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8 <title>Advanced Techniques with mod_rewrite - Apache HTTP Server Version 2.5</title>
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18 <p class="apache">Apache HTTP Server Version 2.5</p>
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20 <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
22 <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 2.5</a> > <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Advanced Techniques with mod_rewrite</h1>
24 <p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
28 <p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
29 <a href="../mod/mod_rewrite.html">reference documentation</a>. It provides
30 a few advanced techniques using mod_rewrite.</p>
34 <div class="warning">Note that many of these examples won't work unchanged in your
35 particular server configuration, so it's important that you understand
36 them, rather than merely cutting and pasting the examples into your
40 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#sharding">URL-based sharding across multiple backends</a></li>
41 <li><img alt="" src="../images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
42 <li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
43 <li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Userdirs</a></li>
44 <li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
45 <li><img alt="" src="../images/down.gif" /> <a href="#time-dependent">Time-Dependent Rewriting</a></li>
46 <li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables Based On URL Parts</a></li>
47 </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>
48 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
50 <h2><a name="sharding" id="sharding">URL-based sharding across multiple backends</a></h2>
58 <p>A common technique for distributing the burden of
59 server load or storage space is called "sharding".
60 When using this method, a front-end server will use the
61 url to consistently "shard" users or objects to separate
68 <p>A mapping is maintained, from users to target servers, in
69 external map files. They look like:</p>
71 <div class="example"><p><code>
72 user1 physical_host_of_user1<br />
73 user2 physical_host_of_user2<br />
77 <p>We put this into a <code>map.users-to-hosts</code> file. The
80 <div class="example"><p><code>
86 <div class="example"><p><code>
87 http://physical_host_of_user1/u/user/anypath
90 <p>thus every URL path need not be valid on every backend physical
91 host. The following ruleset does this for us with the help of the map
92 files assuming that server0 is a default server which will be used if
93 a user has no entry in the map:</p>
95 <pre class="prettyprint lang-config">RewriteEngine on
96 RewriteMap users-to-hosts "txt:/path/to/map.users-to-hosts"
97 RewriteRule "^/u/([^/]+)/?(.*)" "http://${users-to-hosts:$1|server0}/u/$1/$2"</pre>
102 <p>See the <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
103 documentation for more discussion of the syntax of this directive.</p>
105 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
106 <div class="section">
107 <h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
112 <dt>Description:</dt>
115 <p>We wish to dynamically generate content, but store it
116 statically once it is generated. This rule will check for the
117 existence of the static file, and if it's not there, generate
118 it. The static files can be removed periodically, if desired (say,
119 via cron) and will be regenerated on demand.</p>
125 This is done via the following ruleset:
127 <pre class="prettyprint lang-config"># This example is valid in per-directory context only
128 RewriteCond "%{REQUEST_URI}" !-U
129 RewriteRule "^(.+)\.html$" "/regenerate_page.cgi" [PT,L]</pre>
132 <p>The <code>-U</code> operator determines whether the test string
133 (in this case, <code>REQUEST_URI</code>) is a valid URL. It does
134 this via a subrequest. In the event that this subrequest fails -
135 that is, the requested resource doesn't exist - this rule invokes
136 the CGI program <code>/regenerate_page.cgi</code>, which generates
137 the requested resource and saves it into the document directory, so
138 that the next time it is requested, a static copy can be served.</p>
140 <p>In this way, documents that are infrequently updated can be served in
141 static form. if documents need to be refreshed, they can be deleted
142 from the document directory, and they will then be regenerated the
143 next time they are requested.</p>
147 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
148 <div class="section">
149 <h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
154 <dt>Description:</dt>
157 <p>We wish to randomly distribute load across several servers
158 using mod_rewrite.</p>
164 <p>We'll use <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code> and a list of servers
