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18 <p class="apache">Apache HTTP Server Version 2.5</p>
19 <img alt="" src="./images/feather.gif" /></div>
20 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
22 <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>
24 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
25 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
26 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
29 <p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>,
30 <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.
31 It describes how to use the Apache HTTP Server's caching features to accelerate web and
32 proxy serving, while avoiding common problems and misconfigurations.</p>
34 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
35 <li><img alt="" src="./images/down.gif" /> <a href="#http-caching">Three-state RFC2616 HTTP caching</a></li>
36 <li><img alt="" src="./images/down.gif" /> <a href="#socache-caching">Two-state Key/Value Shared Object Caching</a></li>
37 <li><img alt="" src="./images/down.gif" /> <a href="#file-caching">Specialized File Caching</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
40 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
42 <h2><a name="introduction" id="introduction">Introduction</a></h2>
45 <p>The Apache HTTP server offers a range of caching features that
46 are designed to improve the performance of the server in various
50 <dt>Three-state RFC2616 HTTP caching</dt>
52 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
53 and its provider modules
54 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>
55 provide intelligent, HTTP-aware caching. The content itself is stored
56 in the cache, and mod_cache aims to honor all of the various HTTP
57 headers and options that control the cacheability of content
59 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section
61 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
62 is aimed at both simple and complex caching configurations, where
63 you are dealing with proxied content, dynamic local content or
64 have a need to speed up access to local files on a potentially
68 <dt>Two-state key/value shared object caching</dt>
70 The <a href="socache.html">shared object cache API</a> (socache)
71 and its provider modules provide a
72 server wide key/value based shared object cache. These modules
73 are designed to cache low level data such as SSL sessions and
74 authentication credentials. Backends allow the data to be stored
75 server wide in shared memory, or datacenter wide in a cache such
76 as memcache or distcache.
79 <dt>Specialized file caching</dt>
81 <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>
82 offers the ability to pre-load
83 files into memory on server startup, and can improve access
84 times and save file handles on files that are accessed often,
85 as there is no need to go to disk on each request.
89 <p>To get the most from this document, you should be familiar with
90 the basics of HTTP, and have read the Users' Guides to
91 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
92 <a href="content-negotiation.html">Content negotiation</a>.</p>
94 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
96 <h2><a name="http-caching" id="http-caching">Three-state RFC2616 HTTP caching</a></h2>
100 <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>
102 <p>The HTTP protocol contains built in support for an in-line caching
104 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
105 described by section 13 of RFC2616</a>, and the
106 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module can be used to take advantage of
109 <p>Unlike a simple two state key/value cache where the content
110 disappears completely when no longer fresh, an HTTP cache includes
111 a mechanism to retain stale content, and to ask the origin server
112 whether this stale content has changed and if not, make it fresh
115 <p>An entry in an HTTP cache exists in one of three states:</p>
120 If the content is new enough (younger than its <strong>freshness
121 lifetime</strong>), it is considered <strong>fresh</strong>. An
122 HTTP cache is free to serve fresh content without making any
123 calls to the origin server at all.
127 <p>If the content is too old (older than its <strong>freshness
128 lifetime</strong>), it is considered <strong>stale</strong>. An
129 HTTP cache should contact the origin server and check whether
130 the content is still fresh before serving stale content to a
131 client. The origin server will either respond with replacement
132 content if not still valid, or ideally, the origin server will
133 respond with a code to tell the cache the content is still
134 fresh, without the need to generate or send the content again.
135 The content becomes fresh again and the cycle continues.</p>
137 <p>The HTTP protocol does allow the cache to serve stale data
138 under certain circumstances, such as when an attempt to freshen
139 the data with an origin server has failed with a 5xx error, or
140 when another request is already in the process of freshening
141 the given entry. In these cases a <code>Warning</code> header
142 is added to the response.</p>
144 <dt>Non Existent</dt>
146 If the cache gets full, it reserves the option to delete content
147 from the cache to make space. Content can be deleted at any time,
148 and can be stale or fresh. The <a href="programs/htcacheclean.html">htcacheclean</a> tool can be
149 run on a once off basis, or deployed as a daemon to keep the size
150 of the cache within the given size, or the given number of inodes.
151 The tool attempts to delete stale content before attempting to
152 delete fresh content.
