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21 <manualpage metafile="rewritemap.xml.meta">
22 <parentdocument href="./">Rewrite</parentdocument>
23 <title>Using RewriteMap</title>
26 <p>This document supplements the <module>mod_rewrite</module>
27 <a href="../mod/mod_rewrite.html">reference documentation</a>. It describes
28 the use of the <directive module="mod_rewrite">RewriteMap</directive> directive,
29 and provides examples of each of the various <code>RewriteMap</code> types.</p>
31 <note type="warning">Note that many of these examples won't work unchanged in your
32 particular server configuration, so it's important that you understand
33 them, rather than merely cutting and pasting the examples into your
37 <seealso><a href="../mod/mod_rewrite.html">Module documentation</a></seealso>
38 <seealso><a href="intro.html">mod_rewrite introduction</a></seealso>
39 <seealso><a href="remapping.html">Redirection and remapping</a></seealso>
40 <seealso><a href="access.html">Controlling access</a></seealso>
41 <seealso><a href="vhosts.html">Virtual hosts</a></seealso>
42 <seealso><a href="proxy.html">Proxying</a></seealso>
43 <seealso><a href="advanced.html">Advanced techniques and tricks</a></seealso>
44 <seealso><a href="avoid.html">When not to use mod_rewrite</a></seealso>
46 <section id="introduction">
47 <title>Introduction</title>
50 The <directive module="mod_rewrite">RewriteMap</directive> directive
51 defines an external function which can be called in the context of
52 <directive module="mod_rewrite">RewriteRule</directive> or
53 <directive module="mod_rewrite">RewriteCond</directive> directives to
54 perform rewriting that is too complicated, or too specialized to be
55 performed just by regular expressions. The source of this lookup can
56 be any of the types listed in the sections below, and enumerated in
57 the <directive module="mod_rewrite">RewriteMap</directive> reference
60 <p>The syntax of the <code>RewriteMap</code> directive is as
64 RewriteMap <em>MapName</em> <em>MapType</em>:<em>MapSource</em>
67 <p>The <a id="mapfunc" name="mapfunc"><em>MapName</em></a> is an
68 arbitray name that you assign to the map, and which you will use in
69 directives later on. Arguments are passed to the map via the
74 <code>${</code> <em>MapName</em> <code>:</code> <em>LookupKey</em>
75 <code>}</code> <br/> <code>${</code> <em>MapName</em> <code>:</code>
76 <em>LookupKey</em> <code>|</code> <em>DefaultValue</em> <code>}</code>
80 <p>When such a construct occurs, the map <em>MapName</em> is
81 consulted and the key <em>LookupKey</em> is looked-up. If the
82 key is found, the map-function construct is substituted by
83 <em>SubstValue</em>. If the key is not found then it is
84 substituted by <em>DefaultValue</em> or by the empty string
85 if no <em>DefaultValue</em> was specified.</p>
87 <p>For example, you might define a
88 <directive>RewriteMap</directive> as:</p>
90 RewriteMap examplemap txt:/path/to/file/map.txt
92 <p>You would then be able to use this map in a
93 <directive>RewriteRule</directive> as follows:</p>
95 RewriteRule ^/ex/(.*) ${examplemap:$1}
98 <p>A default value can be specified in the event that nothing is found
102 RewriteRule ^/ex/(.*) ${examplemap:$1|/not_found.html}
105 <note><title>Per-directory and .htaccess context</title>
107 The <code>RewriteMap</code> directive may not be used in
108 <Directory> sections or <code>.htaccess</code> files. You must
109 declare the map in server or virtualhost context. You may use the map,
110 once created, in your <code>RewriteRule</code> and
111 <code>RewriteCond</code> directives in those scopes. You just can't
112 <strong>declare</strong> it in those scopes.
