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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</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:$1|NOTFOUND} [PT]
150 <p>We assume here that the <code>prods.php</code> script knows what
151 to do when it received an argument of <code>id=NOTFOUND</code> when
152 a product is not found in the lookup map.</p>
154 <p>The file <code>/etc/apache2/productmap.txt</code> then contains
157 <example><title>Product to ID map</title>
159 ## productmap.txt - Product to ID map file<br />
169 <p>Thus, when <code>http://example.com/product/television</code> is
170 requested, the <code>RewriteRule</code> is applied, and the request
171 is internally mapped to <code>/prods.php?id=993</code>.</p>
173 <note><title>Note: .htaccess files</title>
174 The example given is crafted to be used in server or virtualhost
175 scope. If you're planning to use this in a <code>.htaccess</code>
176 file, you'll need to remove the leading slash from the rewrite
177 pattern in order for it to match anything:
179 RewriteRule ^product/(.*) /prods.php?id=${product2id:$1|NOTFOUND} [PT]
183 <note><title>Cached lookups</title>
185 The looked-up keys are cached by httpd until the <code>mtime</code>
186 (modified time) of the mapfile changes, or the httpd server is
187 restarted. This ensures better performance on maps that are called
194 <title>rnd: Randomized Plain Text</title>
196 <p>When a MapType of <code>rnd</code> is used, the MapSource is a
197 filesystem path to a plain-text mapping file, each line of which
198 contains a key, and one or more values separated by <code>|</code>.
199 One of these values will be chosen at random if the key is
202 <p>For example, you might use the following map
203 file and directives to provide a random load balancing between
204 several back-end servers, via a reverse-proxy. Images are sent
205 to one of the servers in the 'static' pool, while everything
206 else is sent to one of the 'dynamic' pool.</p>
208 <example><title>Rewrite map file</title>
210 ## map.txt -- rewriting map<br />
213 static www1|www2|www3|www4<br />
217 <example><title>Configuration directives</title>
218 RewriteMap servers rnd:/path/to/file/map.txt<br/>
220 RewriteRule ^/(.*\.(png|gif|jpg)) http://${servers:static}/$1 [NC,P,L]<br/>
221 RewriteRule ^/(.*) http://${servers:dynamic}/$1 [P,L]
224 <p>So, when an image is requested and the first of these rules is
225 matched, <code>RewriteMap</code> looks up the string
226 <code>static</code> in the map file, which returns one of the
227 specified hostnames at random, which is then used in the
228 <code>RewriteRule</code> target.</p>
230 <p>If you wanted to have one of the servers more likely to be chosen
231 (for example, if one of the server has more memory than the others,
232 and so can handle more requests) simply list it more times in the
236 static www1|www1|www2|www3|www4
242 <title>dbm: DBM Hash File</title>
244 <p>When a MapType of <code>dbm</code> is used, the MapSource is a
245 filesystem path to a DBM database file containing key/value pairs to
246 be used in the mapping. This works exactly the same way as the
247 <code>txt</code> map, but is much faster, because a DBM is indexed,
248 whereas a text file is not. This allows more rapid access to the
251 <p>You may optionally specify a particular dbm type:</p>
254 RewriteMap examplemap dbm=sdbm:/etc/apache/mapfile.dbm
257 <p>The type can be sdbm, gdbm, ndbm or db.
258 However, it is recommended that you just use the <a
259 href="../programs/httxt2dbm.html">httxt2dbm</a> utility that is
260 provided with Apache HTTP Server, as it will use the correct DBM library,
261 matching the one that was used when httpd itself was built.</p>
263 <p>To create a dbm file, first create a text map file as described
264 in the <a href="#txt">txt</a> section. Then run
265 <code>httxt2dbm</code>:</p>
268 $ httxt2dbm -i mapfile.txt -o mapfile.map
271 <p>You can then reference the resulting file in your
272 <code>RewriteMap</code> directive:</p>
275 RewriteMap mapname dbm:/etc/apache/mapfile.map
279 <p>Note that with some dbm types, more than one file is generated, with
280 a common base name. For example, you may have two files named
281 <code>mapfile.map.dir</code> and <code>mapfiile.map.pag</code>. This is
282 normal, and you need only use the base name <code>mapfile.map</code> in
283 your <code>RewriteMap</code> directive.</p>
286 <note><title>Cached lookups</title>
288 The looked-up keys are cached by httpd until the <code>mtime</code>
289 (modified time) of the mapfile changes, or the httpd server is
290 restarted. This ensures better performance on maps that are called
298 <title>int: Internal Function</title>
300 <p>When a MapType of <code>int</code> is used, the MapSource is one
301 of the available internal RewriteMap functions. Module authors can provide
302 additional internal functions by registering them with the
303 <code>ap_register_rewrite_mapfunc</code> API.