156 <p>Full details of how HTTP caching works can be found in
157 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
158 Section 13 of RFC2616</a>.</p>
160 <h3>Interaction with the Server</h3>
163 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module hooks into the server in two
164 possible places depending on the value of the
165 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> directive:
169 <dt>Quick handler phase</dt>
171 <p>This phase happens very early on during the request processing,
172 just after the request has been parsed. If the content is
173 found within the cache, it is served immediately and almost
174 all request processing is bypassed.</p>
176 <p>In this scenario, the cache behaves as if it has been "bolted
177 on" to the front of the server.</p>
179 <p>This mode offers the best performance, as the majority of
180 server processing is bypassed. This mode however also bypasses the
181 authentication and authorization phases of server processing, so
182 this mode should be chosen with care when this is important.</p>
184 <dt>Normal handler phase</dt>
186 <p>This phase happens late in the request processing, after all
187 the request phases have completed.</p>
189 <p>In this scenario, the cache behaves as if it has been "bolted
190 on" to the back of the server.</p>
192 <p>This mode offers the most flexibility, as the potential exists
193 for caching to occur at a precisely controlled point in the filter
194 chain, and cached content can be filtered or personalized before
195 being sent to the client.</p>
199 <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
200 will add a <a href="filter.html">filter</a> to the filter stack in order
201 to record the response to the cache, and then stand down, allowing normal
202 request processing to continue. If the content is determined to be
203 cacheable, the content will be saved to the cache for future serving,
204 otherwise the content will be ignored.</p>
206 <p>If the content found within the cache is stale, the
207 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module converts the request into a
208 <strong>conditional request</strong>. If the origin server responds with
209 a normal response, the normal response is cached, replacing the content
210 already cached. If the origin server responds with a 304 Not Modified
211 response, the content is marked as fresh again, and the cached content
212 is served by the filter instead of saving it.</p>
215 <h3>Improving Cache Hits</h3>
218 <p>When a virtual host is known by one of many different server aliases,
219 ensuring that <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is
220 set to <code>On</code> can dramatically improve the ratio of cache hits.
221 This is because the hostname of the virtual-host serving the content is
222 used within the cache key. With the setting set to <code>On</code>
223 virtual-hosts with multiple server names or aliases will not produce
224 differently cached entities, and instead content will be cached as
225 per the canonical hostname.</p>
229 <h3>Freshness Lifetime</h3>
232 <p>Well formed content that is intended to be cached should declare an
233 explicit freshness lifetime with the <code>Cache-Control</code>
234 header's <code>max-age</code> or <code>s-maxage</code> fields, or
235 by including an <code>Expires</code> header.</p>
237 <p>At the same time, the origin server defined freshness lifetime can
238 be overridden by a client when the client presents their own
239 <code>Cache-Control</code> header within the request. In this case,
240 the lowest freshness lifetime between request and response wins.</p>
242 <p>When this freshness lifetime is missing from the request or the
243 response, a default freshness lifetime is applied. The default
244 freshness lifetime for cached entities is one hour, however
245 this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive.</p>
247 <p>If a response does not include an <code>Expires</code> header but does
248 include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
249 can infer a freshness lifetime based on a heuristic, which can be
250 controlled through the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p>
252 <p>For local content, or for remote content that does not define its own
253 <code>Expires</code> header, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
254 fine-tune the freshness lifetime by adding <code>max-age</code> and
255 <code>Expires</code>.</p>
257 <p>The maximum freshness lifetime may also be controlled by using the
258 <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
262 <h3>A Brief Guide to Conditional Requests</h3>
265 <p>When content expires from the cache and becomes stale, rather than
266 pass on the original request, httpd will modify the request to make
267 it conditional instead.</p>
269 <p>When an <code>ETag</code> header exists in the original cached
270 response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
271 <code>If-None-Match</code> header to the request to the origin server.
