2 <!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
3 <?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
4 <!-- $LastChangedRevision$ -->
7 Licensed to the Apache Software Foundation (ASF) under one or more
8 contributor license agreements. See the NOTICE file distributed with
9 this work for additional information regarding copyright ownership.
10 The ASF licenses this file to You under the Apache License, Version 2.0
11 (the "License"); you may not use this file except in compliance with
12 the License. You may obtain a copy of the License at
14 http://www.apache.org/licenses/LICENSE-2.0
16 Unless required by applicable law or agreed to in writing, software
17 distributed under the License is distributed on an "AS IS" BASIS,
18 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
19 See the License for the specific language governing permissions and
20 limitations under the License.
23 <modulesynopsis metafile="mod_cache.xml.meta">
25 <name>mod_cache</name>
26 <description>RFC 2616 compliant HTTP caching filter.</description>
27 <status>Extension</status>
28 <sourcefile>mod_cache.c</sourcefile>
29 <identifier>cache_module</identifier>
32 <note type="warning">This module should be used with care, as when the
33 <directive module="mod_cache">CacheQuickHandler</directive> directive is
34 in its default value of <strong>on</strong>, the <directive
35 module="mod_authz_host">Allow</directive> and <directive
36 module="mod_authz_host">Deny</directive> directives will be circumvented.
37 You should not enable quick handler caching for any content to which you
38 wish to limit access by client host name, address or environment
41 <p><module>mod_cache</module> implements an <a
42 href="http://www.ietf.org/rfc/rfc2616.txt">RFC 2616</a> compliant
43 <strong>HTTP content caching filter</strong>, with support for the caching
44 of content negotiated responses containing the Vary header.</p>
46 <p>RFC 2616 compliant caching provides a mechanism to verify whether
47 stale or expired content is still fresh, and can represent a significant
48 performance boost when the origin server supports <strong>conditional
49 requests</strong> by honouring the
50 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26">If-None-Match</a>
51 HTTP request header. Content is only regenerated from scratch when the content
52 has changed, and not when the cached entry expires.</p>
54 <p>As a filter, <module>mod_cache</module> can be placed in front of
55 content originating from any handler, including <strong>flat
56 files</strong> (served from a slow disk cached on a fast disk), the output
57 of a <strong>CGI script</strong> or <strong>dynamic content
58 generator</strong>, or content <strong>proxied from another
61 <p>In the default configuration, <module>mod_cache</module> inserts the
62 caching filter as far forward as possible within the filter stack,
63 utilising the <strong>quick handler</strong> to bypass all per request
64 processing when returning content to the client. In this mode of
65 operation, <module>mod_cache</module> may be thought of as a caching
66 proxy server bolted to the front of the webserver, while running within
67 the webserver itself.</p>
69 <p>When the quick handler is switched off using the
70 <directive module="mod_cache">CacheQuickHandler</directive> directive,
71 it becomes possible to insert the <strong>CACHE</strong> filter at a
72 point in the filter stack chosen by the administrator. This provides the
73 opportunity to cache content before that content is personalised by the
74 <module>mod_include</module> filter, or optionally compressed by the
75 <module>mod_deflate</module> filter.</p>
77 <p>Under normal operation, <module>mod_cache</module> will respond to
78 and can be controlled by the
79 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9">Cache-Control</a>
81 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32">Pragma</a>
82 headers sent from a client in a request, or from a
83 server within a response. Under exceptional circumstances,
84 <module>mod_cache</module> can be configured to override these headers
85 and force site specific behaviour, however such behaviour will be limited
86 to this cache only, and will not affect the operation of other caches
87 that may exist between the client and server, and as a result is not
88 recommended unless strictly necessary.</p>
90 <p>RFC 2616 allows for the cache to return stale data while the existing
91 stale entry is refreshed from the origin server, and this is supported
92 by <module>mod_cache</module> when the
93 <directive module="mod_cache">CacheLock</directive> directive is suitably
94 configured. Such responses will contain a
95 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
96 HTTP header with a 110 response code. RFC 2616 also allows a cache to return
