mod_file_cache.xml

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<modulesynopsis metafile="mod_file_cache.xml.meta">

<name>mod_file_cache</name>
<description>Caches a static list of files in memory</description>
<status>Experimental</status>
<sourcefile>mod_file_cache.c</sourcefile>
<identifier>file_cache_module</identifier>

<summary>

    <note type="warning">
      This module should be used with care. You can easily create a broken
      site using <module>mod_file_cache</module>, so read this document
      carefully.
    </note>

    <p><em>Caching</em> frequently requested files that change very
    infrequently is a technique for reducing server load.
    <module>mod_file_cache</module> provides two techniques for caching
    frequently requested <em>static</em> files. Through configuration
    directives, you can direct <module>mod_file_cache</module> to either
    open then <code>mmap()</code> a file, or to pre-open a file and save
    the file's open <em>file handle</em>. Both techniques reduce server
    load when processing requests for these files by doing part of the work
    (specifically, the file I/O) for serving the file when the
    server is started rather than during each request.</p>

    <p>Notice: You cannot use this for speeding up CGI programs or
    other files which are served by special content handlers. It
    can only be used for regular files which are usually served by
    the Apache core content handler.</p>

    <p>This module is an extension of and borrows heavily from the
    <code>mod_mmap_static</code> module in Apache 1.3.</p>
</summary>

<section id="using"><title>Using mod_file_cache</title>

    <p><module>mod_file_cache</module> caches a list of statically
    configured files via <directive module="mod_file_cache"
    >MMapFile</directive> or <directive module="mod_file_cache"
    >CacheFile</directive> directives in the main server configuration.</p>

    <p>Not all platforms support both directives. You will receive an error
    message in the server error log if you attempt to use an
    unsupported directive. If given an unsupported directive, the
    server will start but the file will not be cached. On platforms
    that support both directives, you should experiment with both to
    see which works best for you.</p>

    <section><title>MMapFile Directive</title>

      <p>The <directive module="mod_file_cache">MMapFile</directive>
      directive of <module>mod_file_cache</module> maps a list of
      statically configured files into memory through the system call
      <code>mmap()</code>. This system call is available on most modern
      Unix derivatives, but not on all. There are sometimes system-specific
      limits on the size and number of files that can be
      <code>mmap()</code>ed, experimentation is probably the easiest way
      to find out.</p>

      <p>This <code>mmap()</code>ing is done once at server start or
      restart, only. So whenever one of the mapped files changes on the
      filesystem you <em>have</em> to restart the server (see the <a
      href="../stopping.html">Stopping and Restarting</a> documentation).
      To reiterate that point: if the files are modified <em>in place</em>
      without restarting the server you may end up serving requests that
      are completely bogus. You should update files by unlinking the old
      copy and putting a new copy in place. Most tools such as
      <code>rdist</code> and <code>mv</code> do this. The reason why this
      modules doesn't take care of changes to the files is that this check
      would need an extra <code>stat()</code> every time which is a waste
      and against the intent of I/O reduction.</p>
    </section>

    <section><title>CacheFile Directive</title>

      <p>The <directive module="mod_file_cache">CacheFile</directive>
      directive of <module>mod_file_cache</module> opens an active
      <em>handle</em> or <em>file descriptor</em> to the file (or files)
      listed in the configuration directive and places these open file
      handles in the cache. When the file is requested, the server
      retrieves the handle from the cache and passes it to the
      <code>sendfile()</code> (or <code>TransmitFile()</code> on Windows),
      socket API.</p>

      <!-- XXX
      <p>Insert more details about sendfile API...</p>
      -->

      <p>This file handle caching is done once at server start or
      restart, only. So whenever one of the cached files changes on
      the filesystem you <em>have</em> to restart the server (see the
      <a href="../stopping.html">Stopping and Restarting</a>
      documentation). To reiterate that point: if the files are
      modified <em>in place</em> without restarting the server you
      may end up serving requests that are completely bogus. You
      should update files by unlinking the old copy and putting a new
      copy in place. Most tools such as <code>rdist</code> and
      <code>mv</code> do this.</p>
    </section>

    <note><title>Note</title>
      <p>Don't bother asking for a directive which recursively
      caches all the files in a directory. Try this instead... See the 
      <directive module="core">Include</directive> directive, and consider
      this command:</p>

      <example>
        find /www/htdocs -type f -print \<br />
        | sed -e 's/.*/mmapfile &amp;/' &gt; /www/conf/mmap.conf
      </example>
    </note>
</section>

<directivesynopsis>
<name>MMapFile</name>
<description>Map a list of files into memory at startup time</description>
<syntax>MMapFile <var>file-path</var> [<var>file-path</var>] ...</syntax>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>MMapFile</directive> directive maps one or more files
    (given as whitespace separated arguments) into memory at server
    startup time. They are automatically unmapped on a server
    shutdown. When the files have changed on the filesystem at
    least a <code>HUP</code> or <code>USR1</code> signal should be send to
    the server to re-<code>mmap()</code> them.</p>

    <p>Be careful with the <var>file-path</var> arguments: They have
    to literally match the filesystem path Apache's URL-to-filename
    translation handlers create. We cannot compare inodes or other
    stuff to match paths through symbolic links <em>etc.</em>
    because that again would cost extra <code>stat()</code> system
    calls which is not acceptable. This module may or may not work
    with filenames rewritten by <module>mod_alias</module> or
    <module>mod_rewrite</module>.</p>

    <example><title>Example</title>
      MMapFile /usr/local/apache/htdocs/index.html
    </example>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>CacheFile</name>
<description>Cache a list of file handles at startup time</description>
<syntax>CacheFile <var>file-path</var> [<var>file-path</var>] ...</syntax>
<contextlist><context>server config</context></contextlist>

<usage>
    <p>The <directive>CacheFile</directive> directive opens handles to
    one or more files (given as whitespace separated arguments) and
    places these handles into the cache at server startup
    time. Handles to cached files are automatically closed on a server
    shutdown.  When the files have changed on the filesystem, the
    server should be restarted to re-cache them.</p>

    <p>Be careful with the <var>file-path</var> arguments: They have
    to literally match the filesystem path Apache's URL-to-filename
    translation handlers create. We cannot compare inodes or other
    stuff to match paths through symbolic links <em>etc.</em>
    because that again would cost extra <code>stat()</code> system
    calls which is not acceptable. This module may or may not work
    with filenames rewritten by <module>mod_alias</module> or
    <module>mod_rewrite</module>.</p>

    <example><title>Example</title>
      CacheFile /usr/local/apache/htdocs/index.html
    </example>
</usage>
</directivesynopsis>

</modulesynopsis>