignoreheaders">CacheIgnoreHeaders</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorenolastmod">CacheIgnoreNoLastMod</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorequerystring">CacheIgnoreQueryString</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachekeybaseurl">CacheKeyBaseURL</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelastmodifiedfactor">CacheLastModifiedFactor</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelock">CacheLock</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockmaxage">CacheLockMaxAge</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockpath">CacheLockPath</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachemaxexpire">CacheMaxExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheminexpire">CacheMinExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachequickhandler">CacheQuickHandler</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestaleonerror">CacheStaleOnError</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestoreexpired">CacheStoreExpired</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestorenostore">CacheStoreNoStore</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestoreprivate">CacheStorePrivate</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#related">Related Modules and Directives</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#sampleconf">Sample Configuration</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#thunderingherd">Avoiding the Thundering Herd</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#finecontrol">Fine Control with the CACHE Filter</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#status">Cache Status and Logging</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><a href="../caching.html">Caching Guide</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="related" id="related">Related Modules and Directives</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/mod_cache_disk.html#cacheroot">CacheRoot</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cacheminfilesize">CacheMinFileSize</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachemaxfilesize">CacheMaxFileSize</a></code></li></ul></td></tr></table>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="sampleconf" id="sampleconf">Sample Configuration</a></h2>
<div class="example"><h3>Sample httpd.conf</h3><p><code>
#<br />
# Sample Cache Configuration<br />
#<br />
LoadModule cache_module modules/mod_cache.so<br />
<br />
<IfModule mod_cache.c><br />
<span class="indent">
LoadModule cache_disk_module modules/mod_cache_disk.so<br />
<IfModule mod_cache_disk.c><br />
<span class="indent">
CacheRoot c:/cacheroot<br />
CacheEnable disk /<br />
CacheDirLevels 5<br />
CacheDirLength 3<br />
</span>
</IfModule> <br />
<br />
# When acting as a proxy, don't cache the list of security updates<br />
CacheDisable http://security.update.server/update-list/<br />
</span>
</IfModule>
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="thunderingherd" id="thunderingherd">Avoiding the Thundering Herd</a></h2>
<p>When a cached entry becomes stale, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> 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.</p>
<p>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 <strong>thundering herd</strong> of requests to strike the backend
suddenly and unpredicably.</p>
<p>To keep the thundering herd at bay, the <code class="directive">CacheLock</code>
directive can be used to define a directory in which locks are created for
URLs <strong>in flight</strong>. The lock is used as a <strong>hint</strong>
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).
</p>
<h3>Initial caching of an entry</h3>
<p>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.
</p>
<h3>Refreshment of a stale entry</h3>
<p>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.</p>
<h3>Locks and Cache-Control: no-cache</h3>
<p>Locks are used as a <strong>hint only</strong> 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.</p>
<p>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 <code class="directive">CacheLockMaxAge</code> directive, and defaults to 5
seconds.
</p>
<h3>Example configuration</h3>
<div class="example"><h3>Enabling the cache lock</h3><p><code>
#<br />
# Enable the cache lock<br />
#<br />
<IfModule mod_cache.c><br />
<span class="indent">
CacheLock on<br />
CacheLockPath /tmp/mod_cache-lock<br />
CacheLockMaxAge 5<br />
</span>
</IfModule>
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="finecontrol" id="finecontrol">Fine Control with the CACHE Filter</a></h2>
<p>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.</p>
<p>In this mode, the cache <strong>bolts onto</strong> the front of the server,
acting as if a free standing RFC 2616 caching proxy had been placed in front of
the server.</p>
<p>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.</p>
<p>To solve this problem the <code class="directive"><a href="#cachequickhandler ">CacheQuickHandler
</a></code> directive can be set to <strong>off</strong>, and the server will
process all phases normally handled by a non-cached request, including the
<strong>authentication and authorisation</strong> phases.</p>
<p>In addition, the administrator may optionally specify the <strong>precise point
within the filter chain</strong> where caching is to take place by adding the
<strong>CACHE</strong> filter to the output filter chain.</p>
<p>For example, to cache content before applying compression to the response,
place the <strong>CACHE</strong> filter before the <strong>DEFLATE</strong>
filter as in the example below:</p>
<div class="example"><p><code>
# Cache content before optional compression<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;DEFLATE text/plain<br /><br />
</code></p></div>
<p>Another option is to have content cached before personalisation is applied
by <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> (or another content processing filter). In this
example templates containing tags understood by
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> are cached before being parsed:</p>
<div class="example"><p><code>
# Cache content before mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
</code></p></div>
<p>You may place the <strong>CACHE</strong> filter anywhere you wish within the
filter chain. In this example, content is cached after being parsed by
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code>, but before being processed by
<code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code>:</p>
<div class="example"><p><code>
# Cache content between mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html<br /><br />
</code></p></div>
<div class="warning"><h3>Warning:</h3>If the location of the
<strong>CACHE</strong> filter in the filter chain is changed for any reason,
you may need to <strong>flush your cache</strong> to ensure that your data
served remains consistent. <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> is not in a position
to enforce this for you.</div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="status" id="status">Cache Status and Logging</a></h2>
<p>Once <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> 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
<strong>cache-status</strong> key. This reason can be logged by the
<code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code> directive as
follows:</p>
<div class="example"><p><code>
LogFormat "%{cache-status}e ..."