116 <p>The sections that follow describe the various <em>MapType</em>s that
117 may be used, and give examples of each.</p>
121 <title>txt: Plain text maps</title>
123 <p>When a MapType of <code>txt</code>is used, the MapSource is a filesystem path to a
124 plain-text mapping file, containing space-separated key/value pair
125 per line. Optionally, a line may be contain a comment, starting with
128 <p>For example, the following might be valid entries in a map
133 <strong><em>MatchingKey</em> <em>SubstValue</em></strong><br />
134 <strong><em>MatchingKey</em> <em>SubstValue</em></strong> # comment<br />
137 <p>When the RewriteMap is invoked the argument is looked for in the
138 first argument of a line, and, if found, the substitution value is
141 <p>For example, we might use a mapfile to translate product names to
142 product IDs for easier-to-remember URLs, using the following
145 <example><title>Product to ID configuration</title>
146 RewriteMap product2id txt:/etc/apache2/productmap.txt<br />
147 RewriteRule ^/product/(.*) /prods.php?id=${product2id:$1|NOTFOUND} [PT]
150 <p>We assume here that the <code>prods.php</code> script knows what
151 to do when it received an argument of <code>id=NOTFOUND</code> when
152 a product is not found in the lookup map.</p>
154 <p>The file <code>/etc/apache2/productmap.txt</code> then contains
157 <example><title>Product to ID map</title>
160 ## productmap.txt - Product to ID map file
171 <p>Thus, when <code>http://example.com/product/television</code> is
172 requested, the <code>RewriteRule</code> is applied, and the request
173 is internally mapped to <code>/prods.php?id=993</code>.</p>
175 <note><title>Note: .htaccess files</title>
176 The example given is crafted to be used in server or virtualhost
177 scope. If you're planning to use this in a <code>.htaccess</code>
178 file, you'll need to remove the leading slash from the rewrite
179 pattern in order for it to match anything:
181 RewriteRule ^product/(.*) /prods.php?id=${product2id:$1|NOTFOUND} [PT]
185 <note><title>Cached lookups</title>
187 The looked-up keys are cached by httpd until the <code>mtime</code>
188 (modified time) of the mapfile changes, or the httpd server is
189 restarted. This ensures better performance on maps that are called
196 <title>rnd: Randomized Plain Text</title>
198 <p>When a MapType of <code>rnd</code> is used, the MapSource is a
199 filesystem path to a plain-text mapping file, each line of which
200 contains a key, and one or more values separated by <code>|</code>.
201 One of these values will be chosen at random if the key is
204 <p>For example, you might use the following map
205 file and directives to provide a random load balancing between
206 several back-end server, via a reverse-proxy. Images are sent
207 to one of the servers in the 'static' pool, while everything
208 else is sent to one of the 'dynamic' pool.</p>
210 <example><title>Rewrite map file</title>
213 ## map.txt -- rewriting map
216 static www1|www2|www3|www4
221 <example><title>Configuration directives</title>
222 RewriteMap servers rnd:/path/to/file/map.txt<br/>
224 RewriteRule ^/(.*\.(png|gif|jpg)) http://${servers:static}/$1 [NC,P,L]<br/>
225 RewriteRule ^/(.*) http://${servers:dynamic}/$1 [P,L]
228 <p>So, when an image is requested and the first of these rules is
229 matched, <code>RewriteMap</code> looks up the string
230 <code>static</code> in the map file, which returns one of the
231 specified hostnames at random, which is then used in the
232 <code>RewriteRule</code> target.</p>
234 <p>If you wanted to have one of the servers more likely to be chosen
235 (for example, if one of the server has more memory than the others,
236 and so can handle more requests) simply list it more times in the
240 static www1|www1|www2|www3|www4
246 <title>dbm: DBM Hash File</title>
248 <p>When a MapType of <code>dbm</code> is used, the MapSource is a
249 filesystem path to a DBM database file containing key/value pairs to
250 be used in the mapping. This works exactly the same way as the
251 <code>txt</code> map, but is much faster, because a DBM is indexed,
252 whereas a text file is not. This allows more rapid access to the
255 <p>You may optionally specify a particular dbm type:</p>
258 RewriteMap examplemap dbm=sdbm:/etc/apache/mapfile.dbm
261 <p>The type can be sdbm, gdbm, ndbm or db.
262 However, it is recommended that you just use the <a
263 href="../programs/httxt2dbm.html">httxt2dbm</a> utility that is
264 provided with Apache HTTP Server, as it will use the correct DBM library,
265 matching the one that was used when httpd itself was built.</p>
267 <p>To create a dbm file, first create a text map file as described
268 in the <a href="#txt">txt</a> section. Then run
269 <code>httxt2dbm</code>:</p>
272 $ httxt2dbm -i mapfile.txt -o mapfile.map
275 <p>You can then reference the resulting file in your
276 <code>RewriteMap</code> directive:</p>
279 RewriteMap mapname dbm:/etc/apache/mapfile.map
283 <p>Note that with some dbm types, more than one file is generated, with
284 a common base name. For example, you may have two files named
285 <code>mapfile.map.dir</code> and <code>mapfiile.map.pag</code>. This is
286 normal, and you need only use the base name <code>mapfile.map</code> in
287 your <code>RewriteMap</code> directive.</p>
290 <note><title>Cached lookups</title>
292 The looked-up keys are cached by httpd until the <code>mtime</code>
293 (modified time) of the mapfile changes, or the httpd server is
294 restarted. This ensures better performance on maps that are called
302 <title>int: Internal Function</title>
304 <p>When a MapType of <code>int</code> is used, the MapSource is one
305 of the available internal RewriteMap functions. Module authors can provide
306 additional internal functions by registering them with the
307 <code>ap_register_rewrite_mapfunc</code> API.
