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908 lines
51 KiB
XML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head>
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<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
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<title>Caching Guide - Apache HTTP Server Version 2.5</title>
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<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="./style/css/prettify.css" />
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<script src="./style/scripts/prettify.min.js" type="text/javascript">
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</script>
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<link href="./images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="./images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
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<div id="path">
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.5</a></div><div id="page-content"><div id="preamble"><h1>Caching Guide</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
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<a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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<a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div>
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<p>This document supplements the <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>,
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<code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> and <a href="programs/htcacheclean.html">htcacheclean</a> reference documentation.
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It describes how to use the Apache HTTP Server's caching features to accelerate web and
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proxy serving, while avoiding common problems and misconfigurations.</p>
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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#http-caching">Three-state RFC2616 HTTP caching</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#examples">Cache Setup Examples</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#socache-caching">General Two-state Key/Value Shared Object Caching</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#file-caching">Specialized File Caching</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="introduction" id="introduction">Introduction</a> <a title="Permanent link" href="#introduction" class="permalink">¶</a></h2>
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<p>The Apache HTTP server offers a range of caching features that
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are designed to improve the performance of the server in various
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ways.</p>
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<dl>
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<dt>Three-state RFC2616 HTTP caching</dt>
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<dd>
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<code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
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and its provider modules
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<code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code>
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provide intelligent, HTTP-aware caching. The content itself is stored
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in the cache, and mod_cache aims to honor all of the various HTTP
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headers and options that control the cacheability of content
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as described in
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<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section
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13 of RFC2616</a>.
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<code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
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is aimed at both simple and complex caching configurations, where
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you are dealing with proxied content, dynamic local content or
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have a need to speed up access to local files on a potentially
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slow disk.
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</dd>
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<dt>Two-state key/value shared object caching</dt>
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<dd>
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The <a href="socache.html">shared object cache API</a> (socache)
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and its provider modules provide a
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server wide key/value based shared object cache. These modules
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are designed to cache low level data such as SSL sessions and
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authentication credentials. Backends allow the data to be stored
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server wide in shared memory, or datacenter wide in a cache such
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as memcache or distcache.
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</dd>
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<dt>Specialized file caching</dt>
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<dd>
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<code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>
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offers the ability to pre-load
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files into memory on server startup, and can improve access
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times and save file handles on files that are accessed often,
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as there is no need to go to disk on each request.
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</dd>
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</dl>
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<p>To get the most from this document, you should be familiar with
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the basics of HTTP, and have read the Users' Guides to
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<a href="urlmapping.html">Mapping URLs to the Filesystem</a> and
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<a href="content-negotiation.html">Content negotiation</a>.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="http-caching" id="http-caching">Three-state RFC2616 HTTP caching</a> <a title="Permanent link" href="#http-caching" class="permalink">¶</a></h2>
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<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code></li><li><code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code></li><li><code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code></li><li><code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code></li><li><code class="directive"><a href="./mod/mod_negotiation.html#cachenegotiateddocs">CacheNegotiatedDocs</a></code></li></ul></td></tr></table>
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<p>The HTTP protocol contains built in support for an in-line caching
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mechanism
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<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
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described by section 13 of RFC2616</a>, and the
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<code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module can be used to take advantage of
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this.</p>
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<p>Unlike a simple two state key/value cache where the content
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disappears completely when no longer fresh, an HTTP cache includes
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a mechanism to retain stale content, and to ask the origin server
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whether this stale content has changed and if not, make it fresh
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again.</p>
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<p>An entry in an HTTP cache exists in one of three states:</p>
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<dl>
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<dt>Fresh</dt>
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<dd>
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If the content is new enough (younger than its <strong>freshness
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lifetime</strong>), it is considered <strong>fresh</strong>. An
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HTTP cache is free to serve fresh content without making any
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calls to the origin server at all.
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</dd>
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<dt>Stale</dt>
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<dd>
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<p>If the content is too old (older than its <strong>freshness
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lifetime</strong>), it is considered <strong>stale</strong>. An
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HTTP cache should contact the origin server and check whether
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the content is still fresh before serving stale content to a
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client. The origin server will either respond with replacement
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content if not still valid, or ideally, the origin server will
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respond with a code to tell the cache the content is still
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fresh, without the need to generate or send the content again.