97 stale data when the attempt made to refresh the stale data returns an
98 error 500 or above, and this behaviour is supported by default by
99 <module>mod_cache</module>. Such responses will contain a
100 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
101 HTTP header with a 111 response code.</p>
103 <p><module>mod_cache</module> requires the services of one or more
104 storage management modules. One storage management module is included in
105 the base Apache distribution:</p>
107 <dt><module>mod_disk_cache</module></dt>
108 <dd>Implements a disk based storage manager. Headers and bodies are
109 stored separately on disk, in a directory structure derived from the
110 md5 hash of the cached URL. Multiple content negotiated responses can
111 be stored concurrently, however the caching of partial content is not
112 supported by this module. The <program>htcacheclean</program> tool is
113 provided to list cached URLs, remove cached URLs, or to maintain the size
114 of the disk cache within size and inode limits.</dd>
117 <p>Further details, discussion, and examples, are provided in the
118 <a href="../caching.html">Caching Guide</a>.</p>
120 <seealso><a href="../caching.html">Caching Guide</a></seealso>
122 <section id="related"><title>Related Modules and Directives</title>
125 <module>mod_disk_cache</module>
128 <directive module="mod_disk_cache">CacheRoot</directive>
129 <directive module="mod_disk_cache">CacheDirLevels</directive>
130 <directive module="mod_disk_cache">CacheDirLength</directive>
131 <directive module="mod_disk_cache">CacheMinFileSize</directive>
132 <directive module="mod_disk_cache">CacheMaxFileSize</directive>
137 <section id="sampleconf"><title>Sample Configuration</title>
138 <example><title>Sample httpd.conf</title>
140 # Sample Cache Configuration<br />
142 LoadModule cache_module modules/mod_cache.so<br />
144 <IfModule mod_cache.c><br />
146 LoadModule disk_cache_module modules/mod_disk_cache.so<br />
147 <IfModule mod_disk_cache.c><br />
149 CacheRoot c:/cacheroot<br />
150 CacheEnable disk /<br />
151 CacheDirLevels 5<br />
152 CacheDirLength 3<br />
154 </IfModule> <br />
156 # When acting as a proxy, don't cache the list of security updates<br />
157 CacheDisable http://security.update.server/update-list/<br />
163 <section id="thunderingherd"><title>Avoiding the Thundering Herd</title>
164 <p>When a cached entry becomes stale, <module>mod_cache</module> will submit
165 a conditional request to the backend, which is expected to confirm whether the
166 cached entry is still fresh, and send an updated entity if not.</p>
167 <p>A small but finite amount of time exists between the time the cached entity
168 becomes stale, and the time the stale entity is fully refreshed. On a busy
169 server, a significant number of requests might arrive during this time, and
170 cause a <strong>thundering herd</strong> of requests to strike the backend
171 suddenly and unpredicably.</p>
172 <p>To keep the thundering herd at bay, the <directive>CacheLock</directive>
173 directive can be used to define a directory in which locks are created for
174 URLs <strong>in flight</strong>. The lock is used as a <strong>hint</strong>
175 by other requests to either suppress an attempt to cache (someone else has
176 gone to fetch the entity), or to indicate that a stale entry is being refreshed
177 (stale content will be returned in the mean time).
180 <title>Initial caching of an entry</title>
181 <p>When an entity is cached for the first time, a lock will be created for the
182 entity until the response has been fully cached. During the lifetime of the
183 lock, the cache will suppress the second and subsequent attempt to cache the
184 same entity. While this doesn't hold back the thundering herd, it does stop
185 the cache attempting to cache the same entity multiple times simultaneously.
189 <title>Refreshment of a stale entry</title>
190 <p>When an entity reaches its freshness lifetime and becomes stale, a lock
191 will be created for the entity until the response has either been confirmed as
192 still fresh, or replaced by the backend. During the lifetime of the lock, the
193 second and subsequent incoming request will cause stale data to be returned,
194 and the thundering herd is kept at bay.</p>
197 <title>Locks and Cache-Control: no-cache</title>
198 <p>Locks are used as a <strong>hint only</strong> to enable the cache to be
199 more gentle on backend servers, however the lock can be overridden if necessary.
200 If the client sends a request with a Cache-Control header forcing a reload, any
201 lock that may be present will be ignored, and the client's request will be
202 honored immediately and the cached entry refreshed.</p>
203 <p>As a further safety mechanism, locks have a configurable maximum age.
