<!-- $LastChangedRevision$ -->
<!--
- Copyright 2002-2005 The Apache Software Foundation or its licensors, as
- applicable.
-
- Licensed 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
+ 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
<modulesynopsis metafile="mod_cache.xml.meta">
<name>mod_cache</name>
-<description>Content cache keyed to URIs.</description>
+<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 and
- can be used to circumvent <directive
- module="mod_authz_host">Allow</directive> and <directive
- module="mod_authz_host">Deny</directive> directives. You
- should not enable caching for any content to which you wish
- to limit access by client host name, address or environment
- variable.</note>
+ <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 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. Two storage management modules are included in
+ 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. The following storage management modules are included in
the base Apache distribution:</p>
<dl>
- <dt><module>mod_disk_cache</module></dt>
- <dd>implements a disk based storage manager.</dd>
-
- <dt><module>mod_mem_cache</module></dt>
- <dd>implements a memory based storage manager.
- <module>mod_mem_cache</module> can be configured to operate in two
- modes: caching open file descriptors or caching objects in heap storage.
- <module>mod_mem_cache</module> can be used to cache locally generated content
- or to cache backend server content for <module>mod_proxy</module> when
- configured using <directive module="mod_proxy">ProxyPass</directive>
- (aka <dfn>reverse proxy</dfn>)</dd>
+ <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>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>
- <module>mod_mem_cache</module>
+ <module>mod_cache_disk</module>
+ <module>mod_cache_socache</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>
- <directive module="mod_mem_cache">MCacheSize</directive>
- <directive module="mod_mem_cache">MCacheMaxObjectCount</directive>
- <directive module="mod_mem_cache">MCacheMinObjectSize</directive>
- <directive module="mod_mem_cache">MCacheMaxObjectSize</directive>
- <directive module="mod_mem_cache">MCacheRemovalAlgorithm</directive>
- <directive module="mod_mem_cache">MCacheMaxStreamingBuffer</directive>
+ <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>
- #<br />
- # Sample Cache Configuration<br />
- #<br />
- LoadModule cache_module modules/mod_cache.so<br />
- <br />
- <IfModule mod_cache.c><br />
- <indent>
- #LoadModule disk_cache_module modules/mod_disk_cache.so<br />
- # If you want to use mod_disk_cache instead of mod_mem_cache,<br />
- # uncomment the line above and comment out the LoadModule line below.<br />
- <IfModule mod_disk_cache.c><br />
- <indent>
- CacheRoot c:/cacheroot<br />
- CacheEnable disk /<br />
- CacheDirLevels 5<br />
- CacheDirLength 3<br />
- </indent>
- </IfModule> <br />
- <br />
- LoadModule mem_cache_module modules/mod_mem_cache.so<br />
- <IfModule mod_mem_cache.c><br />
- <indent>
- CacheEnable mem /<br />
- MCacheSize 4096<br />
- MCacheMaxObjectCount 100<br />
- MCacheMinObjectSize 1<br />
- MCacheMaxObjectSize 2048<br />
- </indent>
- </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 />
- </indent>
- </IfModule>
+ <highlight language="config">
+#
+# Sample Cache Configuration
+#
+LoadModule cache_module modules/mod_cache.so
+<IfModule mod_cache.c>
+ LoadModule cache_disk_module modules/mod_cache_disk.so
+ <IfModule mod_cache_disk.c>
+ CacheRoot c:/cacheroot
+ CacheEnable disk /
+ CacheDirLevels 5
+ CacheDirLength 3
+ </IfModule>
+
+ # When acting as a proxy, don't cache the list of security updates
+ CacheDisable http://security.update.server/update-list/
+</IfModule>
+ </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
+#
+<IfModule mod_cache.c>
+ CacheLock on
+ CacheLockPath /tmp/mod_cache-lock
+ CacheLockMaxAge 5
+</IfModule>
+ </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>
+<syntax>CacheEnable <var>cache_type</var> [<var>url-string</var>]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
+<context>directory</context>
</contextlist>
+<compatibility>A url-string of '/' applied to forward proxy content in 2.2 and
+ earlier.</compatibility>
<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. <var>cache_type</var> <code> mem</code>
- instructs <module>mod_cache</module> to use the memory based storage
- manager implemented by <module>mod_mem_cache</module>.
+ <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_disk_cache</module>.
