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14 <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>
15 <p class="apache">Apache HTTP Server Version 2.1</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-project/">Documentation</a> > <a href="./">Version 2.1</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></p>
24 <p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>,
25 <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>, <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code>,
26 <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.
27 It describes how to use Apache's caching features to accelerate web and
28 proxy serving, while avoiding common problems and misconfigurations.</p>
30 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
31 <li><img alt="" src="./images/down.gif" /> <a href="#overview">Caching Overview</a></li>
32 <li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
33 <li><img alt="" src="./images/down.gif" /> <a href="#filehandle">File-Handle Caching</a></li>
34 <li><img alt="" src="./images/down.gif" /> <a href="#inmemory">In-Memory Caching</a></li>
35 <li><img alt="" src="./images/down.gif" /> <a href="#disk">Disk-based Caching</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>As of Apache HTTP server version 2.2 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
43 and <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> are no longer marked
44 experimental and are considered suitable for production use. These
45 caching architectures provide a powerful means to accelerate HTTP
46 handling, both as an origin webserver and as a proxy.</p>
48 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> and its provider modules
49 <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> and <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>
50 provide intelligent, HTTP-aware caching. The content itself is stored
51 in the cache, and mod_cache aims to honour all of the various HTTP
52 headers and options that control the cachability of content. It can
53 handle both local and proxied content. <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
54 is aimed at both simple and complex caching configurations, where
55 you are dealing with proxied content, dynamic local content or
56 have a need to speed up access to local files which change with
59 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> on the other hand presents a more
60 basic, but sometimes useful, form of caching. Rather than maintain
61 the complexity of actively ensuring the cachability of URLs,
62 <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> offers file-handle and memory-mapping
63 tricks to keep a cache of files as they were when Apache was last
64 started. As such, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> is aimed at improving
65 the access time to local static files which do not change very
68 <p>As <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> presents a relatively simple
69 caching implementation, apart from the specific sections on <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> and <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code>, the explanations
70 in this guide cover the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching
73 <p>To get the most from this document, you should be familiar with
74 the basics of HTTP, and have read the Users' Guides to
75 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
76 <a href="content-negotiation.html">Content negotiation</a>.</p>
78 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
80 <h2><a name="overview" id="overview">Caching Overview</a></h2>
84 <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_mem_cache.html">mod_mem_cache</a></code></li><li><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code></li><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_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/mod_file_cache.html#mmapstatic">MMapStatic</a></code></li><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#cachefile">CacheFile</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>
86 <p>There are two main stages in <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> that can
87 occur in the lifetime of a request. First, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
88 is a URL mapping module, which means that if a URL has been cached,
89 and the cached version of that URL has not expired, the request will
90 be served directly by <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>.</p>
92 <p>This means that any other stages that might ordinarily happen
93 in the process of serving a request -- for example being handled
94 by <code class="module"><a href="./mod/mod_proxy.html">mod_proxy</a></code>, or <code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code> --
95 won't happen. But then this is the point of caching content in
98 <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
99 will add a <a href="filter.html">filter</a> to the request handling. After
100 Apache has located the content by the usual means, the filter will be run
101 as the content is served. If the content is determined to be cacheable,
102 the content will be saved to the cache for future serving.</p>
104 <p>If the URL is found within the cache, but also found to have expired,
105 the filter is added anyway, but <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will create
106 a conditional request to the backend, to determine if the cached version
107 is still current. If the cached version is still current, its
108 meta-information will be updated and the request will be served from the
109 cache. If the cached version is no longer current, the cached version
110 will be deleted and the filter will save the updated content to the cache
113 <h3>Improving Cache Hits</h3>
116 <p>When caching locally generated content, ensuring that
117 <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is set to
118 <code>On</code> can dramatically improve the ratio of cache hits. This
119 is because the hostname of the virtual-host serving the content forms
120 a part of the cache key. With the setting set to <code>On</code>
121 virtual-hosts with multiple server names or aliases will not produce
122 differently cached entities, and instead content will be cached as
123 per the canonical hostname.</p>
125 <p>Because caching is performed within the URL to filename translation
126 phase, cached documents will only be served in response to URL requests.
