Connie's Convenience Store - About Peter Coad

Selecting Data-Management Objects and Resposibilities Connie’s Convenience Store 87
So why are DM objects so doggone important?
– A DM object knows how to store and retrieve objects from whatever storage
mechanism is to be used, whether flat file, indexed, relational, or
object-oriented.
– A DM object isolates data management complexities from the rest of the
application.
It interacts with local data management facilities and storage devices.
When necessary, it interacts with an object that represents another
system, to find objects located on an interacting system.
– A DM object isolates the transition from an object oriented environment
to a nonobject-oriented environment
– And in systems that need such capability, a DM object could cache results
when an interacting system is down.
For searching
If an object needs to search across all the objects in a class, and if no domain-based collection
can be found to hold those objects, then let the object send a message to a DM
object, asking it to do the search.
For persistence
This point-of-sale system also needs for some of its objects to be persistent, remembered
over time, from one execution of the system to the next.
Here’s the approach. If the underlying storage management is not object-oriented,
then add a DM object for each class with objects that need to be persistent.
In this system, just add these DM objects:
– person DM
– cashier DM
– item DM
– register DM
– cash drawer DM
– sale DM
– sale line item DM
– session DM
– store DM
– tax category DM
– upc DM
– price DM
– payment DM.
Each DM object knows its corresponding PD objects.