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diff --git a/docs/manual/caching.html.en b/docs/manual/caching.html.en new file mode 100644 index 00000000..1d8259a7 --- /dev/null +++ b/docs/manual/caching.html.en @@ -0,0 +1,655 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>Caching Guide - Apache HTTP Server</title> +<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> +<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> +<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /> +<link href="./images/favicon.ico" rel="shortcut icon" /></head> +<body id="manual-page"><div id="page-header"> +<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> +<p class="apache">Apache HTTP Server Version 2.2</p> +<img alt="" src="./images/feather.gif" /></div> +<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div> +<div id="path"> +<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 2.2</a></div><div id="page-content"><div id="preamble"><h1>Caching Guide</h1> +<div class="toplang"> +<p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a></p> +</div> + + <p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>, + <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>, + <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. + It describes how to use Apache's caching features to accelerate web and + proxy serving, while avoiding common problems and misconfigurations.</p> + </div> +<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#overview">Caching Overview</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#filehandle">File-Handle Caching</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#inmemory">In-Memory Caching</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#disk">Disk-based Caching</a></li> +</ul></div> +<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="introduction" id="introduction">Introduction</a></h2> + + + <p>As of Apache HTTP server version 2.2 <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + and <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> are no longer marked + experimental and are considered suitable for production use. These + caching architectures provide a powerful means to accelerate HTTP + handling, both as an origin webserver and as a proxy.</p> + + <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> and its provider modules + <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> + provide intelligent, HTTP-aware caching. The content itself is stored + in the cache, and mod_cache aims to honour all of the various HTTP + headers and options that control the cachability of content. It can + handle both local and proxied content. <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + is aimed at both simple and complex caching configurations, where + you are dealing with proxied content, dynamic local content or + have a need to speed up access to local files which change with + time.</p> + + <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> on the other hand presents a more + basic, but sometimes useful, form of caching. Rather than maintain + the complexity of actively ensuring the cachability of URLs, + <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> offers file-handle and memory-mapping + tricks to keep a cache of files as they were when Apache was last + started. As such, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> is aimed at improving + the access time to local static files which do not change very + often.</p> + + <p>As <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> presents a relatively simple + 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 + in this guide cover the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching + architecture.</p> + + <p>To get the most from this document, you should be familiar with + the basics of HTTP, and have read the Users' Guides to + <a href="urlmapping.html">Mapping URLs to the Filesystem</a> and + <a href="content-negotiation.html">Content negotiation</a>.</p> + + </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="overview" id="overview">Caching Overview</a></h2> + + + + <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> + + <p>There are two main stages in <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> that can + occur in the lifetime of a request. First, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + is a URL mapping module, which means that if a URL has been cached, + and the cached version of that URL has not expired, the request will + be served directly by <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>.</p> + + <p>This means that any other stages that might ordinarily happen + in the process of serving a request -- for example being handled + 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> -- + won't happen. But then this is the point of caching content in + the first place.</p> + + <p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + will add a <a href="filter.html">filter</a> to the request handling. After + Apache has located the content by the usual means, the filter will be run + as the content is served. If the content is determined to be cacheable, + the content will be saved to the cache for future serving.</p> + + <p>If the URL is found within the cache, but also found to have expired, + the filter is added anyway, but <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will create + a conditional request to the backend, to determine if the cached version + is still current. If the cached version is still current, its + meta-information will be updated and the request will be served from the + cache. If the cached version is no longer current, the cached version + will be deleted and the filter will save the updated content to the cache + as it is served.