Walking Through Scenarios

[ Team LiB ]

Walking Through Scenarios To cross-check all these decisions, we have to constantly step through scenarios to confirm that we can solve application problems effectively. Sample Application Feature: Changing the Destination of a Cargo Occasionally a Customer calls up and says, "Oh no! We said to send our cargo to Hackensack, but we really need it in Hoboken." We are here to serve, so the system is required to provide for this change. Delivery Specification is a VALUE OBJECT, so it would be simplest to just to throw it away and get a new one, then use a setter method on Cargo to replace the old one with the new one. Sample Application Feature: Repeat Business The users say that repeated bookings from the same Customers tend to be similar, so they want to use old Cargoes as prototypes for new ones. The application will allow them to find a Cargo in the REPOSITORY and then select a command to create a new Cargo based on the selected one. We'll design this using the PROTOTYPE pattern (Gamma et al. 1995). Cargo is an ENTITY and is the root of an AGGREGATE. Therefore, it must be copied carefully; we need to consider what should happen to each object or attribute enclosed by its AGGREGATE boundary. Let's go over each one: Delivery History: We should create a new, empty one, because the history of the old one doesn't apply. This is the usual case with ENTITIES inside the AGGREGATE boundary. Customer Roles: We should copy the Map (or other collection) that holds the keyed references to Customers, including the keys, because they are likely to play the same roles in the new shipment. But we have to be careful not to copy the Customer objects themselves. We must end up with references to the same Customer objects as the old Cargo object referenced, because they are ENTITIES outside the AGGREGATE boundary. Tracking ID: We must provide a new Tracking ID from the same source as we would when creating a new Cargo from scratch. Notice that we have copied everything inside the Cargo AGGREGATE boundary, we have made some modifications to the copy, but we have affected nothing outside the AGGREGATE boundary at all.

[ Team LiB ]