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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 2.3</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 2.3</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_disk_cache.html">mod_disk_cache</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="#overview">Caching Overview</a></li>
33 <li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
34 <li><img alt="" src="./images/down.gif" /> <a href="#filehandle">File-Handle Caching</a></li>
35 <li><img alt="" src="./images/down.gif" /> <a href="#inmemory">In-Memory Caching</a></li>
36 <li><img alt="" src="./images/down.gif" /> <a href="#disk">Disk-based Caching</a></li>
38 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
40 <h2><a name="introduction" id="introduction">Introduction</a></h2>
43 <p>As of Apache HTTP server version 2.2 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
44 and <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> are no longer marked
45 experimental and are considered suitable for production use. These
46 caching architectures provide a powerful means to accelerate HTTP
47 handling, both as an origin webserver and as a proxy.</p>
49 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> and its provider modules
50 <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>
51 provide intelligent, HTTP-aware caching. The content itself is stored
52 in the cache, and mod_cache aims to honor all of the various HTTP
53 headers and options that control the cachability of content. It can
54 handle both local and proxied content. <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
55 is aimed at both simple and complex caching configurations, where
56 you are dealing with proxied content, dynamic local content or
57 have a need to speed up access to local files which change with
60 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> on the other hand presents a more
61 basic, but sometimes useful, form of caching. Rather than maintain
62 the complexity of actively ensuring the cachability of URLs,
63 <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> offers file-handle and memory-mapping
64 tricks to keep a cache of files as they were when httpd was last
65 started. As such, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> is aimed at improving
66 the access time to local static files which do not change very
69 <p>As <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> presents a relatively simple
70 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#mmapfile">MMapFile</a></code>, the explanations
71 in this guide cover the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching
74 <p>To get the most from this document, you should be familiar with
75 the basics of HTTP, and have read the Users' Guides to
76 <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
77 <a href="content-negotiation.html">Content negotiation</a>.</p>
79 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
81 <h2><a name="overview" id="overview">Caching Overview</a></h2>
85 <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_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#cachefile">CacheFile</a></code></li><li><code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</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>
87 <p>There are two main stages in <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> that can
88 occur in the lifetime of a request. First, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
89 is a URL mapping module, which means that if a URL has been cached,
90 and the cached version of that URL has not expired, the request will
91 be served directly by <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>.</p>
93 <p>This means that any other stages that might ordinarily happen
94 in the process of serving a request -- for example being handled
95 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> --
96 won't happen. But then this is the point of caching content in
99 <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
100 will add a <a href="filter.html">filter</a> to the request handling. After
101 httpd has located the content by the usual means, the filter will be run
102 as the content is served. If the content is determined to be cacheable,
103 the content will be saved to the cache for future serving.</p>
105 <p>If the URL is found within the cache, but also found to have expired,
106 the filter is added anyway, but <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will create
107 a conditional request to the backend, to determine if the cached version
108 is still current. If the cached version is still current, its
109 meta-information will be updated and the request will be served from the
110 cache. If the cached version is no longer current, the cached version
111 will be deleted and the filter will save the updated content to the cache
114 <h3>Improving Cache Hits</h3>
117 <p>When caching locally generated content, ensuring that
118 <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is set to
119 <code>On</code> can dramatically improve the ratio of cache hits. This
120 is because the hostname of the virtual-host serving the content forms
121 a part of the cache key. With the setting set to <code>On</code>
122 virtual-hosts with multiple server names or aliases will not produce
123 differently cached entities, and instead content will be cached as
124 per the canonical hostname.</p>
126 <p>Because caching is performed within the URL to filename translation
127 phase, cached documents will only be served in response to URL requests.