</p> + + <h3>Improving Cache Hits</h3> + + + <p>When caching locally generated content, ensuring that + <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is set to + <code>On</code> can dramatically improve the ratio of cache hits. This + is because the hostname of the virtual-host serving the content forms + a part of the cache key. With the setting set to <code>On</code> + virtual-hosts with multiple server names or aliases will not produce + differently cached entities, and instead content will be cached as + per the canonical hostname.</p> + + <p>Because caching is performed within the URL to filename translation + phase, cached documents will only be served in response to URL requests. + Ordinarily this is of little consequence, but there is one circumstance + in which it matters: If you are using <a href="howto/ssi.html">Server + Side Includes</a>;</p> + + <div class="example"><pre> +<!-- The following include can be cached --> +<!--#include virtual="/footer.html" --> + +<!-- The following include can not be cached --> +<!--#include file="/path/to/footer.html" --></pre></div> + + <p>If you are using Server Side Includes, and want the benefit of speedy + serves from the cache, you should use <code>virtual</code> include + types.</p> + + + <h3>Expiry Periods</h3> + + + <p>The default expiry period for cached entities is one hour, however + this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive. This + default is only used when the original source of the content does not + specify an expire time or time of last modification.</p> + + <p>If a response does not include an <code>Expires</code> header but does + include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + 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> + + <p>For local content, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to + fine-tune the expiry period.</p> + + <p>The maximum expiry period may also be controlled by using the + <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p> + + + + <h3>A Brief Guide to Conditional Requests</h3> + + + <p>When content expires from the cache and is re-requested from the + backend or content provider, rather than pass on the original request, + Aoache will use a conditional request instead.</p> + + <p>HTTP offers a number of headers which allow a client, or cache + to discern between different versions of the same content. For + example if a resource was served with an "Etag:" header, it is + possible to make a conditional request with an "If-Match:" + header. If a resource was served with a "Last-Modified:" header + it is possible to make a conditional request with an + "If-Modified-Since:" header, and so on.</p> + + <p>When such a conditional request is made, the response differs + depending on whether the content matches the conditions. If a request is + made with an "If-Modified-Since:" header, and the content has not been + modified since the time indicated in the request then a terse "304 Not + Modified" response is issued.</p> + + <p>If the content has changed, then it is served as if the request were + not conditional to begin with.</p> + + <p>The benefits of conditional requests in relation to caching are + twofold. Firstly, when making such a request to the backend, if the + content from the backend matches the content in the store, this can be + determined easily and without the overhead of transferring the entire + resource.</p> + + <p>Secondly, conditional requests are usually less strenuous on the + backend. For static files, typically all that is involved is a call + to <code>stat()</code> or similar system call, to see if the file has + changed in size or modification time. As such, even if Apache is + caching local content, even expired content may still be served faster + from the cache if it has not changed. As long as reading from the cache + store is faster than reading from the backend (e.g. an in-memory cache + compared to reading from disk).</p> + + + <h3>What Can be Cached?</h3> + + + <p>As mentioned already, the two styles of caching in Apache work + differently, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> caching maintains file + contents as they were when Apache was started. When a request is + made for a file that is cached by this module, it is intercepted + and the cached file is served.</p> + + <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> caching on the other hand is more + complex. When serving a request, if it has not been cached + previously, the caching module will determine if the content + is cacheable. The conditions for determining cachability of + a response are;</p> + + <ol> + <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> + + <li>The response must have a HTTP status code of 200, 203, 300, 301 or + 410.</li> + + <li>The request must be a HTTP GET request.</li> + + <li>If the request contains an "Authorization:" header, the response + will not be cached.</li> + + <li>If the response contains an "Authorization:" header, it must + also contain an "s-maxage", "must-revalidate" or "public" option + in the "Cache-Control:" header.</li> + + <li>If the URL included a query string (e.g. from a HTML form GET + method) it will not be cached unless the response includes an + "Expires:" header, as per RFC2616 section 13.9.</li> + + <li>If the response has a status of 200 (OK), the response must + also include at least one of the "Etag", "Last-Modified" or + the "Expires" headers, unless the + <code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code> + directive has been used to require otherwise.</li> + + <li>If the response includes the "private" option in a "Cache-Control:" + header, it will not be stored unless the + <code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been + used to require otherwise.</li> + + <li>Likewise, if the response includes the "no-store" option in a + "Cache-Control:" header, it will not be stored unless the + <code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been + used.