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The content becomes fresh again and the cycle continues.</p>
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<p>The HTTP protocol does allow the cache to serve stale data
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under certain circumstances, such as when an attempt to freshen
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the data with an origin server has failed with a 5xx error, or
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when another request is already in the process of freshening
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the given entry. In these cases a <code>Warning</code> header
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is added to the response.</p>
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</dd>
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<dt>Non Existent</dt>
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<dd>
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If the cache gets full, it reserves the option to delete content
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from the cache to make space. Content can be deleted at any time,
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and can be stale or fresh. The <a href="programs/htcacheclean.html">htcacheclean</a> tool can be
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run on a once off basis, or deployed as a daemon to keep the size
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of the cache within the given size, or the given number of inodes.
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The tool attempts to delete stale content before attempting to
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delete fresh content.
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</dd>
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</dl>
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<p>Full details of how HTTP caching works can be found in
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<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">
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Section 13 of RFC2616</a>.</p>
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<h3>Interaction with the Server</h3>
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<p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module hooks into the server in two
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possible places depending on the value of the
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<code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> directive:
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</p>
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<dl>
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<dt>Quick handler phase</dt>
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<dd>
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<p>This phase happens very early on during the request processing,
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just after the request has been parsed. If the content is
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found within the cache, it is served immediately and almost
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all request processing is bypassed.</p>
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<p>In this scenario, the cache behaves as if it has been "bolted
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on" to the front of the server.</p>
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<p>This mode offers the best performance, as the majority of
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server processing is bypassed. This mode however also bypasses the
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authentication and authorization phases of server processing, so
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this mode should be chosen with care when this is important.</p>
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<p> Requests with an "Authorization" header (for example, HTTP Basic
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Authentication) are neither cacheable nor served from the cache
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when <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> is running in this phase.</p>
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</dd>
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<dt>Normal handler phase</dt>
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<dd>
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<p>This phase happens late in the request processing, after all
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the request phases have completed.</p>
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<p>In this scenario, the cache behaves as if it has been "bolted
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on" to the back of the server.</p>
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<p>This mode offers the most flexibility, as the potential exists
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for caching to occur at a precisely controlled point in the filter
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chain, and cached content can be filtered or personalized before
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being sent to the client.</p>
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</dd>
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</dl>
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<p>If the URL is not found within the cache, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
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will add a <a href="filter.html">filter</a> to the filter stack in order
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to record the response to the cache, and then stand down, allowing normal
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request processing to continue. If the content is determined to be
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cacheable, the content will be saved to the cache for future serving,
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otherwise the content will be ignored.</p>
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<p>If the content found within the cache is stale, the
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<code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module converts the request into a
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<strong>conditional request</strong>. If the origin server responds with
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a normal response, the normal response is cached, replacing the content
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already cached. If the origin server responds with a 304 Not Modified
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response, the content is marked as fresh again, and the cached content
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is served by the filter instead of saving it.</p>
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<h3>Improving Cache Hits</h3>
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<p>When a virtual host is known by one of many different server aliases,
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ensuring that <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is
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set to <code>On</code> can dramatically improve the ratio of cache hits.
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This is because the hostname of the virtual-host serving the content is
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used within the cache key. With the setting set to <code>On</code>
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virtual-hosts with multiple server names or aliases will not produce
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differently cached entities, and instead content will be cached as
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per the canonical hostname.</p>
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<h3>Freshness Lifetime</h3>
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<p>Well formed content that is intended to be cached should declare an
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explicit freshness lifetime with the <code>Cache-Control</code>
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header's <code>max-age</code> or <code>s-maxage</code> fields, or
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by including an <code>Expires</code> header.</p>
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<p>At the same time, the origin server defined freshness lifetime can
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be overridden by a client when the client presents their own
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<code>Cache-Control</code> header within the request. In this case,
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the lowest freshness lifetime between request and response wins.</p>
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<p>When this freshness lifetime is missing from the request or the
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response, a default freshness lifetime is applied. The default
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freshness lifetime for cached entities is one hour, however
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this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive.</p>
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<p>If a response does not include an <code>Expires</code> header but does
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include a <code>Last-Modified</code> header, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
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can infer a freshness lifetime based on a heuristic, which can be
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controlled through the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p>
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<p>For local content, or for remote content that does not define its own
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<code>Expires</code> header, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to
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fine-tune the freshness lifetime by adding <code>max-age</code> and
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<code>Expires</code>.</p>
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<p>The maximum freshness lifetime may also be controlled by using the
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<code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>
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<h3>A Brief Guide to Conditional Requests</h3>
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<p>When content expires from the cache and becomes stale, rather than
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pass on the original request, httpd will modify the request to make
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it conditional instead.</p>
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<p>When an <code>ETag</code> header exists in the original cached
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response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
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<code>If-None-Match</code> header to the request to the origin server.
