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<manualpage metafile="avoid.xml.meta">
  <parentdocument href="./">Rewrite</parentdocument>

<title>Advanced Techniques with mod_rewrite</title>

<summary>

<p>This document supplements the <module>mod_rewrite</module> 
<a href="../mod/mod_rewrite.html">reference documentation</a>. It provides 
a few advanced techniques and tricks using mod_rewrite.</p>

<note type="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.</note>

</summary>
<seealso><a href="../mod/mod_rewrite.html">Module documentation</a></seealso>
<seealso><a href="intro.html">mod_rewrite introduction</a></seealso>
<seealso><a href="remapping.html">Redirection and remapping</a></seealso>
<seealso><a href="access.html">Controlling access</a></seealso>
<seealso><a href="vhosts.html">Virtual hosts</a></seealso>
<seealso><a href="proxy.html">Proxying</a></seealso>
<seealso><a href="rewritemap.html">Using RewriteMap</a></seealso>
<!--<seealso><a href="advanced.html">Advanced techniques and tricks</a></seealso>-->
<seealso><a href="avoid.html">When not to use mod_rewrite</a></seealso>

<section id="sharding">

  <title>URL-based sharding accross multiple backends</title>

  <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>

<example>
user1  physical_host_of_user1<br />
user2  physical_host_of_user2<br />
:      :
</example>

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

<example>
/u/user1/anypath
</example>

  <p>to</p>

<example>
http://physical_host_of_user1/u/user/anypath
</example>

      <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>

<example>
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
</example>
    </dd>
  </dl>

</section>

<section id="on-the-fly-content">

  <title>On-the-fly Content-Regeneration</title>

  <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:

<example>
# 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]
</example>

      <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>

</section>

<section id="load-balancing">

  <title>Load Balancing</title>

  <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 <directive
      module="mod_rewrite">RewriteMap</directive> and a list of servers
      to accomplish this.</p>

<example>
RewriteEngine on<br />
RewriteMap lb rnd:/path/to/serverlist.txt<br />
<br />
RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
</example>

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

<example>
## serverlist.txt<br />
<br />
servers one.example.com|two.example.com|three.example.com<br />
</example>

<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 -
<module>mod_proxy_balancer</module> - which is far more flexible and
featureful than anything you can cobble together using mod_rewrite.</p>
   </dd>
  </dl>

</section>

<section id="autorefresh">

  <title>Document With Autorefresh</title>

  <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
      <module>mod_rewrite</module>. 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>

<example>
RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1
</example>

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

<example>
/u/foo/bar/page.html:refresh
</example>

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

<example>
/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
</example>

      <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>

<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></example>
    </dd>
  </dl>

</section>

<section id="structuredhomedirs">

  <title>Structured Userdirs</title>

  <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>

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

</section>

<section id="redirectanchors">

  <title>Redirecting Anchors</title>

  <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>

</section>

<section id="time-dependent">

  <title>Time-Dependent Rewriting</title>

  <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>

<example>
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
</example>

      <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>

      <note type="warning"><module>mod_cache</module>, intermediate proxies
      and browsers may each cache responses and cause the either page to be
      shown outside of the time-window configured.
      <module>mod_expires</module> 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.</note>

    </dd>
  </dl>

</section>

<section id="setenvvars">

  <title>Set Environment Variables Based On URL Parts</title>

  <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>

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

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

<example>
RewriteCond %{ENV:rewritten} =1
</example>

    </dd>
  </dl>

</section>

</manualpage> 
