Chapter Five. A Model Expressed in Software
To compromise in implementation without losing the punch of a MODEL-DRIVEN DESIGN requires a reframing of the basics. Connecting model and implementation has to be done at the detail level. This chapter focuses on those individual model elements, getting them in shape to support the activities in later chapters.
This discussion will start with the issues of designing and streamlining associations. Associations between objects are simple to conceive and to draw, but implementing them is a potential quagmire. Associations illustrate how crucial detailed implementation decisions are to the viability of a MODEL-DRIVEN DESIGN.
Turning to the objects themselves, but continuing to scrutinize the relationship between detailed model choices and implementation concerns, we'll focus on making distinctions among the three patterns of model elements that express the model: ENTITIES, VALUE OBJECTS, and SERVICES.
Defining objects that capture concepts of the domain seems very intuitive on the surface, but serious challenges are lurking in the shades of meaning. Certain distinctions have emerged that clarify the meaning of model elements and tie into a body of design practices for carving out specific kinds of objects.
Does an object represent something with continuity and identity—something that is tracked through different states or even across different implementations? Or is it an attribute that describes the state of something else? This is the basic distinction between an ENTITY and a VALUE OBJECT. Defining objects that clearly follow one pattern or the other makes the objects less ambiguous and lays out the path toward specific choices for robust design.
Then there are those aspects of the domain that are more clearly expressed as actions or operations, rather than as objects. Although it is a slight departure from object-oriented modeling tradition, it is often best to express these as SERVICES, rather than forcing responsibility for an operation onto some ENTITY or VALUE OBJECT. A SERVICE is something that is done for a client on request. In the technical layers of the software, there are many SERVICES. They emerge in the domain also, when some activity is modeled that corresponds to something the software must do, but does not correspond with state.
There are inevitable situations in which the purity of the object model must be compromised, such as for storage in a relational database. This chapter will lay out some guidelines for staying on course when you are forced to deal with these messy realities.
Finally, a discussion of MODULES will drive home the point that every design decision should be motivated by some insight into the domain. The ideas of high cohesion and low coupling, often thought of as technical metrics, can be applied to the concepts themselves. In a MODEL-DRIVEN DESIGN, MODULES are part of the model, and they should reflect concepts in the domain.
This chapter brings together all of these building blocks, which embody the model in software. These ideas are conventional, and the modeling and design biases that follow from them have been written about before. But framing them in this context will help developers create detailed components that will serve the priorities of domaindriven design when tackling the larger model and design issues. Also, a sense of the basic principles will help developers stay on course through the inevitable compromises.
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