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15 <p class="apache">Apache HTTP Server Version 2.5</p>
16 <img alt="" src="./images/feather.gif" /></div>
17 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
19 <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>
21 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
22 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
23 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
26 <p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>,
27 <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.
28 It describes how to use the Apache HTTP Server's caching features to accelerate web and
29 proxy serving, while avoiding common problems and misconfigurations.</p>
31 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
32 <li><img alt="" src="./images/down.gif" /> <a href="#http-caching">Three-state RFC2616 HTTP caching</a></li>
33 <li><img alt="" src="./images/down.gif" /> <a href="#socache-caching">Two-state Key/Value Shared Object Caching</a></li>
34 <li><img alt="" src="./images/down.gif" /> <a href="#file-caching">Specialized File Caching</a></li>
35 <li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
37 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
39 <h2><a name="introduction" id="introduction">Introduction</a></h2>
42 <p>The Apache HTTP server offers a range of caching features that
43 are designed to improve the performance of the server in various
47 <dt>Three-state RFC2616 HTTP caching</dt>
49 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
50 and its provider modules
51 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>
52 provide intelligent, HTTP-aware caching. The content itself is stored
53 in the cache, and mod_cache aims to honor all of the various HTTP
54 headers and options that control the cacheability of content
56 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section
58 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
59 is aimed at both simple and complex caching configurations, where
60 you are dealing with proxied content, dynamic local content or
61 have a need to speed up access to local files on a potentially
65 <dt>Two-state key/value shared object caching</dt>
67 <code class="module"><a href="./mod/mod_socache.html">mod_socache</a></code>
68 and its provider modules provide a
69 server wide key/value based shared object cache. These modules
70 are designed to cache low level data such as SSL sessions and
71 authentication credentials. Backends allow the data to be stored
72 server wide in shared memory, or datacenter wide in a cache such
73 as memcache or distcache.
76 <dt>Specialized file caching</dt>
78 <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>
79 offers the ability to pre-load
80 files into memory on server startup, and can improve access
81 times and save file handles on files that are accessed often,
82 as there is no need to go to disk on each request.
86 <p>To get the most from this document, you should be familiar with
87 the basics of HTTP, and have read the Users' Guides to
88 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
89 <a href="content-negotiation.html">Content negotiation</a>.</p>
91 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
93 <h2><a name="http-caching" id="http-caching">Three-state RFC2616 HTTP caching</a></h2>
97 <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>
99 <p>The HTTP protocol contains built in support for an in-line caching
101 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
102 described by section 13 of RFC2616</a>, and the
103 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module can be used to take advantage of
106 <p>Unlike a simple two state key/value cache where the content
107 disappears completely when no longer fresh, an HTTP cache includes
108 a mechanism to retain stale content, and to ask the origin server
109 whether this stale content has changed and if not, make it fresh
112 <p>An entry in an HTTP cache exists in one of three states:</p>
117 If the content is new enough (younger than its <strong>freshness
118 lifetime</strong>), it is considered <strong>fresh</strong>. An
119 HTTP cache is free to serve fresh content without making any
120 calls to the origin server at all.
124 <p>If the content is too old (older than its <strong>freshness
125 lifetime</strong>), it is considered <strong>stale</strong>. An
126 HTTP cache should contact the origin server and check whether
127 the content is still fresh before serving stale content to a
128 client. The origin server will either respond with replacement
129 content if not still valid, or ideally, the origin server will
130 respond with a code to tell the cache the content is still
131 fresh, without the need to generate or send the content again.
132 The content becomes fresh again and the cycle continues.</p>
134 <p>The HTTP protocol does allow the cache to serve stale data
135 under certain circumstances, such as when an attempt to freshen
136 the data with an origin server has failed with a 5xx error, or
137 when another request is already in the process of freshening
138 the given entry. In these cases a <code>Warning</code> header
139 is added to the response.</p>
141 <dt>Non Existent</dt>
143 If the cache gets full, it reserves the option to delete content
144 from the cache to make space. Content can be deleted at any time,
145 and can be stale or fresh. The <a href="programs/htcacheclean.html">htcacheclean</a> tool can be
146 run on a once off basis, or deployed as a daemon to keep the size
147 of the cache within the given size, or the given number of inodes.
148 The tool attempts to delete stale content before attempting to
149 delete fresh content.