128 Ordinarily this is of little consequence, but there is one circumstance
129 in which it matters: If you are using <a href="howto/ssi.html">Server
130 Side Includes</a>;</p>
132 <div class="example"><pre>
133 <!-- The following include can be cached -->
134 <!--#include virtual="/footer.html" -->
136 <!-- The following include can not be cached -->
137 <!--#include file="/path/to/footer.html" --></pre></div>
139 <p>If you are using Server Side Includes, and want the benefit of speedy
140 serves from the cache, you should use <code>virtual</code> include
144 <h3>Expiry Periods</h3>
147 <p>The default expiry period for cached entities is one hour, however
148 this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive. This
149 default is only used when the original source of the content does not
150 specify an expire time or time of last modification.</p>
152 <p>If a response does not include an <code>Expires</code> header but does
153 include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
154 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>
156 <p>For local content, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
157 fine-tune the expiry period.</p>
159 <p>The maximum expiry period may also be controlled by using the
160 <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
164 <h3>A Brief Guide to Conditional Requests</h3>
167 <p>When content expires from the cache and is re-requested from the
168 backend or content provider, rather than pass on the original request,
169 httpd will use a conditional request instead.</p>
171 <p>HTTP offers a number of headers which allow a client, or cache
172 to discern between different versions of the same content. For
173 example if a resource was served with an "Etag:" header, it is
174 possible to make a conditional request with an "If-None-Match:"
175 header. If a resource was served with a "Last-Modified:" header
176 it is possible to make a conditional request with an
177 "If-Modified-Since:" header, and so on.</p>
179 <p>When such a conditional request is made, the response differs
180 depending on whether the content matches the conditions. If a request is
181 made with an "If-Modified-Since:" header, and the content has not been
182 modified since the time indicated in the request then a terse "304 Not
183 Modified" response is issued.</p>
185 <p>If the content has changed, then it is served as if the request were
186 not conditional to begin with.</p>
188 <p>The benefits of conditional requests in relation to caching are
189 twofold. Firstly, when making such a request to the backend, if the
190 content from the backend matches the content in the store, this can be
191 determined easily and without the overhead of transferring the entire
194 <p>Secondly, conditional requests are usually less strenuous on the
195 backend. For static files, typically all that is involved is a call
196 to <code>stat()</code> or similar system call, to see if the file has
197 changed in size or modification time. As such, even if httpd is
198 caching local content, even expired content may still be served faster
199 from the cache if it has not changed. As long as reading from the cache
200 store is faster than reading from the backend (e.g. <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> with memory disk
201 compared to reading from disk).</p>
204 <h3>What Can be Cached?</h3>
207 <p>As mentioned already, the two styles of caching in httpd work
208 differently, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> caching maintains file
209 contents as they were when httpd was started. When a request is
210 made for a file that is cached by this module, it is intercepted
211 and the cached file is served.</p>
213 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching on the other hand is more
214 complex. When serving a request, if it has not been cached
215 previously, the caching module will determine if the content
216 is cacheable. The conditions for determining cachability of
220 <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>
222 <li>The response must have a HTTP status code of 200, 203, 300, 301 or
225 <li>The request must be a HTTP GET request.</li>
227 <li>If the request contains an "Authorization:" header, the response
228 will not be cached.</li>
230 <li>If the response contains an "Authorization:" header, it must
231 also contain an "s-maxage", "must-revalidate" or "public" option
232 in the "Cache-Control:" header.</li>
234 <li>If the URL included a query string (e.g. from a HTML form GET
235 method) it will not be cached unless the response specifies an
236 explicit expiration by including an "Expires:" header or the max-age
237 or s-maxage directive of the "Cache-Control:" header, as per RFC2616
238 sections 13.9 and 13.2.1.</li>
240 <li>If the response has a status of 200 (OK), the response must
241 also include at least one of the "Etag", "Last-Modified" or
242 the "Expires" headers, or the max-age or s-maxage directive of
243 the "Cache-Control:" header, unless the
244 <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