165 to accomplish this.</p>
167 <pre class="prettyprint lang-config">RewriteEngine on
168 RewriteMap lb "rnd:/path/to/serverlist.txt"
169 RewriteRule "^/(.*)" "http://${lb:servers}/$1" [P,L]</pre>
172 <p><code>serverlist.txt</code> will contain a list of the servers:</p>
174 <div class="example"><p><code>
175 ## serverlist.txt<br />
177 servers one.example.com|two.example.com|three.example.com<br />
180 <p>If you want one particular server to get more of the load than the
181 others, add it more times to the list.</p>
187 <p>Apache comes with a load-balancing module -
188 <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> - which is far more flexible and
189 featureful than anything you can cobble together using mod_rewrite.</p>
193 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
194 <div class="section">
195 <h2><a name="structuredhomedirs" id="structuredhomedirs">Structured Userdirs</a></h2>
200 <dt>Description:</dt>
203 <p>Some sites with thousands of users use a
204 structured homedir layout, <em>i.e.</em> each homedir is in a
205 subdirectory which begins (for instance) with the first
206 character of the username. So, <code>/~larry/anypath</code>
207 is <code>/home/<strong>l</strong>/larry/public_html/anypath</code>
208 while <code>/~waldo/anypath</code> is
209 <code>/home/<strong>w</strong>/waldo/public_html/anypath</code>.</p>
215 <p>We use the following ruleset to expand the tilde URLs
216 into the above layout.</p>
218 <pre class="prettyprint lang-config">RewriteEngine on
219 RewriteRule "^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)" "/home/<strong>$2</strong>/$1/public_html$3"</pre>
224 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
225 <div class="section">
226 <h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>
231 <dt>Description:</dt>
234 <p>By default, redirecting to an HTML anchor doesn't work,
235 because mod_rewrite escapes the <code>#</code> character,
236 turning it into <code>%23</code>. This, in turn, breaks the
243 <p>Use the <code>[NE]</code> flag on the
244 <code>RewriteRule</code>. NE stands for No Escape.
249 <dd>This technique will of course also work with other
250 special characters that mod_rewrite, by default, URL-encodes.</dd>
253 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
254 <div class="section">
255 <h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>
260 <dt>Description:</dt>
263 <p>We wish to use mod_rewrite to serve different content based on
270 <p>There are a lot of variables named <code>TIME_xxx</code>
271 for rewrite conditions. In conjunction with the special
272 lexicographic comparison patterns <code><STRING</code>,
273 <code>>STRING</code> and <code>=STRING</code> we can
274 do time-dependent redirects:</p>
276 <pre class="prettyprint lang-config">RewriteEngine on
277 RewriteCond "%{TIME_HOUR}%{TIME_MIN}" >0700
278 RewriteCond "%{TIME_HOUR}%{TIME_MIN}" <1900
279 RewriteRule "^foo\.html$" "foo.day.html" [L]
280 RewriteRule "^foo\.html$" "foo.night.html"</pre>
283 <p>This provides the content of <code>foo.day.html</code>
284 under the URL <code>foo.html</code> from
285 <code>07:01-18:59</code> and at the remaining time the
286 contents of <code>foo.night.html</code>.</p>
288 <div class="warning"><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>, intermediate proxies
289 and browsers may each cache responses and cause the either page to be
290 shown outside of the time-window configured.
291 <code class="module"><a href="../mod/mod_expires.html">mod_expires</a></code> may be used to control this
292 effect. You are, of course, much better off simply serving the
293 content dynamically, and customizing it based on the time of day.</div>
298 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
299 <div class="section">
300 <h2><a name="setenvvars" id="setenvvars">Set Environment Variables Based On URL Parts</a></h2>
305 <dt>Description:</dt>
308 <p>At time, we want to maintain some kind of status when we
309 perform a rewrite. For example, you want to make a note that
310 you've done that rewrite, so that you can check later to see if a
311 request can via that rewrite. One way to do this is by setting an
312 environment variable.</p>
318 <p>Use the [E] flag to set an environment variable.</p>
320 <pre class="prettyprint lang-config">RewriteEngine on
321 RewriteRule "^/horse/(.*)" "/pony/$1" [E=<strong>rewritten:1</strong>]</pre>
324 <p>Later in your ruleset you might check for this environment
325 variable using a RewriteCond:</p>
327 <pre class="prettyprint lang-config">RewriteCond "%{ENV:rewritten}" =1</pre>
330 <p>Note that environment variables do not survive an external
331 redirect. You might consider using the [CO] flag to set a
338 <div class="bottomlang">
339 <p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
340 </div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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