Chapter Six. The Life Cycle of a Domain Object

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Chapter Six. The Life Cycle of a Domain Object Every object has a life cycle. An object is born, it likely goes through various states, and it eventually dies�being either archived or deleted. Of course, many of these are simple, transient objects, created with an easy call to their constructor, used in some computation, and then abandoned to the garbage collector. There is no need to complicate such objects. But other objects have longer lives, not all of which are spent in active memory. They have complex interdependencies with other objects. They go through changes of state to which invariants apply. Managing these objects presents challenges that can easily derail an attempt at MODEL-DRIVEN DESIGN. Figure 6.1. The life cycle of a domain object The challenges fall into two categories. Maintaining integrity throughout the life cycle Preventing the model from getting swamped by the complexity of managing the life cycle This chapter will address these issues through three patterns. First, AGGREGATES tighten up the model itself by defining clear ownership and boundaries, avoiding a chaotic, tangled web of objects. This pattern is crucial to maintaining integrity in all phases of the life cycle. Next, the focus turns to the beginning of the life cycle, using FACTORIES to create and reconstitute complex objects and AGGREGATES, keeping their internal structure encapsulated. Finally, REPOSITORIES address the middle and end of the life cycle, providing the means of finding and retrieving persistent objects while encapsulating the immense infrastructure involved. Although REPOSITORIES and FACTORIES do not themselves come from the domain, they have meaningful roles in the domain design. These constructs complete the MODEL-DRIVEN DESIGN by giving us accessible handles on the model objects. Modeling AGGREGATES and adding FACTORIES and REPOSITORIES to the design gives us the ability to manipulate the model objects systematically and in meaningful units throughout their life cycle. AGGREGATES mark off the scope within which invariants have to be maintained at every stage of the life cycle. FACTORIES and REPOSITORIES operate on AGGREGATES, encapsulating the complexity of specific life cycle transitions.

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