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20 <p class="apache">Apache HTTP Server Version 2.5</p>
21 <img alt="" src="./images/feather.gif" /></div>
22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
24 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>Caching Guide</h1>
26 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
27 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
28 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
31 <p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>,
32 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> and <a href="programs/htcacheclean.html">htcacheclean</a> reference documentation.
33 It describes how to use the Apache HTTP Server's caching features to accelerate web and
34 proxy serving, while avoiding common problems and misconfigurations.</p>
36 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
37 <li><img alt="" src="./images/down.gif" /> <a href="#http-caching">Three-state RFC2616 HTTP caching</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#socache-caching">Two-state Key/Value Shared Object Caching</a></li>
39 <li><img alt="" src="./images/down.gif" /> <a href="#file-caching">Specialized File Caching</a></li>
40 <li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
41 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
42 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
44 <h2><a name="introduction" id="introduction">Introduction</a></h2>
47 <p>The Apache HTTP server offers a range of caching features that
48 are designed to improve the performance of the server in various
52 <dt>Three-state RFC2616 HTTP caching</dt>
54 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
55 and its provider modules
56 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>
57 provide intelligent, HTTP-aware caching. The content itself is stored
58 in the cache, and mod_cache aims to honor all of the various HTTP
59 headers and options that control the cacheability of content
61 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section
63 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
64 is aimed at both simple and complex caching configurations, where
65 you are dealing with proxied content, dynamic local content or
66 have a need to speed up access to local files on a potentially
70 <dt>Two-state key/value shared object caching</dt>
72 The <a href="socache.html">shared object cache API</a> (socache)
73 and its provider modules provide a
74 server wide key/value based shared object cache. These modules
75 are designed to cache low level data such as SSL sessions and
76 authentication credentials. Backends allow the data to be stored
77 server wide in shared memory, or datacenter wide in a cache such
78 as memcache or distcache.
81 <dt>Specialized file caching</dt>
83 <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>
84 offers the ability to pre-load
85 files into memory on server startup, and can improve access
86 times and save file handles on files that are accessed often,
87 as there is no need to go to disk on each request.
91 <p>To get the most from this document, you should be familiar with
92 the basics of HTTP, and have read the Users' Guides to
93 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
94 <a href="content-negotiation.html">Content negotiation</a>.</p>
96 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
98 <h2><a name="http-caching" id="http-caching">Three-state RFC2616 HTTP caching</a></h2>
102 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code></li><li><code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code></li><li><code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code></li><li><code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code></li><li><code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code></li></ul></td></tr></table>
104 <p>The HTTP protocol contains built in support for an in-line caching
106 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
107 described by section 13 of RFC2616</a>, and the
108 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module can be used to take advantage of
111 <p>Unlike a simple two state key/value cache where the content
112 disappears completely when no longer fresh, an HTTP cache includes
113 a mechanism to retain stale content, and to ask the origin server
114 whether this stale content has changed and if not, make it fresh
117 <p>An entry in an HTTP cache exists in one of three states:</p>
122 If the content is new enough (younger than its <strong>freshness
123 lifetime</strong>), it is considered <strong>fresh</strong>. An
124 HTTP cache is free to serve fresh content without making any
125 calls to the origin server at all.
129 <p>If the content is too old (older than its <strong>freshness
130 lifetime</strong>), it is considered <strong>stale</strong>. An
131 HTTP cache should contact the origin server and check whether
132 the content is still fresh before serving stale content to a
133 client. The origin server will either respond with replacement
134 content if not still valid, or ideally, the origin server will
135 respond with a code to tell the cache the content is still
136 fresh, without the need to generate or send the content again.