153 <p>Full details of how HTTP caching works can be found in
154 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
155 Section 13 of RFC2616</a>.</p>
157 <h3>Interaction with the Server</h3>
160 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module hooks into the server in two
161 possible places depending on the value of the
162 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> directive:
166 <dt>Quick handler phase</dt>
168 <p>This phase happens very early on during the request processing,
169 just after the request has been parsed. If the content is
170 found within the cache, it is served immediately and almost
171 all request processing is bypassed.</p>
173 <p>In this scenario, the cache behaves as if it has been "bolted
174 on" to the front of the server.</p>
176 <p>This mode offers the best performance, as the majority of
177 server processing is bypassed. This mode however also bypasses the
178 authentication and authorization phases of server processing, so
179 this mode should be chosen with care when this is important.</p>
181 <dt>Normal handler phase</dt>
183 <p>This phase happens late in the request processing, after all
184 the request phases have completed.</p>
186 <p>In this scenario, the cache behaves as if it has been "bolted
187 on" to the back of the server.</p>
189 <p>This mode offers the most flexibility, as the potential exists
190 for caching to occur at a precisely controlled point in the filter
191 chain, and cached content can be filtered or personalized before
192 being sent to the client.</p>
196 <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
197 will add a <a href="filter.html">filter</a> to the filter stack in order
198 to record the response to the cache, and then stand down, allowing normal
199 request processing to continue. If the content is determined to be
200 cacheable, the content will be saved to the cache for future serving,
201 otherwise the content will be ignored.</p>
203 <p>If the content found within the cache is stale, the
204 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module converts the request into a
205 <strong>conditional request</strong>. If the origin server responds with
206 a normal response, the normal response is cached, replacing the content
207 already cached. If the origin server responds with a 304 Not Modified
208 response, the content is marked as fresh again, and the cached content
209 is served by the filter instead of saving it.</p>
212 <h3>Improving Cache Hits</h3>
215 <p>When a virtual host is known by one of many different server aliases,
216 ensuring that <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is
217 set to <code>On</code> can dramatically improve the ratio of cache hits.
218 This is because the hostname of the virtual-host serving the content is
219 used within the cache key. With the setting set to <code>On</code>
220 virtual-hosts with multiple server names or aliases will not produce
221 differently cached entities, and instead content will be cached as
222 per the canonical hostname.</p>
226 <h3>Freshness Lifetime</h3>
229 <p>Well formed content that is intended to be cached should declare an
230 explicit freshness lifetime with the <code>Cache-Control</code>
231 header's <code>max-age</code> or <code>s-maxage</code> fields, or
232 by including an <code>Expires</code> header.</p>
234 <p>At the same time, the origin server defined freshness lifetime can
235 be overridden by a client when the client presents their own
236 <code>Cache-Control</code> header within the request. In this case,
237 the lowest freshness lifetime between request and response wins.</p>
239 <p>When this freshness lifetime is missing from the request or the
240 response, a default freshness lifetime is applied. The default
241 freshness lifetime for cached entities is one hour, however
242 this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive.</p>
244 <p>If a response does not include an <code>Expires</code> header but does
245 include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
246 can infer a freshness lifetime based on a heuristic, which can be
247 controlled through the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p>
249 <p>For local content, or for remote content that does not define its own
250 <code>Expires</code> header, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
251 fine-tune the freshness lifetime by adding <code>max-age</code> and
252 <code>Expires</code>.</p>
254 <p>The maximum freshness lifetime may also be controlled by using the
255 <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
259 <h3>A Brief Guide to Conditional Requests</h3>
262 <p>When content expires from the cache and becomes stale, rather than
263 pass on the original request, httpd will modify the request to make
264 it conditional instead.</p>
266 <p>When an <code>ETag</code> header exists in the original cached
267 response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
268 <code>If-None-Match</code> header to the request to the origin server.
