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10 <title>Advanced Techniques with mod_rewrite - Apache HTTP Server Version 2.5</title>
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19 <p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
20 <p class="apache">Apache HTTP Server Version 2.5</p>
21 <img alt="" src="../images/feather.gif" /></div>
22 <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
24 <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>
26 <p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
30 <p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
31 <a href="../mod/mod_rewrite.html">reference documentation</a>. It provides
32 a few advanced techniques using mod_rewrite.</p>
36 <div class="warning">Note that many of these examples won't work unchanged in your
37 particular server configuration, so it's important that you understand
38 them, rather than merely cutting and pasting the examples into your
42 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#sharding">URL-based sharding across multiple backends</a></li>
43 <li><img alt="" src="../images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
44 <li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
45 <li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Userdirs</a></li>
46 <li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
47 <li><img alt="" src="../images/down.gif" /> <a href="#time-dependent">Time-Dependent Rewriting</a></li>
48 <li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables Based On URL Parts</a></li>
49 </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>
50 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
52 <h2><a name="sharding" id="sharding">URL-based sharding across multiple backends</a></h2>
60 <p>A common technique for distributing the burden of
61 server load or storage space is called "sharding".
62 When using this method, a front-end server will use the
63 url to consistently "shard" users or objects to separate
70 <p>A mapping is maintained, from users to target servers, in
71 external map files. They look like:</p>
73 <div class="example"><p><code>
74 user1 physical_host_of_user1<br />
75 user2 physical_host_of_user2<br />
79 <p>We put this into a <code>map.users-to-hosts</code> file. The
82 <div class="example"><p><code>
88 <div class="example"><p><code>
89 http://physical_host_of_user1/u/user/anypath
92 <p>thus every URL path need not be valid on every backend physical
93 host. The following ruleset does this for us with the help of the map
94 files assuming that server0 is a default server which will be used if
95 a user has no entry in the map:</p>
97 <pre class="prettyprint lang-config">RewriteEngine on
98 RewriteMap users-to-hosts "txt:/path/to/map.users-to-hosts"
99 RewriteRule "^/u/([^/]+)/?(.*)" "http://${users-to-hosts:$1|server0}/u/$1/$2"</pre>
104 <p>See the <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
105 documentation for more discussion of the syntax of this directive.</p>
107 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
108 <div class="section">
109 <h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
114 <dt>Description:</dt>
117 <p>We wish to dynamically generate content, but store it
118 statically once it is generated. This rule will check for the
119 existence of the static file, and if it's not there, generate
120 it. The static files can be removed periodically, if desired (say,
121 via cron) and will be regenerated on demand.</p>
127 This is done via the following ruleset:
129 <pre class="prettyprint lang-config"># This example is valid in per-directory context only
130 RewriteCond "%{REQUEST_URI}" !-U
131 RewriteRule "^(.+)\.html$" "/regenerate_page.cgi" [PT,L]</pre>
134 <p>The <code>-U</code> operator determines whether the test string
135 (in this case, <code>REQUEST_URI</code>) is a valid URL. It does
136 this via a subrequest. In the event that this subrequest fails -
137 that is, the requested resource doesn't exist - this rule invokes
138 the CGI program <code>/regenerate_page.cgi</code>, which generates
139 the requested resource and saves it into the document directory, so
140 that the next time it is requested, a static copy can be served.</p>
142 <p>In this way, documents that are infrequently updated can be served in
143 static form. if documents need to be refreshed, they can be deleted
144 from the document directory, and they will then be regenerated the
145 next time they are requested.</p>
149 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
150 <div class="section">
151 <h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
156 <dt>Description:</dt>
159 <p>We wish to randomly distribute load across several servers
160 using mod_rewrite.</p>
166 <p>We'll use <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code> and a list of servers
167 to accomplish this.</p>