204 Once this age has been reached, the lock is removed, and a new request is
205 given the opportunity to create a new lock. This maximum age can be set using
206 the <directive>CacheLockMaxAge</directive> directive, and defaults to 5
211 <title>Example configuration</title>
212 <example><title>Enabling the cache lock</title>
214 # Enable the cache lock<br />
216 <IfModule mod_cache.c><br />
219 CacheLockPath /tmp/mod_cache-lock<br />
220 CacheLockMaxAge 5<br />
227 <section id="finecontrol"><title>Fine Control with the CACHE Filter</title>
228 <p>Under the default mode of cache operation, the cache runs as a quick handler,
229 short circuiting the majority of server processing and offering the highest
230 cache performance available.</p>
232 <p>In this mode, the cache <strong>bolts onto</strong> the front of the server,
233 acting as if a free standing RFC 2616 caching proxy had been placed in front of
236 <p>While this mode offers the best performance, the administrator may find that
237 under certain circumstances they may want to perform further processing on the
238 request after the request is cached, such as to inject personalisation into the
239 cached page, or to apply authorisation restrictions to the content. Under these
240 circumstances, an administrator is often forced to place independent reverse
241 proxy servers either behind or in front of the caching server to achieve this.</p>
243 <p>To solve this problem the <directive module="mod_cache">CacheQuickHandler
244 </directive> directive can be set to <strong>off</strong>, and the server will
245 process all phases normally handled by a non-cached request, including the
246 <strong>authentication and authorisation</strong> phases.</p>
248 <p>In addition, the administrator may optionally specify the <strong>precise point
249 within the filter chain</strong> where caching is to take place by adding the
250 <strong>CACHE</strong> filter to the output filter chain.</p>
252 <p>For example, to cache content before applying compression to the response,
253 place the <strong>CACHE</strong> filter before the <strong>DEFLATE</strong>
254 filter as in the example below:</p>
257 # Cache content before optional compression<br />
258 CacheQuickHandler off<br />
259 AddOutputFilterByType CACHE;DEFLATE text/plain<br /><br />
262 <p>Another option is to have content cached before personalisation is applied
263 by <module>mod_include</module> (or another content processing filter). In this
264 example templates containing tags understood by
265 <module>mod_include</module> are cached before being parsed:</p>
268 # Cache content before mod_include and mod_deflate<br />
269 CacheQuickHandler off<br />
270 AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
273 <p>You may place the <strong>CACHE</strong> filter anywhere you wish within the
274 filter chain. In this example, content is cached after being parsed by
275 <module>mod_include</module>, but before being processed by
276 <module>mod_deflate</module>:</p>
279 # Cache content between mod_include and mod_deflate<br />
280 CacheQuickHandler off<br />
281 AddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html<br /><br />
284 <note type="warning"><title>Warning:</title>If the location of the
285 <strong>CACHE</strong> filter in the filter chain is changed for any reason,
286 you may need to <strong>flush your cache</strong> to ensure that your data
287 served remains consistent. <module>mod_cache</module> is not in a position
288 to enforce this for you.</note>
292 <section id="status"><title>Cache Status and Logging</title>
293 <p>Once <module>mod_cache</module> has made a decision as to whether or not
294 an entity is to be served from cache, the detailed reason for the decision
295 is written to the subprocess environment within the request under the
296 <strong>cache-status</strong> key. This reason can be logged by the
297 <directive module="mod_log_config">LogFormat</directive> directive as
301 LogFormat "%{cache-status}e ..."
304 <p>Based on the caching decision made, the reason is also written to the
305 subprocess environment under one the following three keys, as appropriate:</p>
308 <dt>cache-hit</dt><dd>The response was served from cache.</dd>
309 <dt>cache-revalidate</dt><dd>The response was stale and was successfully
310 revalidated, then served from cache.</dd>
311 <dt>cache-miss</dt><dd>The response was served from the upstream server.</dd>
314 <p>This makes it possible to support conditional logging of cached requests
315 as per the following example:</p>
318 CustomLog cached-requests.log common env=cache-hit<br />
319 CustomLog uncached-requests.log common env=cache-miss<br />
320 CustomLog revalidated-requests.log common env=cache-revalidate<br />
326 <name>CacheEnable</name>
327 <description>Enable caching of specified URLs using a specified storage
328 manager</description>
329 <syntax>CacheEnable <var>cache_type</var> <var>url-string</var></syntax>
330 <contextlist><context>server config</context><context>virtual host</context>
334 <p>The <directive>CacheEnable</directive> directive instructs
335 <module>mod_cache</module> to cache urls at or below
336 <var>url-string</var>. The cache storage manager is specified with the
337 <var>cache_type</var> argument. If the <directive>CacheEnable</directive>
338 directive is placed inside a <directive type="section">Location</directive>
339 directive, the <var>url-string</var> becomes optional.
