<?xml version="1.0"?>

<!DOCTYPE modulesynopsis SYSTEM "/style/modulesynopsis.dtd">

<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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. 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>

<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

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>

<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>

<example>

# Override the base URL of the cache key.<br />

CacheKeyBaseURL http://www.example.com/<br />

</example>

<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>

<example>

# Serve stale data on error.<br />

CacheStaleOnError on<br />

</example>

</usage>

</directivesynopsis>

</modulesynopsis>