<h3>Improving Cache Hits</h3>

      <span class="indent">

      <p>When caching locally generated content, ensuring that  

      <code class="directive"><a href="./mod/core.html#usecanonicalname">UseCanonicalName</a></code> is set to 

      <code>On</code> can dramatically improve the ratio of cache hits. This

      <p>If you are using Server Side Includes, and want the benefit of speedy

      serves from the cache, you should use <code>virtual</code> include

      types.</p>

      </span>

    <h3>Expiry Periods</h3>

      <span class="indent">

      <p>The default expiry period for cached entities is one hour, however 

      this can be easily over-ridden by using the <code class="directive"><a href="./mod/mod_cache.html#cachedefaultexpire">CacheDefaultExpire</a></code> directive. This

      default is only used when the original source of the content does not

      <p>The maximum expiry period may also be controlled by using the

      <code class="directive"><a href="./mod/mod_cache.html#cachemaxexpire">CacheMaxExpire</a></code>.</p>

      </span>

    <h3>A Brief Guide to Conditional Requests</h3>

      <span class="indent">

      <p>When content expires from the cache and is re-requested from the 

      backend or content provider, rather than pass on the original request,

      Aoache will use a conditional request instead.</p>

      from the cache if it has not changed. As long as reading from the cache

      store is faster than reading from the backend (e.g. an in-memory cache 

      compared to reading from disk).</p> 

      </span>

    <h3>What Can be Cached?</h3>

      <span class="indent">

      <p>As mentioned already, the two styles of caching in Apache work 

      differently, <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> caching maintains file 

      contents as they were when Apache was started. When a request is 

        containing the match-all "*".</li>

      </ol>

      </span>

    <h3>What Should Not be Cached?</h3>

      <span class="indent">

      <p>In short, any content which is highly time-sensitive, or which varies

      depending on the particulars of the request that are not covered by

      HTTP negotiation, should not be cached.</p>

      <p>If on the other hand, the content served differs depending on the

      values of various HTTP headers, it is possible that it might be possible

      to cache it intelligently through the use of a "Vary" header.</p>

      </span>

    <h3>Variable/Negotiated Content</h3>

      <span class="indent">

      <p>If a response with a "Vary" header is received by 

      <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> when requesting content by the backend it

      will attempt to handle it intelligently. If possible, 

      <p><code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code> will only serve the cached content to

      requesters with matching accept-language and accept-charset headers

      matching those of the original request.</p>

      </span>

  </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>

    <h3>Local exploits</h3>

      <span class="indent">

      <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

      using <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> you should bear this in mind - 

      ensure you upgrade Apache when security upgrades are announced and 

      run CGI processes as a non-Apache user using <a href="suexec.html">suEXEC</a> if possible.</p>

      </span>

    <h3>Cache Poisoning</h3>

      <span class="indent">

      <p>When running Apache as a caching proxy server, there is also the

      potential for so-called cache poisoning. Cache Poisoning is a broad 

      term for attacks in which an attacker causes the proxy server to 

      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>

      </span>

  </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>

<div class="section">

    <h3>CacheFile</h3>

      <span class="indent">

      <p>The most basic form of caching present in Apache is the file-handle

      caching provided by <code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code>. Rather than caching 

      file-contents, this cache maintains a table of open file descriptors. Files 

      will have been deleted, and not show up on the filesystem, extra free

      space will not be recovered until Apache is stopped and the file

      descriptor closed.</p>

      </span>

    <h3>CacheEnable fd</h3>

      <span class="indent">

      <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> also provides its own file-handle 

      caching scheme, which can be enabled via the 

      <code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code> directive.</p>

      caching is intelligent, and handles will not be maintained beyond

      the expiry time of the cached content.</p>

      </span>

  </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>

<div class="section">

<h2><a name="inmemory" id="inmemory">In-Memory Caching</a></h2>

    <h3>Operating System Caching</h3>

      <span class="indent">

      <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

      deleted or modified, it can automatically remove file contents from the 

      cache when neccessary. This is a big advantage over Apache's in-memory 

      caching which has no way of knowing when a file has changed.</p>

      </span>

    <p>Despite the performance and advantages of automatic operating system

    <h3>MMapStatic Caching</h3>

      <span class="indent">

      <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the 

      <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> directive, which

      allows you to have Apache map a static file's contents into memory at

      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>

      </span>

    <h3>mod_mem_cache Caching</h3>

      <span class="indent">

      <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> provides a HTTP-aware intelligent

      in-memory cache. It also uses heap memory directly, which means that

      even if <var>MMap</var> is not supported on your system, 

# Limit the size of the cache to 1 Megabyte

MCacheSize 1024</pre></div>

      </span>

  </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>

<div class="section">

<h2><a name="disk" id="disk">Disk-based Caching</a></h2>

    <h3>Understanding the Cache-Store</h3>

      <span class="indent">

      <p>To store items in the cache, <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> creates

      a 22 character hash of the url being requested. Thie hash incorporates

      the hostname, protocol, port, path and any CGI arguments to the URL,

      ".vary" directory will be created for the url in question. This 

      directory will have multiple ".data" files corresponding to the

      differently negotiated content.</p>

      </span>

    <h3>Maintaining the Disk Cache</h3>

      <span class="indent">    

      <p>Although <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> will remove cached content