304 The functions that are provided by default are:
308 <li><strong>toupper</strong>:<br/>
309 Converts the key to all upper case.</li>
310 <li><strong>tolower</strong>:<br/>
311 Converts the key to all lower case.</li>
312 <li><strong>escape</strong>:<br/>
313 Translates special characters in the key to
315 <li><strong>unescape</strong>:<br/>
316 Translates hex-encodings in the key back to
317 special characters.</li>
321 To use one of these functions, create a <code>RewriteMap</code> referencing
322 the int function, and then use that in your <code>RewriteRule</code>:
325 <example><title>Redirect a URI to an all-lowercase version of itself</title>
326 RewriteMap lc int:tolower<br />
327 RewriteRule (.*?[A-Z]+.*) ${lc:$1} [R]
331 <p>Please note that the example offered here is for
332 illustration purposes only, and is not a recommendation. If you want
333 to make URLs case-insensitive, consider using
334 <module>mod_speling</module> instead.
340 <section id="prg"><title>prg: External Rewriting Program</title>
342 <p>When a MapType of <code>prg</code> is used, the MapSource is a
343 filesystem path to an executable program which will providing the
344 mapping behavior. This can be a compiled binary file, or a program
345 in an interpreted language such as Perl or Python.</p>
347 <p>This program is started once, when the Apache HTTP Server is
348 started, and then communicates with the rewriting engine via
349 <code>STDIN</code> and <code>STDOUT</code>. That is, for each map
350 function lookup, it expects one argument via <code>STDIN</code>, and
351 should return one new-line terminated response string on
352 <code>STDOUT</code>. If there is no corresponding lookup value, the
353 map program should return the four-character string
354 "<code>NULL</code>" to indicate this.</p>
356 <p>External rewriting programs are not started if they're defined in
357 a context that does not have <directive
358 module="mod_rewrite">RewriteEngine</directive> set to
361 <p>This feature utilizes the <code>rewrite-map</code> mutex,
362 which is required for reliable communication with the program.
363 The mutex mechanism and lock file can be configured with the
364 <directive module="core">Mutex</directive> directive.</p>
366 <p>A simple example is shown here which will replace all dashes with
367 underscores in a request URI.</p>
369 <example><title>Rewrite configuration</title>
370 RewriteMap d2u prg:/www/bin/dash2under.pl<br />
371 RewriteRule - ${d2u:%{REQUEST_URI}}
374 <example><title>dash2under.pl</title>
375 <highlight language="perl">
377 $| = 1; # Turn off I/O buffering
378 while (<STDIN>) {
379 s/-/_/g; # Replace dashes with underscores
385 <note><title>Caution!</title>
387 <li>Keep your rewrite map program as simple as possible. If the program
388 hangs, it will cause httpd to wait indefinitely for a response from the
389 map, which will, in turn, cause httpd to stop responding to
391 <li>Be sure to turn off buffering in your program. In Perl this is done
392 by the second line in the example script: <code>$| = 1;</code> This will
393 of course vary in other languages. Buffered I/O will cause httpd to wait
394 for the output, and so it will hang.</li>
395 <li>Remember that there is only one copy of the program, started at
396 server startup. All requests will need to go through this one bottleneck.
397 This can cause significant slowdowns if many requests must go through
398 this process, or if the script itself is very slow.</li>
406 <title>dbd or fastdbd: SQL Query</title>
408 <p>When a MapType of <code>dbd</code> or <code>fastdbd</code> is
409 used, the MapSource is a SQL SELECT statement that takes a single
410 argument and returns a single value.</p>
412 <p><module>mod_dbd</module> will need to be configured to point at
413 the right database for this statement to be executed.</p>
415 <p>There are two forms of this MapType.
416 Using a MapType of <code>dbd</code> causes the query to be
417 executed with each map request, while using <code>fastdbd</code>
418 caches the database lookups internally. So, while
419 <code>fastdbd</code> is more efficient, and therefore faster, it
420 won't pick up on changes to the database until the server is
423 <p>If a query returns more than one row, a random row from
424 the result set is used.</p>
426 <example><title>Example</title>
427 RewriteMap myquery "fastdbd:SELECT destination FROM rewrite WHERE source = %s"
431 <section id="summary">
432 <title>Summary</title>
434 <p>The <directive>RewriteMap</directive> directive can occur more than
435 once. For each mapping-function use one
436 <directive>RewriteMap</directive> directive to declare its rewriting
439 <p>While you cannot <strong>declare</strong> a map in
440 per-directory context (<code>.htaccess</code> files or
441 <Directory> blocks) it is possible to
442 <strong>use</strong> this map in per-directory context. </p>