269 When a <code>Last-Modified</code> header exists in the original
270 cached response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
271 <code>If-Modified-Since</code> header to the request to the origin
272 server. Performing either of these actions makes the request
273 <strong>conditional</strong>.</p>
275 <p>When a conditional request is received by an origin server, the
276 origin server should check whether the ETag or the Last-Modified
277 parameter has changed, as appropriate for the request. If not, the
278 origin should respond with a terse "304 Not Modified" response. This
279 signals to the cache that the stale content is still fresh should be
280 used for subsequent requests until the content's new freshness lifetime
281 is reached again.</p>
283 <p>If the content has changed, then the content is served as if the
284 request were not conditional to begin with.</p>
286 <p>Conditional requests offer two benefits. Firstly, when making such
287 a request to the origin server, if the content from the origin
288 matches the content in the cache, this can be determined easily and
289 without the overhead of transferring the entire resource.</p>
291 <p>Secondly, a well designed origin server will be designed in such
292 a way that conditional requests will be significantly cheaper to
293 produce than a full response. For static files, typically all that is
294 involved is a call to <code>stat()</code> or similar system call, to
295 see if the file has changed in size or modification time. As such, even
296 local content may still be served faster from the cache if it has not
299 <p>Origin servers should make every effort to support conditional
300 requests as is practical, however if conditional requests are not
301 supported, the origin will respond as if the request was not
302 conditional, and the cache will respond as if the content had changed
303 and save the new content to the cache. In this case, the cache will
304 behave like a simple two state cache, where content is effectively
305 either fresh or deleted.</p>
308 <h3>What Can be Cached?</h3>
311 <p>The full definition of which responses can be cached by an HTTP
313 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4">
314 RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as
318 <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>
320 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
323 <li>The request must be a HTTP GET request.</li>
325 <li>If the response contains an "Authorization:" header, it must
326 also contain an "s-maxage", "must-revalidate" or "public" option
327 in the "Cache-Control:" header, or it won't be cached.</li>
329 <li>If the URL included a query string (e.g. from a HTML form GET
330 method) it will not be cached unless the response specifies an
331 explicit expiration by including an "Expires:" header or the max-age
332 or s-maxage directive of the "Cache-Control:" header, as per RFC2616
333 sections 13.9 and 13.2.1.</li>
335 <li>If the response has a status of 200 (OK), the response must
336 also include at least one of the "Etag", "Last-Modified" or
337 the "Expires" headers, or the max-age or s-maxage directive of
338 the "Cache-Control:" header, unless the
339 <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
340 directive has been used to require otherwise.</li>
342 <li>If the response includes the "private" option in a "Cache-Control:"
343 header, it will not be stored unless the
344 <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
345 used to require otherwise.</li>
347 <li>Likewise, if the response includes the "no-store" option in a
348 "Cache-Control:" header, it will not be stored unless the
349 <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
352 <li>A response will not be stored if it includes a "Vary:" header
353 containing the match-all "*".</li>
357 <h3>What Should Not be Cached?</h3>
360 <p>It should be up to the client creating the request, or the origin
361 server constructing the response to decide whether or not the content
362 should be cacheable or not by correctly setting the
363 <code>Cache-Control</code> header, and <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> should
364 be left alone to honor the wishes of the client or server as appropriate.
367 <p>Content that is time sensitive, or which varies depending on the
368 particulars of the request that are not covered by HTTP negotiation,
369 should not be cached. This content should declare itself uncacheable
370 using the <code>Cache-Control</code> header.</p>
372 <p>If content changes often, expressed by a freshness lifetime of minutes
373 or seconds, the content can still be cached, however it is highly
374 desirable that the origin server supports
375 <strong>conditional requests</strong> correctly to ensure that
376 full responses do not have to be generated on a regular basis.</p>
378 <p>Content that varies based on client provided request headers can be
379 cached through intelligent use of the <code>Vary</code> response
384 <h3>Variable/Negotiated Content</h3>
387 <p>When the origin server is designed to respond with different content
388 based on the value of headers in the request, for example to serve
389 multiple languages at the same URL, HTTP's caching mechanism makes it
390 possible to cache multiple variants of the same page at the same URL.</p>
392 <p>This is done by the origin server adding a <code>Vary</code> header
393 to indicate which headers must be taken into account by a cache when
394 determining whether two variants are different from one another.</p>
396 <p>If for example, a response is received with a vary header such as;</p>
398 <div class="example"><p><code>
399 Vary: negotiate,accept-language,accept-charset
402 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to
403 requesters with accept-language and accept-charset headers
404 matching those of the original request.</p>
406 <p>Multiple variants of the content can be cached side by side,
407 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> uses the <code>Vary</code> header and the
408 corresponding values of the request headers listed by <code>Vary</code>
