This example follows the original Cargo Shipping example that figures in the DDD book, as worked up into a running application by the DDD Sample project:
"One of the most requested aids to coming up to speed on DDD has been a running example application. Starting from a simple set of functions and a model based on the cargo example used in Eric Evans' book, we have built a running application with which to demonstrate a practical implementation of the building block patterns as well as illustrate the impact of aggregates and bounded contexts."
The original example was not event-sourced and was coded in Java. The example below is an event-sourced version of the original coded in Python.
The aggregate Cargo allows new cargo bookings to be made, the destination of the cargo to be changed, a route to be assigned, and for handling events to be registered. It is defined in the more verbose style, using explicit definitions of aggregate events, an explicit "created" method, and command methods that explicitly trigger events by calling trigger_event(). An aggregate projector function is implemented on the aggregate object using @simpledispatchmethod, with an event-specific method registered to handle each type of aggregate event.
Custom value objects such as Location and Itinerary, are defined as part of the domain model, and used in the Cargo aggregate events and methods. For the purpose of simplicity in this example, a fixed collection of routes between locations are also defined, but in practice these would be editable and could be also modelled as event-sourced aggregates.
../../../eventsourcing/examples/cargoshipping/domainmodel.py
The application object BookingService allows new cargo to be booked, cargo details to be presented, the destination of cargo to be changed, choices of possible routes for cargo to be presented, a route to be assigned, and for cargo handling events to be registered.
The Booking application defines and registers custom transcodings for the custom value objects that are defined and used in the domain model.
../../../eventsourcing/examples/cargoshipping/application.py
The interface object BookingService repeats the application methods, allowing new cargo to be booked, cargo details to be presented, the destination of cargo to be changed, choices of possible routes for cargo to be presented, a route to be assigned, and for cargo handling events to be registered.
It allows clients (e.g. a test case, or Web interface) to deal with simple object types that can be easily serialised and deserialised. It interacts with the application using the custom value objects defined in the domain model. For the purposes of testing, we need to simulate the user selecting a preferred itinerary from a list, which we do by picking the first in the list of presented options using the select_preferred_itinerary() function.
../../../eventsourcing/examples/cargoshipping/interface.py
Following the sample project, the test case has two test methods. One test shows an administrator booking a new cargo, viewing the current state of the cargo, and changing the destination. The other test goes further by assigning a route to a cargo booking, tracking the cargo handling events as it is shipped around the world, recovering by assigning a new route after the cargo was unloaded in the wrong place, until finally the cargo is claimed at its correct destination.
../../../eventsourcing/examples/cargoshipping/test.py