- <var>cache_type</var> <code>fd</code> instructs
- <module>mod_cache</module> to use the file descriptor cache implemented
- by <module>mod_mem_cache</module>.</p>
+ 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.</p>
+ 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>
- <example>
- CacheEnable mem /manual<br />
- CacheEnable fd /images<br />
- CacheEnable disk /<br />
- </example>
+ <p>When acting as a forward proxy server, <var>url-string</var> must
+ minimally begin with a protocol for which caching should be enabled.</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>
+ <highlight language="config">
+# Cache content (normal handler only)
+CacheQuickHandler off
+<Location "/foo">
+ CacheEnable disk
+</Location>
+
+# Cache regex (normal handler only)
+CacheQuickHandler off
+<LocationMatch "foo$">
+ CacheEnable disk
+</LocationMatch>
+
+# Cache all but forward proxy url's (normal or quick handler)
+CacheEnable disk /
+
+# Cache FTP-proxied url's (normal or quick handler)
+CacheEnable disk ftp://
+
+# Cache forward proxy content from www.example.org (normal or quick handler)
+CacheEnable disk http://www.example.org/
+ </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></syntax>
+<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>
<var>url-string</var>.</p>
<example><title>Example</title>
+ <highlight language="config">
CacheDisable /local_files
+ </highlight>
</example>
-</usage>
+ <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">
+<Location "/foo">
+ CacheDisable on
+</Location>
+ </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>
+<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
+ seconds for which cacheable HTTP documents will be retained without checking the origin
server. Thus, documents will be out of date at most this number of seconds. This maximum
value is enforced even if an expiry date was supplied with the document.</p>
- <example>
+ <highlight language="config">
CacheMaxExpire 604800
- </example>
+ </highlight>
</usage>
</directivesynopsis>
<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><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
+ seconds for which cacheable HTTP documents will be retained without checking the origin
server. This is only used if no valid expire time was supplied with the document.</p>
- <example>
+ <highlight language="config">
CacheMinExpire 3600
- </example>
+ </highlight>
</usage>
</directivesynopsis>
<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><context>server config</context>
+ <context>virtual host</context>
+ <context>directory</context>
+ <context>.htaccess</context>
</contextlist>
<usage>
with the document. The value specified with the <directive>CacheMaxExpire</directive>
directive does <em>not</em> override this setting.</p>
- <example>
+ <highlight language="config">
CacheDefaultExpire 86400
- </example>
+ </highlight>
</usage>
</directivesynopsis>
header.</description>
<syntax>CacheIgnoreNoLastMod On|Off</syntax>
<default>CacheIgnoreNoLastMod Off</default>
-<contextlist><context>server config</context><context>virtual host</context>
+<contextlist><context>server config</context>
+ <context>virtual host</context>
+ <context>directory</context>
+ <context>.htaccess</context>
</contextlist>
<usage>
<directive>CacheDefaultExpire</directive> directive will be used to
generate an expiration date.</p>
- <example>
+ <highlight language="config">
CacheIgnoreNoLastMod On
- </example>
+ </highlight>
</usage>
</directivesynopsis>
<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</directive>
- On tells the server to attempt to serve the resource from the cache even
- if the request contains no-cache header values. Resources requiring
- authorization will <em>never</em> be cached.</p>
+ 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>
- <example>
+ <highlight language="config">
CacheIgnoreCacheControl On
- </example>
+ </highlight>
<note type="warning"><title>Warning:</title>
This directive will allow serving from the cache even if the client has
<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>
+<contextlist><context>server config</context>
+ <context>virtual host</context>
+ <context>directory</context>
+ <context>.htaccess</context>
</contextlist>
<usage>
<directive>CacheMaxExpire</directive>, then the latter takes
precedence.</p>
- <example>
+ <highlight language="config">
CacheLastModifiedFactor 0.5
- </example>
+ </highlight>
</usage>
</directivesynopsis>
<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>
</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 cacheable 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 cacheable 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>
+<contextlist><context>server config</context>
+ <context>virtual host</context>
+ <context>directory</context>
+ <context>.htaccess</context>
</contextlist>
<usage>
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>
+ private header values.</p>
- <example>
+ <highlight language="config">
CacheStorePrivate On
- </example>
+ </highlight>
<note type="warning"><title>Warning:</title>
This directive will allow caching even if the upstream server has
<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><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>CacheStoreNoCache</directive> directive allows this
- behavior to be overridden. <directive>CacheStoreNoCache</directive> On
+ <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. Resources requiring authorization will
- <em>never</em> be cached.</p>
+ no-store header values.</p>
- <example>
+ <highlight language="config">
CacheStoreNoStore On
- </example>
+ </highlight>
<note type="warning"><title>Warning:</title>
As described in RFC 2616, the no-store directive is intended to
<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 type="section" module="core">Directory</directive>
+ or <directive type="section" module="core">Location</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 type="section" module="core">Directory</directive> or
+ <directive type="section" module="core">Location</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>
projects / apache / blobdiff