272 When a <code>Last-Modified</code> header exists in the original
273 cached response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
274 <code>If-Modified-Since</code> header to the request to the origin
275 server. Performing either of these actions makes the request
276 <strong>conditional</strong>.</p>
278 <p>When a conditional request is received by an origin server, the
279 origin server should check whether the ETag or the Last-Modified
280 parameter has changed, as appropriate for the request. If not, the
281 origin should respond with a terse "304 Not Modified" response. This
282 signals to the cache that the stale content is still fresh should be
283 used for subsequent requests until the content's new freshness lifetime
284 is reached again.</p>
286 <p>If the content has changed, then the content is served as if the
287 request were not conditional to begin with.</p>
289 <p>Conditional requests offer two benefits. Firstly, when making such
290 a request to the origin server, if the content from the origin
291 matches the content in the cache, this can be determined easily and
292 without the overhead of transferring the entire resource.</p>
294 <p>Secondly, a well designed origin server will be designed in such
295 a way that conditional requests will be significantly cheaper to
296 produce than a full response. For static files, typically all that is
297 involved is a call to <code>stat()</code> or similar system call, to
298 see if the file has changed in size or modification time. As such, even
299 local content may still be served faster from the cache if it has not
302 <p>Origin servers should make every effort to support conditional
303 requests as is practical, however if conditional requests are not
304 supported, the origin will respond as if the request was not
305 conditional, and the cache will respond as if the content had changed
306 and save the new content to the cache. In this case, the cache will
307 behave like a simple two state cache, where content is effectively
308 either fresh or deleted.</p>
311 <h3>What Can be Cached?</h3>
314 <p>The full definition of which responses can be cached by an HTTP
316 <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4">
317 RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as
321 <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>
323 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
326 <li>The request must be a HTTP GET request.</li>
328 <li>If the response contains an "Authorization:" header, it must
329 also contain an "s-maxage", "must-revalidate" or "public" option
330 in the "Cache-Control:" header, or it won't be cached.</li>
332 <li>If the URL included a query string (e.g. from a HTML form GET
333 method) it will not be cached unless the response specifies an
334 explicit expiration by including an "Expires:" header or the max-age
335 or s-maxage directive of the "Cache-Control:" header, as per RFC2616
336 sections 13.9 and 13.2.1.</li>
338 <li>If the response has a status of 200 (OK), the response must
339 also include at least one of the "Etag", "Last-Modified" or
340 the "Expires" headers, or the max-age or s-maxage directive of
341 the "Cache-Control:" header, unless the
342 <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
343 directive has been used to require otherwise.</li>
345 <li>If the response includes the "private" option in a "Cache-Control:"
346 header, it will not be stored unless the
347 <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
348 used to require otherwise.</li>
350 <li>Likewise, if the response includes the "no-store" option in a
351 "Cache-Control:" header, it will not be stored unless the
352 <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
355 <li>A response will not be stored if it includes a "Vary:" header
356 containing the match-all "*".</li>
360 <h3>What Should Not be Cached?</h3>
363 <p>It should be up to the client creating the request, or the origin
364 server constructing the response to decide whether or not the content
365 should be cacheable or not by correctly setting the
366 <code>Cache-Control</code> header, and <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> should
367 be left alone to honor the wishes of the client or server as appropriate.
370 <p>Content that is time sensitive, or which varies depending on the
371 particulars of the request that are not covered by HTTP negotiation,
372 should not be cached. This content should declare itself uncacheable
373 using the <code>Cache-Control</code> header.</p>
375 <p>If content changes often, expressed by a freshness lifetime of minutes
376 or seconds, the content can still be cached, however it is highly
377 desirable that the origin server supports
378 <strong>conditional requests</strong> correctly to ensure that
379 full responses do not have to be generated on a regular basis.</p>
381 <p>Content that varies based on client provided request headers can be
382 cached through intelligent use of the <code>Vary</code> response
387 <h3>Variable/Negotiated Content</h3>
390 <p>When the origin server is designed to respond with different content
391 based on the value of headers in the request, for example to serve
392 multiple languages at the same URL, HTTP's caching mechanism makes it
393 possible to cache multiple variants of the same page at the same URL.</p>
395 <p>This is done by the origin server adding a <code>Vary</code> header
396 to indicate which headers must be taken into account by a cache when
397 determining whether two variants are different from one another.</p>
399 <p>If for example, a response is received with a vary header such as;</p>
401 <div class="example"><p><code>
402 Vary: negotiate,accept-language,accept-charset
405 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to
406 requesters with accept-language and accept-charset headers
407 matching those of the original request.</p>
409 <p>Multiple variants of the content can be cached side by side,
410 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> uses the <code>Vary</code> header and the
411 corresponding values of the request headers listed by <code>Vary</code>
