Design an order tracking system in C++ [on hold]

How should I design order tracking system using the below constraints:-

Once an order is received, it will be assigned to a delivery boy and sends notification at every stage of the order such as order received with expected time of delivery, delivery boy assigned, order picked up, order delivered.

The system should be able to horizontally scalable from hundreds of orders to million orders and should perform at almost the same level irrespective of the number of orders received.