path: root/docs/manual/mod/mod_cache.xml

<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->

<!--
 Licensed to the Apache Software Foundation (ASF) under one or more
 contributor license agreements.  See the NOTICE file distributed with
 this work for additional information regarding copyright ownership.
 The ASF licenses this file to You under the Apache License, Version 2.0
 (the "License"); you may not use this file except in compliance with
 the License.  You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
-->

<modulesynopsis metafile="mod_cache.xml.meta">

<name>mod_cache</name>
<description>RFC 2616 compliant HTTP caching filter.</description>
<status>Extension</status>
<sourcefile>mod_cache.c</sourcefile>
<identifier>cache_module</identifier>

<summary>
    <note type="warning">This module should be used with care, as when the
    <directive module="mod_cache">CacheQuickHandler</directive> directive is
    in its default value of <strong>on</strong>, the <directive
    module="mod_access_compat">Allow</directive> and <directive
    module="mod_access_compat">Deny</directive> 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.</note>

    <p><module>mod_cache</module> 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, <module>mod_cache</module> 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, <module>mod_cache</module> 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, <module>mod_cache</module> 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
    <directive module="mod_cache">CacheQuickHandler</directive> 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
    <module>mod_include</module> filter, or optionally compressed by the
    <module>mod_deflate</module> filter.</p>

    <p>Under normal operation, <module>mod_cache</module> 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,
    <module>mod_cache</module> 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 <module>mod_cache</module> when the
    <directive module="mod_cache">CacheLock</directive> 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
    <module>mod_cache</module>. 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><module>mod_cache</module> requires the services of one or more
    storage management modules. One storage management module is included in
    the base Apache distribution:</p>
    <dl>
    <dt><module>mod_cache_disk</module></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 <program>htcacheclean</program> 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>
    <dt><module>mod_cache_socache</module></dt>
    <dd>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.</dd>
    </dl>

    <p>Further details, discussion, and examples, are provided in the
    <a href="../caching.html">Caching Guide</a>.</p>
</summary>
<seealso><a href="../caching.html">Caching Guide</a></seealso>

<section id="related"><title>Related Modules and Directives</title>
    <related>
      <modulelist>
        <module>mod_cache_disk</module>
        <module>mod_cache_socache</module>
      </modulelist>
      <directivelist>
        <directive module="mod_cache_disk">CacheRoot</directive>
        <directive module="mod_cache_disk">CacheDirLevels</directive>
        <directive module="mod_cache_disk">CacheDirLength</directive>
        <directive module="mod_cache_disk">CacheMinFileSize</directive>
        <directive module="mod_cache_disk">CacheMaxFileSize</directive>
        <directive module="mod_cache_socache">CacheSocache</directive>
        <directive module="mod_cache_socache">CacheSocacheMaxTime</directive>
        <directive module="mod_cache_socache">CacheSocacheMinTime</directive>
        <directive module="mod_cache_socache">CacheSocacheMaxSize</directive>
        <directive module="mod_cache_socache">CacheSocacheReadSize</directive>
        <directive module="mod_cache_socache">CacheSocacheReadTime</directive>
      </directivelist>
    </related>
</section>

<section id="sampleconf"><title>Sample Configuration</title>
    <example><title>Sample httpd.conf</title>
    <highlight language="config">
#
# Sample Cache Configuration
#
LoadModule cache_module modules/mod_cache.so
&lt;IfModule mod_cache.c&gt;
    LoadModule cache_disk_module modules/mod_cache_disk.so
    &lt;IfModule mod_cache_disk.c&gt;
        CacheRoot c:/cacheroot
        CacheEnable disk  /
        CacheDirLevels 5
        CacheDirLength 3
    &lt;/IfModule&gt;
    
    # When acting as a proxy, don't cache the list of security updates
    CacheDisable http://security.update.server/update-list/
&lt;/IfModule&gt;
      </highlight>
    </example>
</section>

