protected function define_execution() {
        global $DB;

        $plugins = enrol_get_plugins(true);
        $enrols = $DB->get_records('enrol', array(
                'courseid' => $this->task->get_courseid()));

        // Allow each enrol plugin to add annotations.
        foreach ($enrols as $enrol) {
            if (isset($plugins[$enrol->enrol])) {
                $plugins[$enrol->enrol]->backup_annotate_custom_fields($this, $enrol);
            }
        }
    }