Dispatching is the process of taking the request object, Zend_Controller_Request_Abstract, extracting the module name, controller name, action name, and optional parameters contained in it, and then instantiating a controller and calling an action of that controller. If any of the module, controller, or action are not found, it will use default values for them. Zend_Controller_Dispatcher_Standard specifies index for each of the controller and action defaults and default for the module default value, but allows the developer to change the default values for each using the setDefaultController(), setDefaultAction(), and setDefaultModule() methods, respectively.

Dispatching happens in a loop in the front controller. Before dispatching occurs, the front controller routes the request to find user specified values for the module, controller, action, and optional parameters. It then enters a dispatch loop, dispatching the request.

At the beginning of each iteration, it sets a flag in the request object indicating that the action has been dispatched. If an action or pre/postDispatch plugin resets that flag, the dispatch loop will continue and attempt to dispatch the new request. By changing the controller and/or action in the request and resetting the dispatched flag, the developer may define a chain of requests to perform.

The action controller method that controlls such dispatching is _forward(); call this method from any of the pre/postDispatch() or action methods, providing an action, controller, module, and optionally any additional parameters you may wish to send to the new action:

<?php
public function fooAction()
{
    // forward to another action in the current controller and module:
    $this->_forward('bar', null, null, array('baz' => 'bogus'));
}

public function barAction()
{
    // forward to an action in another controller, FooController::bazAction(),
    // in the current module:
    $this->_forward('baz', 'foo', null, array('baz' => 'bogus'));
}

public function bazAction()
{
    // forward to an action in another controller in another module,
    // Foo_BarController::bazAction():
    $this->_forward('baz', 'bar', 'foo', array('baz' => 'bogus'));
}
?>

Zend_Controller_Front will first call the router to determine the first action in the request. It then enters a dispatch loop, which calls on the dispatcher to dispatch the action.

The dispatcher needs a variety of data in order to do its work - it needs to know how to format controller and action names, where to look for controller class files, whether or not a provided module name is valid, and an API for determining if a given request is even dispatchable based on the other information available.

Zend_Controller_Dispatcher_Interface defines the following methods as required for any dispatcher implementation:

interface Zend_Controller_Dispatcher_Interface
{
    /**
     * Format a string into a controller class name.
     *
     * @param string $unformatted
     * @return string
     */
    public function formatControllerName($unformatted);

    /**
     * Format a string into an action method name.
     *
     * @param string $unformatted
     * @return string
     */
    public function formatActionName($unformatted);

    /**
     * Determine if a request is dispatchable
     *
     * @param  Zend_Controller_Request_Abstract $request
     * @return boolean
     */
    public function isDispatchable(Zend_Controller_Request_Abstract $request);

    /**
     * Set a user parameter (via front controller, or for local use)
     * 
     * @param string $name 
     * @param mixed $value 
     * @return Zend_Controller_Dispatcher_Interface
     */
    public function setParam($name, $value);

    /**
     * Set an array of user parameters
     * 
     * @param array $params 
     * @return Zend_Controller_Dispatcher_Interface
     */
    public function setParams(array $params);

    /**
     * Retrieve a single user parameter
     * 
     * @param string $name 
     * @return mixed
     */
    public function getParam($name);

    /**
     * Retrieve all user parameters
     * 
     * @return array
     */
    public function getParams();

    /**
     * Clear the user parameter stack, or a single user parameter
     *
     * @param null|string|array single key or array of keys for params to clear
     * @return Zend_Controller_Dispatcher_Interface
     */
    public function clearParams($name = null);

    /**
     * Set the response object to use, if any
     * 
     * @param Zend_Controller_Response_Abstract|null $response 
     * @return void
     */
    public function setResponse(Zend_Controller_Response_Abstract $response = null);

    /**
     * Retrieve the response object, if any
     * 
     * @return Zend_Controller_Response_Abstract|null
     */
    public function getResponse();

    /**
     * Add a controller directory to the controller directory stack
     * 
     * @param string $path 
     * @param string $args
     * @return Zend_Controller_Dispatcher_Interface
     */
    public function addControllerDirectory($path, $args = null);

    /**
     * Set the directory (or directories) where controller files are stored
     *
     * @param string|array $dir 
     * @return Zend_Controller_Dispatcher_Interface
     */
    public function setControllerDirectory($path);

    /**
     * Return the currently set directory(ies) for controller file lookup
     * 
     * @return array
     */
    public function getControllerDirectory();

    /**
     * Dispatch a request to a (module/)controller/action.
     *
     * @param  Zend_Controller_Request_Abstract $request
     * @param  Zend_Controller_Response_Abstract $response
     * @return Zend_Controller_Request_Abstract|boolean
     */
    public function dispatch(Zend_Controller_Request_Abstract $request, Zend_Controller_Response_Abstract $response);

    /**
     * Whether or not a given module is valid
     * 
     * @param string $module 
     * @return boolean
     */
    public function isValidModule($module);
}
?>

In most cases, however, you should simply extend the abstract class Zend_Controller_Dispatcher_Abstract, in which each of these have already been defined, or Zend_Controller_Dispatcher_Standard to modify functionality of the standard dispatcher.

Possible reasons to subclass the dispatcher include a desire to use a different class or method naming schema in your action controllers, or a desire to use a different dispatching paradigm such as dispatching to action files under controller directories (instead of dispatching to class methods).