If you are going to design a system, you basically need to identify prioritized main use cases and quality attribute scenarios among other forces to design the system. Together with a problem domain model that shows the inner constituents, and a context model that integrates the system into its environment, the design can evolve in a evolutionary way. As a result of the design activities, architects are able to introduce subsystems as well as their relations according to functional or qualitative drivers. And, of course, interfaces start to appear, each of them defining one specific and explicit role of the subsystem.
A subsystem is itself integrated into an environment – the enclosing system under development. So the subsystem can also act as a system. Thus the same principles apply for the subsystem acting as a system itself – you may even define use cases and quality scenarios for the subsystem with use cases and scenarios being derived as a subset of the “outer” use cases and scenarios. After this step, “subsubsystems” are created, often coined “components”.
What we do here is applying the same design principle in a top-down manner.
But is it useful or possible to apply the principle infinitely? No, because at some level the solution domain is shining through. Solution domains tend to introduce their own composition techniques such as assemblies, bundles, EJBs, services, classes and objects. If the top-down design approach reaches this level, designers must map the architecture artifacts to the solution domain. Maybe, we should call this level architectural twilight zone or the problem-solution-boundary
Side remark: To overcome the twilight zone, we could introduce DSLs and use Model-Driven Software Development.
As a rule of thumb, we typically obtain 2 sublevels (subsystems, subsubsystems) as architects. If less, the design is too abstract and vague. If more, we are introducing too many details.
One of the core challenges in this context is the fact that there might be different strategies to view a domain and thus different ways to cut a system into subsystems.
Partitioning a system into subsystems independent of the hierarchy level is influenced by functional aspects and the problem domain. Thus, subsystems should introduce meaningful subdomains of the surrounding problem domain. With other words: methods like Domain-Driven Design together with some extensions can help nicely.
No matter what you do, there will always be crosscutting qualities and topics. The same observation can be made when structuring an organization into divisions, departments, groups. Have you ever seen an organization without overlapping units? The introduction of crosscutting concerns may introduce new sub^nsystems, add new interfaces or even change their implementation depending on the invasiveness of the concrete concern. Each concern can be considered a subordinate view mixed into the subsystem respectively its domain.
Architecture design is basically fractal design up to two levels of depth. The priorities of use cases and quality scenarios as well as their properties (strategic versus tactical) define how and in which order the functional model needs to be refined hierarchically by integrating scenario-based views.
Of course, one person’s solution domain can be another person’s problem domain which is why exceptions to the aforementioned rule might apply.