127 Ordinarily this is of little consequence, but there is one circumstance
128 in which it matters: If you are using <a href="howto/ssi.html">Server
129 Side Includes</a>;</p>
131 <div class="example"><pre>
132 <!-- The following include can be cached -->
133 <!--#include virtual="/footer.html" -->
135 <!-- The following include can not be cached -->
136 <!--#include file="/path/to/footer.html" --></pre></div>
138 <p>If you are using Server Side Includes, and want the benefit of speedy
139 serves from the cache, you should use <code>virtual</code> include
143 <h3>Expiry Periods</h3>
146 <p>The default expiry period for cached entities is one hour, however
147 this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive. This
148 default is only used when the original source of the content does not
149 specify an expire time or time of last modification.</p>
151 <p>If a response does not include an <code>Expires</code> header but does
152 include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
153 can infer an expiry period based on the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p>
155 <p>For local content, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
156 fine-tune the expiry period.</p>
158 <p>The maximum expiry period may also be controlled by using the
159 <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
163 <h3>A Brief Guide to Conditional Requests</h3>
166 <p>When content expires from the cache and is re-requested from the
167 backend or content provider, rather than pass on the original request,
168 Aoache will use a conditional request instead.</p>
170 <p>HTTP offers a number of headers which allow a client, or cache
171 to discern between different versions of the same content. For
172 example if a resource was served with an "Etag:" header, it is
173 possible to make a conditional request with an "If-Match:"
174 header. If a resource was served with a "Last-Modified:" header
175 it is possible to make a conditional request with an
176 "If-Modified-Since:" header, and so on.</p>
178 <p>When such a conditional request is made, the response differs
179 depending on whether the content matches the conditions. If a request is
180 made with an "If-Modified-Since:" header, and the content has not been
181 modified since the time indicated in the request then a terse "304 Not
182 Modified" response is issued.</p>
184 <p>If the content has changed, then it is served as if the request were
185 not conditional to begin with.</p>
187 <p>The benefits of conditional requests in relation to caching are
188 twofold. Firstly, when making such a request to the backend, if the
189 content from the backend matches the content in the store, this can be
190 determined easily and without the overhead of transferring the entire
193 <p>Secondly, conditional requests are usually less strenuous on the
194 backend. For static files, typically all that is involved is a call
195 to <code>stat()</code> or similar system call, to see if the file has
196 changed in size or modification time. As such, even if Apache is
197 caching local content, even expired content may still be served faster
198 from the cache if it has not changed. As long as reading from the cache
199 store is faster than reading from the backend (e.g. an in-memory cache
200 compared to reading from disk).</p>
203 <h3>What Can be Cached?</h3>
206 <p>As mentioned already, the two styles of caching in Apache work
207 differently, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> caching maintains file
208 contents as they were when Apache was started. When a request is
209 made for a file that is cached by this module, it is intercepted
210 and the cached file is served.</p>
212 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching on the other hand is more
213 complex. When serving a request, if it has not been cached
214 previously, the caching module will determine if the content
215 is cacheable. The conditions for determining cachability of
219 <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>
221 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
224 <li>The request must be a HTTP GET request.</li>
226 <li>If the request contains an "Authorization:" header, the response
227 will not be cached.</li>
229 <li>If the response contains an "Authorization:" header, it must
230 also contain an "s-maxage", "must-revalidate" or "public" option
231 in the "Cache-Control:" header.</li>
233 <li>If the URL included a query string (e.g. from a HTML form GET
234 method) it will not be cached unless the response includes an
235 "Expires:" header, as per RFC2616 section 13.9.</li>
237 <li>If the response has a status of 200 (OK), the response must
238 also include at least one of the "Etag", "Last-Modified" or
239 the "Expires" headers, unless the
240 <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
241 directive has been used to require otherwise.</li>
243 <li>If the response includes the "private" option in a "Cache-Control:"
244 header, it will not be stored unless the
245 <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
246 used to require otherwise.</li>
248 <li>Likewise, if the response includes the "no-store" option in a
249 "Cache-Control:" header, it will not be stored unless the
250 <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
253 <li>A response will not be stored if it includes a "Vary:" header
254 containing the match-all "*".</li>
258 <h3>What Should Not be Cached?</h3>
