Using RewriteMap
This document supplements the mod_rewrite
reference documentation. It describes
the use of the RewriteMap directive,
and provides examples of each of the various RewriteMap types.
See also
Introduction
The RewriteMap directive
defines an external function which can be called in the context of
RewriteRule or
RewriteCond directives to
perform rewriting that is too complicated, or too specialized to be
performed just by regular expressions. The source of this lookup can
be any of the types listed in the sections below, and enumerated in
the RewriteMap reference
documentation.
The syntax of the RewriteMap directive is as
follows:
RewriteMap MapName MapType:MapSource
The MapName is an arbitray name that you assign to the map, and which you will use in directives later on. Arguments are passed to the map via the following syntax:
${ MapName : LookupKey
} ${ MapName :
LookupKey | DefaultValue }
When such a construct occurs, the map MapName is consulted and the key LookupKey is looked-up. If the key is found, the map-function construct is substituted by SubstValue. If the key is not found then it is substituted by DefaultValue or by the empty string if no DefaultValue was specified.
For example, you might define a
RewriteMap as:
RewriteMap examplemap txt:/path/to/file/map.txt
You would then be able to use this map in a
RewriteRule as follows:
RewriteRule ^/ex/(.*) ${examplemap:$1}
A default value can be specified in the event that nothing is found in the map:
RewriteRule ^/ex/(.*) ${examplemap:$1|/not_found.html}
Per-directory and .htaccess context
The RewriteMap directive may not be used in
<Directory> sections or .htaccess files. You must
declare the map in server or virtualhost context. You may use the map,
once created, in your RewriteRule and
RewriteCond directives in those scopes. You just can't
declare it in those scopes.
The sections that follow describe the various MapTypes that may be used, and give examples of each.
txt: Plain text maps
When a MapType of txt is used, the MapSource is a filesystem path to a
plain-text mapping file, containing space-separated key/value pair
per line. Optionally, a line may be contain a comment, starting with
a '#' character.
For example, the following might be valid entries in a map file.
# Comment line
MatchingKey SubstValue
MatchingKey SubstValue # comment
When the RewriteMap is invoked the argument is looked for in the first argument of a line, and, if found, the substitution value is returned.
For example, we might use a mapfile to translate product names to product IDs for easier-to-remember URLs, using the following recipe:
Product to ID configuration
RewriteMap product2id txt:/etc/apache2/productmap.txt
RewriteRule ^/product/(.*) /prods.php?id=${product2id:$1|NOTFOUND} [PT]
We assume here that the prods.php script knows what
to do when it received an argument of id=NOTFOUND when
a product is not found in the lookup map.
The file /etc/apache2/productmap.txt then contains
the following:
Product to ID map
##
## productmap.txt - Product to ID map file
##
television 993
stereo 198
fishingrod 043
basketball 418
telephone 328
Thus, when http://example.com/product/television is
requested, the RewriteRule is applied, and the request
is internally mapped to /prods.php?id=993.
Note: .htaccess files
The example given is crafted to be used in server or virtualhost scope. If you're planning to use this in a.htaccess
file, you'll need to remove the leading slash from the rewrite
pattern in order for it to match anything:
RewriteRule ^product/(.*) /prods.php?id=${product2id:$1|NOTFOUND} [PT]
Cached lookups
The looked-up keys are cached by httpd until the mtime
(modified time) of the mapfile changes, or the httpd server is
restarted. This ensures better performance on maps that are called
by many requests.
rnd: Randomized Plain Text
When a MapType of rnd is used, the MapSource is a
filesystem path to a plain-text mapping file, each line of which
contains a key, and one or more values separated by |.
One of these values will be chosen at random if the key is
matched.
For example, you might use the following map file and directives to provide a random load balancing between several back-end server, via a reverse-proxy. Images are sent to one of the servers in the 'static' pool, while everything else is sent to one of the 'dynamic' pool.
Rewrite map file
##
## map.txt -- rewriting map
##
static www1|www2|www3|www4
dynamic www5|www6
Configuration directives
RewriteMap servers rnd:/path/to/file/map.txt
RewriteRule ^/(.*\.(png|gif|jpg)) http://${servers:static}/$1 [NC,P,L]
RewriteRule ^/(.*) http://${servers:dynamic}/$1 [P,L]
So, when an image is requested and the first of these rules is
matched, RewriteMap looks up the string
static in the map file, which returns one of the
specified hostnames at random, which is then used in the
RewriteRule target.