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When a <code>Last-Modified</code> header exists in the original
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cached response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an
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<code>If-Modified-Since</code> header to the request to the origin
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server. Performing either of these actions makes the request
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<strong>conditional</strong>.</p>
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<p>When a conditional request is received by an origin server, the
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origin server should check whether the ETag or the Last-Modified
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parameter has changed, as appropriate for the request. If not, the
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origin should respond with a terse "304 Not Modified" response. This
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signals to the cache that the stale content is still fresh should be
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used for subsequent requests until the content's new freshness lifetime
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is reached again.</p>
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<p>If the content has changed, then the content is served as if the
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request were not conditional to begin with.</p>
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<p>Conditional requests offer two benefits. Firstly, when making such
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a request to the origin server, if the content from the origin
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matches the content in the cache, this can be determined easily and
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without the overhead of transferring the entire resource.</p>
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<p>Secondly, a well designed origin server will be designed in such
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a way that conditional requests will be significantly cheaper to
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produce than a full response. For static files, typically all that is
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involved is a call to <code>stat()</code> or similar system call, to
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see if the file has changed in size or modification time. As such, even
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local content may still be served faster from the cache if it has not
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changed.</p>
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<p>Origin servers should make every effort to support conditional
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requests as is practical, however if conditional requests are not
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supported, the origin will respond as if the request was not
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conditional, and the cache will respond as if the content had changed
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and save the new content to the cache. In this case, the cache will
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behave like a simple two state cache, where content is effectively
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either fresh or deleted.</p>
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<h3>What Can be Cached?</h3>
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<p>The full definition of which responses can be cached by an HTTP
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cache is defined in
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<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4">
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RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as
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follows:</p>
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<ol>
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<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>
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<li>If the response has an HTTP status code other than 200, 203, 300,
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301 or 410 it must also specify an "Expires" or "Cache-Control" header.
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</li>
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<li>The request must be a HTTP GET request.</li>
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<li>If the response contains an "Authorization:" header, it must
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also contain an "s-maxage", "must-revalidate" or "public" option
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in the "Cache-Control:" header, or it won't be cached.</li>
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<li>If the URL included a query string (e.g. from a HTML form GET
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method) it will not be cached unless the response specifies an
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explicit expiration by including an "Expires:" header or the max-age
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or s-maxage directive of the "Cache-Control:" header, as per RFC2616
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sections 13.9 and 13.2.1.</li>
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<li>If the response has a status of 200 (OK), the response must
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also include at least one of the "Etag", "Last-Modified" or
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the "Expires" headers, or the max-age or s-maxage directive of
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the "Cache-Control:" header, unless the
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<code class="directive"><a href="./mod/mod_cache.html#cacheignorenolastmod">CacheIgnoreNoLastMod</a></code>
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directive has been used to require otherwise.</li>
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<li>If the response includes the "private" option in a "Cache-Control:"
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header, it will not be stored unless the
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<code class="directive"><a href="./mod/mod_cache.html#cachestoreprivate">CacheStorePrivate</a></code> has been
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used to require otherwise.</li>
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<li>Likewise, if the response includes the "no-store" option in a
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"Cache-Control:" header, it will not be stored unless the
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<code class="directive"><a href="./mod/mod_cache.html#cachestorenostore">CacheStoreNoStore</a></code> has been
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used.</li>
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<li>A response will not be stored if it includes a "Vary:" header
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containing the match-all "*".</li>
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</ol>
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<h3>What Should Not be Cached?</h3>
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|
|
<p>It should be up to the client creating the request, or the origin
|
|
server constructing the response to decide whether or not the content
|
|
should be cacheable or not by correctly setting the
|
|
<code>Cache-Control</code> header, and <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> should
|
|
be left alone to honor the wishes of the client or server as appropriate.