412 to decide on which of many variants to return to the client.</p>
415 <h3><a name="disk" id="disk">Caching to Disk</a></h3>
418 <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module relies on specific backend store
419 implementations in order to manage the cache, and for caching to disk
420 <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> is provided to support this.</p>
422 <p>Typically the module will be configured as so;</p>
424 <pre class="prettyprint lang-config">
425 CacheRoot "/var/cache/apache/"
432 <p>Importantly, as the cached files are locally stored, operating system
433 in-memory caching will typically be applied to their access also. So
434 although the files are stored on disk, if they are frequently accessed
435 it is likely the operating system will ensure that they are actually
436 served from memory.</p>
440 <h3>Understanding the Cache-Store</h3>
443 <p>To store items in the cache, <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> creates
444 a 22 character hash of the URL being requested. This hash incorporates
445 the hostname, protocol, port, path and any CGI arguments to the URL,
446 as well as elements defined by the Vary header to ensure that multiple
447 URLs do not collide with one another.</p>
449 <p>Each character may be any one of 64-different characters, which mean
450 that overall there are 64^22 possible hashes. For example, a URL might
451 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
452 as a prefix for the naming of the files specific to that URL within
453 the cache, however first it is split up into directories as per
454 the <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code> and
455 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
458 <p><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
459 specifies how many levels of subdirectory there should be, and
460 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
461 specifies how many characters should be in each directory. With
462 the example settings given above, the hash would be turned into
464 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
466 <p>The overall aim of this technique is to reduce the number of
467 subdirectories or files that may be in a particular directory,
468 as most file-systems slow down as this number increases. With
470 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
471 there can at most be 64 subdirectories at any particular level.
472 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
473 Unless you have a good reason not to, using a setting of "1"
474 for <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
478 <code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
479 depends on how many files you anticipate to store in the cache.
480 With the setting of "2" used in the above example, a grand
481 total of 4096 subdirectories can ultimately be created. With
482 1 million files cached, this works out at roughly 245 cached
483 URLs per directory.</p>
485 <p>Each URL uses at least two files in the cache-store. Typically
486 there is a ".header" file, which includes meta-information about
487 the URL, such as when it is due to expire and a ".data" file
488 which is a verbatim copy of the content to be served.</p>
490 <p>In the case of a content negotiated via the "Vary" header, a
491 ".vary" directory will be created for the URL in question. This
492 directory will have multiple ".data" files corresponding to the
493 differently negotiated content.</p>
496 <h3>Maintaining the Disk Cache</h3>
499 <p>The <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> module makes no attempt to
500 regulate the amount of disk space used by the cache, although it
501 will gracefully stand down on any disk error and behave as if the
502 cache was never present.</p>
504 <p>Instead, provided with httpd is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which allows you
505 to clean the cache periodically. Determining how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to
506 use for the cache is somewhat complex and trial and error may be needed to
507 select optimal values.</p>
509 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
510 operation. It can be run as persistent daemon, or periodically from
511 cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
512 or more to process very large (tens of gigabytes) caches and if you are
513 running it from cron it is recommended that you determine how long a typical
514 run takes, to avoid running more than one instance at a time.</p>
516 <p>It is also recommended that an appropriate "nice" level is chosen for
517 htcacheclean so that the tool does not cause excessive disk io while the
518 server is running.</p>
521 <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
522 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
523 cache growth / clean sequence.</p>
525 <p>Because <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> does not itself pay attention
526 to how much space is used you should ensure that
527 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
528 leave enough "grow room" following a clean.</p>
531 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
532 <div class="section">
533 <h2><a name="socache-caching" id="socache-caching">Two-state Key/Value Shared Object Caching</a></h2>
537 <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>
539 <p>The Apache HTTP server offers a low level shared object cache for
540 caching information such as SSL sessions, or authentication credentials,
541 within the <a href="socache.html">socache</a> interface.</p>
543 <p>Additional modules are provided for each implementation, offering the
544 following backends:</p>
547 <dt><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></dt>
548 <dd>DBM based shared object cache.</dd>
549 <dt><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></dt>
550 <dd>Distcache based shared object cache.</dd>
551 <dt><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></dt>
552 <dd>Memcache based shared object cache.</dd>
553 <dt><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></dt>