261 <p>In short, any content which is highly time-sensitive, or which varies
262 depending on the particulars of the request that are not covered by
263 HTTP negotiation, should not be cached.</p>
265 <p>If you have dynamic content which changes depending on the IP address
266 of the requester, or changes every 5 minutes, it should almost certainly
269 <p>If on the other hand, the content served differs depending on the
270 values of various HTTP headers, it is possible that it might be possible
271 to cache it intelligently through the use of a "Vary" header.</p>
274 <h3>Variable/Negotiated Content</h3>
277 <p>If a response with a "Vary" header is received by
278 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> when requesting content by the backend it
279 will attempt to handle it intelligently. If possible,
280 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will detect the headers attributed in the
281 "Vary" response in future requests and serve the correct cached
284 <p>If for example, a response is received with a vary header such as;</p>
286 <div class="example"><p><code>
287 Vary: negotiate,accept-language,accept-charset
290 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to
291 requesters with matching accept-language and accept-charset headers
292 matching those of the original request.</p>
295 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
296 <div class="section">
297 <h2><a name="security" id="security">Security Considerations</a></h2>
300 <h3>Local exploits</h3>
303 <p>As requests to end-users can be served from the cache, the cache
304 itself can become a target for those wishing to deface or interfere with
305 content. It is important to bear in mind that the cache must at all
306 times be writable by the user which Apache is running as. This is in
307 stark contrast to the usually recommended situation of maintaining
308 all content unwritable by the Apache user.</p>
310 <p>If the Apache user is compromised, for example through a flaw in
311 a CGI process, it is possible that the cache may be targeted. When
312 using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>, it is relatively easy to
313 insert or modify a cached entity.</p>
315 <p>This presents a somewhat elevated risk in comparison to the other
316 types of attack it is possible to make as the Apache user. If you are
317 using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> you should bear this in mind -
318 ensure you upgrade Apache when security upgrades are announced and
319 run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>
323 <h3>Cache Poisoning</h3>
326 <p>When running Apache as a caching proxy server, there is also the
327 potential for so-called cache poisoning. Cache Poisoning is a broad
328 term for attacks in which an attacker causes the proxy server to
329 retrieve incorrect (and usually undesirable) content from the backend.
332 <p>For example if the DNS servers used by your system running Apache
333 are vulnerable to DNS cache poisoning, an attacker may be able to control
334 where Apache connects to when requesting content from the origin server.
335 Another example is so-called HTTP request-smuggling attacks.</p>
337 <p>This document is not the correct place for an in-depth discussion
338 of HTTP request smuggling (instead, try your favourite search engine)
339 however it is important to be aware that it is possible to make
340 a series of requests, and to exploit a vulnerability on an origin
341 webserver such that the attacker can entirely control the content
342 retrieved by the proxy.</p>
344 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
345 <div class="section">
346 <h2><a name="filehandle" id="filehandle">File-Handle Caching</a></h2>
349 <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><li><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_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_cache.html#cacheenable">CacheEnable</a></code></li><li><code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code></li></ul></td></tr></table>
351 <p>The act of opening a file can itself be a source of delay, particularly
352 on network filesystems. By maintaining a cache of open file descriptors
353 for commonly served files, Apache can avoid this delay. Currently Apache
354 provides two different implementations of File-Handle Caching.</p>
359 <p>The most basic form of caching present in Apache is the file-handle
360 caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching
361 file-contents, this cache maintains a table of open file descriptors. Files
362 to be cached in this manner are specified in the configuration file using
363 the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
367 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive
368 instructs Apache to open the file when Apache is started and to re-use
369 this file-handle for all subsequent access to this file.</p>
371 <div class="example"><pre>CacheFile /usr/local/apache2/htdocs/index.html</pre></div>
373 <p>If you intend to cache a large number of files in this manner, you
374 must ensure that your operating system's limit for the number of open
375 files is set appropriately.</p>
377 <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
378 does not cause the file-contents to be cached per-se, it does mean
379 that if the file changes while Apache is running these changes will
380 not be picked up. The file will be consistently served as it was
381 when Apache was started.</p>