</code></p></div>
<p>Based on the caching decision made, the reason is also written to the
subprocess environment under one the following three keys, as appropriate:</p>
<dl>
<dt>cache-hit</dt><dd>The response was served from cache.</dd>
<dt>cache-revalidate</dt><dd>The response was stale and was successfully
revalidated, then served from cache.</dd>
<dt>cache-miss</dt><dd>The response was served from the upstream server.</dd>
</dl>
<p>This makes it possible to support conditional logging of cached requests
as per the following example:</p>
<div class="example"><p><code>
CustomLog cached-requests.log common env=cache-hit<br />
CustomLog uncached-requests.log common env=cache-miss<br />
CustomLog revalidated-requests.log common env=cache-revalidate<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDefaultExpire" id="CacheDefaultExpire">CacheDefaultExpire</a> <a name="cachedefaultexpire" id="cachedefaultexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The default duration to cache a document when no expiry date is specified.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDefaultExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheDefaultExpire 3600 (one hour)</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheDefaultExpire</code> 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 <code class="directive">CacheMaxExpire</code>
directive does <em>not</em> override this setting.</p>
<div class="example"><p><code>
CacheDefaultExpire 86400
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDetailHeader" id="CacheDetailHeader">CacheDetailHeader</a> <a name="cachedetailheader" id="cachedetailheader">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Add an X-Cache-Detail header to the response.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDetailHeader <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheDetailHeader off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachedetailheader">CacheDetailHeader</a></code> directive
is switched on, an <strong>X-Cache-Detail</strong> header will be added to the response
containing the detailed reason for a particular caching decision.</p>
<p>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 <code>Cache-Control</code> and other headers have been correctly
used by the service and client.</p>
<p>If the normal handler is used, this directive may appear within a
<code class="directive"><a href="../mod/core.html#<directory>"><Directory></a></code> or
<code class="directive"><a href="../mod/core.html#<location>"><Location></a></code> directive. If the quick handler
is used, this directive must appear within a server or virtual host context, otherwise
the setting will be ignored.</p>
<div class="example"><p><code>
# Enable the X-Cache-Detail header<br />
CacheDetailHeader on<br />
</code></p></div>
<div class="example"><p><code>
X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDisable" id="CacheDisable">CacheDisable</a> <a name="cachedisable" id="cachedisable">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Disable caching of specified URLs</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDisable <var>url-string</var> | <var>on</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheDisable</code> directive instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to <em>not</em> cache urls at or below
<var>url-string</var>.</p>
<div class="example"><h3>Example</h3><p><code>
CacheDisable /local_files
</code></p></div>
<p>If used in a <code class="directive"><Location></code> 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.</p>
<div class="example"><h3>Example</h3><p><code>
<Location /foo><br />
CacheDisable on<br />
</Location><br />
</code></p></div>
<p> The <code>no-cache</code> environment variable can be set to
disable caching on a finer grained set of resources in versions
2.2.12 and later.</p>
<h3>See also</h3>
<ul>
<li><a href="../env.html">Environment Variables in Apache</a></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheEnable" id="CacheEnable">CacheEnable</a> <a name="cacheenable" id="cacheenable">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enable caching of specified URLs using a specified storage
manager</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheEnable <var>cache_type</var> <var>url-string</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheEnable</code> directive instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to cache urls at or below
<var>url-string</var>. The cache storage manager is specified with the
<var>cache_type</var> argument. If the <code class="directive">CacheEnable</code>
directive is placed inside a <code class="directive"><Location></code>
directive, the <var>url-string</var> becomes optional.
