Refactoring the shipping application code to use DDD factories

Question

I was trying to find examples for using DDD factories and I came across the shipping application from Eric Evans' book.

However when I checked the BookingService the code to create a Cargo had this comment at line 37:

34    public TrackingId bookNewCargo(final UnLocode originUnLocode,
35                                   final UnLocode destinationUnLocode,
36                                   final Date arrivalDeadline) {
37      // TODO modeling this as a cargo factory might be suitable
38      final TrackingId trackingId = cargoRepository.nextTrackingId();
39      final Location origin = locationRepository.find(originUnLocode);
40      final Location destination = locationRepository.find(destinationUnLocode);
41      final RouteSpecification routeSpecification = new RouteSpecification(origin, destination, arrivalDeadline);
42  
43      final Cargo cargo = new Cargo(trackingId, routeSpecification);
44  
45      cargoRepository.store(cargo);
46      logger.info("Booked new cargo with tracking id " + cargo.trackingId().idString());
47  
48      return cargo.trackingId();
49    }

How should the DDD factory be used here? Which parts exactly should be refactored to use the factory in these piece of code?

Answer 1

Lines 38 to 43 could be classed as domain logic as this is how you create a valid cargo. Given that the booking service is an application service it should be delegating to the domain layer to create a cargo. Maybe the code would look like so...

37      // TODO modeling this as a cargo factory might be suitable
38      final TrackingId trackingId = cargoRepository.nextTrackingId();
39      final Location origin = locationRepository.find(originUnLocode);
40      final Location destination = locationRepository.find(destinationUnLocode);
42  
43      final Cargo cargo = CargoFactory.create(trackingId, origin, destination);

Check out my free book on DDD https://leanpub.com/Practicing-DDD for more info on factory implementations.