|
|
</p>
|
|
|
|
<p>Content that is time sensitive, or which varies depending on the
|
|
particulars of the request that are not covered by HTTP negotiation,
|
|
should not be cached. This content should declare itself uncacheable
|
|
using the <code>Cache-Control</code> header.</p>
|
|
|
|
<p>If content changes often, expressed by a freshness lifetime of minutes
|
|
or seconds, the content can still be cached, however it is highly
|
|
desirable that the origin server supports
|
|
<strong>conditional requests</strong> correctly to ensure that
|
|
full responses do not have to be generated on a regular basis.</p>
|
|
|
|
<p>Content that varies based on client provided request headers can be
|
|
cached through intelligent use of the <code>Vary</code> response
|
|
header.</p>
|
|
|
|
|
|
|
|
<h3>Variable/Negotiated Content</h3>
|
|
|
|
|
|
<p>When the origin server is designed to respond with different content
|
|
based on the value of headers in the request, for example to serve
|
|
multiple languages at the same URL, HTTP's caching mechanism makes it
|
|
possible to cache multiple variants of the same page at the same URL.</p>
|
|
|
|
<p>This is done by the origin server adding a <code>Vary</code> header
|
|
to indicate which headers must be taken into account by a cache when
|
|
determining whether two variants are different from one another.</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 accept-language and accept-charset headers
|
|
matching those of the original request.</p>
|
|
|
|
<p>Multiple variants of the content can be cached side by side,
|
|
<code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> uses the <code>Vary</code> header and the
|
|
corresponding values of the request headers listed by <code>Vary</code>
|
|
to decide on which of many variants to return to the client.</p>
|
|
|
|
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="examples" id="examples">Cache Setup Examples</a> <a title="Permanent link" href="#examples" class="permalink">¶</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_cache_disk.html">mod_cache_disk</a></code></li><li><code class="module"><a href="./mod/mod_cache_socache.html">mod_cache_socache</a></code></li><li><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</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_disk.html#cacheroot">CacheRoot</a></code></li><li><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code></li><li><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code></li><li><code class="directive"><a href="./mod/mod_cache_socache.html#cachesocache">CacheSocache</a></code></li></ul></td></tr></table>
|
|
|
|
<h3><a name="disk" id="disk">Caching to Disk</a></h3>
|
|
|
|
|
|
<p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module relies on specific backend store
|
|
implementations in order to manage the cache, and for caching to disk
|
|
<code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> is provided to support this.</p>
|
|
|
|
<p>Typically the module will be configured as so;</p>
|
|
|
|
<pre class="prettyprint lang-config">CacheRoot "/var/cache/apache/"
|
|
CacheEnable disk /
|
|
CacheDirLevels 2
|
|
CacheDirLength 1</pre>
|
|
|
|
|
|
<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_cache_disk.html">mod_cache_disk</a></code> creates
|
|
a 22 character hash of the URL being requested. This hash incorporates
|
|
the hostname, protocol, port, path and any CGI arguments to the URL,
|
|
as well as elements defined by the Vary header to ensure that multiple
|
|
URLs do not collide with one another.</p>
|
|
|
|
<p>Each character may be any one of 64-different characters, which mean
|
|
that overall there are 64^22 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_cache_disk.html#cachedirlevels">CacheDirLevels</a></code> and
|
|
<code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code>
|
|
directives.</p>
|
|
|
|
<p><code class="directive"><a href="./mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code>
|
|
specifies how many levels of subdirectory there should be, and
|
|
<code class="directive"><a href="./mod/mod_cache_disk.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_cache_disk.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_cache_disk.html#cachedirlength">CacheDirLength</a></code>
|
|
is recommended.</p>
|
|
|
|
<p>Setting
|
|
<code class="directive"><a href="./mod/mod_cache_disk.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>The <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> module makes no attempt to
|
|
regulate the amount of disk space used by the cache, although it
|
|
will gracefully stand down on any disk error and behave as if the
|
|
cache was never present.</p>
|
|
|
|
<p>Instead, provided with httpd is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which 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>It is also recommended that an appropriate "nice" level is chosen for
|
|
htcacheclean so that the tool does not cause excessive disk io while the
|
|
server is running.</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_cache_disk.html">mod_cache_disk</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>
|
|
|
|
|
|
<h3><a name="memcache" id="memcache">Caching to memcached</a></h3>
|
|
|
|
|
|
<p>Using the <code class="module"><a href="./mod/mod_cache_socache.html">mod_cache_socache</a></code> module, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
