GCS Amplitude
GCS Amplitude

Advanced Techniques with mod_rewrite

This document supplements the modrewritereference documentation. It provides a few advanced techniques using modrewrite.

A common technique for distributing the burden of server load or storage space is called "sharding". When using this method, a front-end server will use the url to consistently "shard" users or objects to separate backend servers.

A mapping is maintained, from users to target servers, in external map files. They look like:

http://physicalhostof_user1/u/user/anypath

thus every URL path need not be valid on every backend physical host. The following ruleset does this for us with the help of the map files assuming that server0 is a default server which will be used if a user has no entry in the map:

RewriteEngine on RewriteMap users-to-hosts txt:/path/to/map.users-to-hosts RewriteRule ^/u/([^/]+)/?(.*) http://${users-to-hosts:$1|server0}/u/$1/$2

documentation for more discussion of the syntax of this directive.RewriteMap

We wish to dynamically generate content, but store it statically once it is generated. This rule will check for the existence of the static file, and if it's not there, generate it. The static files can be removed periodically, if desired (say, via cron) and will be regenerated on demand.

operator determines whether the test string (in this case, REQUEST_URI

) is a valid URL. It does this via a subrequest. In the event that this subrequest fails - that is, the requested resource doesn't exist - this rule invokes the CGI program /regenerate_page.cgi

, which generates the requested resource and saves it into the document directory, so that the next time it is requested, a static copy can be served.

In this way, documents that are infrequently updated can be served in static form. if documents need to be refreshed, they can be deleted from the document directory, and they will then be regenerated the next time they are requested.

We wish to randomly distribute load across several servers using mod_rewrite.

RewriteEngine on RewriteMap lb rnd:/path/to/serverlist.txt RewriteRule ^/(.*) http://${lb:servers}/$1 [P,L]

servers one.example.com|two.example.com|three.example.com

If you want one particular server to get more of the load than the others, add it more times to the list.

- which is far more flexible and featureful than anything you can cobble together using modrewrite.modproxy_balancer

Some sites with thousands of users use a structured homedir layout, i.e. each homedir is in a subdirectory which begins (for instance) with the first character of the username. So, /~larry/anypath

while l/larry/public_html/anypath/~waldo/anypath

We use the following ruleset to expand the tilde URLs into the above layout.

RewriteEngine on RewriteRule ^/~(([a-z])[a-z0-9]+)(.*) /home/$2/$1/public_html$3

By default, redirecting to an HTML anchor doesn't work, because mod_rewrite escapes the #

character, turning it into %23

. This, in turn, breaks the redirection.

. NE stands for No Escape.

We wish to use mod_rewrite to serve different content based on the time of day.

There are a lot of variables named TIME_xxx

for rewrite conditions. In conjunction with the special lexicographic comparison patterns

RewriteEngine on RewriteCond %{TIMEHOUR}%{TIMEMIN} >0700 RewriteCond %{TIMEHOUR}%{TIMEMIN} <1900 RewriteRule ^foo\.html$ foo.day.html [L] RewriteRule ^foo\.html$ foo.night.html

from 07:01-18:59

and at the remaining time the contents of foo.night.html

, intermediate proxies and browsers may each cache responses and cause the either page to be shown outside of the time-window configured. mod_expires

may be used to control this effect. You are, of course, much better off simply serving the content dynamically, and customizing it based on the time of day.At time, we want to maintain some kind of status when we perform a rewrite. For example, you want to make a note that you've done that rewrite, so that you can check later to see if a request can via that rewrite. One way to do this is by setting an environment variable.

RewriteEngine on RewriteRule ^/horse/(.*) /pony/$1 [E=rewritten:1]

Note that environment variables do not survive an external redirect. You might consider using the [CO] flag to set a cookie.

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Apache HTTP Server Version 2.4

Advanced Techniques with mod_rewrite

Available Languages: en | fr

This document supplements the mod_rewrite reference documentation. It provides a few advanced techniques using mod_rewrite.

mod_rewrite

See also

URL-based sharding across multiple backends

On-the-fly Content-Regeneration

Load Balancing

Structured Userdirs

Redirecting Anchors

Time-Dependent Rewriting

Set Environment Variables Based On URL Parts