137 The content becomes fresh again and the cycle continues.</p>
139 <p>The HTTP protocol does allow the cache to serve stale data
140 under certain circumstances, such as when an attempt to freshen
141 the data with an origin server has failed with a 5xx error, or
142 when another request is already in the process of freshening
143 the given entry. In these cases a <code>Warning</code> header
144 is added to the response.</p>
146 <dt>Non Existent</dt>
148 If the cache gets full, it reserves the option to delete content
149 from the cache to make space. Content can be deleted at any time,
150 and can be stale or fresh. The <a href="programs/htcacheclean.html">htcacheclean</a> tool can be
151 run on a once off basis, or deployed as a daemon to keep the size
152 of the cache within the given size, or the given number of inodes.
153 The tool attempts to delete stale content before attempting to
154 delete fresh content.
158 <p>Full details of how HTTP caching works can be found in
159 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
160 Section 13 of RFC2616</a>.</p>
162 <h3>Interaction with the Server</h3>
165 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module hooks into the server in two
166 possible places depending on the value of the
167 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> directive:
171 <dt>Quick handler phase</dt>
173 <p>This phase happens very early on during the request processing,
174 just after the request has been parsed. If the content is
175 found within the cache, it is served immediately and almost
176 all request processing is bypassed.</p>
178 <p>In this scenario, the cache behaves as if it has been "bolted
179 on" to the front of the server.</p>
181 <p>This mode offers the best performance, as the majority of
182 server processing is bypassed. This mode however also bypasses the
183 authentication and authorization phases of server processing, so
184 this mode should be chosen with care when this is important.</p>
186 <p> Requests with an "Authorization" header (for example, HTTP Basic
187 Authentication) are neither cacheable nor served from the cache
188 when <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> is running in this phase.</p>
190 <dt>Normal handler phase</dt>
192 <p>This phase happens late in the request processing, after all
193 the request phases have completed.</p>
195 <p>In this scenario, the cache behaves as if it has been "bolted
196 on" to the back of the server.</p>
198 <p>This mode offers the most flexibility, as the potential exists
199 for caching to occur at a precisely controlled point in the filter
200 chain, and cached content can be filtered or personalized before
201 being sent to the client.</p>
205 <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
206 will add a <a href="filter.html">filter</a> to the filter stack in order
207 to record the response to the cache, and then stand down, allowing normal
208 request processing to continue. If the content is determined to be
209 cacheable, the content will be saved to the cache for future serving,
210 otherwise the content will be ignored.</p>
212 <p>If the content found within the cache is stale, the
213 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module converts the request into a
214 <strong>conditional request</strong>. If the origin server responds with
215 a normal response, the normal response is cached, replacing the content
216 already cached. If the origin server responds with a 304 Not Modified
217 response, the content is marked as fresh again, and the cached content
218 is served by the filter instead of saving it.</p>
221 <h3>Improving Cache Hits</h3>
224 <p>When a virtual host is known by one of many different server aliases,
225 ensuring that <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is
226 set to <code>On</code> can dramatically improve the ratio of cache hits.
227 This is because the hostname of the virtual-host serving the content is
228 used within the cache key. With the setting set to <code>On</code>
229 virtual-hosts with multiple server names or aliases will not produce
230 differently cached entities, and instead content will be cached as
231 per the canonical hostname.</p>
235 <h3>Freshness Lifetime</h3>
238 <p>Well formed content that is intended to be cached should declare an
239 explicit freshness lifetime with the <code>Cache-Control</code>
240 header's <code>max-age</code> or <code>s-maxage</code> fields, or
241 by including an <code>Expires</code> header.</p>
243 <p>At the same time, the origin server defined freshness lifetime can
244 be overridden by a client when the client presents their own
245 <code>Cache-Control</code> header within the request. In this case,
246 the lowest freshness lifetime between request and response wins.</p>
248 <p>When this freshness lifetime is missing from the request or the
249 response, a default freshness lifetime is applied. The default
250 freshness lifetime for cached entities is one hour, however
251 this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive.</p>
253 <p>If a response does not include an <code>Expires</code> header but does
254 include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
255 can infer a freshness lifetime based on a heuristic, which can be
256 controlled through the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p>
258 <p>For local content, or for remote content that does not define its own
259 <code>Expires</code> header, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
260 fine-tune the freshness lifetime by adding <code>max-age</code> and
261 <code>Expires</code>.</p>
263 <p>The maximum freshness lifetime may also be controlled by using the
264 <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
268 <h3>A Brief Guide to Conditional Requests</h3>
271 <p>When content expires from the cache and becomes stale, rather than
272 pass on the original request, httpd will modify the request to make
273 it conditional instead.</p>
275 <p>When an <code>ETag</code> header exists in the original cached
276 response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
277 <code>If-None-Match</code> header to the request to the origin server.
