Minor nitpick - We've attempted to standardize on American spelling.

[apache] / docs / manual / mod / mod_cache.xml

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

<name>mod_cache</name>
<description>Content cache keyed to URIs.</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_authz_host">Allow</directive> and <directive 
    module="mod_authz_host">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 HTTP
    content cache that can be used to cache either local or proxied content.
    <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_disk_cache</module></dt>
    <dd>implements a disk based storage manager.</dd>
    </dl>

    <p>Content is stored in and retrieved from the cache using URI based keys. Content with
    access protection is not cached.</p>
    <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_disk_cache</module>
      </modulelist>
      <directivelist>
        <directive module="mod_disk_cache">CacheRoot</directive>
        <directive module="mod_disk_cache">CacheDirLevels</directive>
        <directive module="mod_disk_cache">CacheDirLength</directive>
        <directive module="mod_disk_cache">CacheMinFileSize</directive>
        <directive module="mod_disk_cache">CacheMaxFileSize</directive>
      </directivelist>
    </related>
</section>

<section id="sampleconf"><title>Sample Configuration</title>
    <example><title>Sample httpd.conf</title>
      #<br />
      # Sample Cache Configuration<br />
      #<br />
      LoadModule cache_module modules/mod_cache.so<br />
      <br />
      &lt;IfModule mod_cache.c&gt;<br />
      <indent>
        LoadModule disk_cache_module modules/mod_disk_cache.so<br />
        &lt;IfModule mod_disk_cache.c&gt;<br />
        <indent>
          CacheRoot c:/cacheroot<br />
          CacheEnable disk  /<br />
          CacheDirLevels 5<br />
          CacheDirLength 3<br />
        </indent>
        &lt;/IfModule&gt; <br />
        <br />
        # When acting as a proxy, don't cache the list of security updates<br />
        CacheDisable http://security.update.server/update-list/<br />
      </indent>
      &lt;/IfModule&gt;
    </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 unpredicably.</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>
      #<br />
      # Enable the cache lock<br />
      #<br />
      &lt;IfModule mod_cache.c&gt;<br />
      <indent>
        CacheLock on<br />
        CacheLockPath /tmp/mod_cache-lock<br />
        CacheLockMaxAge 5<br />
      </indent>
      &lt;/IfModule&gt;
    </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 RFC2616 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 <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 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>

  <example>
    # Cache content before optional compression<br />
    CacheQuickHandler off<br />
    AddOutputFilterByType CACHE;DEFLATE text/plain<br /><br />
  </example>

  <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>

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

  <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>

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

  <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>

  <example>
    LogFormat "%{cache-status}e ..."
  </example>

  <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>

  <example>
    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 />
  </example>

</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>
</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. If the <directive>CacheEnable</directive>
    directive is placed inside a <directive type="section">Location</directive>
    directive, the <var>url-string</var> becomes optional.
    <var>cache_type</var> <code>disk</code> instructs
    <module>mod_cache</module> to use the disk based storage manager
    implemented by <module>mod_disk_cache</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.</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>
 
    <example>
      # Cache proxied url's<br />
      CacheEnable  disk  /<br /><br />
      # Cache FTP-proxied url's<br />
      CacheEnable  disk  ftp://<br /><br />
      # Cache content from www.apache.org<br />
      CacheEnable  disk  http://www.apache.org/<br />
    </example>

    <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>

    <example>
      # Match www.apache.org, and fooapache.org<br />
      CacheEnable  disk  http://*apache.org/<br />
      # Match www.apache.org, but not fooapache.org<br />
      CacheEnable  disk  http://.apache.org/<br />
    </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>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>
</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>
      CacheDisable /local_files
    </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>
      &lt;Location /foo&gt;<br />
        CacheDisable on<br />
      &lt;/Location&gt;<br />
    </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>
</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>

    <example>
      CacheMaxExpire 604800
    </example>
</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>
</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>


    <example>
      CacheMinExpire 3600
    </example>
</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>
</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>

    <example>
      CacheDefaultExpire 86400
    </example>
</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>
</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>

    <example>
      CacheIgnoreNoLastMod On
    </example>
</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.  Resources requiring
    authorization will <em>never</em> be cached.</p>

    <example>
      CacheIgnoreCacheControl On
    </example>

    <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>

    <example>
      CacheIgnoreQueryString On
    </example>

</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>
</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>

    <example>
      CacheLastModifiedFactor 0.5
    </example>
</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>
      CacheIgnoreHeaders Set-Cookie
    </example>

    <example><title>Example 2</title>
      CacheIgnoreHeaders None
    </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>
      CacheIgnoreURLSessionIdentifiers jsessionid
    </example>

    <example><title>Example 2</title>
      CacheIgnoreURLSessionIdentifiers None
    </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>
</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>

    <example>
      CacheStoreExpired On
    </example>
</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>
</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.  Resources requiring authorization will
       <em>never</em> be cached.</p>

    <example>
      CacheStorePrivate On
    </example>

    <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>
</contextlist>

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

    <example>
      CacheStoreNoStore On
    </example>

    <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>
<compatibility>Available in Apache 2.2.15 and later</compatibility>

<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 system temp directory.</p>

  <example>
    # Enable cache lock<br />
    CacheLock on<br /><br />
  </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheLockPath</name>
<description>Set the lock path directory.</description>
<syntax>CacheLockPath <var>directory</var></syntax>
<default>CacheLockPath /tmp/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. 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>

</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>

  <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 authorisation.</p>

    <example>
      # Run cache as a normal handler<br />
      CacheQuickHandler off<br /><br />
    </example>

    <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>

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

    <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>

  <example>
    # Enable the X-Cache header<br />
    CacheHeader on<br />
  </example>

  <example>
    X-Cache: HIT from localhost<br />
  </example>

</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>

  <example>
    # Enable the X-Cache-Detail header<br />
    CacheDetailHeader on<br />
  </example>

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

</usage>
</directivesynopsis>

</modulesynopsis>