docs/manual/rewrite/advanced.html.en - httpd - Git at Google

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 <title>Advanced Techniques with mod_rewrite - Apache HTTP Server</title>
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 <p class="apache">Apache HTTP Server Version 2.3</p>
 <img alt="" src="../images/feather.gif" /></div>
 <div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
 <div id="path">
 <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <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">&nbsp;en&nbsp;</a></p>
 </div>


 <p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
 <a href="../mod/mod_rewrite.html">reference documentation</a>. It provides
 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>
 <li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#autorefresh">Document With Autorefresh</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Userdirs</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#time-dependent">Time-Dependent Rewriting</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables Based On URL Parts</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="vhosts.html">Virtual hosts</a></li><li><a href="proxy.html">Proxying</a></li><li><a href="rewritemap.html">Using RewriteMap</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul></div>
 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></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"><p><code>
 user1  physical_host_of_user1<br />
 user2  physical_host_of_user2<br />
 :      :
 </code></p></div>

   <p>We put this into a <code>map.users-to-hosts</code> file. The
     aim is to map;</p>

 <div class="example"><p><code>
 /u/user1/anypath
 </code></p></div>

   <p>to</p>

 <div class="example"><p><code>
 http://physical_host_of_user1/u/user/anypath
 </code></p></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"><p><code>
 RewriteEngine on<br />
 <br />
 RewriteMap      users-to-hosts   txt:/path/to/map.users-to-hosts<br />
 <br />
 RewriteRule   ^/u/<strong>([^/]+)</strong>/?(.*)   http://<strong>${users-to-hosts:$1|server0}</strong>/u/$1/$2
 </code></p></div>
     </dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <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"><p><code>
 # This example is valid in per-directory context only<br />
 RewriteCond %{REQUEST_FILENAME}   <strong>!-s</strong><br />
 RewriteRule ^page\.<strong>html</strong>$          page.<strong>cgi</strong>   [T=application/x-httpd-cgi,L]
 </code></p></div>

       <p>Here a request for <code>page.html</code> leads to an
       internal run of a corresponding <code>page.cgi</code> if
       <code>page.html</code> is missing or has filesize
       null. The trick here is that <code>page.cgi</code> is a
       CGI script which (additionally to its <code>STDOUT</code>)
       writes its output to the file <code>page.html</code>.
       Once it has completed, the server sends out
       <code>page.html</code>. When the webmaster wants to force
       a refresh of the contents, he just removes
       <code>page.html</code> (typically from <code>cron</code>).</p>
     </dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <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"><p><code>
 RewriteEngine on<br />
 RewriteMap lb rnd:/path/to/serverlist.txt<br />
 <br />
 RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
 </code></p></div>

 <p><code>serverlist.txt</code> will contain a list of the servers:</p>

 <div class="example"><p><code>
 ## serverlist.txt<br />
 <br />
 servers one.example.com|two.example.com|three.example.com<br />
 </code></p></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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <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"><p><code>
 RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1
 </code></p></div>

       <p>Now when we reference the URL</p>

 <div class="example"><p><code>
 /u/foo/bar/page.html:refresh
 </code></p></div>

       <p>this leads to the internal invocation of the URL</p>

 <div class="example"><p><code>
 /internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
 </code></p></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>
 #!/sw/bin/perl
 ##
 ##  nph-refresh -- NPH/CGI script for auto refreshing pages
 ##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
 ##
 $| = 1;

 #   split the QUERY_STRING variable
 @pairs = split(/&amp;/, $ENV{'QUERY_STRING'});
 foreach $pair (@pairs) {
 ($name, $value) = split(/=/, $pair);
 $name =~ tr/A-Z/a-z/;
 $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 "HTTP/1.0 200 OK\n";
 print "Content-type: text/html\n\n";
 print "&amp;lt;b&amp;gt;ERROR&amp;lt;/b&amp;gt;: No file given\n";
 exit(0);
 }
 if (! -f $QS_f) {
 print "HTTP/1.0 200 OK\n";
 print "Content-type: text/html\n\n";
 print "&amp;lt;b&amp;gt;ERROR&amp;lt;/b&amp;gt;: File $QS_f not found\n";
 exit(0);
 }

 sub print_http_headers_multipart_begin {
 print "HTTP/1.0 200 OK\n";
 $bound = "ThisRandomString12345";
 print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
 &amp;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-type: text/html\n";
 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", $size);
 open(FP, "&amp;lt;$file");
 $bytes = sysread(FP, $buffer, $size);
 close(FP);
 return $buffer;
 }