340 <var>cache_type</var> <code>disk</code> instructs
341 <module>mod_cache</module> to use the disk based storage manager
342 implemented by <module>mod_disk_cache</module>.</p>
343 <p>In the event that the URL space overlaps between different
344 <directive>CacheEnable</directive> directives (as in the example below),
345 each possible storage manager will be run until the first one that
346 actually processes the request. The order in which the storage managers are
347 run is determined by the order of the <directive>CacheEnable</directive>
348 directives in the configuration file.</p>
350 <p>When acting as a forward proxy server, <var>url-string</var> can
351 also be used to specify remote sites and proxy protocols which
352 caching should be enabled for.</p>
355 # Cache proxied url's<br />
356 CacheEnable disk /<br /><br />
357 # Cache FTP-proxied url's<br />
358 CacheEnable disk ftp://<br /><br />
359 # Cache content from www.apache.org<br />
360 CacheEnable disk http://www.apache.org/<br />
363 <p>A hostname starting with a <strong>"*"</strong> matches all hostnames with
364 that suffix. A hostname starting with <strong>"."</strong> matches all
365 hostnames containing the domain components that follow.</p>
368 # Match www.apache.org, and fooapache.org<br />
369 CacheEnable disk http://*apache.org/<br />
370 # Match www.apache.org, but not fooapache.org<br />
371 CacheEnable disk http://.apache.org/<br />
374 <p> The <code>no-cache</code> environment variable can be set to
375 disable caching on a finer grained set of resources in versions
376 2.2.12 and later.</p>
379 <seealso><a href="../env.html">Environment Variables in Apache</a></seealso>
383 <name>CacheDisable</name>
384 <description>Disable caching of specified URLs</description>
385 <syntax>CacheDisable <var>url-string</var> | <var>on</var></syntax>
386 <contextlist><context>server config</context><context>virtual host</context>
390 <p>The <directive>CacheDisable</directive> directive instructs
391 <module>mod_cache</module> to <em>not</em> cache urls at or below
392 <var>url-string</var>.</p>
394 <example><title>Example</title>
395 CacheDisable /local_files
398 <p>If used in a <directive type="section">Location</directive> directive,
399 the path needs to be specified below the Location, or if the word "on"
400 is used, caching for the whole location will be disabled.</p>
402 <example><title>Example</title>
403 <Location /foo><br />
404 CacheDisable on<br />
405 </Location><br />
408 <p> The <code>no-cache</code> environment variable can be set to
409 disable caching on a finer grained set of resources in versions
410 2.2.12 and later.</p>
413 <seealso><a href="../env.html">Environment Variables in Apache</a></seealso>
416 <name>CacheMaxExpire</name>
417 <description>The maximum time in seconds to cache a document</description>
418 <syntax>CacheMaxExpire <var>seconds</var></syntax>
419 <default>CacheMaxExpire 86400 (one day)</default>
420 <contextlist><context>server config</context>
421 <context>virtual host</context>
422 <context>directory</context>
423 <context>.htaccess</context>
427 <p>The <directive>CacheMaxExpire</directive> directive specifies the maximum number of
428 seconds for which cachable HTTP documents will be retained without checking the origin
429 server. Thus, documents will be out of date at most this number of seconds. This maximum
430 value is enforced even if an expiry date was supplied with the document.</p>
433 CacheMaxExpire 604800
439 <name>CacheMinExpire</name>
440 <description>The minimum time in seconds to cache a document</description>
441 <syntax>CacheMinExpire <var>seconds</var></syntax>
442 <default>CacheMinExpire 0</default>
443 <contextlist><context>server config</context>
444 <context>virtual host</context>
445 <context>directory</context>
446 <context>.htaccess</context>
450 <p>The <directive>CacheMinExpire</directive> directive specifies the minimum number of
451 seconds for which cachable HTTP documents will be retained without checking the origin
452 server. This is only used if no valid expire time was supplied with the document.</p>
462 <name>CacheDefaultExpire</name>
463 <description>The default duration to cache a document when no expiry date is specified.</description>
464 <syntax>CacheDefaultExpire <var>seconds</var></syntax>
465 <default>CacheDefaultExpire 3600 (one hour)</default>
466 <contextlist><context>server config</context>
467 <context>virtual host</context>
468 <context>directory</context>
469 <context>.htaccess</context>
473 <p>The <directive>CacheDefaultExpire</directive> directive specifies a default time,
474 in seconds, to cache a document if neither an expiry date nor last-modified date are provided
475 with the document. The value specified with the <directive>CacheMaxExpire</directive>
476 directive does <em>not</em> override this setting.</p>
479 CacheDefaultExpire 86400
485 <name>CacheIgnoreNoLastMod</name>
486 <description>Ignore the fact that a response has no Last Modified
487 header.</description>
488 <syntax>CacheIgnoreNoLastMod On|Off</syntax>
489 <default>CacheIgnoreNoLastMod Off</default>
490 <contextlist><context>server config</context>
491 <context>virtual host</context>
492 <context>directory</context>
493 <context>.htaccess</context>
497 <p>Ordinarily, documents without a last-modified date are not cached.
