Combining Large-Scale Structures and B'OUNDED' C'ONTEXTS'

[ Team LiB ]

Combining Large-Scale Structures and BOUNDED CONTEXTS Figure 17.1. The three basic principles of strategic design (context, distillation, and large-scale structure) are not substitutes for each other; they are complementary and interact in many ways. For example, a large-scale structure can exist within one BOUNDED CONTEXT, or it can cut across many of them and organize the CONTEXT MAP. The previous examples of RESPONSIBILITY LAYERS were confined to one BOUNDED CONTEXT. This is the easiest way to explain the idea, and it's a common use of the pattern. In such a simple scenario, the meanings of layer names are restricted to that CONTEXT, as are the names of model elements or subsystem interfaces that exist within that CONTEXT. Figure 17.2. Structuring a model within a single BOUNDED CONTEXT Such a local structure can be useful in a very complicated but unified model, raising the complexity ceiling on how much can be maintained in a single BOUNDED CONTEXT. But on many projects, the greater challenge is to understand how disparate parts fit together. They may be partitioned into separate CONTEXTS, but what part does each play in the whole integrated system and how do the parts relate to each other? Then the large-scale structure can be used to organize the CONTEXT MAP. In this case, the terminology of the structure applies to the whole project (or at least some clearly bounded part of it). Figure 17.3. Structure imposed on the relationships of components of distinct BOUNDED CONTEXTS Suppose you want to adopt RESPONSIBILITY LAYERS, but you have a legacy system whose organization is inconsistent with your desired large-scale structure. Do you have to give up your LAYERS? No, but you have to acknowledge the actual place the legacy has within the structure. In fact, it may help to characterize the legacy. The SERVICES the legacy provides may in fact be confined to only a few LAYERS. To be able to say that the legacy system fits within particular RESPONSIBILITY LAYERS concisely describes a key aspect of its scope and role. Figure 17.4. A structure that allows some components to span layers If the legacy subsystem's capabilities are being accessed through a FACADE, you may be able to design each SERVICE offered by the FACADE to fit within one layer. The interior of the Shipping Coordination application, being a legacy in this example, is presented as an undifferentiated mass. But a team on a project with a well-established large-scale structure spanning the CONTEXT MAP could choose, within their CONTEXT, to order their model by the same familiar LAYERS. Figure 17.5. The same structure applied within a CONTEXT and across the CONTEXT MAP as a whole Of course, because each BOUNDED CONTEXT is its own name space, one structure could be used to organize the model within one CONTEXT, while another was used in a neighboring CONTEXT, and still another organized the CONTEXT MAP. However, going too far down that path can erode the value of the large-scale structure as a unifying set of concepts for the project.

[ Team LiB ]