mod_file_cache.xml revision e942c741056732f50da2074b36fe59805d370650
<?xml version="1.0"?>
<modulesynopsis>
<name>mod_file_cache</name>
<description>Caches a static list of files in memory</description>
<status>Experimental</status>
<identifier>file_cache_module</identifier>
<summary>
<note type="warning">
This module should be used with care. You can easily
create a broken site using mod_file_cache, so read this
document carefully.
</note>
<p><em>Caching</em> frequently requested files that change very
infrequently is a technique for reducing server load.
mod_file_cache provides two techniques for caching frequently
requested <em>static</em> files. Through configuration
directives, you can direct mod_file_cache to either open then
mmap()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
mod_mmap_static module in Apache 1.3.</p>
</summary>
<section><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. For example, Apache
on Windows does not currently support the <directive
module="mod_file_cache">MMapStatic</directive> directive, while
other platforms, like AIX, support both. 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 derivates, but not on all. There are sometimes
system-specific limits on the size and number of files that can be
mmap()d, experimentation is probably the easiest way to find
out.</p>
<p>This mmap()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
sendfile() (or TransmitFile() on Windows), socket API.</p>
<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
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> Don't bother asking for a 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:
<example>
</example>
</note>
</section>
<directivesynopsis>
<name>MMapFile</name>
<syntax>MMapFile <em>file-path</em> [<em>file-path</em>] ...</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 HUP or USR1 signal should be send to the server to
re-mmap them.</p>
<p>Be careful with the <em>file-path</em> 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>
</example>
</usage>
</directivesynopsis>
<directivesynopsis>
<name>CacheFile</name>
<syntax>CacheFile
<em>file-path</em> [<em>file-path</em>] ...</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 to re-cache them.</p>
<p>Be careful with the <em>file-path</em> 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>
</example>
</usage>
</directivesynopsis>
</modulesynopsis>