Here is an example of a Bootstrap loading a session class then loading a database class and using a db configuration from the application config.
<?php
class Bootstrap extends Yaf_Bootstrap_Abstract
{
    public function _initSession(Yaf_Dispatcher $dispatcher)
    {
        $session = new Vendor\Session();
        $session->start();
    }
    public function _initDatabase(Yaf_Dispatcher $dispatcher)
    {
        $config = Yaf_Application::app()->getConfig()->application->database;
        Yaf_Registry::set('db', Vendor\Database($config));
    }
}
?>