554 <dd>Shared memory based shared object cache.</dd>
557 <h3><a name="mod_authn_socache-caching" id="mod_authn_socache-caching">Caching Authentication Credentials</a></h3>
560 <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>
562 <p>The <code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code> module allows the result of
563 authentication to be cached, relieving load on authentication backends.</p>
567 <h3><a name="mod_ssl-caching" id="mod_ssl-caching">Caching SSL Sessions</a></h3>
570 <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>
572 <p>The <code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code> module uses the <code>socache</code> interface
573 to provide a session cache and a stapling cache.</p>
577 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
578 <div class="section">
579 <h2><a name="file-caching" id="file-caching">Specialized File Caching</a></h2>
583 <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>
585 <p>On platforms where a filesystem might be slow, or where file
586 handles are expensive, the option exists to pre-load files into
587 memory on startup.</p>
589 <p>On systems where opening files is slow, the option exists to
590 open the file on startup and cache the file handle. These
591 options can help on systems where access to static files is
594 <h3><a name="filehandle" id="filehandle">File-Handle Caching</a></h3>
597 <p>The act of opening a file can itself be a source of delay, particularly
598 on network filesystems. By maintaining a cache of open file descriptors
599 for commonly served files, httpd can avoid this delay. Currently httpd
600 provides one implementation of File-Handle Caching.</p>
605 <p>The most basic form of caching present in httpd is the file-handle
606 caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching
607 file-contents, this cache maintains a table of open file descriptors. Files
608 to be cached in this manner are specified in the configuration file using
609 the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
613 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive
614 instructs httpd to open the file when it is started and to re-use
615 this file-handle for all subsequent access to this file.</p>
617 <div class="example"><p><code>
618 CacheFile /usr/local/apache2/htdocs/index.html
621 <p>If you intend to cache a large number of files in this manner, you
622 must ensure that your operating system's limit for the number of open
623 files is set appropriately.</p>
625 <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
626 does not cause the file-contents to be cached per-se, it does mean
627 that if the file changes while httpd is running these changes will
628 not be picked up. The file will be consistently served as it was
629 when httpd was started.</p>
631 <p>If the file is removed while httpd is running, it will continue
632 to maintain an open file descriptor and serve the file as it was when
633 httpd was started. This usually also means that although the file
634 will have been deleted, and not show up on the filesystem, extra free
635 space will not be recovered until httpd is stopped and the file
636 descriptor closed.</p>
641 <h3><a name="inmemory" id="inmemory">In-Memory Caching</a></h3>
644 <p>Serving directly from system memory is universally the fastest method
645 of serving content. Reading files from a disk controller or, even worse,
646 from a remote network is orders of magnitude slower. Disk controllers
647 usually involve physical processes, and network access is limited by
648 your available bandwidth. Memory access on the other hand can take mere
651 <p>System memory isn't cheap though, byte for byte it's by far the most
652 expensive type of storage and it's important to ensure that it is used
653 efficiently. By caching files in memory you decrease the amount of
654 memory available on the system. As we'll see, in the case of operating
655 system caching, this is not so much of an issue, but when using
656 httpd's own in-memory caching it is important to make sure that you
657 do not allocate too much memory to a cache. Otherwise the system
658 will be forced to swap out memory, which will likely degrade
661 <h4>Operating System Caching</h4>
664 <p>Almost all modern operating systems cache file-data in memory managed
665 directly by the kernel. This is a powerful feature, and for the most
666 part operating systems get it right. For example, on Linux, let's look at
667 the difference in the time it takes to read a file for the first time
668 and the second time;</p>
670 <div class="example"><pre>
671 colm@coroebus:~$ time cat testfile > /dev/null
675 colm@coroebus:~$ time cat testfile > /dev/null
678 sys 0m0.000s</pre></div>
680 <p>Even for this small file, there is a huge difference in the amount
681 of time it takes to read the file. This is because the kernel has cached
682 the file contents in memory.</p>
684 <p>By ensuring there is "spare" memory on your system, you can ensure
685 that more and more file-contents will be stored in this cache. This
686 can be a very efficient means of in-memory caching, and involves no
687 extra configuration of httpd at all.</p>
689 <p>Additionally, because the operating system knows when files are
690 deleted or modified, it can automatically remove file contents from the
691 cache when necessary. This is a big advantage over httpd's in-memory
692 caching which has no way of knowing when a file has changed.</p>
695 <p>Despite the performance and advantages of automatic operating system
696 caching there are some circumstances in which in-memory caching may be
697 better performed by httpd.</p>
699 <h4>MMapFile Caching</h4>
702 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the
703 <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code> directive, which
704 allows you to have httpd map a static file's contents into memory at
705 start time (using the mmap system call). httpd will use the in-memory
706 contents for all subsequent accesses to this file.</p>
708 <div class="example"><p><code>
709 MMapFile /usr/local/apache2/htdocs/index.html