278 When a <code>Last-Modified</code> header exists in the original
279 cached response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
280 <code>If-Modified-Since</code> header to the request to the origin
281 server. Performing either of these actions makes the request
282 <strong>conditional</strong>.</p>
284 <p>When a conditional request is received by an origin server, the
285 origin server should check whether the ETag or the Last-Modified
286 parameter has changed, as appropriate for the request. If not, the
287 origin should respond with a terse "304 Not Modified" response. This
288 signals to the cache that the stale content is still fresh should be
289 used for subsequent requests until the content's new freshness lifetime
290 is reached again.</p>
292 <p>If the content has changed, then the content is served as if the
293 request were not conditional to begin with.</p>
295 <p>Conditional requests offer two benefits. Firstly, when making such
296 a request to the origin server, if the content from the origin
297 matches the content in the cache, this can be determined easily and
298 without the overhead of transferring the entire resource.</p>
300 <p>Secondly, a well designed origin server will be designed in such
301 a way that conditional requests will be significantly cheaper to
302 produce than a full response. For static files, typically all that is
303 involved is a call to <code>stat()</code> or similar system call, to
304 see if the file has changed in size or modification time. As such, even
305 local content may still be served faster from the cache if it has not
308 <p>Origin servers should make every effort to support conditional
309 requests as is practical, however if conditional requests are not
310 supported, the origin will respond as if the request was not
311 conditional, and the cache will respond as if the content had changed
312 and save the new content to the cache. In this case, the cache will
313 behave like a simple two state cache, where content is effectively
314 either fresh or deleted.</p>
317 <h3>What Can be Cached?</h3>
320 <p>The full definition of which responses can be cached by an HTTP
322 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4">
323 RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as
327 <li>Caching must be enabled for this URL. See the <code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code> and <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code> directives.</li>
329 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
332 <li>The request must be a HTTP GET request.</li>
334 <li>If the response contains an "Authorization:" header, it must
335 also contain an "s-maxage", "must-revalidate" or "public" option
336 in the "Cache-Control:" header, or it won't be cached.</li>
338 <li>If the URL included a query string (e.g. from a HTML form GET
339 method) it will not be cached unless the response specifies an
340 explicit expiration by including an "Expires:" header or the max-age
341 or s-maxage directive of the "Cache-Control:" header, as per RFC2616
342 sections 13.9 and 13.2.1.</li>
344 <li>If the response has a status of 200 (OK), the response must
345 also include at least one of the "Etag", "Last-Modified" or
346 the "Expires" headers, or the max-age or s-maxage directive of
347 the "Cache-Control:" header, unless the
348 <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
349 directive has been used to require otherwise.</li>
351 <li>If the response includes the "private" option in a "Cache-Control:"
352 header, it will not be stored unless the
353 <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
354 used to require otherwise.</li>
356 <li>Likewise, if the response includes the "no-store" option in a
357 "Cache-Control:" header, it will not be stored unless the
358 <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
361 <li>A response will not be stored if it includes a "Vary:" header
362 containing the match-all "*".</li>
366 <h3>What Should Not be Cached?</h3>
369 <p>It should be up to the client creating the request, or the origin
370 server constructing the response to decide whether or not the content
371 should be cacheable or not by correctly setting the
372 <code>Cache-Control</code> header, and <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> should
373 be left alone to honor the wishes of the client or server as appropriate.