</li> + + <li>A response will not be stored if it includes a "Vary:" header + containing the match-all "*".</li> + </ol> + + + <h3>What Should Not be Cached?</h3> + + + <p>In short, any content which is highly time-sensitive, or which varies + depending on the particulars of the request that are not covered by + HTTP negotiation, should not be cached.</p> + + <p>If you have dynamic content which changes depending on the IP address + of the requester, or changes every 5 minutes, it should almost certainly + not be cached.</p> + + <p>If on the other hand, the content served differs depending on the + values of various HTTP headers, it is possible that it might be possible + to cache it intelligently through the use of a "Vary" header.</p> + + + <h3>Variable/Negotiated Content</h3> + + + <p>If a response with a "Vary" header is received by + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> when requesting content by the backend it + will attempt to handle it intelligently. If possible, + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will detect the headers attributed in the + "Vary" response in future requests and serve the correct cached + response.</p> + + <p>If for example, a response is received with a vary header such as;</p> + + <div class="example"><p><code> +Vary: negotiate,accept-language,accept-charset + </code></p></div> + + <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to + requesters with matching accept-language and accept-charset headers + matching those of the original request.</p> + + + </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="security" id="security">Security Considerations</a></h2> + + + <h3>Authorisation, Access & and Control</h3> + + + <p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> is very much like having a built + in reverse-proxy. Requests will be served by the caching module unless + it determines that the backend should be queried. When caching local + resources, this drastically changes the security model of Apache.</p> + + <p>As traversing a filesystem hierarchy to examine potential + <code>.htaccess</code> files would be a very expensive operation, + partially defeating the point of caching (to speed up requests), + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> makes no decision about whether a cached + entity is authorised for serving. In other words; if + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> has cached some content, it will be served + from the cache as long as that content has not expired.</p> + + <p>If, for example, your configuration permits access to a resource by IP + address you should ensure that this content is not cached. You can do this by + using the <code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code> + directive, or <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code>. Left unchecked, + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> - very much like a reverse proxy - would cache + the content when served and then serve it to any client, on any IP + address.</p> + + + <h3>Local exploits</h3> + + + <p>As requests to end-users can be served from the cache, the cache + itself can become a target for those wishing to deface or interfere with + content. It is important to bear in mind that the cache must at all + times be writable by the user which Apache is running as. This is in + stark contrast to the usually recommended situation of maintaining + all content unwritable by the Apache user.</p> + + <p>If the Apache user is compromised, for example through a flaw in + a CGI process, it is possible that the cache may be targeted. When + using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code>, it is relatively easy to + insert or modify a cached entity.</p> + + <p>This presents a somewhat elevated risk in comparison to the other + types of attack it is possible to make as the Apache user. If you are + using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> you should bear this in mind - + ensure you upgrade Apache when security upgrades are announced and + run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p> + + + + <h3>Cache Poisoning</h3> + + + <p>When running Apache as a caching proxy server, there is also the + potential for so-called cache poisoning. Cache Poisoning is a broad + term for attacks in which an attacker causes the proxy server to + retrieve incorrect (and usually undesirable) content from the backend. + </p> + + <p>For example if the DNS servers used by your system running Apache + are vulnerable to DNS cache poisoning, an attacker may be able to control + where Apache connects to when requesting content from the origin server. + Another example is so-called HTTP request-smuggling attacks.</p> + + <p>This document is not the correct place for an in-depth discussion + of HTTP request smuggling (instead, try your favourite search engine) + however it is important to be aware that it is possible to make + a series of requests, and to exploit a vulnerability on an origin + webserver such that the attacker can entirely control the content + retrieved by the proxy.</p> + + </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="filehandle" id="filehandle">File-Handle Caching</a></h2> + + + <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> + + <p>The act of opening a file can itself be a source of delay, particularly + on network filesystems. By maintaining a cache of open file descriptors + for commonly served files, Apache can avoid this delay. Currently Apache + provides two different implementations of File-Handle Caching.</p> + + <h3>CacheFile</h3> + + + <p>The most basic form of caching present in Apache is the file-handle + caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching + file-contents, this cache maintains a table of open file descriptors. Files + to be cached in this manner are specified in the configuration file using + the <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> + directive.