245 directive has been used to require otherwise.</li>
247 <li>If the response includes the "private" option in a "Cache-Control:"
248 header, it will not be stored unless the
249 <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
250 used to require otherwise.</li>
252 <li>Likewise, if the response includes the "no-store" option in a
253 "Cache-Control:" header, it will not be stored unless the
254 <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
257 <li>A response will not be stored if it includes a "Vary:" header
258 containing the match-all "*".</li>
262 <h3>What Should Not be Cached?</h3>
265 <p>In short, any content which is highly time-sensitive, or which varies
266 depending on the particulars of the request that are not covered by
267 HTTP negotiation, should not be cached.</p>
269 <p>If you have dynamic content which changes depending on the IP address
270 of the requester, or changes every 5 minutes, it should almost certainly
273 <p>If on the other hand, the content served differs depending on the
274 values of various HTTP headers, it might be possible
275 to cache it intelligently through the use of a "Vary" header.</p>
278 <h3>Variable/Negotiated Content</h3>
281 <p>If a response with a "Vary" header is received by
282 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> when requesting content by the backend it
283 will attempt to handle it intelligently. If possible,
284 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will detect the headers attributed in the
285 "Vary" response in future requests and serve the correct cached
288 <p>If for example, a response is received with a vary header such as;</p>
290 <div class="example"><p><code>
291 Vary: negotiate,accept-language,accept-charset
294 <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to
295 requesters with accept-language and accept-charset headers
296 matching those of the original request.</p>
298 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
299 <div class="section">
300 <h2><a name="security" id="security">Security Considerations</a></h2>
303 <h3>Authorization and Access Control</h3>
306 <p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> is very much like having a built
307 in reverse-proxy. Requests will be served by the caching module unless
308 it determines that the backend should be queried. When caching local
309 resources, this drastically changes the security model of httpd.</p>
311 <p>As traversing a filesystem hierarchy to examine potential
312 <code>.htaccess</code> files would be a very expensive operation,
313 partially defeating the point of caching (to speed up requests),
314 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> makes no decision about whether a cached
315 entity is authorised for serving. In other words; if
316 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> has cached some content, it will be served
317 from the cache as long as that content has not expired.</p>
319 <p>If, for example, your configuration permits access to a resource by IP
320 address you should ensure that this content is not cached. You can do this
321 by using the <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code>
322 directive, or <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code>. Left unchecked,
323 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> - very much like a reverse proxy - would cache
324 the content when served and then serve it to any client, on any IP
328 <h3>Local exploits</h3>
331 <p>As requests to end-users can be served from the cache, the cache
332 itself can become a target for those wishing to deface or interfere with
333 content. It is important to bear in mind that the cache must at all
334 times be writable by the user which httpd is running as. This is in
335 stark contrast to the usually recommended situation of maintaining
336 all content unwritable by the Apache user.</p>
338 <p>If the Apache user is compromised, for example through a flaw in
339 a CGI process, it is possible that the cache may be targeted. When
340 using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>, it is relatively easy to
341 insert or modify a cached entity.</p>
343 <p>This presents a somewhat elevated risk in comparison to the other
344 types of attack it is possible to make as the Apache user. If you are
345 using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> you should bear this in mind -
346 ensure you upgrade httpd when security upgrades are announced and
347 run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>
351 <h3>Cache Poisoning</h3>
354 <p>When running httpd as a caching proxy server, there is also the
355 potential for so-called cache poisoning. Cache Poisoning is a broad
356 term for attacks in which an attacker causes the proxy server to
357 retrieve incorrect (and usually undesirable) content from the backend.
360 <p>For example if the DNS servers used by your system running
362 are vulnerable to DNS cache poisoning, an attacker may be able to control
363 where httpd connects to when requesting content from the origin server.