|
|
can cache data from a variety of implementations (aka: "providers"). Using the
|
|
<code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code> module, for example, one can specify that
|
|
<a href="http://memcached.org">memcached</a> is to be used as the
|
|
the backend storage mechanism.</p>
|
|
|
|
<p>Typically the module will be configured as so:</p>
|
|
|
|
<pre class="prettyprint lang-config">CacheEnable socache /
|
|
CacheSocache memcache:memcd.example.com:11211</pre>
|
|
|
|
|
|
<p>Additional <code>memcached</code> servers can be specified by
|
|
appending them to the end of the <code>CacheSocache memcache:</code>
|
|
line separated by commas:</p>
|
|
|
|
<pre class="prettyprint lang-config">CacheEnable socache /
|
|
CacheSocache memcache:mem1.example.com:11211,mem2.example.com:11212</pre>
|
|
|
|
|
|
<p>This format is also used with the other various <code class="module"><a href="./mod/mod_cache_socache.html">mod_cache_socache</a></code>
|
|
providers. For example:</p>
|
|
|
|
<pre class="prettyprint lang-config">CacheEnable socache /
|
|
CacheSocache shmcb:/path/to/datafile(512000)</pre>
|
|
|
|
|
|
<pre class="prettyprint lang-config">CacheEnable socache /
|
|
CacheSocache dbm:/path/to/datafile</pre>
|
|
|
|
|
|
|
|
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="socache-caching" id="socache-caching">General Two-state Key/Value Shared Object Caching</a> <a title="Permanent link" href="#socache-caching" class="permalink">¶</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_authn_socache.html">mod_authn_socache</a></code></li><li><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></li><li><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></li><li><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></li><li><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></li><li><code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authn_socache.html#authncachesocache">AuthnCacheSOCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslsessioncache">SSLSessionCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslstaplingcache">SSLStaplingCache</a></code></li></ul></td></tr></table>
|
|
|
|
<p>The Apache HTTP server offers a low level shared object cache for
|
|
caching information such as SSL sessions, or authentication credentials,
|
|
within the <a href="socache.html">socache</a> interface.</p>
|
|
|
|
<p>Additional modules are provided for each implementation, offering the
|
|
following backends:</p>
|
|
|
|
<dl>
|
|
<dt><code class="module"><a href="./mod/mod_socache_dbm.html">mod_socache_dbm</a></code></dt>
|
|
<dd>DBM based shared object cache.</dd>
|
|
<dt><code class="module"><a href="./mod/mod_socache_dc.html">mod_socache_dc</a></code></dt>
|
|
<dd>Distcache based shared object cache.</dd>
|
|
<dt><code class="module"><a href="./mod/mod_socache_memcache.html">mod_socache_memcache</a></code></dt>
|
|
<dd>Memcache based shared object cache.</dd>
|
|
<dt><code class="module"><a href="./mod/mod_socache_shmcb.html">mod_socache_shmcb</a></code></dt>
|
|
<dd>Shared memory based shared object cache.</dd>
|
|
</dl>
|
|
|
|
<h3><a name="mod_authn_socache-caching" id="mod_authn_socache-caching">Caching Authentication Credentials</a></h3>
|
|
|
|
|
|
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_authn_socache.html#authncachesocache">AuthnCacheSOCache</a></code></li></ul></td></tr></table>
|
|
|
|
<p>The <code class="module"><a href="./mod/mod_authn_socache.html">mod_authn_socache</a></code> module allows the result of
|
|
authentication to be cached, relieving load on authentication backends.</p>
|
|
|
|
|
|
|
|
<h3><a name="mod_ssl-caching" id="mod_ssl-caching">Caching SSL Sessions</a></h3>
|
|
|
|
|
|
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_ssl.html#sslsessioncache">SSLSessionCache</a></code></li><li><code class="directive"><a href="./mod/mod_ssl.html#sslstaplingcache">SSLStaplingCache</a></code></li></ul></td></tr></table>
|
|
|
|
<p>The <code class="module"><a href="./mod/mod_ssl.html">mod_ssl</a></code> module uses the <code>socache</code> interface
|
|
to provide a session cache and a stapling cache.</p>
|
|
|
|
|
|
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="file-caching" id="file-caching">Specialized File Caching</a> <a title="Permanent link" href="#file-caching" class="permalink">¶</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></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code></li><li><code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code></li></ul></td></tr></table>
|
|
|
|
<p>On platforms where a filesystem might be slow, or where file
|
|
handles are expensive, the option exists to pre-load files into
|
|
memory on startup.</p>
|
|
|
|
<p>On systems where opening files is slow, the option exists to
|
|
open the file on startup and cache the file handle. These
|
|
options can help on systems where access to static files is
|
|
slow.</p>
|
|
|
|
<h3><a name="filehandle" id="filehandle">File-Handle Caching</a></h3>
|
|
|
|
|
|
<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, httpd can avoid this delay. Currently httpd
|
|
provides one implementation of File-Handle Caching.</p>
|
|
|
|
<h4>CacheFile</h4>
|
|
|
|
|
|
<p>The most basic form of caching present in httpd 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 httpd to open the file when it is started and to re-use