383 <p>If the file is removed while Apache is running, Apache will continue
384 to maintain an open file descriptor and serve the file as it was when
385 Apache was started. This usually also means that although the file
386 will have been deleted, and not show up on the filesystem, extra free
387 space will not be recovered until Apache is stopped and the file
388 descriptor closed.</p>
391 <h3>CacheEnable fd</h3>
394 <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> also provides its own file-handle
395 caching scheme, which can be enabled via the
396 <code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code> directive.</p>
398 <div class="example"><pre>CacheEnable fd /</pre></div>
400 <p>As with all of <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> this type of file-handle
401 caching is intelligent, and handles will not be maintained beyond
402 the expiry time of the cached content.</p>
404 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
405 <div class="section">
406 <h2><a name="inmemory" id="inmemory">In-Memory Caching</a></h2>
409 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code></li><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_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/mod_file_cache.html#mmapstatic">MMapStatic</a></code></li></ul></td></tr></table>
411 <p>Serving directly from system memory is universally the fastest method
412 of serving content. Reading files from a disk controller or, even worse,
413 from a remote network is orders of magnitude slower. Disk controllers
414 usually involve physical processes, and network access is limited by
415 your available bandwidth. Memory access on the other hand can take mere
418 <p>System memory isn't cheap though, byte for byte it's by far the most
419 expensive type of storage and it's important to ensure that it is used
420 efficiently. By caching files in memory you decrease the amount of
421 memory available on the system. As we'll see, in the case of operating
422 system caching, this is not so much of an issue, but when using
423 Apache's own in-memory caching it is important to make sure that you
424 do not allocate too much memory to a cache. Otherwise the system
425 will be forced to swap out memory, which will likely degrade
428 <h3>Operating System Caching</h3>
431 <p>Almost all modern operating systems cache file-data in memory managed
432 directly by the kernel. This is a powerful feature, and for the most
433 part operating systems get it right. For example, on Linux, let's look at
434 the difference in the time it takes to read a file for the first time
435 and the second time;</p>
437 <div class="example"><pre>
438 colm@coroebus:~$ time cat testfile > /dev/null
442 colm@coroebus:~$ time cat testfile > /dev/null
445 sys 0m0.000s</pre></div>
447 <p>Even for this small file, there is a huge difference in the amount
448 of time it takes to read the file. This is because the kernel has cached
449 the file contents in memory.</p>
451 <p>By ensuring there is "spare" memory on your system, you can ensure
452 that more and more file-contents will be stored in this cache. This
453 can be a very efficient means of in-memory caching, and involves no
454 extra configuration of Apache at all.</p>
456 <p>Additionally, because the operating system knows when files are
457 deleted or modified, it can automatically remove file contents from the
458 cache when neccessary. This is a big advantage over Apache's in-memory
459 caching which has no way of knowing when a file has changed.</p>
462 <p>Despite the performance and advantages of automatic operating system
463 caching there are some circumstances in which in-memory caching may be
464 better performed by Apache.</p>
466 <p>Firstly, an operating system can only cache files it knows about. If
467 you are running Apache as a proxy server, the files you are caching are
468 not locally stored but remotely served. If you still want the unbeatable
469 speed of in-memory caching, Apache's own memory caching is needed.</p>
471 <h3>MMapStatic Caching</h3>
474 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the
475 <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> directive, which
476 allows you to have Apache map a static file's contents into memory at
477 start time (using the mmap system call). Apache will use the in-memory
478 contents for all subsequent accesses to this file.</p>
480 <div class="example"><pre>MMapStatic /usr/local/apache2/htdocs/index.html</pre></div>
483 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
484 changes in these files will not be picked up by Apache after it has
487 <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code>
488 directive does not keep track of how much memory it allocates, so
489 you must ensure not to over-use the directive. Each Apache child
490 process will replicate this memory, so it is critically important
491 to ensure that the files mapped are not so large as to cause the
492 system to swap memory.</p>
495 <h3>mod_mem_cache Caching</h3>
498 <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> provides a HTTP-aware intelligent
499 in-memory cache. It also uses heap memory directly, which means that
500 even if <var>MMap</var> is not supported on your system,
501 <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> may still be able to perform caching.</p>
503 <p>Caching of this type is enabled via;</p>
505 <div class="example"><pre>
506 # Enable memory caching
509 # Limit the size of the cache to 1 Megabyte