409 to decide on which of many variants to return to the client.</p>
412 <h3><a name="disk" id="disk">Caching to Disk</a></h3>
415 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module relies on specific backend store
416 implementations in order to manage the cache, and for caching to disk
417 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> is provided to support this.</p>
419 <p>Typically the module will be configured as so;</p>
421 <div class="example"><p><code>
422 CacheRoot /var/cache/apache/<br />
423 CacheEnable disk /<br />
424 CacheDirLevels 2<br />
428 <p>Importantly, as the cached files are locally stored, operating system
429 in-memory caching will typically be applied to their access also. So
430 although the files are stored on disk, if they are frequently accessed
431 it is likely the operating system will ensure that they are actually
432 served from memory.</p>
436 <h3>Understanding the Cache-Store</h3>
439 <p>To store items in the cache, <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> creates
440 a 22 character hash of the URL being requested. This hash incorporates
441 the hostname, protocol, port, path and any CGI arguments to the URL,
442 as well as elements defined by the Vary header to ensure that multiple
443 URLs do not collide with one another.</p>
445 <p>Each character may be any one of 64-different characters, which mean
446 that overall there are 64^22 possible hashes. For example, a URL might
447 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
448 as a prefix for the naming of the files specific to that URL within
449 the cache, however first it is split up into directories as per
450 the <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code> and
451 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
454 <p><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
455 specifies how many levels of subdirectory there should be, and
456 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
457 specifies how many characters should be in each directory. With
458 the example settings given above, the hash would be turned into
460 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
462 <p>The overall aim of this technique is to reduce the number of
463 subdirectories or files that may be in a particular directory,
464 as most file-systems slow down as this number increases. With
466 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
467 there can at most be 64 subdirectories at any particular level.
468 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
469 Unless you have a good reason not to, using a setting of "1"
470 for <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
474 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
475 depends on how many files you anticipate to store in the cache.
476 With the setting of "2" used in the above example, a grand
477 total of 4096 subdirectories can ultimately be created. With
478 1 million files cached, this works out at roughly 245 cached
479 URLs per directory.</p>
481 <p>Each URL uses at least two files in the cache-store. Typically
482 there is a ".header" file, which includes meta-information about
483 the URL, such as when it is due to expire and a ".data" file
484 which is a verbatim copy of the content to be served.</p>
486 <p>In the case of a content negotiated via the "Vary" header, a
487 ".vary" directory will be created for the URL in question. This
488 directory will have multiple ".data" files corresponding to the
489 differently negotiated content.</p>
492 <h3>Maintaining the Disk Cache</h3>
495 <p>The <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> module makes no attempt to
496 regulate the amount of disk space used by the cache, although it
497 will gracefully stand down on any disk error and behave as if the
498 cache was never present.</p>
500 <p>Instead, provided with httpd is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which allows you
501 to clean the cache periodically. Determining how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to
502 use for the cache is somewhat complex and trial and error may be needed to
503 select optimal values.</p>
505 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
506 operation. It can be run as persistent daemon, or periodically from
507 cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
508 or more to process very large (tens of gigabytes) caches and if you are
509 running it from cron it is recommended that you determine how long a typical
510 run takes, to avoid running more than one instance at a time.</p>
512 <p>It is also recommended that an appropriate "nice" level is chosen for
513 htcacheclean so that the tool does not cause excessive disk io while the
514 server is running.</p>
517 <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
518 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
519 cache growth / clean sequence.</p>
521 <p>Because <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> does not itself pay attention
522 to how much space is used you should ensure that
523 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
524 leave enough "grow room" following a clean.</p>
527 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
528 <div class="section">
529 <h2><a name="socache-caching" id="socache-caching">Two-state Key/Value Shared Object Caching</a></h2>
533 <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>
535 <p>The Apache HTTP server offers a low level shared object cache for
536 caching information such as SSL sessions, or authentication credentials,
537 within the <a href="socache.html">socache</a> interface.</p>
539 <p>Additional modules are provided for each implementation, offering the
540 following backends:</p>
543 <dt><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></dt>
544 <dd>DBM based shared object cache.</dd>
545 <dt><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></dt>
546 <dd>Distcache based shared object cache.</dd>
547 <dt><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></dt>
548 <dd>Memcache based shared object cache.</dd>
549 <dt><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></dt>
550 <dd>Shared memory based shared object cache.</dd>
