How Restrictive Should a Structure Be?

[ Team LiB ]

How Restrictive Should a Structure Be? The large-scale structure patterns discussed in this chapter range from the very loose SYSTEM METAPHOR to the restrictive PLUGGABLE COMPONENT FRAMEWORK. Other structures are possible, of course, and even within a general structural pattern, there is a lot of choice about how restrictive to make the rules. For example, RESPONSIBILITY LAYERS dictate a kind of factoring of model concepts and their dependencies, but you could add rules that would specify communication patterns between the layers. Consider a manufacturing plant where software directs each part to a machine where it is processed according to some recipe. The correct process is ordered from a Policy layer and executed in an Operations layer. But inevitably there will be mistakes made on the factory floor. The actual situation will not be consistent with the rules of the software. Now, an Operations layer must reflect the world as it is, which means that when a part is occasionally put in the wrong machine, that information must be accepted unconditionally. Somehow, this exceptional condition needs to be communicated to a higher layer. A decision-making layer can then use other policies to correct the situation, perhaps by rerouting the part to a repair process or by scrapping it. But Operations does not know anything about higher layers. The communication has to be done in a way that doesn't create two-way dependencies from the lower layers to the higher ones. Typically, this signaling would be done through some kind of event mechanism. The Operations objects would generate events whenever their state changed. Policy layer objects would listen for events of interest from the lower layers. When an event occurred that violated a rule, the rule would execute an action (part of the rule's definition) that makes the appropriate response, or it might generate an event for the benefit of some still higher layer. In the banking example, the values of assets change (Operations), shifting the values of segments of a portfolio. When these values exceed portfolio allocation limits (Policy), perhaps a trader is alerted, who can buy or sell assets to redress the balance. We could figure this out on a case-by-case basis, or we could decide on a consistent pattern for everyone to follow in interactions of objects of particular layers. A more restrictive structure increases uniformity, making the design easier to interpret. If the structure fits, the rules are likely to push developers toward good designs. Disparate pieces are likely to fit together better. On the other hand, the restrictions may take away flexibility that developers need. Very particular communication paths might be impractical to apply across BOUNDED CONTEXTS, especially in different implementation technologies, in a heterogeneous system. So you have to fight the temptation to build frameworks and regiment the implementation of the large-scale structure. The most important contribution of the large-scale structure is conceptual coherence, and giving insight into the domain. Each structural rule should make development easier.

[ Team LiB ]