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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<HTML>
<HEAD>
<TITLE>Apache module mod_file_cache</TITLE>
</HEAD>
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<H1 ALIGN="CENTER">Module mod_file_cache</H1>
<P>
<STRONG>This module should be used with care. You can easily create a
broken site using mod_file_cache, so read this document
carefully.</STRONG>
</P>
<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>
<CODE>mod_file_cache</CODE> is not compiled into the server by
default. To use <CODE>mod_file_cache</CODE> you have to enable the
following line in the server build <CODE>Configuration</CODE> file:
<PRE>
AddModule modules/experimental/mod_file_cache.o
</PRE>
</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>
<H2>Summary</H2>
<P>
<CODE>mod_file_cache</CODE> caches a list of statically configured
files via <CODE>MMapFile</CODE> or <CODE>CacheFile</CODE> directives
in the main server configuration.
</P>
<P>
Not all platforms support both directives. For
example, Apache on Windows does not currently support the MMapStatic
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>
<H3><CODE>MmapFile</CODE> Directive </H3>
<P>
The <CODE>MmapFile</CODE> directive of <CODE>mod_file_cache</CODE>
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>
<H3><CODE>CacheFile</CODE> Directive </H3>
<P>
The <CODE>CacheFile</CODE> directive of <CODE>mod_file_cache</CODE>
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 <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>
<H2>Directives</H2>
<UL>
<LI><A HREF="#mmapfile">MMapFile</A>
</LI>
<LI><A HREF="#cachefile">CacheFile</A>
</LI>
</UL>
<HR>
<H2><A NAME="mmapfile">MMapFile</A></H2>
<P>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> MMapFile <EM>filename</EM> [<em>filename</em>] ...
<BR>
<A
HREF="directive-dict.html#Default"
REL="Help"
><STRONG>Default:</STRONG></A> <EM>None</EM>
<BR>
<A
HREF="directive-dict.html#Context"
REL="Help"
><STRONG>Context:</STRONG></A> server-config
<BR>
<A
HREF="directive-dict.html#Override"
REL="Help"
><STRONG>Override:</STRONG></A> <EM>Not applicable</EM>
<BR>
<A
HREF="directive-dict.html#Status"
REL="Help"
><STRONG>Status:</STRONG></A> Experimental
<BR>
<A
HREF="directive-dict.html#Module"
REL="Help"
><STRONG>Module:</STRONG></A> mod_file_cache
<BR>
<A
HREF="directive-dict.html#Compatibility"
REL="Help"
><STRONG>Compatibility:</STRONG></A> Only in Apache 1.3 (via
mod_mmap_statis) or later.
<P>
The <CODE>MMapFile</CODE> 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>filename</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 <CODE>mod_alias</CODE> or
<CODE>mod_rewrite</CODE>.
</P>
Example:
<PRE>
MMapFile /usr/local/apache/htdocs/index.html
</PRE>
<hr>
<H2><A NAME="cachefile">CacheFile</A></H2>
<P>
<A
HREF="directive-dict.html#Syntax"
REL="Help"
><STRONG>Syntax:</STRONG></A> CacheFile <EM>filename</EM> [<em>filename</em>] ...
<BR>
<A
HREF="directive-dict.html#Default"
REL="Help"
><STRONG>Default:</STRONG></A> <EM>None</EM>
<BR>
<A
HREF="directive-dict.html#Context"
REL="Help"
><STRONG>Context:</STRONG></A> server-config
<BR>
<A
HREF="directive-dict.html#Override"
REL="Help"
><STRONG>Override:</STRONG></A> <EM>Not applicable</EM>
<BR>
<A
HREF="directive-dict.html#Status"
REL="Help"
><STRONG>Status:</STRONG></A> Experimental
<BR>
<A
HREF="directive-dict.html#Module"
REL="Help"
><STRONG>Module:</STRONG></A> mod_file_cache
<BR>
<A
HREF="directive-dict.html#Compatibility"
REL="Help"
><STRONG>Compatibility:</STRONG></A> Only available in Apache 2.0 or later.
<P>
The <CODE>CacheFile</CODE> 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>filename</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 <CODE>mod_alias</CODE> or
<CODE>mod_rewrite</CODE>.
</P>
Example:
<PRE>
CacheFile /usr/local/apache/htdocs/index.html
</PRE>
<P>
<STRONG>Note</STRONG>: don't bother asking for a for a directive which
recursively caches all the files in a directory. Try this
instead...
See the <A HREF="core.html#include">Include</A> directive, and consider this command:
<PRE>
find /www/htdocs -type f -print \
| sed -e 's/.*/mmapfile &/' > /www/conf/mmap.conf
</PRE>
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