364 Another example is so-called HTTP request-smuggling attacks.</p>
366 <p>This document is not the correct place for an in-depth discussion
367 of HTTP request smuggling (instead, try your favourite search engine)
368 however it is important to be aware that it is possible to make
369 a series of requests, and to exploit a vulnerability on an origin
370 webserver such that the attacker can entirely control the content
371 retrieved by the proxy.</p>
373 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
374 <div class="section">
375 <h2><a name="filehandle" id="filehandle">File-Handle Caching</a></h2>
378 <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></ul></td></tr></table>
380 <p>The act of opening a file can itself be a source of delay, particularly
381 on network filesystems. By maintaining a cache of open file descriptors
382 for commonly served files, httpd can avoid this delay. Currently
384 provides one implementation of File-Handle Caching.</p>
389 <p>The most basic form of caching present in httpd is the file-handle
390 caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching
391 file-contents, this cache maintains a table of open file descriptors. Files
392 to be cached in this manner are specified in the configuration file using
393 the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
397 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive
398 instructs httpd to open the file when it is started and to re-use
399 this file-handle for all subsequent access to this file.</p>
401 <div class="example"><pre>CacheFile /usr/local/apache2/htdocs/index.html</pre></div>
403 <p>If you intend to cache a large number of files in this manner, you
404 must ensure that your operating system's limit for the number of open
405 files is set appropriately.</p>
407 <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code>
408 does not cause the file-contents to be cached per-se, it does mean
409 that if the file changes while httpd is running these changes will
410 not be picked up. The file will be consistently served as it was
411 when httpd was started.</p>
413 <p>If the file is removed while httpd is running, it will continue
414 to maintain an open file descriptor and serve the file as it was when
415 httpd was started. This usually also means that although the file
416 will have been deleted, and not show up on the filesystem, extra free
417 space will not be recovered until httpd is stopped and the file
418 descriptor closed.</p>
421 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
422 <div class="section">
423 <h2><a name="inmemory" id="inmemory">In-Memory Caching</a></h2>
426 <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_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#mmapfile">MMapFile</a></code></li></ul></td></tr></table>
428 <p>Serving directly from system memory is universally the fastest method
429 of serving content. Reading files from a disk controller or, even worse,
430 from a remote network is orders of magnitude slower. Disk controllers
431 usually involve physical processes, and network access is limited by
432 your available bandwidth. Memory access on the other hand can take mere
435 <p>System memory isn't cheap though, byte for byte it's by far the most
436 expensive type of storage and it's important to ensure that it is used
437 efficiently. By caching files in memory you decrease the amount of
438 memory available on the system. As we'll see, in the case of operating
439 system caching, this is not so much of an issue, but when using
440 httpd's own in-memory caching it is important to make sure that you
441 do not allocate too much memory to a cache. Otherwise the system
442 will be forced to swap out memory, which will likely degrade
445 <h3>Operating System Caching</h3>
448 <p>Almost all modern operating systems cache file-data in memory managed
449 directly by the kernel. This is a powerful feature, and for the most
450 part operating systems get it right. For example, on Linux, let's look at
451 the difference in the time it takes to read a file for the first time
452 and the second time;</p>
454 <div class="example"><pre>
455 colm@coroebus:~$ time cat testfile > /dev/null
459 colm@coroebus:~$ time cat testfile > /dev/null
462 sys 0m0.000s</pre></div>
464 <p>Even for this small file, there is a huge difference in the amount
465 of time it takes to read the file. This is because the kernel has cached
466 the file contents in memory.</p>
468 <p>By ensuring there is "spare" memory on your system, you can ensure
469 that more and more file-contents will be stored in this cache. This
470 can be a very efficient means of in-memory caching, and involves no
471 extra configuration of httpd at all.</p>
473 <p>Additionally, because the operating system knows when files are
474 deleted or modified, it can automatically remove file contents from the
475 cache when neccessary. This is a big advantage over httpd's in-memory
476 caching which has no way of knowing when a file has changed.</p>
479 <p>Despite the performance and advantages of automatic operating system
480 caching there are some circumstances in which in-memory caching may be
481 better performed by httpd.</p>
483 <h3>MMapFile Caching</h3>
486 <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the
487 <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code> directive, which
488 allows you to have httpd map a static file's contents into memory at
489 start time (using the mmap system call). httpd will use the in-memory
490 contents for all subsequent accesses to this file.</p>
492 <div class="example"><pre>MMapFile /usr/local/apache2/htdocs/index.html</pre></div>
495 <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
496 changes in these files will not be picked up by httpd after it has