<var>cache_type</var> <code>disk</code> instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to use the disk based storage manager
implemented by <code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code>.</p>
<p>In the event that the URL space overlaps between different
<code class="directive">CacheEnable</code> 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 <code class="directive">CacheEnable</code>
directives in the configuration file.</p>
<p>When acting as a forward proxy server, <var>url-string</var> can
also be used to specify remote sites and proxy protocols which
caching should be enabled for.</p>
<div class="example"><p><code>
# Cache proxied url's<br />
CacheEnable disk /<br /><br />
# Cache FTP-proxied url's<br />
CacheEnable disk ftp://<br /><br />
# Cache content from www.example.org<br />
CacheEnable disk http://www.example.org/<br />
</code></p></div>
<p>A hostname starting with a <strong>"*"</strong> matches all hostnames with
that suffix. A hostname starting with <strong>"."</strong> matches all
hostnames containing the domain components that follow.</p>
<div class="example"><p><code>
# Match www.example.org, and fooexample.org<br />
CacheEnable disk http://*example.org/<br />
# Match www.example.org, but not fooexample.org<br />
CacheEnable disk http://.example.org/<br />
</code></p></div>
<p> The <code>no-cache</code> environment variable can be set to
disable caching on a finer grained set of resources in versions
2.2.12 and later.</p>
<h3>See also</h3>
<ul>
<li><a href="../env.html">Environment Variables in Apache</a></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheHeader" id="CacheHeader">CacheHeader</a> <a name="cacheheader" id="cacheheader">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Add an X-Cache header to the response.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheHeader <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheHeader off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cacheheader">CacheHeader</a></code> directive
is switched on, an <strong>X-Cache</strong> 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 <code class="directive"><a href="../mod/core.html#<directory>"><Directory></a></code>
or <code class="directive"><a href="../mod/core.html#<location>"><Location></a></code> directive. If the quick
handler is used, this directive must appear within a server or virtual host
context, otherwise the setting will be ignored.</p>
<dl>
<dt><strong>HIT</strong></dt><dd>The entity was fresh, and was served from
cache.</dd>
<dt><strong>REVALIDATE</strong></dt><dd>The entity was stale, was successfully
revalidated and was served from cache.</dd>
<dt><strong>MISS</strong></dt><dd>The entity was fetched from the upstream
server and was not served from cache.</dd>
</dl>
<div class="example"><p><code>
# Enable the X-Cache header<br />
CacheHeader on<br />
</code></p></div>
<div class="example"><p><code>
X-Cache: HIT from localhost<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreCacheControl" id="CacheIgnoreCacheControl">CacheIgnoreCacheControl</a> <a name="cacheignorecachecontrol" id="cacheignorecachecontrol">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore request to not serve cached content to client</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreCacheControl On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreCacheControl Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, requests containing a Cache-Control: no-cache or
Pragma: no-cache header value will not be served from the cache. The
<code class="directive">CacheIgnoreCacheControl</code> directive allows this
behavior to be overridden. <code class="directive">CacheIgnoreCacheControl On</code>
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 <em>never</em> be cached.</p>
<div class="example"><p><code>
CacheIgnoreCacheControl On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
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.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cachestoreprivate">CacheStorePrivate</a></code></li>
<li><code class="directive"><a href="#cachestorenostore">CacheStoreNoStore</a></code></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreHeaders" id="CacheIgnoreHeaders">CacheIgnoreHeaders</a> <a name="cacheignoreheaders" id="cacheignoreheaders">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Do not store the given HTTP header(s) in the cache.
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreHeaders <var>header-string</var> [<var>header-string</var>] ...</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreHeaders None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>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 <em>any</em> case regardless of the
setting of <code class="directive">CacheIgnoreHeaders</code>:</p>
<ul>
<li><code>Connection</code></li>
<li><code>Keep-Alive</code></li>
<li><code>Proxy-Authenticate</code></li>
<li><code>Proxy-Authorization</code></li>
<li><code>TE</code></li>
<li><code>Trailers</code></li>
<li><code>Transfer-Encoding</code></li>
<li><code>Upgrade</code></li>
</ul>
<p><code class="directive">CacheIgnoreHeaders</code> 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.</p>
<p><code class="directive">CacheIgnoreHeaders</code> 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), <code class="directive">CacheIgnoreHeaders</code> can be set to
<code>None</code>.</p>
<div class="example"><h3>Example 1</h3><p><code>
CacheIgnoreHeaders Set-Cookie
</code></p></div>
<div class="example"><h3>Example 2</h3><p><code>
CacheIgnoreHeaders None
</code></p></div>
<div class="warning"><h3>Warning:</h3>
If headers like <code>Expires</code> which are needed for proper cache
management are not stored due to a
<code class="directive">CacheIgnoreHeaders</code> setting, the behaviour of
mod_cache is undefined.
