Building Applications - eXist: A NoSQL Database and Application Development Platform - page 200

$exist:path
The part of the URL after the part that led to the controller. For example:
a/b/c.xq .
$exist:resource
The part of the URL after the last / character, usually pointing to a resource. For
example: c.xq .
$exist:controller
The part of the URL leading from the prefix (see below) to the controller script.
For example: /exist-book/building-applications/show-controller-variables .
$exist:prefix
The URL prefix that caused the URL rewriting controller to become active. This
is defined in the controller-config.xml configuration file. For example: /apps .
$exist:root
The root path used for finding the controller, as defined in the controller-
config.xml configuration file. This path can be on the filesystem or in the data‐
base. In our example it is xmldb:exist:///db .
Figure 9-2 summarizes all this.
Figure 9-2. Going from a URL to controller variables
The Controller's Output XML Format
A URL rewriting controller must output an XML fragment. This fragment deter‐
mines what eXist will do next.