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<title>Advanced Techniques with mod_rewrite - Apache HTTP Server</title>
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<div class="toplang">
<p><span>Available Languages: </span><a href="../en/rewrite/avoid.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../fr/rewrite/avoid.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
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<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 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><ul class="seealso"><li><a href="#comments_section">Comments</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>

<pre class="prettyprint lang-config">
RewriteEngine on
RewriteMap      users-to-hosts   txt:/path/to/map.users-to-hosts
RewriteRule   ^/u/([^/]+)/?(.*)   http://${users-to-hosts:$1|server0}/u/$1/$2
</pre>

    </dd>
  </dl>

  <p>See the <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
  documentation for more discussion of the syntax of this directive.</p>

</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:

<pre class="prettyprint lang-config">
# This example is valid in per-directory context only
RewriteCond %{REQUEST_URI}   !-U
RewriteRule ^(.+)\.html$          /regenerate_page.cgi   [PT,L]
</pre>


    <p>The <code>-U</code> operator determines whether the test string
    (in this case, <code>REQUEST_URI</code>) is a valid URL. It does
    this via a subrequest. In the event that this subrequest fails -
    that is, the requested resource doesn't exist - this rule invokes
    the CGI program <code>/regenerate_page.cgi</code>, which generates
    the requested resource and saves it into the document directory, so
    that the next time it is requested, a static copy can be served.</p>

    <p>In this way, documents that are infrequently updated can be served in
    static form. if documents need to be refreshed, they can be deleted
    from the document directory, and they will then be regenerated the
    next time they are requested.</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>

<pre class="prettyprint lang-config">
RewriteEngine on
RewriteMap lb rnd:/path/to/serverlist.txt
RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]
</pre>


<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>

<pre class="prettyprint lang-config">
RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1
</pre>


      <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>

<pre class="prettyprint lang-perl">
#!/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 "&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";
    &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", $size );
    open( FP, "&lt;$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>

    </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>

<pre class="prettyprint lang-config">
RewriteEngine on
RewriteRule   ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)  /home/<strong>$2</strong>/$1/public_html$3
</pre>

    </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>

<pre class="prettyprint lang-config">
RewriteEngine on
RewriteCond   %{TIME_HOUR}%{TIME_MIN} &gt;0700
RewriteCond   %{TIME_HOUR}%{TIME_MIN} &lt;1900
RewriteRule   ^foo\.html$             foo.day.html [L]
RewriteRule   ^foo\.html$             foo.night.html
</pre>


      <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>

<pre class="prettyprint lang-config">
RewriteEngine on
RewriteRule   ^/horse/(.*)   /pony/$1 [E=<strong>rewritten:1</strong>]
</pre>


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

<pre class="prettyprint lang-config">
RewriteCond %{ENV:rewritten} =1
</pre>


    <p>Note that environment variables do not survive an external
    redirect. You might consider using the [CO] flag to set a
    cookie.</p>

    </dd>
  </dl>

</div></div>
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<a href="../fr/rewrite/avoid.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
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