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreNoLastMod" id="CacheIgnoreNoLastMod">CacheIgnoreNoLastMod</a> <a name="cacheignorenolastmod" id="cacheignorenolastmod">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore the fact that a response has no Last Modified
header.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreNoLastMod On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreNoLastMod Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, documents without a last-modified date are not cached.
Under some circumstances the last-modified date is removed (during
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> processing for example) or not provided
at all. The <code class="directive">CacheIgnoreNoLastMod</code> 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
<code class="directive">CacheDefaultExpire</code> directive will be used to
generate an expiration date.</p>
<div class="example"><p><code>
CacheIgnoreNoLastMod On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreQueryString" id="CacheIgnoreQueryString">CacheIgnoreQueryString</a> <a name="cacheignorequerystring" id="cacheignorequerystring">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore query string when caching</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreQueryString On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreQueryString Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>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
<code class="directive">CacheIgnoreQueryString</code> 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.</p>
<div class="example"><p><code>
CacheIgnoreQueryString On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreURLSessionIdentifiers" id="CacheIgnoreURLSessionIdentifiers">CacheIgnoreURLSessionIdentifiers</a> <a name="cacheignoreurlsessionidentifiers" id="cacheignoreurlsessionidentifiers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore defined session identifiers encoded in the URL when caching
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreURLSessionIdentifiers <var>identifier</var> [<var>identifier</var>] ...</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreURLSessionIdentifiers None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Sometimes applications encode the session identifier into the URL like in the following
Examples:
</p>
<ul>
<li><code>/someapplication/image.gif;jsessionid=123456789</code></li>
<li><code>/someapplication/image.gif?PHPSESSIONID=12345678</code></li>
</ul>
<p>This causes cachable resources to be stored separately for each session, which
is often not desired. <code class="directive">CacheIgnoreURLSessionIdentifiers</code> 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.
</p>
<p><code>CacheIgnoreURLSessionIdentifiers None</code> clears the list of ignored
identifiers. Otherwise, each identifier is added to the list.</p>
<div class="example"><h3>Example 1</h3><p><code>
CacheIgnoreURLSessionIdentifiers jsessionid
</code></p></div>
<div class="example"><h3>Example 2</h3><p><code>
CacheIgnoreURLSessionIdentifiers None
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheKeyBaseURL" id="CacheKeyBaseURL">CacheKeyBaseURL</a> <a name="cachekeybaseurl" id="cachekeybaseurl">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Override the base URL of reverse proxied cache keys.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheKeyBaseURL <var>URL</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheKeyBaseURL http://example.com</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachekeybaseurl">CacheKeyBaseURL</a></code> 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.</p>
<div class="example"><p><code>
# Override the base URL of the cache key.<br />
CacheKeyBaseURL http://www.example.com/<br />
</code></p></div>
<div class="warning">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.</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLastModifiedFactor" id="CacheLastModifiedFactor">CacheLastModifiedFactor</a> <a name="cachelastmodifiedfactor" id="cachelastmodifiedfactor">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The factor used to compute an expiry date based on the
LastModified date.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLastModifiedFactor <var>float</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLastModifiedFactor 0.1</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>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
<code class="directive">CacheLastModifiedFactor</code> directive specifies a
<var>factor</var> to be used in the generation of this expiry date
according to the following formula:
<code>expiry-period = time-since-last-modified-date * <var>factor</var>
expiry-date = current-date + expiry-period</code>
For example, if the document was last modified 10 hours ago, and
<var>factor</var> 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
<code class="directive">CacheMaxExpire</code>, then the latter takes
precedence.</p>
<div class="example"><p><code>
CacheLastModifiedFactor 0.5
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLock" id="CacheLock">CacheLock</a> <a name="cachelock" id="cachelock">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enable the thundering herd lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLock <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLock off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.2.15 and later</td></tr>
</table>
<p>The <code class="directive">CacheLock</code> directive enables the thundering herd lock
for the given URL space.</p>
<p>In a minimal configuration the following directive is all that is needed to
enable the thundering herd lock in the default system temp directory.</p>
<div class="example"><p><code>
# Enable cache lock<br />
CacheLock on<br /><br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockMaxAge" id="CacheLockMaxAge">CacheLockMaxAge</a> <a name="cachelockmaxage" id="cachelockmaxage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the maximum possible age of a cache lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockMaxAge <var>integer</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockMaxAge 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockMaxAge</code> directive specifies the maximum
age of any cache lock.</p>