498 Under some circumstances the last-modified date is removed (during
499 <module>mod_include</module> processing for example) or not provided
500 at all. The <directive>CacheIgnoreNoLastMod</directive> directive
501 provides a way to specify that documents without last-modified dates
502 should be considered for caching, even without a last-modified date.
503 If neither a last-modified date nor an expiry date are provided with
504 the document then the value specified by the
505 <directive>CacheDefaultExpire</directive> directive will be used to
506 generate an expiration date.</p>
509 CacheIgnoreNoLastMod On
515 <name>CacheIgnoreCacheControl</name>
516 <description>Ignore request to not serve cached content to client</description>
517 <syntax>CacheIgnoreCacheControl On|Off</syntax>
518 <default>CacheIgnoreCacheControl Off</default>
519 <contextlist><context>server config</context><context>virtual host</context>
523 <p>Ordinarily, requests containing a Cache-Control: no-cache or
524 Pragma: no-cache header value will not be served from the cache. The
525 <directive>CacheIgnoreCacheControl</directive> directive allows this
526 behavior to be overridden. <directive>CacheIgnoreCacheControl On</directive>
527 tells the server to attempt to serve the resource from the cache even
528 if the request contains no-cache header values. Resources requiring
529 authorization will <em>never</em> be cached.</p>
532 CacheIgnoreCacheControl On
535 <note type="warning"><title>Warning:</title>
536 This directive will allow serving from the cache even if the client has
537 requested that the document not be served from the cache. This might
538 result in stale content being served.
541 <seealso><directive module="mod_cache">CacheStorePrivate</directive></seealso>
542 <seealso><directive module="mod_cache">CacheStoreNoStore</directive></seealso>
546 <name>CacheIgnoreQueryString</name>
547 <description>Ignore query string when caching</description>
548 <syntax>CacheIgnoreQueryString On|Off</syntax>
549 <default>CacheIgnoreQueryString Off</default>
550 <contextlist><context>server config</context><context>virtual host</context>
554 <p>Ordinarily, requests with query string parameters are cached separately
555 for each unique query string. This is according to RFC 2616/13.9 done only
556 if an expiration time is specified. The
557 <directive>CacheIgnoreQueryString</directive> directive tells the cache to
558 cache requests even if no expiration time is specified, and to reply with
559 a cached reply even if the query string differs. From a caching point of
560 view the request is treated as if having no query string when this
561 directive is enabled.</p>
564 CacheIgnoreQueryString On
571 <name>CacheLastModifiedFactor</name>
572 <description>The factor used to compute an expiry date based on the
573 LastModified date.</description>
574 <syntax>CacheLastModifiedFactor <var>float</var></syntax>
575 <default>CacheLastModifiedFactor 0.1</default>
576 <contextlist><context>server config</context>
577 <context>virtual host</context>
578 <context>directory</context>
579 <context>.htaccess</context>
583 <p>In the event that a document does not provide an expiry date but does
584 provide a last-modified date, an expiry date can be calculated based on
585 the time since the document was last modified. The
586 <directive>CacheLastModifiedFactor</directive> directive specifies a
587 <var>factor</var> to be used in the generation of this expiry date
588 according to the following formula:
590 <code>expiry-period = time-since-last-modified-date * <var>factor</var>
591 expiry-date = current-date + expiry-period</code>
593 For example, if the document was last modified 10 hours ago, and
594 <var>factor</var> is 0.1 then the expiry-period will be set to
595 10*0.1 = 1 hour. If the current time was 3:00pm then the computed
596 expiry-date would be 3:00pm + 1hour = 4:00pm.
598 If the expiry-period would be longer than that set by
599 <directive>CacheMaxExpire</directive>, then the latter takes
603 CacheLastModifiedFactor 0.5
609 <name>CacheIgnoreHeaders</name>
610 <description>Do not store the given HTTP header(s) in the cache.