</p> + + <p>The + <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive + instructs Apache to open the file when Apache is started and to re-use + this file-handle for all subsequent access to this file.</p> + + <div class="example"><pre>CacheFile /usr/local/apache2/htdocs/index.html</pre></div> + + <p>If you intend to cache a large number of files in this manner, you + must ensure that your operating system's limit for the number of open + files is set appropriately.</p> + + <p>Although using <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> + does not cause the file-contents to be cached per-se, it does mean + that if the file changes while Apache is running these changes will + not be picked up. The file will be consistently served as it was + when Apache was started.</p> + + <p>If the file is removed while Apache is running, Apache will continue + to maintain an open file descriptor and serve the file as it was when + Apache was started. This usually also means that although the file + will have been deleted, and not show up on the filesystem, extra free + space will not be recovered until Apache is stopped and the file + descriptor closed.</p> + + + <h3>CacheEnable fd</h3> + + + <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> also provides its own file-handle + caching scheme, which can be enabled via the + <code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code> directive.</p> + + <div class="example"><pre>CacheEnable fd /</pre></div> + + <p>As with all of <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> this type of file-handle + caching is intelligent, and handles will not be maintained beyond + the expiry time of the cached content.</p> + + </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="inmemory" id="inmemory">In-Memory Caching</a></h2> + + + <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> + + <p>Serving directly from system memory is universally the fastest method + of serving content. Reading files from a disk controller or, even worse, + from a remote network is orders of magnitude slower. Disk controllers + usually involve physical processes, and network access is limited by + your available bandwidth. Memory access on the other hand can take mere + nano-seconds.</p> + + <p>System memory isn't cheap though, byte for byte it's by far the most + expensive type of storage and it's important to ensure that it is used + efficiently. By caching files in memory you decrease the amount of + memory available on the system. As we'll see, in the case of operating + system caching, this is not so much of an issue, but when using + Apache's own in-memory caching it is important to make sure that you + do not allocate too much memory to a cache. Otherwise the system + will be forced to swap out memory, which will likely degrade + performance.</p> + + <h3>Operating System Caching</h3> + + + <p>Almost all modern operating systems cache file-data in memory managed + directly by the kernel. This is a powerful feature, and for the most + part operating systems get it right. For example, on Linux, let's look at + the difference in the time it takes to read a file for the first time + and the second time;</p> + + <div class="example"><pre> +colm@coroebus:~$ time cat testfile > /dev/null +real 0m0.065s +user 0m0.000s +sys 0m0.001s +colm@coroebus:~$ time cat testfile > /dev/null +real 0m0.003s +user 0m0.003s +sys 0m0.000s</pre></div> + + <p>Even for this small file, there is a huge difference in the amount + of time it takes to read the file. This is because the kernel has cached + the file contents in memory.</p> + + <p>By ensuring there is "spare" memory on your system, you can ensure + that more and more file-contents will be stored in this cache. This + can be a very efficient means of in-memory caching, and involves no + extra configuration of Apache at all.</p> + + <p>Additionally, because the operating system knows when files are + deleted or modified, it can automatically remove file contents from the + cache when neccessary. This is a big advantage over Apache's in-memory + caching which has no way of knowing when a file has changed.</p> + + + <p>Despite the performance and advantages of automatic operating system + caching there are some circumstances in which in-memory caching may be + better performed by Apache.</p> + + <p>Firstly, an operating system can only cache files it knows about. If + you are running Apache as a proxy server, the files you are caching are + not locally stored but remotely served. If you still want the unbeatable + speed of in-memory caching, Apache's own memory caching is needed.</p> + + <h3>MMapStatic Caching</h3> + + + <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the + <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> directive, which + allows you to have Apache map a static file's contents into memory at + start time (using the mmap system call). Apache will use the in-memory + contents for all subsequent accesses to this file.</p> + + <div class="example"><pre>MMapStatic /usr/local/apache2/htdocs/index.html</pre></div> + + <p>As with the + <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any + changes in these files will not be picked up by Apache after it has + started.</p> + + <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> + directive does not keep track of how much memory it allocates, so + you must ensure not to over-use the directive. Each Apache child + process will replicate this memory, so it is critically important + to ensure that the files mapped are not so large as to cause the + system to swap memory.