//check scheduled actions
    function scheduler() {
        $date = date("Y-m-d H:i:s");
        //echo $date;
        $last_run = get_option('last_run');
        //if one hour passed since last run
        if (strtotime($last_run)+ 3600 < strtotime($date)) {
            //echo "1 hour passed!";
            update_option('last_run', $date);
            $this->run_scheduled_actions();
            return true;
        }
        else {
            return false;
        }

    }

    //scheduled actions
    function run_scheduled_actions() {
        global $wpdb;
        //get all the books from the database
        $table_name = $wpdb->prefix . 'posts';
        $books = $wpdb->get_results("SELECT * FROM `$table_name` WHERE `post_type` = 'book';", ARRAY_A);
        foreach ($books as $book) {
            $book_id = $book['ID'];
            //echo $book_id;
            $user_id = $this->get_book_user_author($book_id);
            //get tracking ids
            $tracking_id_warehouse = get_post_meta($book_id, 'tracking_id_aftership', true);
            $tracking_id_seller = get_post_meta($book_id, 'seller_tracking_number', true);
            //get old valuss
            $tracking_status_warehouse = get_post_meta($book_id, 'tracking_status_warehouse', true);
            $tracking_status_seller = get_post_meta($book_id, 'tracking_status_seller', true);
            //get new values
            if (!empty($tracking_id_warehouse)) {
                $tracking_status_warehouse_new_full = $this->check_tracking_status($tracking_id_warehouse);
                $tracking_status_warehouse_new = $tracking_status_warehouse_new_full['data']['tracking']['tag'];
                //if tracking status changed, update it in the meta. same if it is first-run
                if ($tracking_status_warehouse != $tracking_status_warehouse_new) {
                    update_post_meta($book_id, 'tracking_status_warehouse', $tracking_status_warehouse_new);
                    $this->mail_on_track_update($user_id, $tracking_status_warehouse_new, $book_id);
                }
            }
            if (!empty($tracking_id_seller)) {
                $tracking_status_seller_new_full = $this->check_tracking_status($tracking_id_warehouse);
                $tracking_status_seller_new = $tracking_status_seller_new_full['data']['tracking']['tag'];
                if ($tracking_status_seller != $tracking_status_seller_new) {
                    update_post_meta($book_id, 'tracking_status_seller', $tracking_status_seller_new);
                    $this->mail_on_track_update($user_id, $tracking_status_seller_new, $book_id);
                }
            }
            //end with checks
        }
        //end with books loop
    }