612 <syntax>CacheIgnoreHeaders <var>header-string</var> [<var>header-string</var>] ...</syntax>
613 <default>CacheIgnoreHeaders None</default>
614 <contextlist><context>server config</context><context>virtual host</context>
618 <p>According to RFC 2616, hop-by-hop HTTP headers are not stored in
619 the cache. The following HTTP headers are hop-by-hop headers and thus
620 do not get stored in the cache in <em>any</em> case regardless of the
621 setting of <directive>CacheIgnoreHeaders</directive>:</p>
624 <li><code>Connection</code></li>
625 <li><code>Keep-Alive</code></li>
626 <li><code>Proxy-Authenticate</code></li>
627 <li><code>Proxy-Authorization</code></li>
628 <li><code>TE</code></li>
629 <li><code>Trailers</code></li>
630 <li><code>Transfer-Encoding</code></li>
631 <li><code>Upgrade</code></li>
634 <p><directive>CacheIgnoreHeaders</directive> specifies additional HTTP
635 headers that should not to be stored in the cache. For example, it makes
636 sense in some cases to prevent cookies from being stored in the cache.</p>
638 <p><directive>CacheIgnoreHeaders</directive> takes a space separated list
639 of HTTP headers that should not be stored in the cache. If only hop-by-hop
640 headers not should be stored in the cache (the RFC 2616 compliant
641 behaviour), <directive>CacheIgnoreHeaders</directive> can be set to
642 <code>None</code>.</p>
644 <example><title>Example 1</title>
645 CacheIgnoreHeaders Set-Cookie
648 <example><title>Example 2</title>
649 CacheIgnoreHeaders None
652 <note type="warning"><title>Warning:</title>
653 If headers like <code>Expires</code> which are needed for proper cache
654 management are not stored due to a
655 <directive>CacheIgnoreHeaders</directive> setting, the behaviour of
656 mod_cache is undefined.
662 <name>CacheIgnoreURLSessionIdentifiers</name>
663 <description>Ignore defined session identifiers encoded in the URL when caching
665 <syntax>CacheIgnoreURLSessionIdentifiers <var>identifier</var> [<var>identifier</var>] ...</syntax>
666 <default>CacheIgnoreURLSessionIdentifiers None</default>
667 <contextlist><context>server config</context><context>virtual host</context>
671 <p>Sometimes applications encode the session identifier into the URL like in the following
675 <li><code>/someapplication/image.gif;jsessionid=123456789</code></li>
676 <li><code>/someapplication/image.gif?PHPSESSIONID=12345678</code></li>
678 <p>This causes cachable resources to be stored separately for each session, which
679 is often not desired. <directive>CacheIgnoreURLSessionIdentifiers</directive> lets
680 define a list of identifiers that are removed from the key that is used to identify
681 an entity in the cache, such that cachable resources are not stored separately for
684 <p><code>CacheIgnoreURLSessionIdentifiers None</code> clears the list of ignored
685 identifiers. Otherwise, each identifier is added to the list.</p>
687 <example><title>Example 1</title>
688 CacheIgnoreURLSessionIdentifiers jsessionid
691 <example><title>Example 2</title>
692 CacheIgnoreURLSessionIdentifiers None
699 <name>CacheStoreExpired</name>
700 <description>Attempt to cache responses that the server reports as expired</description>
701 <syntax>CacheStoreExpired On|Off</syntax>
702 <default>CacheStoreExpired Off</default>
703 <contextlist><context>server config</context>
704 <context>virtual host</context>
705 <context>directory</context>
706 <context>.htaccess</context>
710 <p>Since httpd 2.2.4, responses which have already expired are not
711 stored in the cache. The <directive>CacheStoreExpired</directive>
712 directive allows this behavior to be overridden.
713 <directive>CacheStoreExpired</directive> On
714 tells the server to attempt to cache the resource if it is stale.
715 Subsequent requests would trigger an If-Modified-Since request of
716 the origin server, and the response may be fulfilled from cache
717 if the backend resource has not changed.</p>
726 <name>CacheStorePrivate</name>
727 <description>Attempt to cache responses that the server has marked as private</description>
728 <syntax>CacheStorePrivate On|Off</syntax>
729 <default>CacheStorePrivate Off</default>
730 <contextlist><context>server config</context>
731 <context>virtual host</context>
732 <context>directory</context>
733 <context>.htaccess</context>
737 <p>Ordinarily, responses with Cache-Control: private header values will not
738 be stored in the cache. The <directive>CacheStorePrivate</directive>
739 directive allows this behavior to be overridden.
740 <directive>CacheStorePrivate</directive> On
741 tells the server to attempt to cache the resource even if it contains
742 private header values. Resources requiring authorization will
743 <em>never</em> be cached.</p>
749 <note type="warning"><title>Warning:</title>
750 This directive will allow caching even if the upstream server has
751 requested that the resource not be cached. This directive is only
752 ideal for a 'private' cache.
