My Domain is my Castle

One of the most important (first) steps in architecture modeling encompasses the description of the domain model. This model introduces all entities relevant within the current domain as well as their relationships and interactions. It represents the main language all stakeholders should understand. All further activities in architecture modeling basically take the domain model and enrich it with additional infrastructure entities. 

Sounds very abstract, right? Let me give you a concrete example. Suppose, we are going to develop a Web store. What are the typical objects that appear in the problem space and are well known to all stakeholders?

For example, I'd expect entities such as:

• web store user: someone accessing the web store
• customer: someone interested to buy items
• shopping cart: used to add, remove, pay goods
• product catalog: presents all available products classified by categories and allows to search for these products using different types of query
• customer database: place where all customer information is stored
• order processing: responsible to process orders

If you think about this example further, you'll recognize there are some typical relationships between domain objects. For example: a web store user could be a customer or an administrator. A customer typically owns at most one shopping cart at the same time.

You also can easily defer some interactions between the entities. It is obvious there must be a relationship between the shopping cart and the product catalog, because the information about purchased items is read and updated from the product catalog. Of course, the shopping cart needs to interact with the order processing system after the customer has pressed the order button.

With other words: when thinking about how use cases would be mapped to sequence or interaction diagrams, the knowledge about the domain model is essential. It is the step taking you from a black box perspective to a gray box perspective.

There are different ways to express such a domain model. You could invent a graphical representation or use a textual language instead. If a domain object model gets formalized, we call this a DSL (Domain-Specific Language). In this case, engineers could even provide generators that map from the problem domain to the solution domain, i.e., that map problem domain objects and relations to solution domain objects and relations.

Using domain models offers huge advantages:

• it introduces a common vocabulary among all stakeholders which supports effective discussions and often prevents a steep learning curve for domain dummies
• it reduces the chance of missing to address important parts of a domain
• it eases architecture modeling significantly
• it helps focusing on the problem domain instead of always diving into the solution domain
• when enriched with domain patterns (analysis), it boosts productivity

It is neither necessary nor useful trying to come up with a complete domain model in the first place. In most cases, there will be an initial, maybe incomplete, domain model which engineers together with the other roles will evolve over time.

There are several examples, where the domain model is already available. Think about GUIs, compilers, and some kinds of healthcare domains. In these domains, it is useless and a complete waste of time to come up with your own domain model.

 

So, whenever you are being involved in a new software architecture, always mind the domain object model! It will be your best friend.