|
|
this file-handle for all subsequent access to this file.</p>
|
|
|
|
<pre class="prettyprint lang-config">CacheFile /usr/local/apache2/htdocs/index.html</pre>
|
|
|
|
|
|
<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 httpd is running these changes will
|
|
not be picked up. The file will be consistently served as it was
|
|
when httpd was started.</p>
|
|
|
|
<p>If the file is removed while httpd is running, it will continue
|
|
to maintain an open file descriptor and serve the file as it was when
|
|
httpd 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 httpd is stopped and the file
|
|
descriptor closed.</p>
|
|
|
|
|
|
|
|
|
|
<h3><a name="inmemory" id="inmemory">In-Memory Caching</a></h3>
|
|
|
|
|
|
<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
|
|
httpd'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>
|
|
|
|
<h4>Operating System Caching</h4>
|
|
|
|
|
|
<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 httpd 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 necessary. This is a big advantage over httpd'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 httpd.</p>
|
|
|
|
<h4>MMapFile Caching</h4>
|
|
|
|
|
|
<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#mmapfile">MMapFile</a></code> directive, which
|
|
allows you to have httpd map a static file's contents into memory at
|
|
start time (using the mmap system call). httpd will use the in-memory
|
|
contents for all subsequent accesses to this file.</p>
|
|
|
|
<pre class="prettyprint lang-config">MMapFile /usr/local/apache2/htdocs/index.html</pre>
|
|
|
|
|
|
<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 httpd after it has
|
|
started.</p>
|
|
|
|
<p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapfile">MMapFile</a></code>
|
|
directive does not keep track of how much memory it allocates, so
|
|
you must ensure not to over-use the directive. Each httpd 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>
|
|
|
|
|
|
|
|
</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> <a title="Permanent link" href="#security" class="permalink">¶</a></h2>
|
|
|
|
|
|
<h3>Authorization and Access Control</h3>
|
|
|
|
|
|
<p>Using <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> in its default state where
|
|
<code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> is set to
|
|
<code>On</code> is very much like having a caching reverse-proxy bolted
|
|
to the front of the server. Requests will be served by the caching module
|
|
unless it determines that the origin server should be queried just as an
|
|
external cache would, and this drastically changes the security model of
|
|
httpd.</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>
|
|
|
|
<p>When the <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code>
|
|
directive is set to <code>Off</code>, the full set of request processing
|
|
phases are executed and the security model remains unchanged.</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 httpd 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_cache_disk.html">mod_cache_disk</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_cache_disk.html">mod_cache_disk</a></code> you should bear this in mind -
|
|
ensure you upgrade httpd 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 httpd 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 origin
|
|
server.</p>
|
|
|
|
<p>For example if the DNS servers used by your system running httpd
|
|
are vulnerable to DNS cache poisoning, an attacker may be able to control
|
|
where httpd 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>
|
|
|
|
|
|
<h3>Denial of Service / Cachebusting</h3>
|
|
|
|
|
|
<p>The Vary mechanism allows multiple variants of the same URL to be
|
|
cached side by side. Depending on header values provided by the client,
|
|
the cache will select the correct variant to return to the client. This
|
|
mechanism can become a problem when an attempt is made to vary on a
|
|
header that is known to contain a wide range of possible values under
|
|
normal use, for example the <code>User-Agent</code> header. Depending
|
|
on the popularity of the particular web site thousands or millions of
|
|
duplicate cache entries could be created for the same URL, crowding
|
|
out other entries in the cache.</p>
|
|
|
|
<p>In other cases, there may be a need to change the URL of a particular
|
|
resource on every request, usually by adding a "cachebuster" string to
|
|
the URL. If this content is declared cacheable by a server for a
|
|
significant freshness lifetime, these entries can crowd out
|
|
legitimate entries in a cache. While <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>
|
|
provides a
|
|
<code class="directive"><a href="./mod/mod_cache.html#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></code>
|
|
directive, this directive should be used with care to ensure that
|
|
downstream proxy or browser caches aren't subjected to the same denial
|
|
of service issue.</p>
|
|
|
|
</div></div>
|
|
<div class="bottomlang">
|
|
<p><span>Available Languages: </span><a href="./en/caching.html" title="English"> en </a> |
|
|
<a href="./fr/caching.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
|
|
<a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
|
|
</div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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