<section id="thunderingherd"><title>Avoiding the Thundering Herd</title>
  <p>When a cached entry becomes stale, <module>mod_cache</module> 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 unpredictably.</p>
  <p>To keep the thundering herd at bay, the <directive>CacheLock</directive>
  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>
  <section>
    <title>Initial caching of an entry</title>
    <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>
  </section>
  <section>
    <title>Refreshment of a stale entry</title>
    <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>
  </section>
  <section>
    <title>Locks and Cache-Control: no-cache</title>
    <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 <directive>CacheLockMaxAge</directive> directive, and defaults to 5
    seconds.
    </p>
  </section>
  <section>
    <title>Example configuration</title>
    <example><title>Enabling the cache lock</title>
    <highlight language="config">
#
# Enable the cache lock
#
&lt;IfModule mod_cache.c&gt;
    CacheLock on
    CacheLockPath /tmp/mod_cache-lock
    CacheLockMaxAge 5
&lt;/IfModule&gt;
      </highlight>
    </example>
  </section>
</section>

<section id="finecontrol"><title>Fine Control with the CACHE Filter</title>
  <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 authorization 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 <directive module="mod_cache">CacheQuickHandler
  </directive> 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 authorization</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>

  <highlight language="config">
# Cache content before optional compression
CacheQuickHandler off
AddOutputFilterByType CACHE;DEFLATE text/plain
  </highlight>

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

  <highlight language="config">
# Cache content before mod_include and mod_deflate
CacheQuickHandler off
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html
  </highlight>

  <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
  <module>mod_include</module>, but before being processed by
  <module>mod_deflate</module>:</p>

  <highlight language="config">
# Cache content between mod_include and mod_deflate
CacheQuickHandler off
AddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html
  </highlight>

  <note type="warning"><title>Warning:</title>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. <module>mod_cache</module> is not in a position
  to enforce this for you.</note>

</section>

<section id="status"><title>Cache Status and Logging</title>
  <p>Once <module>mod_cache</module> 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
  <directive module="mod_log_config">LogFormat</directive> directive as
  follows:</p>

  <highlight language="config">
    LogFormat "%{cache-status}e ..."
  </highlight>

  <p>Based on the caching decision made, the reason is also written to the
  subprocess environment under one the following four 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>
    <dt>cache-invalidate</dt><dd>The cached entity was invalidated by a request
      method other than GET or HEAD.</dd>
  </dl>

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

  <highlight language="config">
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
  </highlight>

  <p>For module authors, a hook called <var>cache_status</var> is available,
  allowing modules to respond to the caching outcomes above in customised
  ways.</p>
</section>

<directivesynopsis>
<name>CacheEnable</name>
<description>Enable caching of specified URLs using a specified storage
manager</description>
<syntax>CacheEnable <var>cache_type</var> [<var>url-string</var>]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context>
</contextlist>

<usage>
    <p>The <directive>CacheEnable</directive> directive instructs
    <module>mod_cache</module> to cache urls at or below
    <var>url-string</var>. The cache storage manager is specified with the
    <var>cache_type</var> argument. The <directive>CacheEnable</directive>
    directive can alternatively be placed inside either
    <directive type="section">Location</directive> or
    <directive type="section">LocationMatch</directive> sections to indicate
    the content is cacheable.
    <var>cache_type</var> <code>disk</code> instructs
    <module>mod_cache</module> to use the disk based storage manager
    implemented by <module>mod_cache_disk</module>. <var>cache_type</var>
    <code>socache</code> instructs <module>mod_cache</module> to use the
    shared object cache based storage manager implemented by
    <module>mod_cache_socache</module>.</p>
    <p>In the event that the URL space overlaps between different
    <directive>CacheEnable</directive> 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 <directive>CacheEnable</directive>
    directives in the configuration file. <directive>CacheEnable</directive>
    directives within <directive type="section">Location</directive> or
    <directive type="section">LocationMatch</directive> sections are processed
    before globally defined <directive>CacheEnable</directive> directives.</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>

    <highlight language="config">
# Cache content (normal handler only)
CacheQuickHandler off
&lt;Location /foo&gt;
    CacheEnable disk
&lt;/Location&gt;

# Cache regex (normal handler only)
CacheQuickHandler off
&lt;LocationMatch foo$&gt;
    CacheEnable disk
&lt;/LocationMatch&gt;

# Cache proxied url's (normal or quick handler)
CacheEnable  disk  /

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

# Cache content from www.example.org (normal or quick handler)
CacheEnable  disk  http://www.example.org/
    </highlight>

    <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>

    <highlight language="config">
# 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/
    </highlight>

    <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>

</usage>
<seealso><a href="../env.html">Environment Variables in Apache</a></seealso>
</directivesynopsis>