553 <h3><a name="mod_authn_socache-caching" id="mod_authn_socache-caching">Caching Authentication Credentials</a></h3>
556 <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>
558 <p>The <code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code> module allows the result of
559 authentication to be cached, relieving load on authentication backends.</p>
563 <h3><a name="mod_ssl-caching" id="mod_ssl-caching">Caching SSL Sessions</a></h3>
566 <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>
568 <p>The <code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code> module uses the <code>socache</code> interface
569 to provide a session cache and a stapling cache.</p>
573 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
574 <div class="section">
575 <h2><a name="file-caching" id="file-caching">Specialized File Caching</a></h2>
579 <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>
581 <p>On platforms where a filesystem might be slow, or where file
582 handles are expensive, the option exists to pre-load files into
583 memory on startup.</p>
585 <p>On systems where opening files is slow, the option exists to
586 open the file on startup and cache the file handle. These
587 options can help on systems where access to static files is
590 <h3><a name="filehandle" id="filehandle">File-Handle Caching</a></h3>
593 <p>The act of opening a file can itself be a source of delay, particularly
594 on network filesystems. By maintaining a cache of open file descriptors
595 for commonly served files, httpd can avoid this delay. Currently httpd
596 provides one implementation of File-Handle Caching.</p>
601 <p>The most basic form of caching present in httpd is the file-handle
602 caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching
603 file-contents, this cache maintains a table of open file descriptors. Files
604 to be cached in this manner are specified in the configuration file using
605 the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
609 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive
610 instructs httpd to open the file when it is started and to re-use
611 this file-handle for all subsequent access to this file.</p>
613 <div class="example"><p><code>
614 CacheFile /usr/local/apache2/htdocs/index.html
617 <p>If you intend to cache a large number of files in this manner, you
618 must ensure that your operating system's limit for the number of open
619 files is set appropriately.</p>
621 <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
622 does not cause the file-contents to be cached per-se, it does mean
623 that if the file changes while httpd is running these changes will
624 not be picked up. The file will be consistently served as it was
625 when httpd was started.</p>
627 <p>If the file is removed while httpd is running, it will continue
628 to maintain an open file descriptor and serve the file as it was when
629 httpd was started. This usually also means that although the file
630 will have been deleted, and not show up on the filesystem, extra free
631 space will not be recovered until httpd is stopped and the file
632 descriptor closed.</p>
637 <h3><a name="inmemory" id="inmemory">In-Memory Caching</a></h3>
640 <p>Serving directly from system memory is universally the fastest method
641 of serving content. Reading files from a disk controller or, even worse,
642 from a remote network is orders of magnitude slower. Disk controllers
643 usually involve physical processes, and network access is limited by
644 your available bandwidth. Memory access on the other hand can take mere
647 <p>System memory isn't cheap though, byte for byte it's by far the most
648 expensive type of storage and it's important to ensure that it is used
649 efficiently. By caching files in memory you decrease the amount of
650 memory available on the system. As we'll see, in the case of operating
651 system caching, this is not so much of an issue, but when using
652 httpd's own in-memory caching it is important to make sure that you
653 do not allocate too much memory to a cache. Otherwise the system
654 will be forced to swap out memory, which will likely degrade
657 <h4>Operating System Caching</h4>
660 <p>Almost all modern operating systems cache file-data in memory managed
661 directly by the kernel. This is a powerful feature, and for the most
662 part operating systems get it right. For example, on Linux, let's look at
663 the difference in the time it takes to read a file for the first time
664 and the second time;</p>
666 <div class="example"><pre>
667 colm@coroebus:~$ time cat testfile > /dev/null
671 colm@coroebus:~$ time cat testfile > /dev/null
674 sys 0m0.000s</pre></div>
676 <p>Even for this small file, there is a huge difference in the amount
677 of time it takes to read the file. This is because the kernel has cached
678 the file contents in memory.</p>
680 <p>By ensuring there is "spare" memory on your system, you can ensure
681 that more and more file-contents will be stored in this cache. This
682 can be a very efficient means of in-memory caching, and involves no
683 extra configuration of httpd at all.</p>
685 <p>Additionally, because the operating system knows when files are
686 deleted or modified, it can automatically remove file contents from the
687 cache when necessary. This is a big advantage over httpd's in-memory
688 caching which has no way of knowing when a file has changed.</p>
691 <p>Despite the performance and advantages of automatic operating system
692 caching there are some circumstances in which in-memory caching may be
693 better performed by httpd.</p>
695 <h4>MMapFile Caching</h4>
698 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the
699 <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code> directive, which
700 allows you to have httpd map a static file's contents into memory at
701 start time (using the mmap system call). httpd will use the in-memory
702 contents for all subsequent accesses to this file.</p>
704 <div class="example"><p><code>
705 MMapFile /usr/local/apache2/htdocs/index.html
709 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
710 changes in these files will not be picked up by httpd after it has
