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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-07 02:04:06 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-07 02:04:06 +0000 |
| commit | 5dff2d61cc1c27747ee398e04d8e02843aabb1f8 (patch) | |
| tree | a67c336b406c8227bac912beb74a1ad3cdc55100 /docs/manual/caching.html.en | |
| parent | Initial commit. (diff) | |
| download | apache2-5dff2d61cc1c27747ee398e04d8e02843aabb1f8.tar.xz apache2-5dff2d61cc1c27747ee398e04d8e02843aabb1f8.zip | |
Adding upstream version 2.4.38.upstream/2.4.38upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/manual/caching.html.en')
| -rw-r--r-- | docs/manual/caching.html.en | 908 |
1 files changed, 908 insertions, 0 deletions
diff --git a/docs/manual/caching.html.en b/docs/manual/caching.html.en new file mode 100644 index 0000000..fc38c1b --- /dev/null +++ b/docs/manual/caching.html.en @@ -0,0 +1,908 @@ +<?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> +<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> +<!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>Caching Guide - Apache HTTP Server Version 2.4</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 rel="stylesheet" type="text/css" href="./style/css/prettify.css" /> +<script src="./style/scripts/prettify.min.js" type="text/javascript"> +</script> + +<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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p> +<p class="apache">Apache HTTP Server Version 2.4</p> +<img alt="" src="./images/feather.png" /></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 2.4</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="Français"> fr </a> | +<a href="./tr/caching.html" hreflang="tr" rel="alternate" title="Türkçe"> 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_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. + It describes how to use the Apache HTTP Server's caching features to accelerate web and + proxy serving, while avoiding common problems and misconfigurations.</p> + </div> +<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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="#http-caching">Three-state RFC2616 HTTP caching</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#examples">Cache Setup Examples</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#socache-caching">General Two-state Key/Value Shared Object Caching</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#file-caching">Specialized File Caching</a></li> +<li><img alt="" src="./images/down.gif" /> <a href="#security">Security Considerations</a></li> +</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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>The Apache HTTP server offers a range of caching features that + are designed to improve the performance of the server in various + ways.</p> + + <dl> + <dt>Three-state RFC2616 HTTP caching</dt> + <dd> + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> + and its provider modules + <code class="module"><a href="./mod/mod_cache_disk.html">mod_cache_disk</a></code> + provide intelligent, HTTP-aware caching. The content itself is stored + in the cache, and mod_cache aims to honor all of the various HTTP + headers and options that control the cacheability of content + as described in + <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html">Section + 13 of RFC2616</a>. + <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 on a potentially + slow disk. + </dd> + + <dt>Two-state key/value shared object caching</dt> + <dd> + The <a href="socache.html">shared object cache API</a> (socache) + and its provider modules provide a + server wide key/value based shared object cache. These modules + are designed to cache low level data such as SSL sessions and + authentication credentials. Backends allow the data to be stored + server wide in shared memory, or datacenter wide in a cache such + as memcache or distcache. + </dd> + + <dt>Specialized file caching</dt> + <dd> + <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> + offers the ability to pre-load + files into memory on server startup, and can improve access + times and save file handles on files that are accessed often, + as there is no need to go to disk on each request. + </dd> + </dl> + + <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="http-caching" id="http-caching">Three-state RFC2616 HTTP 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_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> + + <p>The HTTP protocol contains built in support for an in-line caching + mechanism + <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html"> + described by section 13 of RFC2616</a>, and the + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module can be used to take advantage of + this.</p> + + <p>Unlike a simple two state key/value cache where the content + disappears completely when no longer fresh, an HTTP cache includes + a mechanism to retain stale content, and to ask the origin server + whether this stale content has changed and if not, make it fresh + again.</p> + + <p>An entry in an HTTP cache exists in one of three states:</p> + + <dl> + <dt>Fresh</dt> + <dd> + If the content is new enough (younger than its <strong>freshness + lifetime</strong>), it is considered <strong>fresh</strong>. An + HTTP cache is free to serve fresh content without making any + calls to the origin server at all. + </dd> + <dt>Stale</dt> + <dd> + <p>If the content is too old (older than its <strong>freshness + lifetime</strong>), it is considered <strong>stale</strong>. An + HTTP cache should contact the origin server and check whether + the content is still fresh before serving stale content to a + client. The origin server will either respond with replacement + content if not still valid, or ideally, the origin server will + respond with a code to tell the cache the content is still + fresh, without the need to generate or send the content again. + The content becomes fresh again and the cycle continues.</p> + + <p>The HTTP protocol does allow the cache to serve stale data + under certain circumstances, such as when an attempt to freshen + the data with an origin server has failed with a 5xx error, or + when another request is already in the process of freshening + the given entry. In these cases a <code>Warning</code> header + is added to the response.</p> + </dd> + <dt>Non Existent</dt> + <dd> + If the cache gets full, it reserves the option to delete content + from the cache to make space. Content can be deleted at any time, + and can be stale or fresh. The <a href="programs/htcacheclean.html">htcacheclean</a> tool can be + run on a once off basis, or deployed as a daemon to keep the size + of the cache within the given size, or the given number of inodes. + The tool attempts to delete stale content before attempting to + delete fresh content. + </dd> + </dl> + + <p>Full details of how HTTP caching works can be found in + <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html"> + Section 13 of RFC2616</a>.</p> + + <h3>Interaction with the Server</h3> + + + <p>The <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module hooks into the server in two + possible places depending on the value of the + <code class="directive"><a href="./mod/mod_cache.html#cachequickhandler">CacheQuickHandler</a></code> directive: + </p> + + <dl> + <dt>Quick handler phase</dt> + <dd> + <p>This phase happens very early on during the request processing, + just after the request has been parsed. If the content is + found within the cache, it is served immediately and almost + all request processing is bypassed.