376 <p>Content that is time sensitive, or which varies depending on the
377 particulars of the request that are not covered by HTTP negotiation,
378 should not be cached. This content should declare itself uncacheable
379 using the <code>Cache-Control</code> header.</p>
381 <p>If content changes often, expressed by a freshness lifetime of minutes
382 or seconds, the content can still be cached, however it is highly
383 desirable that the origin server supports
384 <strong>conditional requests</strong> correctly to ensure that
385 full responses do not have to be generated on a regular basis.</p>
387 <p>Content that varies based on client provided request headers can be
388 cached through intelligent use of the <code>Vary</code> response
393 <h3>Variable/Negotiated Content</h3>
396 <p>When the origin server is designed to respond with different content
397 based on the value of headers in the request, for example to serve
398 multiple languages at the same URL, HTTP's caching mechanism makes it
399 possible to cache multiple variants of the same page at the same URL.</p>
401 <p>This is done by the origin server adding a <code>Vary</code> header
402 to indicate which headers must be taken into account by a cache when
403 determining whether two variants are different from one another.</p>
405 <p>If for example, a response is received with a vary header such as;</p>
407 <div class="example"><p><code>
408 Vary: negotiate,accept-language,accept-charset
411 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to
412 requesters with accept-language and accept-charset headers
413 matching those of the original request.</p>
415 <p>Multiple variants of the content can be cached side by side,
416 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> uses the <code>Vary</code> header and the
417 corresponding values of the request headers listed by <code>Vary</code>
418 to decide on which of many variants to return to the client.</p>
421 <h3><a name="disk" id="disk">Caching to Disk</a></h3>
424 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module relies on specific backend store
425 implementations in order to manage the cache, and for caching to disk
426 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> is provided to support this.</p>
428 <p>Typically the module will be configured as so;</p>
430 <pre class="prettyprint lang-config">CacheRoot "/var/cache/apache/"
433 CacheDirLength 1</pre>
436 <p>Importantly, as the cached files are locally stored, operating system
437 in-memory caching will typically be applied to their access also. So
438 although the files are stored on disk, if they are frequently accessed
439 it is likely the operating system will ensure that they are actually
440 served from memory.</p>
444 <h3>Understanding the Cache-Store</h3>
447 <p>To store items in the cache, <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> creates
448 a 22 character hash of the URL being requested. This hash incorporates
449 the hostname, protocol, port, path and any CGI arguments to the URL,
450 as well as elements defined by the Vary header to ensure that multiple
451 URLs do not collide with one another.</p>
453 <p>Each character may be any one of 64-different characters, which mean
454 that overall there are 64^22 possible hashes. For example, a URL might
455 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
456 as a prefix for the naming of the files specific to that URL within
457 the cache, however first it is split up into directories as per
458 the <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code> and
459 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
462 <p><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
463 specifies how many levels of subdirectory there should be, and
464 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
465 specifies how many characters should be in each directory. With
466 the example settings given above, the hash would be turned into
468 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
470 <p>The overall aim of this technique is to reduce the number of
471 subdirectories or files that may be in a particular directory,
472 as most file-systems slow down as this number increases. With
474 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
475 there can at most be 64 subdirectories at any particular level.
476 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
477 Unless you have a good reason not to, using a setting of "1"
478 for <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
482 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
483 depends on how many files you anticipate to store in the cache.
