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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> |
| <p class="apache">Apache HTTP Server Version 2.5</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 2.5</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"><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="#socache-caching">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>The response must have a HTTP status code of 200, 203, 300, 301 or |
| 410.</li> |
| |
| <li>The request must be a HTTP GET request.</li> |
| |
| <li>If the 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> |
| |
| |
| <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> |
| |
| |
| </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">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. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again 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 Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div> |
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