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<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->
<!--
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file distributed with
this work for additional information regarding copyright ownership.
The ASF licenses this file to You under the Apache License, Version 2.0
(the "License"); you may not use this file except in compliance with
the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<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 &/' > /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>
<highlight language="config">
MMapFile /usr/local/apache/htdocs/index.html
</highlight>
</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>
<highlight language="config">
CacheFile /usr/local/apache/htdocs/index.html
</highlight>
</example>
</usage>
</directivesynopsis>
</modulesynopsis>
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