484 With the setting of "2" used in the above example, a grand
485 total of 4096 subdirectories can ultimately be created. With
486 1 million files cached, this works out at roughly 245 cached
487 URLs per directory.</p>
489 <p>Each URL uses at least two files in the cache-store. Typically
490 there is a ".header" file, which includes meta-information about
491 the URL, such as when it is due to expire and a ".data" file
492 which is a verbatim copy of the content to be served.</p>
494 <p>In the case of a content negotiated via the "Vary" header, a
495 ".vary" directory will be created for the URL in question. This
496 directory will have multiple ".data" files corresponding to the
497 differently negotiated content.</p>
500 <h3>Maintaining the Disk Cache</h3>
503 <p>The <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> module makes no attempt to
504 regulate the amount of disk space used by the cache, although it
505 will gracefully stand down on any disk error and behave as if the
506 cache was never present.</p>
508 <p>Instead, provided with httpd is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which allows you
509 to clean the cache periodically. Determining how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to
510 use for the cache is somewhat complex and trial and error may be needed to
511 select optimal values.</p>
513 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
514 operation. It can be run as persistent daemon, or periodically from
515 cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
516 or more to process very large (tens of gigabytes) caches and if you are
517 running it from cron it is recommended that you determine how long a typical
518 run takes, to avoid running more than one instance at a time.</p>
520 <p>It is also recommended that an appropriate "nice" level is chosen for
521 htcacheclean so that the tool does not cause excessive disk io while the
522 server is running.</p>
525 <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
526 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
527 cache growth / clean sequence.</p>
529 <p>Because <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> does not itself pay attention
530 to how much space is used you should ensure that
531 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
532 leave enough "grow room" following a clean.</p>
535 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
536 <div class="section">
537 <h2><a name="socache-caching" id="socache-caching">Two-state Key/Value Shared Object Caching</a></h2>
541 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code></li><li><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></li><li><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></li><li><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></li><li><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></li><li><code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authn_socache.html#authncachesocache">AuthnCacheSOCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslsessioncache">SSLSessionCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslstaplingcache">SSLStaplingCache</a></code></li></ul></td></tr></table>
543 <p>The Apache HTTP server offers a low level shared object cache for
544 caching information such as SSL sessions, or authentication credentials,
545 within the <a href="socache.html">socache</a> interface.</p>
547 <p>Additional modules are provided for each implementation, offering the
548 following backends:</p>
551 <dt><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></dt>
552 <dd>DBM based shared object cache.</dd>
553 <dt><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></dt>
554 <dd>Distcache based shared object cache.</dd>
555 <dt><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></dt>
556 <dd>Memcache based shared object cache.</dd>
557 <dt><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></dt>
558 <dd>Shared memory based shared object cache.</dd>
561 <h3><a name="mod_authn_socache-caching" id="mod_authn_socache-caching">Caching Authentication Credentials</a></h3>
564 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authn_socache.html#authncachesocache">AuthnCacheSOCache</a></code></li></ul></td></tr></table>
566 <p>The <code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code> module allows the result of
567 authentication to be cached, relieving load on authentication backends.</p>
571 <h3><a name="mod_ssl-caching" id="mod_ssl-caching">Caching SSL Sessions</a></h3>
574 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_ssl.html#sslsessioncache">SSLSessionCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslstaplingcache">SSLStaplingCache</a></code></li></ul></td></tr></table>
576 <p>The <code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code> module uses the <code>socache</code> interface
577 to provide a session cache and a stapling cache.</p>
581 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
582 <div class="section">
583 <h2><a name="file-caching" id="file-caching">Specialized File Caching</a></h2>
587 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code></li><li><code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code></li></ul></td></tr></table>
589 <p>On platforms where a filesystem might be slow, or where file
590 handles are expensive, the option exists to pre-load files into
591 memory on startup.</p>
593 <p>On systems where opening files is slow, the option exists to
594 open the file on startup and cache the file handle. These