308 The functions that are provided by default are:
312 <li><strong>toupper</strong>:<br/>
313 Converts the key to all upper case.</li>
314 <li><strong>tolower</strong>:<br/>
315 Converts the key to all lower case.</li>
316 <li><strong>escape</strong>:<br/>
317 Translates special characters in the key to
319 <li><strong>unescape</strong>:<br/>
320 Translates hex-encodings in the key back to
321 special characters.</li>
325 To use one of these functions, create a <code>RewriteMap</code> referencing
326 the int function, and then use that in your <code>RewriteRule</code>:
329 <example><title>Redirect a URI to an all-lowercase version of itself</title>
330 RewriteMap lc int:tolower<br />
331 RewriteRule (.*[A-Z]+.*) ${lc:$1} [R]
335 <p>Please note that the example offered here is for
336 illustration purposes only, and is not a recommendation. If you want
337 to make URLs case-insensitive, consider using
338 <module>mod_speling</module> instead.
344 <section id="prg"><title>prg: External Rewriting Program</title>
346 <p>When a MapType of <code>prg</code> is used, the MapSource is a
347 filesystem path to an executable program which will providing the
348 mapping behavior. This can be a compiled binary file, or a program
349 in an interpreted language such as Perl or Python.</p>
351 <p>This program is started once, when the Apache HTTP Server is
352 started, and then communicates with the rewriting engine via
353 <code>STDIN</code> and <code>STDOUT</code>. That is, for each map
354 function lookup, it expects one argument via <code>STDIN</code>, and
355 should return one new-line terminated response string on
356 <code>STDOUT</code>. If there is no corresponding lookup value, the
357 map program should return the four-character string
358 "<code>NULL</code>" to indicate this.</p>
360 <p>External rewriting programs are not started if they're defined in
361 a context that does not have <directive
362 module="mod_rewrite">RewriteEngine</directive> set to
365 <p>This feature utilizes the <code>rewrite-map</code> mutex,
366 which is required for reliable communication with the program.
367 The mutex mechanism and lock file can be configured with the
368 <directive module="core">Mutex</directive> directive.</p>
370 <p>A simple example is shown here which will replace all dashes with
371 underscores in a request URI.</p>
373 <example><title>Rewrite configuration</title>
374 RewriteMap d2u prg:/www/bin/dash2under.pl<br />
375 RewriteRule - ${d2u:%{REQUEST_URI}}
378 <example><title>dash2under.pl</title>
379 #!/usr/bin/perl<br />
380 $| = 1; # Turn off I/O buffering<br />
381 while (<STDIN>) {<br />
383 s/-/_/g; # Replace dashes with underscores<br />
389 <note><title>Caution!</title>
391 <li>Keep your rewrite map program as simple as possible. If the program
392 hangs, it will cause httpd to wait indefinitely for a response from the
393 map, which will, in turn, cause httpd to stop responding to
395 <li>Be sure to turn off buffering in your program. In Perl this is done
396 by the second line in the example script: <code>$| = 1;</code> This will
397 of course vary in other languages. Buffered I/O will cause httpd to wait
398 for the output, and so it will hang.</li>
399 <li>Remember that there is only one copy of the program, started at
400 server startup. All requests will need to go through this one bottleneck.
401 This can cause significant slowdowns if many requests must go through
402 this process, or if the script itself is very slow.</li>
410 <title>dbd or fastdbd: SQL Query</title>
412 <p>When a MapType of <code>dbd</code> or <code>fastdbd</code> is
413 used, the MapSource is a SQL SELECT statement that takes a single
414 argument and returns a single value.</p>
416 <p><module>mod_dbd</module> will need to be configured to point at
417 the right database for this statement to be executed.</p>
419 <p>There are two forms of this MapType.
420 Using a MapType of <code>dbd</code> causes the query to be
421 executed with each map request, while using <code>fastdbd</code>
422 caches the database lookups internally. So, while
423 <code>fastdbd</code> is more efficient, and therefore faster, it
424 won't pick up on changes to the database until the server is
427 <p>If a query returns more than one row, a random row from
428 the result set is used.</p>
430 <example><title>Example</title>
431 RewriteMap myquery "fastdbd:SELECT destination FROM rewrite WHERE source = %s"
435 <section id="summary">
436 <title>Summary</title>
438 <p>The <directive>RewriteMap</directive> directive can occur more than
439 once. For each mapping-function use one
440 <directive>RewriteMap</directive> directive to declare its rewriting
443 <p>While you cannot <strong>declare</strong> a map in
444 per-directory context (<code>.htaccess</code> files or
445 <Directory> blocks) it is possible to
446 <strong>use</strong> this map in per-directory context. </p>