      as it is expired, it does not maintain any information on the total

      size of the cache or how little free space may be left.</p>

      <a href="programs/htcacheclean.html">htcacheclean</a> is configured to 

      leave enough "grow room" following a clean.</p>

      </span>

  </div></div>

<div class="bottomlang">

    <section>

      <title>Improving Cache Hits</title>

      <indent>

      <p>When caching locally generated content, ensuring that  

      <directive module="core">UseCanonicalName</directive> is set to 

      <code>On</code> can dramatically improve the ratio of cache hits. This

      <p>If you are using Server Side Includes, and want the benefit of speedy

      serves from the cache, you should use <code>virtual</code> include

      types.</p>

      </indent>

    </section>

    <section>

      <title>Expiry Periods</title>

      <indent>

      <p>The default expiry period for cached entities is one hour, however 

      this can be easily over-ridden by using the <directive 

      module="mod_cache">CacheDefaultExpire</directive> directive. This

      <p>The maximum expiry period may also be controlled by using the

      <directive module="mod_cache">CacheMaxExpire</directive>.</p>

      </indent>

    </section>

    <section>

      <title>A Brief Guide to Conditional Requests</title>

      <indent>

      <p>When content expires from the cache and is re-requested from the 

      backend or content provider, rather than pass on the original request,

      Aoache will use a conditional request instead.</p>

      from the cache if it has not changed. As long as reading from the cache

      store is faster than reading from the backend (e.g. an in-memory cache 

      compared to reading from disk).</p> 

      </indent>

    </section>

    <section>

      <title>What Can be Cached?</title>

      <indent>

      <p>As mentioned already, the two styles of caching in Apache work 

      differently, <module>mod_file_cache</module> caching maintains file 

      contents as they were when Apache was started. When a request is 

        <li>A response will not be stored if it includes a "Vary:" header

        containing the match-all "*".</li>

      </ol>

      </indent>

    </section>

    <section>

      <title>What Should Not be Cached?</title>

      <indent>

      <p>In short, any content which is highly time-sensitive, or which varies

      depending on the particulars of the request that are not covered by

      HTTP negotiation, should not be cached.</p>

      <p>If on the other hand, the content served differs depending on the

      values of various HTTP headers, it is possible that it might be possible

      to cache it intelligently through the use of a "Vary" header.</p>

      </indent>

    </section>

    <section>

      <title>Variable/Negotiated Content</title>

      <indent>

      <p>If a response with a "Vary" header is received by 

      <module>mod_cache</module> when requesting content by the backend it

      will attempt to handle it intelligently. If possible, 

      <p><module>mod_cache</module> will only serve the cached content to

      requesters with matching accept-language and accept-charset headers

      matching those of the original request.</p>

      </indent>

    </section>

  </section>

    <section>

      <title>Local exploits</title>

      <indent>

      <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

      ensure you upgrade Apache when security upgrades are announced and 

      run CGI processes as a non-Apache user using <a 

      href="suexec.html">suEXEC</a> if possible.</p>

      </indent>

    </section>

    <section>

      <title>Cache Poisoning</title>

      <indent>

      <p>When running Apache as a caching proxy server, there is also the

      potential for so-called cache poisoning. Cache Poisoning is a broad 

      term for attacks in which an attacker causes the proxy server to 

      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>

      </indent>

    </section>

  </section>

    <section>

      <title>CacheFile</title>

      <indent>

      <p>The most basic form of caching present in Apache is the file-handle

      caching provided by <module>mod_file_cache</module>. Rather than caching 

      file-contents, this cache maintains a table of open file descriptors. Files 

      will have been deleted, and not show up on the filesystem, extra free

      space will not be recovered until Apache is stopped and the file

      descriptor closed.</p>

      </indent>

    </section>

    <section>

      <title>CacheEnable fd</title>

      <indent>

      <p><module>mod_mem_cache</module> also provides its own file-handle 

      caching scheme, which can be enabled via the 

      <directive module="mod_cache">CacheEnable</directive> directive.</p>

      <p>As with all of <module>mod_cache</module> this type of file-handle

      caching is intelligent, and handles will not be maintained beyond

      the expiry time of the cached content.</p>

      </indent>

    </section>

  </section>

    <section>

      <title>Operating System Caching</title>

      <indent>

      <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

      deleted or modified, it can automatically remove file contents from the 

      cache when neccessary. This is a big advantage over Apache's in-memory 

      caching which has no way of knowing when a file has changed.</p>

      </indent>

    </section>

    <p>Despite the performance and advantages of automatic operating system

    <section>

      <title>MMapStatic Caching</title>

      <indent>

      <p><module>mod_file_cache</module> provides the 

      <directive module="mod_file_cache">MMapStatic</directive> directive, which

      allows you to have Apache map a static file's contents into memory at

      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>

      </indent>

    </section>

    <section>

      <title>mod_mem_cache Caching</title>

      <indent>

      <p><module>mod_mem_cache</module> provides a HTTP-aware intelligent

      in-memory cache. It also uses heap memory directly, which means that

      even if <var>MMap</var> is not supported on your system, 

# Limit the size of the cache to 1 Megabyte

MCacheSize 1024</pre>

      </example>

      </indent>

    </section>

  </section>

    <section>

      <title>Understanding the Cache-Store</title>

      <indent>

      <p>To store items in the cache, <module>mod_disk_cache</module> creates

      a 22 character hash of the url being requested. Thie hash incorporates

      the hostname, protocol, port, path and any CGI arguments to the URL,

      ".vary" directory will be created for the url in question. This 

      directory will have multiple ".data" files corresponding to the

      differently negotiated content.</p>

      </indent>

    </section>

    <section>

      <title>Maintaining the Disk Cache</title>

      <indent>    

      <p>Although <module>mod_disk_cache</module> will remove cached content

      as it is expired, it does not maintain any information on the total

      size of the cache or how little free space may be left.</p>

      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>

      </indent>

    </section>

  </section>