713 <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code>
714 directive does not keep track of how much memory it allocates, so
715 you must ensure not to over-use the directive. Each httpd child
716 process will replicate this memory, so it is critically important
717 to ensure that the files mapped are not so large as to cause the
718 system to swap memory.</p>
722 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
723 <div class="section">
724 <h2><a name="security" id="security">Security Considerations</a></h2>
727 <h3>Authorization and Access Control</h3>
730 <p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> in its default state where
731 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> is set to
732 <code>On</code> is very much like having a caching reverse-proxy bolted
733 to the front of the server. Requests will be served by the caching module
734 unless it determines that the origin server should be queried just as an
735 external cache would, and this drastically changes the security model of
738 <p>As traversing a filesystem hierarchy to examine potential
739 <code>.htaccess</code> files would be a very expensive operation,
740 partially defeating the point of caching (to speed up requests),
741 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> makes no decision about whether a cached
742 entity is authorised for serving. In other words; if
743 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> has cached some content, it will be served
744 from the cache as long as that content has not expired.</p>
746 <p>If, for example, your configuration permits access to a resource by IP
747 address you should ensure that this content is not cached. You can do this
748 by using the <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code>
749 directive, or <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code>. Left unchecked,
750 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> - very much like a reverse proxy - would cache
751 the content when served and then serve it to any client, on any IP
754 <p>When the <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code>
755 directive is set to <code>Off</code>, the full set of request processing
756 phases are executed and the security model remains unchanged.</p>
759 <h3>Local exploits</h3>
762 <p>As requests to end-users can be served from the cache, the cache
763 itself can become a target for those wishing to deface or interfere with
764 content. It is important to bear in mind that the cache must at all
765 times be writable by the user which httpd is running as. This is in
766 stark contrast to the usually recommended situation of maintaining
767 all content unwritable by the Apache user.</p>
769 <p>If the Apache user is compromised, for example through a flaw in
770 a CGI process, it is possible that the cache may be targeted. When
771 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>, it is relatively easy to
772 insert or modify a cached entity.</p>
774 <p>This presents a somewhat elevated risk in comparison to the other
775 types of attack it is possible to make as the Apache user. If you are
776 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> you should bear this in mind -
777 ensure you upgrade httpd when security upgrades are announced and
778 run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>
782 <h3>Cache Poisoning</h3>
785 <p>When running httpd as a caching proxy server, there is also the
786 potential for so-called cache poisoning. Cache Poisoning is a broad
787 term for attacks in which an attacker causes the proxy server to
788 retrieve incorrect (and usually undesirable) content from the origin
791 <p>For example if the DNS servers used by your system running httpd
792 are vulnerable to DNS cache poisoning, an attacker may be able to control
793 where httpd connects to when requesting content from the origin server.
794 Another example is so-called HTTP request-smuggling attacks.</p>
796 <p>This document is not the correct place for an in-depth discussion
797 of HTTP request smuggling (instead, try your favourite search engine)
798 however it is important to be aware that it is possible to make
799 a series of requests, and to exploit a vulnerability on an origin
800 webserver such that the attacker can entirely control the content
801 retrieved by the proxy.</p>
804 <h3>Denial of Service / Cachebusting</h3>
807 <p>The Vary mechanism allows multiple variants of the same URL to be
808 cached side by side. Depending on header values provided by the client,
809 the cache will select the correct variant to return to the client. This
810 mechanism can become a problem when an attempt is made to vary on a
811 header that is known to contain a wide range of possible values under
812 normal use, for example the <code>User-Agent</code> header. Depending
813 on the popularity of the particular web site thousands or millions of
814 duplicate cache entries could be created for the same URL, crowding
815 out other entries in the cache.</p>
817 <p>In other cases, there may be a need to change the URL of a particular
818 resource on every request, usually by adding a "cachebuster" string to
819 the URL. If this content is declared cacheable by a server for a
820 significant freshness lifetime, these entries can crowd out
821 legitimate entries in a cache. While <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
823 <code class="directive"><a href="./mod/mod_cache.html#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></code>
824 directive, this directive should be used with care to ensure that
825 downstream proxy or browser caches aren't subjected to the same denial
826 of service issue.</p>
829 <div class="bottomlang">
830 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
831 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
832 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
833 </div><div id="footer">
834 <p class="apache">Copyright 2011 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
835 <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div>
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