169 <pre class="prettyprint lang-config">RewriteEngine on
170 RewriteMap lb "rnd:/path/to/serverlist.txt"
171 RewriteRule "^/(.*)" "http://${lb:servers}/$1" [P,L]</pre>
174 <p><code>serverlist.txt</code> will contain a list of the servers:</p>
176 <div class="example"><p><code>
177 ## serverlist.txt<br />
179 servers one.example.com|two.example.com|three.example.com<br />
182 <p>If you want one particular server to get more of the load than the
183 others, add it more times to the list.</p>
189 <p>Apache comes with a load-balancing module -
190 <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> - which is far more flexible and
191 featureful than anything you can cobble together using mod_rewrite.</p>
195 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
196 <div class="section">
197 <h2><a name="structuredhomedirs" id="structuredhomedirs">Structured Userdirs</a></h2>
202 <dt>Description:</dt>
205 <p>Some sites with thousands of users use a
206 structured homedir layout, <em>i.e.</em> each homedir is in a
207 subdirectory which begins (for instance) with the first
208 character of the username. So, <code>/~larry/anypath</code>
209 is <code>/home/<strong>l</strong>/larry/public_html/anypath</code>
210 while <code>/~waldo/anypath</code> is
211 <code>/home/<strong>w</strong>/waldo/public_html/anypath</code>.</p>
217 <p>We use the following ruleset to expand the tilde URLs
218 into the above layout.</p>
220 <pre class="prettyprint lang-config">RewriteEngine on
221 RewriteRule "^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)" "/home/<strong>$2</strong>/$1/public_html$3"</pre>
226 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
227 <div class="section">
228 <h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>
233 <dt>Description:</dt>
236 <p>By default, redirecting to an HTML anchor doesn't work,
237 because mod_rewrite escapes the <code>#</code> character,
238 turning it into <code>%23</code>. This, in turn, breaks the
245 <p>Use the <code>[NE]</code> flag on the
246 <code>RewriteRule</code>. NE stands for No Escape.
251 <dd>This technique will of course also work with other
252 special characters that mod_rewrite, by default, URL-encodes.</dd>
255 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
256 <div class="section">
257 <h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>
262 <dt>Description:</dt>
265 <p>We wish to use mod_rewrite to serve different content based on
272 <p>There are a lot of variables named <code>TIME_xxx</code>
273 for rewrite conditions. In conjunction with the special
274 lexicographic comparison patterns <code><STRING</code>,
275 <code>>STRING</code> and <code>=STRING</code> we can
276 do time-dependent redirects:</p>
278 <pre class="prettyprint lang-config">RewriteEngine on
279 RewriteCond "%{TIME_HOUR}%{TIME_MIN}" >0700
280 RewriteCond "%{TIME_HOUR}%{TIME_MIN}" <1900
281 RewriteRule "^foo\.html$" "foo.day.html" [L]
282 RewriteRule "^foo\.html$" "foo.night.html"</pre>
285 <p>This provides the content of <code>foo.day.html</code>
286 under the URL <code>foo.html</code> from
287 <code>07:01-18:59</code> and at the remaining time the
288 contents of <code>foo.night.html</code>.</p>
290 <div class="warning"><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>, intermediate proxies
291 and browsers may each cache responses and cause the either page to be
292 shown outside of the time-window configured.
293 <code class="module"><a href="../mod/mod_expires.html">mod_expires</a></code> may be used to control this
294 effect. You are, of course, much better off simply serving the
295 content dynamically, and customizing it based on the time of day.</div>
300 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
301 <div class="section">
302 <h2><a name="setenvvars" id="setenvvars">Set Environment Variables Based On URL Parts</a></h2>
307 <dt>Description:</dt>
310 <p>At time, we want to maintain some kind of status when we
311 perform a rewrite. For example, you want to make a note that
312 you've done that rewrite, so that you can check later to see if a
313 request can via that rewrite. One way to do this is by setting an
314 environment variable.</p>
320 <p>Use the [E] flag to set an environment variable.</p>
322 <pre class="prettyprint lang-config">RewriteEngine on
323 RewriteRule "^/horse/(.*)" "/pony/$1" [E=<strong>rewritten:1</strong>]</pre>
326 <p>Later in your ruleset you might check for this environment
327 variable using a RewriteCond:</p>
329 <pre class="prettyprint lang-config">RewriteCond "%{ENV:rewritten}" =1</pre>
332 <p>Note that environment variables do not survive an external
333 redirect. You might consider using the [CO] flag to set a
340 <div class="bottomlang">
341 <p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
342 </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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