<directivesynopsis>
<name>CacheDisable</name>
<description>Disable caching of specified URLs</description>
<syntax>CacheDisable <var>url-string</var> | <var>on</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>CacheDisable</directive> directive instructs
    <module>mod_cache</module> to <em>not</em> cache urls at or below
    <var>url-string</var>.</p>

    <example><title>Example</title>
    <highlight language="config">
      CacheDisable /local_files
      </highlight>
    </example>

    <p>If used in a <directive type="section">Location</directive> 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>

    <example><title>Example</title>
    <highlight language="config">
&lt;Location /foo&gt;
    CacheDisable on
&lt;/Location&gt;
      </highlight>
    </example>

    <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>

</usage>
<seealso><a href="../env.html">Environment Variables in Apache</a></seealso>
</directivesynopsis>
<directivesynopsis>
<name>CacheMaxExpire</name>
<description>The maximum time in seconds to cache a document</description>
<syntax>CacheMaxExpire <var>seconds</var></syntax>
<default>CacheMaxExpire 86400 (one day)</default>
<contextlist><context>server config</context>
    <context>virtual host</context>
    <context>directory</context>
    <context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>CacheMaxExpire</directive> 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>

    <highlight language="config">
      CacheMaxExpire 604800
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheMinExpire</name>
<description>The minimum time in seconds to cache a document</description>
<syntax>CacheMinExpire <var>seconds</var></syntax>
<default>CacheMinExpire 0</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>CacheMinExpire</directive> 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>


    <highlight language="config">
      CacheMinExpire 3600
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheDefaultExpire</name>
<description>The default duration to cache a document when no expiry date is specified.</description>
<syntax>CacheDefaultExpire <var>seconds</var></syntax>
<default>CacheDefaultExpire 3600 (one hour)</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>CacheDefaultExpire</directive> 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 <directive>CacheMaxExpire</directive>
    directive does <em>not</em> override this setting.</p>

    <highlight language="config">
      CacheDefaultExpire 86400
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheIgnoreNoLastMod</name>
<description>Ignore the fact that a response has no Last Modified
header.</description>
<syntax>CacheIgnoreNoLastMod On|Off</syntax>
<default>CacheIgnoreNoLastMod Off</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>Ordinarily, documents without a last-modified date are not cached.
    Under some circumstances the last-modified date is removed (during
    <module>mod_include</module> processing for example) or not provided
    at all. The <directive>CacheIgnoreNoLastMod</directive> 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
    <directive>CacheDefaultExpire</directive> directive will be used to
    generate an expiration date.</p>

    <highlight language="config">
      CacheIgnoreNoLastMod On
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheIgnoreCacheControl</name>
<description>Ignore request to not serve cached content to client</description>
<syntax>CacheIgnoreCacheControl On|Off</syntax>
<default>CacheIgnoreCacheControl Off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <p>Ordinarily, requests containing a Cache-Control: no-cache or
    Pragma: no-cache header value will not be served from the cache.  The
    <directive>CacheIgnoreCacheControl</directive> directive allows this
    behavior to be overridden.  <directive>CacheIgnoreCacheControl On</directive>
    tells the server to attempt to serve the resource from the cache even
    if the request contains no-cache header values.</p>  

    <highlight language="config">
      CacheIgnoreCacheControl On
    </highlight>

    <note type="warning"><title>Warning:</title>
       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.
    </note>
</usage>
<seealso><directive module="mod_cache">CacheStorePrivate</directive></seealso>
<seealso><directive module="mod_cache">CacheStoreNoStore</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>CacheIgnoreQueryString</name>
<description>Ignore query string when caching</description>
<syntax>CacheIgnoreQueryString On|Off</syntax>
<default>CacheIgnoreQueryString Off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <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
    <directive>CacheIgnoreQueryString</directive> 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>

    <highlight language="config">
      CacheIgnoreQueryString On
    </highlight>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheLastModifiedFactor</name>
<description>The factor used to compute an expiry date based on the
LastModified date.</description>
<syntax>CacheLastModifiedFactor <var>float</var></syntax>
<default>CacheLastModifiedFactor 0.1</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <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
    <directive>CacheLastModifiedFactor</directive> 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
    <directive>CacheMaxExpire</directive>, then the latter takes
    precedence.</p>