</p> + + <p>In this scenario, the cache behaves as if it has been "bolted + on" to the front of the server.</p> + + <p>This mode offers the best performance, as the majority of + server processing is bypassed. This mode however also bypasses the + authentication and authorization phases of server processing, so + this mode should be chosen with care when this is important.</p> + + <p> Requests with an "Authorization" header (for example, HTTP Basic + Authentication) are neither cacheable nor served from the cache + when <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> is running in this phase.</p> + </dd> + <dt>Normal handler phase</dt> + <dd> + <p>This phase happens late in the request processing, after all + the request phases have completed.</p> + + <p>In this scenario, the cache behaves as if it has been "bolted + on" to the back of the server.</p> + + <p>This mode offers the most flexibility, as the potential exists + for caching to occur at a precisely controlled point in the filter + chain, and cached content can be filtered or personalized before + being sent to the client.</p> + </dd> + </dl> + + <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 filter stack in order + to record the response to the cache, and then stand down, allowing normal + request processing to continue. If the content is determined to be + cacheable, the content will be saved to the cache for future serving, + otherwise the content will be ignored.</p> + + <p>If the content found within the cache is stale, the + <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> module converts the request into a + <strong>conditional request</strong>. If the origin server responds with + a normal response, the normal response is cached, replacing the content + already cached. If the origin server responds with a 304 Not Modified + response, the content is marked as fresh again, and the cached content + is served by the filter instead of saving it.</p> + + + <h3>Improving Cache Hits</h3> + + + <p>When a virtual host is known by one of many different server aliases, + 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 is + used within 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> + + + + <h3>Freshness Lifetime</h3> + + + <p>Well formed content that is intended to be cached should declare an + explicit freshness lifetime with the <code>Cache-Control</code> + header's <code>max-age</code> or <code>s-maxage</code> fields, or + by including an <code>Expires</code> header.</p> + + <p>At the same time, the origin server defined freshness lifetime can + be overridden by a client when the client presents their own + <code>Cache-Control</code> header within the request. In this case, + the lowest freshness lifetime between request and response wins.</p> + + <p>When this freshness lifetime is missing from the request or the + response, a default freshness lifetime is applied. The default + freshness lifetime 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.</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 a freshness lifetime based on a heuristic, which can be + controlled through the use of the <code class="directive"><a href="./mod/mod_cache.html#cachelastmodifiedfactor">CacheLastModifiedFactor</a></code> directive.</p> + + <p>For local content, or for remote content that does not define its own + <code>Expires</code> header, <code class="module"><a href="./mod/mod_expires.html">mod_expires</a></code> may be used to + fine-tune the freshness lifetime by adding <code>max-age</code> and + <code>Expires</code>.</p> + + <p>The maximum freshness lifetime 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 becomes stale, rather than + pass on the original request, httpd will modify the request to make + it conditional instead.</p> + + <p>When an <code>ETag</code> header exists in the original cached + response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an + <code>If-None-Match</code> header to the request to the origin server. + When a <code>Last-Modified</code> header exists in the original + cached response, <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will add an + <code>If-Modified-Since</code> header to the request to the origin + server. Performing either of these actions makes the request + <strong>conditional</strong>.</p> + + <p>When a conditional request is received by an origin server, the + origin server should check whether the ETag or the Last-Modified + parameter has changed, as appropriate for the request. If not, the + origin should respond with a terse "304 Not Modified" response. This + signals to the cache that the stale content is still fresh should be + used for subsequent requests until the content's new freshness lifetime + is reached again.</p> + + <p>If the content has changed, then the content is served as if the + request were not conditional to begin with.</p> + + <p>Conditional requests offer two benefits. Firstly, when making such + a request to the origin server, if the content from the origin + matches the content in the cache, this can be determined easily and + without the overhead of transferring the entire resource.</p> + + <p>Secondly, a well designed origin server will be designed in such + a way that conditional requests will be significantly cheaper to + produce than a full response. 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 + local content may still be served faster from the cache if it has not + changed.</p> + + <p>Origin servers should make every effort to support conditional + requests as is practical, however if conditional requests are not + supported, the origin will respond as if the request was not + conditional, and the cache will respond as if the content had changed + and save the new content to the cache. In this case, the cache will + behave like a simple two state cache, where content is effectively + either fresh or deleted.</p> + + + <h3>What Can be Cached?</h3> + + + <p>The full definition of which responses can be cached by an HTTP + cache is defined in + <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4"> + RFC2616 Section 13.4 Response Cacheability</a>, and can be summed up as + follows:</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>If the response has an HTTP status code other than 200, 203, 300, + 301 or 410 it must also specify an "Expires" or "Cache-Control" header. + </li> + + <li>The request must be a HTTP GET request.</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, or it won't be cached.</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 specifies an + explicit expiration by including an "Expires:" header or the max-age + or s-maxage directive of the "Cache-Control:" header, as per RFC2616 + sections 13.9 and 13.2.1.</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, or the max-age or s-maxage directive of + the "Cache-Control:" header, 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>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></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></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></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></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. 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