499 <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code>
500 directive does not keep track of how much memory it allocates, so
501 you must ensure not to over-use the directive. Each httpd child
502 process will replicate this memory, so it is critically important
503 to ensure that the files mapped are not so large as to cause the
504 system to swap memory.</p>
506 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
507 <div class="section">
508 <h2><a name="disk" id="disk">Disk-based Caching</a></h2>
511 <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>
513 <p><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> provides a disk-based caching mechanism
514 for <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>. This cache is intelligent and content will
515 be served from the cache only as long as it is considered valid.</p>
517 <p>Typically the module will be configured as so;</p>
519 <div class="example"><pre>
520 CacheRoot /var/cache/apache/
523 CacheDirLength 1</pre></div>
525 <p>Importantly, as the cached files are locally stored, operating system
526 in-memory caching will typically be applied to their access also. So
527 although the files are stored on disk, if they are frequently accessed
528 it is likely the operating system will ensure that they are actually
529 served from memory.</p>
531 <h3>Understanding the Cache-Store</h3>
534 <p>To store items in the cache, <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> creates
535 a 22 character hash of the URL being requested. This hash incorporates
536 the hostname, protocol, port, path and any CGI arguments to the URL,
537 to ensure that multiple URLs do not collide.</p>
539 <p>Each character may be any one of 64-different characters, which mean
540 that overall there are 64^22 possible hashes. For example, a URL might
541 be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
542 as a prefix for the naming of the files specific to that URL within
543 the cache, however first it is split up into directories as per
544 the <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> and
545 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
548 <p><code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code>
549 specifies how many levels of subdirectory there should be, and
550 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
551 specifies how many characters should be in each directory. With
552 the example settings given above, the hash would be turned into
554 <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>
556 <p>The overall aim of this technique is to reduce the number of
557 subdirectories or files that may be in a particular directory,
558 as most file-systems slow down as this number increases. With
560 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
561 there can at most be 64 subdirectories at any particular level.
562 With a setting of 2 there can be 64 * 64 subdirectories, and so on.
563 Unless you have a good reason not to, using a setting of "1"
564 for <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
568 <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code>
569 depends on how many files you anticipate to store in the cache.
570 With the setting of "2" used in the above example, a grand
571 total of 4096 subdirectories can ultimately be created. With
572 1 million files cached, this works out at roughly 245 cached
573 URLs per directory.</p>
575 <p>Each URL uses at least two files in the cache-store. Typically
576 there is a ".header" file, which includes meta-information about
577 the URL, such as when it is due to expire and a ".data" file
578 which is a verbatim copy of the content to be served.</p>
580 <p>In the case of a content negotiated via the "Vary" header, a
581 ".vary" directory will be created for the URL in question. This
582 directory will have multiple ".data" files corresponding to the
583 differently negotiated content.</p>
586 <h3>Maintaining the Disk Cache</h3>
589 <p>Although <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> will remove cached content
590 as it is expired, it does not maintain any information on the total
591 size of the cache or how little free space may be left.</p>
593 <p>Instead, provided with httpd is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which, as the name
594 suggests, allows you to clean the cache periodically. Determining
595 how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to
596 use for the cache is somewhat complex and trial and error may be needed to
597 select optimal values.</p>
599 <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of
600 operation. It can be run as persistent daemon, or periodically from
601 cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour
602 or more to process very large (tens of gigabytes) caches and if you are
603 running it from cron it is recommended that you determine how long a typical
604 run takes, to avoid running more than one instance at a time.</p>
607 <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
608 <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
609 cache growth / clean sequence.</p>
611 <p>Because <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> does not itself pay attention
612 to how much space is used you should ensure that
613 <a href="programs/htcacheclean.html">htcacheclean</a> is configured to
614 leave enough "grow room" following a clean.</p>
618 <div class="bottomlang">
619 <p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
620 <a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
621 <a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
622 </div><div id="footer">
623 <p class="apache">Copyright 2010 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>
624 <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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