</p> + + + <h3>mod_mem_cache Caching</h3> + + + <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> provides a HTTP-aware intelligent + in-memory cache. It also uses heap memory directly, which means that + even if <var>MMap</var> is not supported on your system, + <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> may still be able to perform caching.</p> + + <p>Caching of this type is enabled via;</p> + + <div class="example"><pre> +# Enable memory caching +CacheEnable mem / + +# Limit the size of the cache to 1 Megabyte +MCacheSize 1024</pre></div> + + </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> +<div class="section"> +<h2><a name="disk" id="disk">Disk-based Caching</a></h2> + + + <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> + + <p><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> provides a disk-based caching mechanism + 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> + this cache is intelligent and content will be served from the cache only + as long as it is considered valid.</p> + + <p>Typically the module will be configured as so;</p> + + <div class="example"><pre> +CacheRoot /var/cache/apache/ +CacheEnable disk / +CacheDirLevels 2 +CacheDirLength 1</pre></div> + + <p>Importantly, as the cached files are locally stored, operating system + in-memory caching will typically be applied to their access also. So + although the files are stored on disk, if they are frequently accessed + it is likely the operating system will ensure that they are actually + served from memory.</p> + + <h3>Understanding the Cache-Store</h3> + + + <p>To store items in the cache, <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> creates + a 22 character hash of the url being requested. Thie hash incorporates + the hostname, protocol, port, path and any CGI arguments to the URL, + to ensure that multiple URLs do not collide.</p> + + <p>Each character may be any one of 64-different characters, which mean + that overall there are 22^64 possible hashes. For example, a URL might + be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used + as a prefix for the naming of the files specific to that url within + the cache, however first it is split up into directories as per + the <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> and + <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code> + directives.</p> + + <p><code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> + specifies how many levels of subdirectory there should be, and + <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code> + specifies how many characters should be in each directory. With + the example settings given above, the hash would be turned into + a filename prefix as + <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p> + + <p>The overall aim of this technique is to reduce the number of + subdirectories or files that may be in a particular directory, + as most file-systems slow down as this number increases. With + setting of "1" for + <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code> + there can at most be 64 subdirectories at any particular level. + With a setting of 2 there can be 64 * 64 subdirectories, and so on. + Unless you have a good reason not to, using a setting of "1" + for <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code> + is recommended.</p> + + <p>Setting + <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> + depends on how many files you anticipate to store in the cache. + With the setting of "2" used in the above example, a grand + total of 4096 subdirectories can ultimately be created. With + 1 million files cached, this works out at roughly 245 cached + urls per directory.</p> + + <p>Each url uses at least two files in the cache-store. Typically + there is a ".header" file, which includes meta-information about + the url, such as when it is due to expire and a ".data" file + which is a verbatim copy of the content to be served.</p> + + <p>In the case of a content negotiated via the "Vary" header, a + ".vary" directory will be created for the url in question. This + directory will have multiple ".data" files corresponding to the + differently negotiated content.</p> + + + <h3>Maintaining the Disk Cache</h3> + + + <p>Although <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> will remove cached content + as it is expired, it does not maintain any information on the total + size of the cache or how little free space may be left.</p> + + <p>Instead, provided with Apache is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which, as the name + suggests, allows you to clean the cache periodically. Determining + how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to + use for the cache is somewhat complex and trial and error may be needed to + select optimal values.</p> + + <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of + operation. It can be run as persistent daemon, or periodically from + cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour + or more to process very large (tens of gigabytes) caches and if you are + running it from cron it is recommended that you determine how long a typical + run takes, to avoid running more than one instance at a time.</p> + + <p class="figure"> + <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br /> + <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical + cache growth / clean sequence.</p> + + <p>Because <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> does not itself pay attention + to how much space is used you should ensure that + <a href="programs/htcacheclean.html">htcacheclean</a> is configured to + leave enough "grow room" following a clean.</p> + + + </div></div> +<div class="bottomlang"> +<p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a></p> +</div><div id="footer"> +<p class="apache">Copyright 2006 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> +<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> +</body></html>
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