 $buffer = &amp;readfile($QS_f);
 &amp;print_http_headers_multipart_begin;
 &amp;displayhtml($buffer);

 sub mystat {
 local($file) = $_[0];
 local($time);

 ($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
 return $mtime;
 }

 $mtimeL = &amp;mystat($QS_f);
 $mtime = $mtime;
 for ($n = 0; $n &amp;lt; $QS_n; $n++) {
 while (1) {
     $mtime = &amp;mystat($QS_f);
     if ($mtime ne $mtimeL) {
         $mtimeL = $mtime;
         sleep(2);
         $buffer = &amp;readfile($QS_f);
         &amp;print_http_headers_multipart_next;
         &amp;displayhtml($buffer);
         sleep(5);
         $mtimeL = &amp;mystat($QS_f);
         last;
     }
     sleep($QS_s);
 }
 }

 &amp;print_http_headers_multipart_end;

 exit(0);

 ##EOF##
 </pre></div>
     </dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="structuredhomedirs" id="structuredhomedirs">Structured Userdirs</a></h2>


   <dl>
     <dt>Description:</dt>

     <dd>
       <p>Some sites with thousands of users use a
       structured homedir layout, <em>i.e.</em> each homedir is in a
       subdirectory which begins (for instance) with the first
       character of the username. So, <code>/~larry/anypath</code>
       is <code>/home/<strong>l</strong>/larry/public_html/anypath</code>
       while <code>/~waldo/anypath</code> is
       <code>/home/<strong>w</strong>/waldo/public_html/anypath</code>.</p>
     </dd>

     <dt>Solution:</dt>

     <dd>
       <p>We use the following ruleset to expand the tilde URLs
       into the above layout.</p>

 <div class="example"><p><code>
 RewriteEngine on<br />
 RewriteRule   ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)  /home/<strong>$2</strong>/$1/public_html$3
 </code></p></div>
     </dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>


   <dl>
     <dt>Description:</dt>

     <dd>
     <p>By default, redirecting to an HTML anchor doesn't work,
     because mod_rewrite escapes the <code>#</code> character,
     turning it into <code>%23</code>. This, in turn, breaks the
     redirection.</p>
     </dd>

     <dt>Solution:</dt>

     <dd>
       <p>Use the <code>[NE]</code> flag on the
       <code>RewriteRule</code>. NE stands for No Escape.
       </p>
     </dd>

     <dt>Discussion:</dt>
     <dd>This technique will of course also work with other
     special characters that mod_rewrite, by default, URL-encodes.</dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>


   <dl>
     <dt>Description:</dt>

     <dd>
       <p>We wish to use mod_rewrite to serve different content based on
       the time of day.</p>
     </dd>

     <dt>Solution:</dt>

     <dd>
       <p>There are a lot of variables named <code>TIME_xxx</code>
       for rewrite conditions. In conjunction with the special
       lexicographic comparison patterns <code>&lt;STRING</code>,
       <code>&gt;STRING</code> and <code>=STRING</code> we can
       do time-dependent redirects:</p>

 <div class="example"><p><code>
 RewriteEngine on<br />
 RewriteCond   %{TIME_HOUR}%{TIME_MIN} &gt;0700<br />
 RewriteCond   %{TIME_HOUR}%{TIME_MIN} &lt;1900<br />
 RewriteRule   ^foo\.html$             foo.day.html [L]<br />
 RewriteRule   ^foo\.html$             foo.night.html
 </code></p></div>

       <p>This provides the content of <code>foo.day.html</code>
       under the URL <code>foo.html</code> from
       <code>07:01-18:59</code> and at the remaining time the
       contents of <code>foo.night.html</code>.</p>

       <div class="warning"><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>, intermediate proxies
       and browsers may each cache responses and cause the either page to be
       shown outside of the time-window configured.
       <code class="module"><a href="../mod/mod_expires.html">mod_expires</a></code> may be used to control this
       effect. You are, of course, much better off simply serving the
       content dynamically, and customizing it based on the time of day.</div>

     </dd>
   </dl>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="setenvvars" id="setenvvars">Set Environment Variables Based On URL Parts</a></h2>


   <dl>
     <dt>Description:</dt>

     <dd>
       <p>At time, we want to maintain some kind of status when we
       perform a rewrite. For example, you want to make a note that
       you've done that rewrite, so that you can check later to see if a
       request can via that rewrite. One way to do this is by setting an
       environment variable.</p>
     </dd>

