Rebuild.

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<manualpage metafile="advanced.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 using mod_rewrite.</p>

<!--
I question whether anything remailing in this document qualifies as
"advanced". It's probably time to take inventory of the examples that we
have in the various docs, and consider a reorg of the stuff in this
directory. Again.
-->

<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</a></seealso>-->
<seealso><a href="avoid.html">When not to use mod_rewrite</a></seealso>

<section id="sharding">

  <title>URL-based sharding across 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>

<highlight language="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"
</highlight>
    </dd>
  </dl>

  <p>See the <directive module="mod_rewrite">RewriteMap</directive>
  documentation for more discussion of the syntax of this directive.</p>

</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:

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

    <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>

</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>

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

<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="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>

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

<highlight language="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"
</highlight>

      <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>

<highlight language="config">
RewriteEngine on
RewriteRule   "^/horse/(.*)"   "/pony/$1" [E=<strong>rewritten:1</strong>]
</highlight>

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

<highlight language="config">
RewriteCond "%{ENV:rewritten}"  =1
</highlight>

    <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>

</section>

</manualpage>