Basic rules related to an order management system

Order Management System

Let's build a very simple order management system having orders from pending up to completed state.

Requirements

• An order has a control number and a state.

• When the order is added to the system, it has a pending state.

• I want to be able to start the progress of an order.

• I want to be able to complete an order.

• I want to filter orders by number control and state.

Invariants

• A pending order can only go to in progress state.

• An in progress order can only go to completed.

• Completed orders are done and should preserve its state.