    <highlight language="config">
      CacheLastModifiedFactor 0.5
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheIgnoreHeaders</name>
<description>Do not store the given HTTP header(s) in the cache.
</description>
<syntax>CacheIgnoreHeaders <var>header-string</var> [<var>header-string</var>] ...</syntax>
<default>CacheIgnoreHeaders None</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <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 <directive>CacheIgnoreHeaders</directive>:</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><directive>CacheIgnoreHeaders</directive> 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><directive>CacheIgnoreHeaders</directive> 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), <directive>CacheIgnoreHeaders</directive> can be set to
    <code>None</code>.</p>

    <example><title>Example 1</title>
    <highlight language="config">
      CacheIgnoreHeaders Set-Cookie
      </highlight>
    </example>

    <example><title>Example 2</title>
    <highlight language="config">
      CacheIgnoreHeaders None
      </highlight>
    </example>

    <note type="warning"><title>Warning:</title>
      If headers like <code>Expires</code> which are needed for proper cache
      management are not stored due to a
      <directive>CacheIgnoreHeaders</directive> setting, the behaviour of
      mod_cache is undefined.
    </note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheIgnoreURLSessionIdentifiers</name>
<description>Ignore defined session identifiers encoded in the URL when caching
</description>
<syntax>CacheIgnoreURLSessionIdentifiers <var>identifier</var> [<var>identifier</var>] ...</syntax>
<default>CacheIgnoreURLSessionIdentifiers None</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
    <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. <directive>CacheIgnoreURLSessionIdentifiers</directive> 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>

    <example><title>Example 1</title>
    <highlight language="config">
      CacheIgnoreURLSessionIdentifiers jsessionid
      </highlight>
    </example>

    <example><title>Example 2</title>
    <highlight language="config">
      CacheIgnoreURLSessionIdentifiers None
      </highlight>
    </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheStoreExpired</name>
<description>Attempt to cache responses that the server reports as expired</description>
<syntax>CacheStoreExpired On|Off</syntax>
<default>CacheStoreExpired Off</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>Since httpd 2.2.4, responses which have already expired are not
       stored in the cache.  The <directive>CacheStoreExpired</directive>
       directive allows this behavior to be overridden.
       <directive>CacheStoreExpired</directive> 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>

    <highlight language="config">
      CacheStoreExpired On
    </highlight>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheStorePrivate</name>
<description>Attempt to cache responses that the server has marked as private</description>
<syntax>CacheStorePrivate On|Off</syntax>
<default>CacheStorePrivate Off</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>Ordinarily, responses with Cache-Control: private header values will not
       be stored in the cache.  The <directive>CacheStorePrivate</directive>
       directive allows this behavior to be overridden.
       <directive>CacheStorePrivate</directive> On
       tells the server to attempt to cache the resource even if it contains
       private header values.</p>

    <highlight language="config">
      CacheStorePrivate On
    </highlight>

    <note type="warning"><title>Warning:</title>
       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.
    </note>
</usage>
<seealso><directive module="mod_cache">CacheIgnoreCacheControl</directive></seealso>
<seealso><directive module="mod_cache">CacheStoreNoStore</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>CacheStoreNoStore</name>
<description>Attempt to cache requests or responses that have been marked as no-store.</description>
<syntax>CacheStoreNoStore On|Off</syntax>
<default>CacheStoreNoStore Off</default>
<contextlist><context>server config</context>
  <context>virtual host</context>
  <context>directory</context>
  <context>.htaccess</context>
</contextlist>

<usage>
    <p>Ordinarily, requests or responses with Cache-Control: no-store header
       values will not be stored in the cache.  The
       <directive>CacheStoreNoStore</directive> directive allows this
       behavior to be overridden.  <directive>CacheStoreNoStore</directive> On
       tells the server to attempt to cache the resource even if it contains
       no-store header values.</p>

    <highlight language="config">
      CacheStoreNoStore On
    </highlight>

    <note type="warning"><title>Warning:</title>
       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.
    </note>
</usage>
<seealso><directive module="mod_cache">CacheIgnoreCacheControl</directive></seealso>
<seealso><directive module="mod_cache">CacheStorePrivate</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>CacheLock</name>
<description>Enable the thundering herd lock.</description>
<syntax>CacheLock <var>on|off</var></syntax>
<default>CacheLock off</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
  <p>The <directive>CacheLock</directive> 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 run-time file directory.</p>