If you wanted to have one of the servers more likely to be chosen (for example, if one of the server has more memory than the others, and so can handle more requests) simply list it more times in the map file.
static www1|www1|www2|www3|www4
dbm: DBM Hash File
When a MapType of dbm is used, the MapSource is a
filesystem path to a DBM database file containing key/value pairs to
be used in the mapping. This works exactly the same way as the
txt map, but is much faster, because a DBM is indexed,
whereas a text file is not. This allows more rapid access to the
desired key.
You may optionally specify a particular dbm type:
RewriteMap examplemap dbm=sdbm:/etc/apache/mapfile.dbm
The type can be sdbm, gdbm, ndbm or db. However, it is recommended that you just use the httxt2dbm utility that is provided with Apache HTTP Server, as it will use the correct DBM library, matching the one that was used when httpd itself was built.
To create a dbm file, first create a text map file as described
in the txt section. Then run
httxt2dbm:
$ httxt2dbm -i mapfile.txt -o mapfile.map
You can then reference the resulting file in your
RewriteMap directive:
RewriteMap mapname dbm:/etc/apache/mapfile.map
Note that with some dbm types, more than one file is generated, with
a common base name. For example, you may have two files named
mapfile.map.dir and mapfiile.map.pag. This is
normal, and you need only use the base name mapfile.map in
your RewriteMap directive.
Cached lookups
The looked-up keys are cached by httpd until the mtime
(modified time) of the mapfile changes, or the httpd server is
restarted. This ensures better performance on maps that are called
by many requests.
int: Internal Function
When a MapType of int is used, the MapSource is one
of the available internal RewriteMap functions. Module authors can provide
additional internal functions by registering them with the
ap_register_rewrite_mapfunc API.
The functions that are provided by default are:
- toupper:
Converts the key to all upper case. - tolower:
Converts the key to all lower case. - escape:
Translates special characters in the key to hex-encodings. - unescape:
Translates hex-encodings in the key back to special characters.
To use one of these functions, create a RewriteMap referencing
the int function, and then use that in your RewriteRule:
Redirect a URI to an all-lowercase version of itself
RewriteMap lc int:tolower
RewriteRule (.*[A-Z]+.*) ${lc:$1} [R]
Please note that the example offered here is for
illustration purposes only, and is not a recommendation. If you want
to make URLs case-insensitive, consider using
mod_speling instead.
prg: External Rewriting Program
When a MapType of prg is used, the MapSource is a
filesystem path to an executable program which will providing the
mapping behavior. This can be a compiled binary file, or a program
in an interpreted language such as Perl or Python.
This program is started once, when the Apache HTTP Server is
started, and then communicates with the rewriting engine via
STDIN and STDOUT. That is, for each map
function lookup, it expects one argument via STDIN, and
should return one new-line terminated response string on
STDOUT. If there is no corresponding lookup value, the
map program should return the four-character string
"NULL" to indicate this.
External rewriting programs are not started if they're defined in
a context that does not have RewriteEngine set to
on.
A simple example is shown here which will replace all dashes with underscores in a request URI.
Rewrite configuration
RewriteMap d2u prg:/www/bin/dash2under.pl
RewriteRule - ${d2u:%{REQUEST_URI}}
dash2under.pl
#!/usr/bin/perl
$| = 1; # Turn off I/O buffering
while (<STDIN>) {
s/-/_/g; # Replace dashes with underscores
print $_;
}
Use a RewriteLock!
When using a prg: RewriteMap, you should use a
RewriteLock. Failure to do so
will result in an error message in the log file, and may result in a
race condition on concurrent requests.
Caution!
- Keep your rewrite map program as simple as possible. If the program hangs, it will cause httpd to wait indefinitely for a response from the map, which will, in turn, cause httpd to stop responding to requests.
- Be sure to turn off buffering in your program. In Perl this is done
by the second line in the example script:
$| = 1;This will of course vary in other languages. Buffered I/O will cause httpd to wait for the output, and so it will hang. - Remember that there is only one copy of the program, started at server startup. All requests will need to go through this one bottleneck. This can cause significant slowdowns if many requests must go through this process, or if the script itself is very slow.
Summary
The RewriteMap directive can occur more than
once. For each mapping-function use one
RewriteMap directive to declare its rewriting
mapfile.
While you cannot declare a map in
per-directory context (.htaccess files or
<Directory> blocks) it is possible to
use this map in per-directory context.