<p>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.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockPath" id="CacheLockPath">CacheLockPath</a> <a name="cachelockpath" id="cachelockpath">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the lock path directory.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockPath <var>directory</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockPath /tmp/mod_cache-lock</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockPath</code> 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.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheMaxExpire" id="CacheMaxExpire">CacheMaxExpire</a> <a name="cachemaxexpire" id="cachemaxexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The maximum time in seconds to cache a document</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheMaxExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheMaxExpire 86400 (one day)</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheMaxExpire</code> 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.</p>
<div class="example"><p><code>
CacheMaxExpire 604800
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheMinExpire" id="CacheMinExpire">CacheMinExpire</a> <a name="cacheminexpire" id="cacheminexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The minimum time in seconds to cache a document</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheMinExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheMinExpire 0</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheMinExpire</code> 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.</p>
<div class="example"><p><code>
CacheMinExpire 3600
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheQuickHandler" id="CacheQuickHandler">CacheQuickHandler</a> <a name="cachequickhandler" id="cachequickhandler">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Run the cache from the quick handler.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheQuickHandler <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheQuickHandler on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive
controls the phase in which the cache is handled.</p>
<p>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 <strong>bolts onto</strong> the front of the server, and the
majority of server processing is avoided.</p>
<p>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.</p>
<div class="example"><p><code>
# Run cache as a normal handler<br />
CacheQuickHandler off<br /><br />
</code></p></div>
<p>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 <strong>CACHE</strong> filter to
the chain.</p>
<div class="example"><p><code>
# Cache content before mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
</code></p></div>
<p>If the CACHE filter is specified more than once, the last instance will
apply.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStaleOnError" id="CacheStaleOnError">CacheStaleOnError</a> <a name="cachestaleonerror" id="cachestaleonerror">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Serve stale content in place of 5xx responses.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStaleOnError <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStaleOnError on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachestaleonerror">CacheStaleOnError</a></code> 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.</p>
<div class="example"><p><code>
# Serve stale data on error.<br />
CacheStaleOnError on<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStoreExpired" id="CacheStoreExpired">CacheStoreExpired</a> <a name="cachestoreexpired" id="cachestoreexpired">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache responses that the server reports as expired</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStoreExpired On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStoreExpired Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Since httpd 2.2.4, responses which have already expired are not
stored in the cache. The <code class="directive">CacheStoreExpired</code>
directive allows this behavior to be overridden.
<code class="directive">CacheStoreExpired</code> 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.</p>
<div class="example"><p><code>
CacheStoreExpired On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStoreNoStore" id="CacheStoreNoStore">CacheStoreNoStore</a> <a name="cachestorenostore" id="cachestorenostore">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache requests or responses that have been marked as no-store.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStoreNoStore On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStoreNoStore Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, requests or responses with Cache-Control: no-store header
values will not be stored in the cache. The
<code class="directive">CacheStoreNoCache</code> directive allows this
behavior to be overridden. <code class="directive">CacheStoreNoCache</code> On
tells the server to attempt to cache the resource even if it contains
no-store header values. Resources requiring authorization will
<em>never</em> be cached.</p>
<div class="example"><p><code>
CacheStoreNoStore On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
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.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></code></li>
<li><code class="directive"><a href="#cachestoreprivate">CacheStorePrivate</a></code></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStorePrivate" id="CacheStorePrivate">CacheStorePrivate</a> <a name="cachestoreprivate" id="cachestoreprivate">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache responses that the server has marked as private</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStorePrivate On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStorePrivate Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, responses with Cache-Control: private header values will not
be stored in the cache. The <code class="directive">CacheStorePrivate</code>
directive allows this behavior to be overridden.
<code class="directive">CacheStorePrivate</code> On
tells the server to attempt to cache the resource even if it contains
private header values. Resources requiring authorization will
<em>never</em> be cached.</p>
<div class="example"><p><code>
CacheStorePrivate On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
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.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></code></li>
<li><code class="directive"><a href="#cachestorenostore">CacheStoreNoStore</a></code></li>
</ul>
</div>
</div>
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