  <highlight language="config">
# Enable cache lock
CacheLock on
  </highlight>

  <p>Locks consist of empty files that only exist for stale URLs in flight, so this
  is significantly less resource intensive than the traditional disk cache.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheLockPath</name>
<description>Set the lock path directory.</description>
<syntax>CacheLockPath <var>directory</var></syntax>
<default>CacheLockPath mod_cache-lock</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
  <p>The <directive>CacheLockPath</directive> directive allows you to specify the
  directory in which the locks are created.  If <var>directory</var> is not an absolute
  path, the location specified will be relative to the value of 
  <directive module="core">DefaultRuntimeDir</directive>.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheLockMaxAge</name>
<description>Set the maximum possible age of a cache lock.</description>
<syntax>CacheLockMaxAge <var>integer</var></syntax>
<default>CacheLockMaxAge 5</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>

<usage>
  <p>The <directive>CacheLockMaxAge</directive> 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>

</usage>
</directivesynopsis>

<directivesynopsis>
  <name>CacheQuickHandler</name>
  <description>Run the cache from the quick handler.</description>
  <syntax>CacheQuickHandler <var>on|off</var></syntax>
  <default>CacheQuickHandler on</default>
  <contextlist><context>server config</context><context>virtual host</context>
  </contextlist>
  <compatibility>Apache HTTP Server 2.3.3 and later</compatibility>

  <usage>
    <p>The <directive module="mod_cache">CacheQuickHandler</directive> 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 authorization.</p>

    <highlight language="config">
# Run cache as a normal handler
CacheQuickHandler off
    </highlight>

    <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>

    <highlight language="config">
# Cache content before mod_include and mod_deflate
CacheQuickHandler off
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html
    </highlight>

    <p>If the CACHE filter is specified more than once, the last instance will
    apply.</p>

  </usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheHeader</name>
<description>Add an X-Cache header to the response.</description>
<syntax>CacheHeader <var>on|off</var></syntax>
<default>CacheHeader off</default>
<contextlist><context>server config</context>
    <context>virtual host</context>
    <context>directory</context>
    <context>.htaccess</context>
</contextlist>
<compatibility>Available in Apache 2.3.9 and later</compatibility>

<usage>
  <p>When the <directive module="mod_cache">CacheHeader</directive> 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 <directive module="core">&lt;Directory&gt;</directive>
  or <directive module="core">&lt;Location&gt;</directive> 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>

  <highlight language="config">
# Enable the X-Cache header
CacheHeader on
  </highlight>

  <highlight language="config">
    X-Cache: HIT from localhost
  </highlight>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheDetailHeader</name>
<description>Add an X-Cache-Detail header to the response.</description>
<syntax>CacheDetailHeader <var>on|off</var></syntax>
<default>CacheDetailHeader off</default>
<contextlist><context>server config</context>
      <context>virtual host</context>
      <context>directory</context>
      <context>.htaccess</context>
</contextlist>
<compatibility>Available in Apache 2.3.9 and later</compatibility>

<usage>
  <p>When the <directive module="mod_cache">CacheDetailHeader</directive> 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
  <directive module="core">&lt;Directory&gt;</directive> or
  <directive module="core">&lt;Location&gt;</directive> 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>

  <highlight language="config">
# Enable the X-Cache-Detail header
CacheDetailHeader on
  </highlight>

  <example>
    X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost<br />
  </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheKeyBaseURL</name>
<description>Override the base URL of reverse proxied cache keys.</description>
<syntax>CacheKeyBaseURL <var>URL</var></syntax>
<default>CacheKeyBaseURL http://example.com</default>
<contextlist><context>server config</context>
<context>virtual host</context>
</contextlist>
<compatibility>Available in Apache 2.3.9 and later</compatibility>

<usage>
  <p>When the <directive module="mod_cache">CacheKeyBaseURL</directive> 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>

  <highlight language="config">
# Override the base URL of the cache key.
CacheKeyBaseURL http://www.example.com/
  </highlight>

  <note type="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.</note>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheStaleOnError</name>
<description>Serve stale content in place of 5xx responses.</description>
<syntax>CacheStaleOnError <var>on|off</var></syntax>
<default>CacheStaleOnError on</default>
<contextlist><context>server config</context>
    <context>virtual host</context>
    <context>directory</context>
    <context>.htaccess</context>
</contextlist>
<compatibility>Available in Apache 2.3.9 and later</compatibility>

<usage>
  <p>When the <directive module="mod_cache">CacheStaleOnError</directive> 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>

  <highlight language="config">
# Serve stale data on error.
CacheStaleOnError on
  </highlight>

</usage>
</directivesynopsis>

</modulesynopsis>