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<title>Advanced Techniques with mod_rewrite - Apache HTTP Server Version 2.4</title>
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<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 2.4</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/advanced.html" title="English"> en </a> |
<a href="../fr/rewrite/advanced.html" hreflang="fr" rel="alternate" title="Français"> fr </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 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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#sharding">URL-based sharding across 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="#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><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 across 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 />
# ... and so on
</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/$2"</pre>
</dd>
</dl>
<p>See the <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
documentation and the <a href="./rewritemap.html">RewriteMap HowTo</a>
for more discussion of the syntax of this directive.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>We wish to dynamically generate content, but store it
statically once it is generated. This rule will check for the
existence of the static file, and if it's not there, generate
it. The static files can be removed periodically, if desired (say,
via cron) and will be regenerated on demand.</p>
</dd>
<dt>Solution:</dt>
<dd>
This is done via the following ruleset:
<pre class="prettyprint lang-config"># This example is valid in per-directory context only
RewriteCond "%{REQUEST_URI}" "!-U"
RewriteRule "^(.+)\.html$" "/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="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$3"</pre>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>By default, redirecting to an HTML anchor doesn't work,
because mod_rewrite escapes the <code>#</code> character,
turning it into <code>%23</code>. This, in turn, breaks the
redirection.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>Use the <code>[NE]</code> flag on the
<code>RewriteRule</code>. NE stands for No Escape.
</p>
</dd>
<dt>Discussion:</dt>
<dd>This technique will of course also work with other
special characters that mod_rewrite, by default, URL-encodes.</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>We wish to use mod_rewrite to serve different content based on
the time of day.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>There are a lot of variables named <code>TIME_xxx</code>
for rewrite conditions. In conjunction with the special
lexicographic comparison patterns <code><STRING</code>,
<code>>STRING</code> and <code>=STRING</code> we can
do time-dependent redirects:</p>
<pre class="prettyprint lang-config">RewriteEngine on
RewriteCond "%{TIME_HOUR}%{TIME_MIN}" ">0700"
RewriteCond "%{TIME_HOUR}%{TIME_MIN}" "<1900"
RewriteRule "^foo\.html$" "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 times, 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 came 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. For per-directory and htaccess rewrites, where the final
substitution is processed as an internal redirect, environment
variables from the previous round of rewriting are prefixed with
"REDIRECT_". </p>
</dd>
</dl>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a> |
<a href="../fr/rewrite/advanced.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
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