755 <seealso><directive module="mod_cache">CacheIgnoreCacheControl</directive></seealso>
756 <seealso><directive module="mod_cache">CacheStoreNoStore</directive></seealso>
760 <name>CacheStoreNoStore</name>
761 <description>Attempt to cache requests or responses that have been marked as no-store.</description>
762 <syntax>CacheStoreNoStore On|Off</syntax>
763 <default>CacheStoreNoStore Off</default>
764 <contextlist><context>server config</context>
765 <context>virtual host</context>
766 <context>directory</context>
767 <context>.htaccess</context>
771 <p>Ordinarily, requests or responses with Cache-Control: no-store header
772 values will not be stored in the cache. The
773 <directive>CacheStoreNoCache</directive> directive allows this
774 behavior to be overridden. <directive>CacheStoreNoCache</directive> On
775 tells the server to attempt to cache the resource even if it contains
776 no-store header values. Resources requiring authorization will
777 <em>never</em> be cached.</p>
783 <note type="warning"><title>Warning:</title>
784 As described in RFC 2616, the no-store directive is intended to
785 "prevent the inadvertent release or retention of sensitive information
786 (for example, on backup tapes)." Enabling this option could store
787 sensitive information in the cache. You are hereby warned.
790 <seealso><directive module="mod_cache">CacheIgnoreCacheControl</directive></seealso>
791 <seealso><directive module="mod_cache">CacheStorePrivate</directive></seealso>
795 <name>CacheLock</name>
796 <description>Enable the thundering herd lock.</description>
797 <syntax>CacheLock <var>on|off</var></syntax>
798 <default>CacheLock off</default>
799 <contextlist><context>server config</context><context>virtual host</context>
801 <compatibility>Available in Apache 2.2.15 and later</compatibility>
804 <p>The <directive>CacheLock</directive> directive enables the thundering herd lock
805 for the given URL space.</p>
807 <p>In a minimal configuration the following directive is all that is needed to
808 enable the thundering herd lock in the default system temp directory.</p>
811 # Enable cache lock<br />
812 CacheLock on<br /><br />
819 <name>CacheLockPath</name>
820 <description>Set the lock path directory.</description>
821 <syntax>CacheLockPath <var>directory</var></syntax>
822 <default>CacheLockPath /tmp/mod_cache-lock</default>
823 <contextlist><context>server config</context><context>virtual host</context>
827 <p>The <directive>CacheLockPath</directive> directive allows you to specify the
828 directory in which the locks are created. By default, the system's temporary
829 folder is used. Locks consist of empty files that only exist for stale URLs
830 in flight, so is significantly less resource intensive than the traditional
837 <name>CacheLockMaxAge</name>
838 <description>Set the maximum possible age of a cache lock.</description>
839 <syntax>CacheLockMaxAge <var>integer</var></syntax>
840 <default>CacheLockMaxAge 5</default>
841 <contextlist><context>server config</context><context>virtual host</context>
845 <p>The <directive>CacheLockMaxAge</directive> directive specifies the maximum
846 age of any cache lock.</p>
848 <p>A lock older than this value in seconds will be ignored, and the next
849 incoming request will be given the opportunity to re-establish the lock.
850 This mechanism prevents a slow client taking an excessively long time to refresh
857 <name>CacheQuickHandler</name>
858 <description>Run the cache from the quick handler.</description>
859 <syntax>CacheQuickHandler <var>on|off</var></syntax>
860 <default>CacheQuickHandler on</default>
861 <contextlist><context>server config</context><context>virtual host</context>
865 <p>The <directive module="mod_cache">CacheQuickHandler</directive> directive
866 controls the phase in which the cache is handled.</p>
868 <p>In the default enabled configuration, the cache operates within the quick
869 handler phase. This phase short circuits the majority of server processing,
870 and represents the most performant mode of operation for a typical server.