595 options can help on systems where access to static files is
598 <h3><a name="filehandle" id="filehandle">File-Handle Caching</a></h3>
601 <p>The act of opening a file can itself be a source of delay, particularly
602 on network filesystems. By maintaining a cache of open file descriptors
603 for commonly served files, httpd can avoid this delay. Currently httpd
604 provides one implementation of File-Handle Caching.</p>
609 <p>The most basic form of caching present in httpd is the file-handle
610 caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching
611 file-contents, this cache maintains a table of open file descriptors. Files
612 to be cached in this manner are specified in the configuration file using
613 the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
617 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive
618 instructs httpd to open the file when it is started and to re-use
619 this file-handle for all subsequent access to this file.</p>
621 <pre class="prettyprint lang-config">CacheFile /usr/local/apache2/htdocs/index.html</pre>
624 <p>If you intend to cache a large number of files in this manner, you
625 must ensure that your operating system's limit for the number of open
626 files is set appropriately.</p>
628 <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
629 does not cause the file-contents to be cached per-se, it does mean
630 that if the file changes while httpd is running these changes will
631 not be picked up. The file will be consistently served as it was
632 when httpd was started.</p>
634 <p>If the file is removed while httpd is running, it will continue
635 to maintain an open file descriptor and serve the file as it was when
636 httpd was started. This usually also means that although the file
637 will have been deleted, and not show up on the filesystem, extra free
638 space will not be recovered until httpd is stopped and the file
639 descriptor closed.</p>
644 <h3><a name="inmemory" id="inmemory">In-Memory Caching</a></h3>
647 <p>Serving directly from system memory is universally the fastest method
648 of serving content. Reading files from a disk controller or, even worse,
649 from a remote network is orders of magnitude slower. Disk controllers
650 usually involve physical processes, and network access is limited by
651 your available bandwidth. Memory access on the other hand can take mere
654 <p>System memory isn't cheap though, byte for byte it's by far the most
655 expensive type of storage and it's important to ensure that it is used
656 efficiently. By caching files in memory you decrease the amount of
657 memory available on the system. As we'll see, in the case of operating
658 system caching, this is not so much of an issue, but when using
659 httpd's own in-memory caching it is important to make sure that you
660 do not allocate too much memory to a cache. Otherwise the system
661 will be forced to swap out memory, which will likely degrade
664 <h4>Operating System Caching</h4>
667 <p>Almost all modern operating systems cache file-data in memory managed
668 directly by the kernel. This is a powerful feature, and for the most
669 part operating systems get it right. For example, on Linux, let's look at
670 the difference in the time it takes to read a file for the first time
671 and the second time;</p>
673 <div class="example"><pre>colm@coroebus:~$ time cat testfile > /dev/null
677 colm@coroebus:~$ time cat testfile > /dev/null
680 sys 0m0.000s</pre></div>
682 <p>Even for this small file, there is a huge difference in the amount
683 of time it takes to read the file. This is because the kernel has cached
684 the file contents in memory.</p>
686 <p>By ensuring there is "spare" memory on your system, you can ensure
687 that more and more file-contents will be stored in this cache. This
688 can be a very efficient means of in-memory caching, and involves no
689 extra configuration of httpd at all.</p>
691 <p>Additionally, because the operating system knows when files are
692 deleted or modified, it can automatically remove file contents from the
693 cache when necessary. This is a big advantage over httpd's in-memory
694 caching which has no way of knowing when a file has changed.</p>
697 <p>Despite the performance and advantages of automatic operating system
698 caching there are some circumstances in which in-memory caching may be
699 better performed by httpd.</p>
701 <h4>MMapFile Caching</h4>
704 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the
705 <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code> directive, which
706 allows you to have httpd map a static file's contents into memory at
707 start time (using the mmap system call). httpd will use the in-memory
708 contents for all subsequent accesses to this file.</p>
710 <pre class="prettyprint lang-config">MMapFile /usr/local/apache2/htdocs/index.html</pre>
714 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
715 changes in these files will not be picked up by httpd after it has
718 <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code>
719 directive does not keep track of how much memory it allocates, so
720 you must ensure not to over-use the directive. Each httpd child
721 process will replicate this memory, so it is critically important
722 to ensure that the files mapped are not so large as to cause the
723 system to swap memory.</p>
727 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
728 <div class="section">
729 <h2><a name="security" id="security">Security Considerations</a></h2>
