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<?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> |<a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Fran鏰is"> fr </a> |<a href="./tr/caching.html" hreflang="tr" rel="alternate" title="T黵k鏴"> tr </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, Apache 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-None-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>⌨️ 快捷键说明
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