     <dt>Solution:</dt>

     <dd>
       <p>Use the [E] flag to set an environment variable.</p>

 <div class="example"><p><code>
 RewriteEngine on<br />
 RewriteRule   ^/horse/(.*)   /pony/$1 [E=<strong>rewritten:1</strong>]
 </code></p></div>

     <p>Later in your ruleset you might check for this environment
     variable using a RewriteCond:</p>

 <div class="example"><p><code>
 RewriteCond %{ENV:rewritten} =1
 </code></p></div>

     </dd>
   </dl>

 </div></div>
 <div class="bottomlang">
 <p><span>Available Languages: </span><a href="../en/rewrite/avoid.html" title="English">&nbsp;en&nbsp;</a></p>
 </div><div id="footer">
 <p class="apache">Copyright 2011 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
 <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></div>
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	<?xml version="1.0" encoding="ISO-8859-1"?>
	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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	<title>Advanced Techniques with mod_rewrite - Apache HTTP Server</title>
	<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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	<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
	<link href="../images/favicon.ico" rel="shortcut icon" /></head>
	<body id="manual-page"><div id="page-header">
	<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>
	<img alt="" src="../images/feather.gif" /></div>
	<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
	<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 href="../mod/mod_rewrite.html">reference documentation</a>. It provides
	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>
	<li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#autorefresh">Document With Autorefresh</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Userdirs</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#time-dependent">Time-Dependent Rewriting</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables Based On URL Parts</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="vhosts.html">Virtual hosts</a></li><li><a href="proxy.html">Proxying</a></li><li><a href="rewritemap.html">Using RewriteMap</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul></div>
	<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></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"><p><code>
	user1 physical_host_of_user1<br />
	user2 physical_host_of_user2<br />
	: :
	</code></p></div>

	<p>We put this into a <code>map.users-to-hosts</code> file. The
	aim is to map;</p>

	<div class="example"><p><code>
	/u/user1/anypath
	</code></p></div>

	<p>to</p>

	<div class="example"><p><code>
	http://physical_host_of_user1/u/user/anypath
	</code></p></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"><p><code>
	RewriteEngine on<br />
	<br />
	RewriteMap users-to-hosts txt:/path/to/map.users-to-hosts<br />
	<br />
	RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${users-to-hosts:$1\|server0}</strong>/u/$1/$2
	</code></p></div>
	</dd>
	</dl>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<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"><p><code>
	# This example is valid in per-directory context only<br />
	RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong><br />
	RewriteRule ^page\.<strong>html</strong>$ page.<strong>cgi</strong> [T=application/x-httpd-cgi,L]
	</code></p></div>

	<p>Here a request for <code>page.html</code> leads to an
	internal run of a corresponding <code>page.cgi</code> if
	<code>page.html</code> is missing or has filesize
	null. The trick here is that <code>page.cgi</code> is a
	CGI script which (additionally to its <code>STDOUT</code>)
	writes its output to the file <code>page.html</code>.
	Once it has completed, the server sends out
	<code>page.html</code>. When the webmaster wants to force
	a refresh of the contents, he just removes
	<code>page.html</code> (typically from <code>cron</code>).</p>
	</dd>
	</dl>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<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"><p><code>
	RewriteEngine on<br />
	RewriteMap lb rnd:/path/to/serverlist.txt<br />
	<br />
	RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
	</code></p></div>

	<p><code>serverlist.txt</code> will contain a list of the servers:</p>

	<div class="example"><p><code>
	## serverlist.txt<br />
	<br />
	servers one.example.com\|two.example.com\|three.example.com<br />
	</code></p></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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<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"><p><code>
	RewriteRule ^(/[uge]/[^/]+/?.*):refresh /internal/cgi/apache/nph-refresh?f=$1
	</code></p></div>

	<p>Now when we reference the URL</p>

	<div class="example"><p><code>
	/u/foo/bar/page.html:refresh
	</code></p></div>

	<p>this leads to the internal invocation of the URL</p>

	<div class="example"><p><code>
	/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
	</code></p></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>
	#!/sw/bin/perl
	##
	## nph-refresh -- NPH/CGI script for auto refreshing pages
	## 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 =~ tr/A-Z/a-z/;
	$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 "HTTP/1.0 200 OK\n";
	print "Content-type: text/html\n\n";
	print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
	exit(0);
	}
	if (! -f $QS_f) {
	print "HTTP/1.0 200 OK\n";
	print "Content-type: text/html\n\n";
	print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
	exit(0);
	}

	sub print_http_headers_multipart_begin {
	print "HTTP/1.0 200 OK\n";
	$bound = "ThisRandomString12345";
	print "Content-type: multipart/x-mixed-replace;boundary=$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-type: text/html\n";
	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 class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="structuredhomedirs" id="structuredhomedirs">Structured Userdirs</a></h2>



