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<title>When not to use mod_rewrite - Apache HTTP Server</title>
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<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>When not to use 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></div>
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<p><span>Available Languages: </span><a href="/en/rewrite/avoid.html" title="English"> en </a></p>
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