732 <h3>Authorization and Access Control</h3>
735 <p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> in its default state where
736 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> is set to
737 <code>On</code> is very much like having a caching reverse-proxy bolted
738 to the front of the server. Requests will be served by the caching module
739 unless it determines that the origin server should be queried just as an
740 external cache would, and this drastically changes the security model of
743 <p>As traversing a filesystem hierarchy to examine potential
744 <code>.htaccess</code> files would be a very expensive operation,
745 partially defeating the point of caching (to speed up requests),
746 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> makes no decision about whether a cached
747 entity is authorised for serving. In other words; if
748 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> has cached some content, it will be served
749 from the cache as long as that content has not expired.</p>
751 <p>If, for example, your configuration permits access to a resource by IP
752 address you should ensure that this content is not cached. You can do this
753 by using the <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code>
754 directive, or <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code>. Left unchecked,
755 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> - very much like a reverse proxy - would cache
756 the content when served and then serve it to any client, on any IP
759 <p>When the <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code>
760 directive is set to <code>Off</code>, the full set of request processing
761 phases are executed and the security model remains unchanged.</p>
764 <h3>Local exploits</h3>
767 <p>As requests to end-users can be served from the cache, the cache
768 itself can become a target for those wishing to deface or interfere with
769 content. It is important to bear in mind that the cache must at all
770 times be writable by the user which httpd is running as. This is in
771 stark contrast to the usually recommended situation of maintaining
772 all content unwritable by the Apache user.</p>
774 <p>If the Apache user is compromised, for example through a flaw in
775 a CGI process, it is possible that the cache may be targeted. When
776 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>, it is relatively easy to
777 insert or modify a cached entity.</p>
779 <p>This presents a somewhat elevated risk in comparison to the other
780 types of attack it is possible to make as the Apache user. If you are
781 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> you should bear this in mind -
782 ensure you upgrade httpd when security upgrades are announced and
783 run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>
787 <h3>Cache Poisoning</h3>
790 <p>When running httpd as a caching proxy server, there is also the
791 potential for so-called cache poisoning. Cache Poisoning is a broad
792 term for attacks in which an attacker causes the proxy server to
793 retrieve incorrect (and usually undesirable) content from the origin
796 <p>For example if the DNS servers used by your system running httpd
797 are vulnerable to DNS cache poisoning, an attacker may be able to control
798 where httpd connects to when requesting content from the origin server.
799 Another example is so-called HTTP request-smuggling attacks.</p>
801 <p>This document is not the correct place for an in-depth discussion
802 of HTTP request smuggling (instead, try your favourite search engine)
803 however it is important to be aware that it is possible to make
804 a series of requests, and to exploit a vulnerability on an origin
805 webserver such that the attacker can entirely control the content
806 retrieved by the proxy.</p>
809 <h3>Denial of Service / Cachebusting</h3>
812 <p>The Vary mechanism allows multiple variants of the same URL to be
813 cached side by side. Depending on header values provided by the client,
814 the cache will select the correct variant to return to the client. This
815 mechanism can become a problem when an attempt is made to vary on a
816 header that is known to contain a wide range of possible values under
817 normal use, for example the <code>User-Agent</code> header. Depending
818 on the popularity of the particular web site thousands or millions of
819 duplicate cache entries could be created for the same URL, crowding
820 out other entries in the cache.</p>
822 <p>In other cases, there may be a need to change the URL of a particular
823 resource on every request, usually by adding a "cachebuster" string to
824 the URL. If this content is declared cacheable by a server for a
825 significant freshness lifetime, these entries can crowd out
826 legitimate entries in a cache. While <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
828 <code class="directive"><a href="./mod/mod_cache.html#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></code>
829 directive, this directive should be used with care to ensure that
830 downstream proxy or browser caches aren't subjected to the same denial
831 of service issue.</p>
834 <div class="bottomlang">
835 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
836 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
837 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
838 </div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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