Hi and welcome to Akelos.

In order to observe all instances of a Person model you can use create app/models/person_observer.php with the code

class PersonObserver extends AkObserver
{
    // Use predefined callbacks
    public function afterCreate(&$Person) { }
    public function afterSave(&$Person){ }
    public function afterValidationOnCreate(&$Person){ }
    public function afterValidationOnUpdate(&$Person){ }
    public function beforeSave(&$Person){ }
    public function beforeCreate(&$Person){  }
    public function beforeValidationOnCreate(&$Person){ }
    public function beforeValidation(&$Person){ }
    public function afterValidation(&$Person) { }
}

You can also use the common observer pattern methods update/notify.

I hope that helped.