Requirements Part II - The Art of Multi-Facet Design
Requirements help to decide which paths to take at specific points. This implies that requirements constrain the solution space. If multiple requirements are "applicable" at a specific point, priorities define the precedence which is why we must assign unique priorities to them. Not very complex so far, right?
But how do we really implement a solution architecture?
The first thing we obtain is a functional object model consisting of all the entities in the domain as well as their relationships. For instance, if we are going to build a Web shop, this model would probably consist of concepts such as customer, shopping cart, order system, product catalog, payment system. It is highly recommendable to define a domain model (domain-driven design) first to achieve a sound understanding and a common vocabulary among all stakeholders. Use cases are the means to drive this activity. They help designing the external boundaries of the system to be built but also the internal components required to implement the use cases. Obviously, functional requirements also have priorities which is why the use case priorities should drive the process.
Then we need to take into account infrastructural issues such as distribution, security, ... According to their priorities, we start with the most important infrastructure which needs to integrate into and with the functional model. For example, if we need to add distribution-specific infrastructure to our Web shop, we would introduce a broker-based architecture with customer browsers being the clients and the web shop being the server. Now let us assume, security is the next important requirements. We are now going to integrate an appropriate securiy infrastructure into our result we achieved after embedding the functional entities into the distributed infrastructure. We will add security components such as firewalls, identity management functionality, ... We continue with this approach until we have considered all operational requirements. What we did is extending the functional model (problem domain driven) with non-functional infrastructure parts (solution domain driven).
In the third step we add the infrastructure required by developmental qualities such as configurability, extensibility, and the like. This means that all those strategies, interceptors, configuration interfaces will be added at the end.
What we reached is an onion model with the functional architecture being the core part, the layers derived from the operational/infrastructural requirements being the middle part, and the developmental infrastructures being the outer part.
If you think about it, this is exactly what a building architect or other engineering disciplines do. Our resulting software architecture represents an integration of different functional and non-functional perspectives which is why I call it Multi-Facet Design.
But what about Software Product Line Engineering, In SPLE we will have to introduce a Commality/Variability analysis in order to determine all commanlities and variants. Variants will be considered as variations of a commonality and hence can be defined in terms of a commonality. Sounds abstract, but let me introduce an example. If we take our Web shop, we could define that we need to support different payment options (credit card, bank transfer, Paypal) depending on the customer. The variability is now defined as possible payment options. But the commonality in this example is the payment system itself. As a consequence, we need to consider the payment component in the core design, but specify the exact variations in the developmental design. For example, we could introduce the strategy pattern for the payment system to allow different options.
What about embedded systems?
All stringent constraints emerging from embedded systems such as scarce memory resources or CPU limitations or limited battery life will be handled as high priority requirements. One of the consequences is that static configurations are always preferred over dynamic configurations in such systems to allow determinism and control of QoS properties. Needless to say that realtime capabilities place additional burden to the engineer.