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<title>Advanced Techniques with mod_rewrite - Apache HTTP Server</title>
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<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.html">Directives</a> | <a href="/faq/">FAQ</a> | <a href="/glossary.html">Glossary</a> | <a href="/sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.3</p>
<div id="path">
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Advanced Techniques with mod_rewrite</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="/en/rewrite/avoid.html" title="English"> en </a></p>
</div>
<p>This document supplements the <code class="module"><a href="/mod/mod_rewrite.html">mod_rewrite</a></code>
a few advanced techniques and tricks using mod_rewrite.</p>
<div class="warning">Note that many of these examples won't work unchanged in your
particular server configuration, so it's important that you understand
them, rather than merely cutting and pasting the examples into your
configuration.</div>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#sharding">URL-based sharding accross multiple backends</a></li>
<li><img alt="" src="/images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
</ul><h3>See also</h3><ul class="seealso"><li><a href="/mod/mod_rewrite.html">Module documentation</a></li><li><a href="intro.html">mod_rewrite introduction</a></li><li><a href="remapping.html">Redirection and remapping</a></li><li><a href="access.html">Controlling access</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul></div>
<div class="section">
<h2><a name="sharding" id="sharding">URL-based sharding accross multiple backends</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>A common technique for distributing the burden of
server load or storage space is called "sharding".
When using this method, a front-end server will use the
url to consistently "shard" users or objects to separate
backend servers.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>A mapping is maintained, from users to target servers, in
external map files. They look like:</p>
<div class="example"><pre>
user1 physical_host_of_user1
user2 physical_host_of_user2
: :
</pre></div>
aim is to map;</p>
<div class="example"><pre>
</pre></div>
<p>to</p>
<div class="example"><pre>
</pre></div>
<p>thus every URL path need not be valid on every backend physical
host. The following ruleset does this for us with the help of the map
files assuming that server0 is a default server which will be used if
a user has no entry in the map):</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${users-to-hosts:$1|server0}</strong>/u/$1/$2
</pre></div>
</dd>
</dl>
<div class="section">
<h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>We wish to dynamically generate content, but store it
statically once it is generated. This rule will check for the
existence of the static file, and if it's not there, generate
it. The static files can be removed periodically, if desired (say,
via cron) and will be regenerated on demand.</p>
</dd>
<dt>Solution:</dt>
<dd>
This is done via the following ruleset:
<div class="example"><pre>
# This example is valid in per-directory context only
RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong>
</pre></div>
CGI script which (additionally to its <code>STDOUT</code>)
Once it has completed, the server sends out
a refresh of the contents, he just removes
</dd>
</dl>
<div class="section">
<h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>We wish to randomly distribute load across several servers
using mod_rewrite.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We'll use <code class="directive"><a href="/mod/mod_rewrite.html#rewritemap">RewriteMap</a></code> and a list of servers
to accomplish this.</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
</pre></div>
<div class="example"><pre>
</pre></div>
<p>If you want one particular server to get more of the load than the
others, add it more times to the list.</p>
</dd>
<dt>Discussion</dt>
<dd>
<p>Apache comes with a load-balancing module -
<code class="module"><a href="/mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> - which is far more flexible and
featureful than anything you can cobble together using mod_rewrite.</p>
</dd>
</dl>
<div class="section">
<h2><a name="autorefresh" id="autorefresh">Document With Autorefresh</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>Wouldn't it be nice, while creating a complex web page, if
the web browser would automatically refresh the page every
time we save a new version from within our editor?
Impossible?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>No! We just combine the MIME multipart feature, the
web server NPH feature, and the URL manipulation power of
<code class="module"><a href="/mod/mod_rewrite.html">mod_rewrite</a></code>. First, we establish a new
URL feature: Adding just <code>:refresh</code> to any
URL causes the 'page' to be refreshed every time it is
updated on the filesystem.</p>
<div class="example"><pre>
</pre></div>
<p>Now when we reference the URL</p>
<div class="example"><pre>
</pre></div>
<p>this leads to the internal invocation of the URL</p>
<div class="example"><pre>
</pre></div>
<p>The only missing part is the NPH-CGI script. Although
one would usually say "left as an exercise to the reader"
;-) I will provide this, too.</p>
<div class="example"><pre>
##
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
# split the QUERY_STRING variable
@pairs = split(/&/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
($name, $value) = split(/=/, $pair);
$name = 'QS_' . $name;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
exit(0);
}
if (! -f $QS_f) {
print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
exit(0);
}
sub print_http_headers_multipart_begin {
$bound = "ThisRandomString12345";
print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
&print_http_headers_multipart_next;
}
sub print_http_headers_multipart_next {
print "\n--$bound\n";
}
sub print_http_headers_multipart_end {
print "\n--$bound--\n";
}
sub displayhtml {
local($buffer) = @_;
$len = length($buffer);
print "Content-length: $len\n\n";
print $buffer;
}
sub readfile {
local($file) = @_;
local(*FP, $size, $buffer, $bytes);
($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
$size = sprintf("%d", $size);
open(FP, "&lt;$file");
$bytes = sysread(FP, $buffer, $size);
close(FP);
return $buffer;
}
$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);
sub mystat {
local($file) = $_[0];
local($time);
($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
return $mtime;
}
$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &lt; $QS_n; $n++) {
while (1) {
$mtime = &mystat($QS_f);
if ($mtime ne $mtimeL) {
$mtimeL = $mtime;
sleep(2);
$buffer = &readfile($QS_f);
&print_http_headers_multipart_next;
&displayhtml($buffer);
sleep(5);
$mtimeL = &mystat($QS_f);
last;
}
sleep($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre></div>
</dd>
</dl>
</div></div>
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