713 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
714 changes in these files will not be picked up by httpd after it has
717 <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code>
718 directive does not keep track of how much memory it allocates, so
719 you must ensure not to over-use the directive. Each httpd child
720 process will replicate this memory, so it is critically important
721 to ensure that the files mapped are not so large as to cause the
722 system to swap memory.</p>
726 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
727 <div class="section">
728 <h2><a name="security" id="security">Security Considerations</a></h2>
731 <h3>Authorization and Access Control</h3>
734 <p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> in its default state where
735 <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> is set to
736 <code>On</code> is very much like having a caching reverse-proxy bolted
737 to the front of the server. Requests will be served by the caching module
738 unless it determines that the origin server should be queried just as an
739 external cache would, and this drastically changes the security model of
742 <p>As traversing a filesystem hierarchy to examine potential
743 <code>.htaccess</code> files would be a very expensive operation,
744 partially defeating the point of caching (to speed up requests),
745 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> makes no decision about whether a cached
746 entity is authorised for serving. In other words; if
747 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> has cached some content, it will be served
748 from the cache as long as that content has not expired.</p>
750 <p>If, for example, your configuration permits access to a resource by IP
751 address you should ensure that this content is not cached. You can do this
752 by using the <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code>
753 directive, or <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code>. Left unchecked,
754 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> - very much like a reverse proxy - would cache
755 the content when served and then serve it to any client, on any IP
758 <p>When the <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code>
759 directive is set to <code>Off</code>, the full set of request processing
760 phases are executed and the security model remains unchanged.</p>
763 <h3>Local exploits</h3>
766 <p>As requests to end-users can be served from the cache, the cache
767 itself can become a target for those wishing to deface or interfere with
768 content. It is important to bear in mind that the cache must at all
769 times be writable by the user which httpd is running as. This is in
770 stark contrast to the usually recommended situation of maintaining
771 all content unwritable by the Apache user.</p>
773 <p>If the Apache user is compromised, for example through a flaw in
774 a CGI process, it is possible that the cache may be targeted. When
775 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>, it is relatively easy to
776 insert or modify a cached entity.</p>
778 <p>This presents a somewhat elevated risk in comparison to the other
779 types of attack it is possible to make as the Apache user. If you are
780 using <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> you should bear this in mind -
781 ensure you upgrade httpd when security upgrades are announced and
782 run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>
786 <h3>Cache Poisoning</h3>
789 <p>When running httpd as a caching proxy server, there is also the
790 potential for so-called cache poisoning. Cache Poisoning is a broad
791 term for attacks in which an attacker causes the proxy server to
792 retrieve incorrect (and usually undesirable) content from the origin
795 <p>For example if the DNS servers used by your system running httpd
796 are vulnerable to DNS cache poisoning, an attacker may be able to control
797 where httpd connects to when requesting content from the origin server.
798 Another example is so-called HTTP request-smuggling attacks.</p>
800 <p>This document is not the correct place for an in-depth discussion
801 of HTTP request smuggling (instead, try your favourite search engine)
802 however it is important to be aware that it is possible to make
803 a series of requests, and to exploit a vulnerability on an origin
804 webserver such that the attacker can entirely control the content
805 retrieved by the proxy.</p>
808 <h3>Denial of Service / Cachebusting</h3>
811 <p>The Vary mechanism allows multiple variants of the same URL to be
812 cached side by side. Depending on header values provided by the client,
813 the cache will select the correct variant to return to the client. This
814 mechanism can become a problem when an attempt is made to vary on a
815 header that is known to contain a wide range of possible values under
816 normal use, for example the <code>User-Agent</code> header. Depending
817 on the popularity of the particular web site thousands or millions of
818 duplicate cache entries could be created for the same URL, crowding
819 out other entries in the cache.</p>
821 <p>In other cases, there may be a need to change the URL of a particular
822 resource on every request, usually by adding a "cachebuster" string to
823 the URL. If this content is declared cacheable by a server for a
824 significant freshness lifetime, these entries can crowd out
825 legitimate entries in a cache. While <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
827 <code class="directive"><a href="./mod/mod_cache.html#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></code>
828 directive, this directive should be used with care to ensure that
829 downstream proxy or browser caches aren't subjected to the same denial
830 of service issue.</p>
833 <div class="bottomlang">
834 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
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837 </div><div id="footer">
838 <p class="apache">Copyright 2012 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>
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