871 The cache <strong>bolts onto</strong> the front of the server, and the
872 majority of server processing is avoided.</p>
874 <p>When disabled, the cache operates as a normal handler, and is subject to
875 the full set of phases when handling a server request. While this mode is
876 slower than the default, it allows the cache to be used in cases where full
877 processing is required, such as when content is subject to authorisation.</p>
880 # Run cache as a normal handler<br />
881 CacheQuickHandler off<br /><br />
884 <p>It is also possible, when the quick handler is disabled, for the
885 administrator to choose the precise location within the filter chain where
886 caching is to be performed, by adding the <strong>CACHE</strong> filter to
890 # Cache content before mod_include and mod_deflate<br />
891 CacheQuickHandler off<br />
892 AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
895 <p>If the CACHE filter is specified more than once, the last instance will
902 <name>CacheHeader</name>
903 <description>Add an X-Cache header to the response.</description>
904 <syntax>CacheHeader <var>on|off</var></syntax>
905 <default>CacheHeader off</default>
906 <contextlist><context>server config</context>
907 <context>virtual host</context>
908 <context>directory</context>
909 <context>.htaccess</context>
911 <compatibility>Available in Apache 2.3.9 and later</compatibility>
914 <p>When the <directive module="mod_cache">CacheHeader</directive> directive
915 is switched on, an <strong>X-Cache</strong> header will be added to the response
916 with the cache status of this response. If the normal handler is used, this
917 directive may appear within a <directive module="core"><Directory></directive>
918 or <directive module="core"><Location></directive> directive. If the quick
919 handler is used, this directive must appear within a server or virtual host
920 context, otherwise the setting will be ignored.</p>
923 <dt><strong>HIT</strong></dt><dd>The entity was fresh, and was served from
925 <dt><strong>REVALIDATE</strong></dt><dd>The entity was stale, was successfully
926 revalidated and was served from cache.</dd>
927 <dt><strong>MISS</strong></dt><dd>The entity was fetched from the upstream
928 server and was not served from cache.</dd>
932 # Enable the X-Cache header<br />
937 X-Cache: HIT from localhost<br />
944 <name>CacheDetailHeader</name>
945 <description>Add an X-Cache-Detail header to the response.</description>
946 <syntax>CacheDetailHeader <var>on|off</var></syntax>
947 <default>CacheDetailHeader off</default>
948 <contextlist><context>server config</context>
949 <context>virtual host</context>
950 <context>directory</context>
951 <context>.htaccess</context>
953 <compatibility>Available in Apache 2.3.9 and later</compatibility>
956 <p>When the <directive module="mod_cache">CacheDetailHeader</directive> directive
957 is switched on, an <strong>X-Cache-Detail</strong> header will be added to the response
958 containing the detailed reason for a particular caching decision.</p>
960 <p>It can be useful during development of cached RESTful services to have additional
961 information about the caching decision written to the response headers, so as to
962 confirm whether <code>Cache-Control</code> and other headers have been correctly
963 used by the service and client.</p>
965 <p>If the normal handler is used, this directive may appear within a
966 <directive module="core"><Directory></directive> or
967 <directive module="core"><Location></directive> directive. If the quick handler
968 is used, this directive must appear within a server or virtual host context, otherwise
969 the setting will be ignored.</p>
972 # Enable the X-Cache-Detail header<br />
973 CacheDetailHeader on<br />
977 X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost<br />
984 <name>CacheKeyBaseURL</name>
985 <description>Override the base URL of reverse proxied cache keys.</description>
986 <syntax>CacheKeyBaseURL <var>URL</var></syntax>
987 <default>CacheKeyBaseURL http://example.com</default>
988 <contextlist><context>server config</context>
989 <context>virtual host</context>
991 <compatibility>Available in Apache 2.3.9 and later</compatibility>
994 <p>When the <directive module="mod_cache">CacheKeyBaseURL</directive> directive
995 is specified, the URL provided will be used as the base URL to calculate
996 the URL of the cache keys in the reverse proxy configuration. When not specified,
997 the scheme, hostname and port of the current virtual host is used to construct
998 the cache key. When a cluster of machines is present, and all cached entries
999 should be cached beneath the same cache key, a new base URL can be specified
1000 with this directive.</p>
1003 # Override the base URL of the cache key.<br />
1004 CacheKeyBaseURL http://www.example.com/<br />
1007 <note type="warning">Take care when setting this directive. If two separate virtual
1008 hosts are accidentally given the same base URL, entries from one virtual host
1009 will be served to the other.</note>
1012 </directivesynopsis>
1015 <name>CacheStaleOnError</name>
1016 <description>Serve stale content in place of 5xx responses.</description>
1017 <syntax>CacheStaleOnError <var>on|off</var></syntax>
1018 <default>CacheStaleOnError on</default>
1019 <contextlist><context>server config</context>
1020 <context>virtual host</context>
1021 <context>directory</context>
1022 <context>.htaccess</context>
1024 <compatibility>Available in Apache 2.3.9 and later</compatibility>
1027 <p>When the <directive module="mod_cache">CacheStaleOnError</directive> directive
1028 is switched on, and when stale data is available in the cache, the cache will
1029 respond to 5xx responses from the backend by returning the stale data instead of
1030 the 5xx response. While the Cache-Control headers sent by clients will be respected,
1031 and the raw 5xx responses returned to the client on request, the 5xx response so
1032 returned to the client will not invalidate the content in the cache.</p>
1035 # Serve stale data on error.<br />
1036 CacheStaleOnError on<br />
1040 </directivesynopsis>