It used Front Controller pattern.  zend also use singleton pattern.
=> routeStartup: This function is called before Zend_Controller_Front calls on the router to evaluate the request.
=> routeShutdown: This function  is called after the router finishes routing the request.
=> dispatchLoopStartup: This is called before Zend_Controller_Front enters its dispatch loop.
=> preDispatch: called before an action is dispatched by the dispatcher.
=> postDispatch: is called after an action is dispatched by the dispatcher.