Using RewriteMap

Available Languages: en | + fr

+ + +

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.

+ +

Note that many of these examples won't work unchanged in your +particular server configuration, so it's important that you understand +them, rather than merely cutting and pasting the examples into your +configuration.

+ +

Introduction
int: Internal Function
txt: Plain text maps
rnd: Randomized Plain Text
dbm: DBM Hash File
prg: External Rewriting Program
dbd or fastdbd: SQL Query
Summary

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 + arbitrary 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 can 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.

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 "(.*)" "${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. +

+ +

txt: Plain text maps

+ + +

When a MapType of txt is used, the MapSource is a filesystem path to a + plain-text mapping file, containing one space-separated key/value pair + per line. Optionally, a line may contain a comment, starting with + a '#' character.

+ +

A valid text rewrite map file will have the following syntax:

+ +

+ # 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 can 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 can use the following map + file and directives to provide a random load balancing between + several back-end servers, 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 mapfile.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. +

+ +

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.

+ +

By default, external rewriting programs are run as the + user:group who started httpd. This can be changed on UNIX systems + by passing user name and group name as third argument to + RewriteMap in the + username:groupname format.

+ +

This feature utilizes the rewrite-map mutex, + which is required for reliable communication with the program. + The mutex mechanism and lock file can be configured with the + Mutex directive.

+ +

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" apache:apache
+RewriteRule "-" "${d2u:%{REQUEST_URI}}"

+ + +

dash2under.pl

#!/usr/bin/perl
+$| = 1; # Turn off I/O buffering
+while (<STDIN>) {
+    s/-/_/g; # Replace dashes with underscores
+    print $_;
+}

+ + +

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.

+ +

dbd or fastdbd: SQL Query

+ + +

When a MapType of dbd or fastdbd is + used, the MapSource is a SQL SELECT statement that takes a single + argument and returns a single value.

+ +

mod_dbd will need to be configured to point at + the right database for this statement to be executed.

+ +

There are two forms of this MapType. + Using a MapType of dbd causes the query to be + executed with each map request, while using fastdbd + caches the database lookups internally. So, while + fastdbd is more efficient, and therefore faster, it + won't pick up on changes to the database until the server is + restarted.

+ +

If a query returns more than one row, a random row from + the result set is used.

+ +

Example

RewriteMap myquery "fastdbd:SELECT destination FROM rewrite WHERE source = %s"

+ +

Note

The query name is passed to the database driver as a label for + an SQL prepared statement, and will therefore need to follow any rules + (such as case-sensitivity) required for your database.

+ +

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.

+ +

Using RewriteMap

See also

Introduction

Per-directory and .htaccess context

int: Internal Function

txt: Plain text maps

Product to ID map

Note: .htaccess files

Cached lookups

rnd: Randomized Plain Text

Rewrite map file

dbm: DBM Hash File

Cached lookups

prg: External Rewriting Program

Caution!

dbd or fastdbd: SQL Query

Example

Note

Summary

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