510 MCacheSize 1024</pre></div>
512 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
513 <div class="section">
514 <h2><a name="disk" id="disk">Disk-based Caching</a></h2>
517 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</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></ul></td></tr></table>
519 <p><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> provides a disk-based caching mechanism
520 for <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>. As with <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code>
521 this cache is intelligent and content will be served from the cache only
522 as long as it is considered valid.</p>
524 <p>Typically the module will be configured as so;</p>
526 <div class="example"><pre>
527 CacheRoot /var/cache/apache/
530 CacheDirLength 1</pre></div>
532 <p>Importantly, as the cached files are locally stored, operating system
533 in-memory caching will typically be applied to their access also. So
534 although the files are stored on disk, if they are frequently accessed
535 it is likely the operating system will ensure that they are actually
536 served from memory.</p>
538 <h3>Understanding the Cache-Store</h3>
541 <p>To store items in the cache, <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> creates
542 a 22 character hash of the url being requested. Thie hash incorporates
543 the hostname, protocol, port, path and any CGI arguments to the URL,
544 to ensure that multiple URLs do not collide.</p>
546 <p>Each character may be any one of 64-different characters, which mean
547 that overall there are 22^64 possible hashes. For example, a URL might
548 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
549 as a prefix for the naming of the files specific to that url within
550 the cache, however first it is split up into directories as per
551 the <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> and
552 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
555 <p><code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code>
556 specifies how many levels of subdirectory there should be, and
557 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
558 specifies how many characters should be in each directory. With
559 the example settings given above, the hash would be turned into
561 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
563 <p>The overall aim of this technique is to reduce the number of
564 subdirectories or files that may be in a particular directory,
565 as most file-systems slow down as this number increases. With
567 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
568 there can at most be 64 subdirectories at any particular level.
569 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
570 Unless you have a good reason not to, using a setting of "1"
571 for <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
575 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code>
576 depends on how many files you anticipate to store in the cache.
577 With the setting of "2" used in the above example, a grand
578 total of 4096 subdirectories can ultimately be created. With
579 1 million files cached, this works out at roughly 245 cached
580 urls per directory.</p>
582 <p>Each url uses at least two files in the cache-store. Typically
583 there is a ".header" file, which includes meta-information about
584 the url, such as when it is due to expire and a ".data" file
585 which is a verbatim copy of the content to be served.</p>
587 <p>In the case of a content negotiated via the "Vary" header, a
588 ".vary" directory will be created for the url in question. This
589 directory will have multiple ".data" files corresponding to the
590 differently negotiated content.</p>
593 <h3>Maintaining the Disk Cache</h3>
596 <p>Although <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> will remove cached content
597 as it is expired, it does not maintain any information on the total
598 size of the cache or how little free space may be left.</p>
600 <p>Instead, provided with Apache is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which, as the name
601 suggests, allows you to clean the cache periodically. Determining
602 how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to
603 use for the cache is somewhat complex and trial and error may be needed to
604 select optimal values.</p>
606 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
607 operation. It can be run as persistent daemon, or periodically from
608 cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
609 or more to process very large (tens of gigabytes) caches and if you are
610 running it from cron it is recommended that you determine how long a typical
611 run takes, to avoid running more than one instance at a time.</p>
614 <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
615 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
616 cache growth / clean sequence.</p>
618 <p>Because <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> does not itself pay attention
619 to how much space is used you should ensure that
620 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
621 leave enough "grow room" following a clean.</p>
625 <div class="bottomlang">
626 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a></p>
627 </div><div id="footer">
628 <p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
629 <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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