	<dl>
	<dt>Description:</dt>

	<dd>
	<p>Some sites with thousands of users use a
	structured homedir layout, <em>i.e.</em> each homedir is in a
	subdirectory which begins (for instance) with the first
	character of the username. So, <code>/~larry/anypath</code>
	is <code>/home/<strong>l</strong>/larry/public_html/anypath</code>
	while <code>/~waldo/anypath</code> is
	<code>/home/<strong>w</strong>/waldo/public_html/anypath</code>.</p>
	</dd>

	<dt>Solution:</dt>

	<dd>
	<p>We use the following ruleset to expand the tilde URLs
	into the above layout.</p>

	<div class="example"><p><code>
	RewriteEngine on<br />
	RewriteRule ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*) /home/<strong>$2</strong>/$1/public_html$3
	</code></p></div>
	</dd>
	</dl>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>



	<dl>
	<dt>Description:</dt>

	<dd>
	<p>By default, redirecting to an HTML anchor doesn't work,
	because mod_rewrite escapes the <code>#</code> character,
	turning it into <code>%23</code>. This, in turn, breaks the
	redirection.</p>
	</dd>

	<dt>Solution:</dt>

	<dd>
	<p>Use the <code>[NE]</code> flag on the
	<code>RewriteRule</code>. NE stands for No Escape.
	</p>
	</dd>

	<dt>Discussion:</dt>
	<dd>This technique will of course also work with other
	special characters that mod_rewrite, by default, URL-encodes.</dd>
	</dl>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>



	<dl>
	<dt>Description:</dt>

	<dd>
	<p>We wish to use mod_rewrite to serve different content based on
	the time of day.</p>
	</dd>

	<dt>Solution:</dt>

	<dd>
	<p>There are a lot of variables named <code>TIME_xxx</code>
	for rewrite conditions. In conjunction with the special
	lexicographic comparison patterns <code><STRING</code>,
	<code>>STRING</code> and <code>=STRING</code> we can
	do time-dependent redirects:</p>

	<div class="example"><p><code>
	RewriteEngine on<br />
	RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700<br />
	RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900<br />
	RewriteRule ^foo\.html$ foo.day.html [L]<br />
	RewriteRule ^foo\.html$ foo.night.html
	</code></p></div>

	<p>This provides the content of <code>foo.day.html</code>
	under the URL <code>foo.html</code> from
	<code>07:01-18:59</code> and at the remaining time the
	contents of <code>foo.night.html</code>.</p>

	<div class="warning"><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>, intermediate proxies
	and browsers may each cache responses and cause the either page to be
	shown outside of the time-window configured.
	<code class="module"><a href="../mod/mod_expires.html">mod_expires</a></code> may be used to control this
	effect. You are, of course, much better off simply serving the
	content dynamically, and customizing it based on the time of day.</div>

	</dd>
	</dl>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="setenvvars" id="setenvvars">Set Environment Variables Based On URL Parts</a></h2>



	<dl>
	<dt>Description:</dt>

	<dd>
	<p>At time, we want to maintain some kind of status when we
	perform a rewrite. For example, you want to make a note that
	you've done that rewrite, so that you can check later to see if a
	request can via that rewrite. One way to do this is by setting an
	environment variable.</p>
	</dd>

	<dt>Solution:</dt>

	<dd>
	<p>Use the [E] flag to set an environment variable.</p>

	<div class="example"><p><code>
	RewriteEngine on<br />
	RewriteRule ^/horse/(.*) /pony/$1 [E=<strong>rewritten:1</strong>]
	</code></p></div>

	<p>Later in your ruleset you might check for this environment
	variable using a RewriteCond:</p>

	<div class="example"><p><code>
	RewriteCond %{ENV:rewritten} =1
	</code></p></div>

	</dd>
	</dl>

	</div></div>
	<div class="bottomlang">
	<p><span>Available Languages: </span><a href="../en/rewrite/avoid.html" title="English"> en </a></p>
	</div><div id="footer">
	<p class="apache">Copyright 2011 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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