Bounded contexts

Now we are entering the main part of DDD: To model the domain.

Context

Context - The environment in which a word or proposition that determines it's meaning.

Bounded contexts are a breakdown of the domain. All major projects covering different sub-models and when the code based on the different models are combined, it can become complex. Communication to and from, and within the team gets confusing. It is often not clear in which context one model should not be applied. The model must be divided in limited contexts with clear interfaces to other parts of the system. Each bounded context shall be independent of the others.

When a number of people working in the same bounded context, there is a risk model becomes fragmented. The more people who are inside, the greater the problems. This breaks down the system into smaller contexts and eventually lose the integration and coherence. There must be a process of merging all code and other artifacts with automated testing. Use the ubiquitous language in order to hammer out a common understanding of the model and concepts. Continuous integration is an essential part of the DDD.

Implementations that have grown during time with no good analysis of the bounded contexts often has a design pattern that could be called BBoM – Big Ball of Mud. The borders between the parts of the server are unclear. Even the responsibilities between the client and the server are entangled. There is no good way to see the protocols.

Context maps

A bounded context leaves some problems in the absence of global perspective. The relationships with other parts of the model may still be vague and constant change.

Developers of other teams may lack knowledge of your context's limits and will unconsciously make changes that blurs the edges or complicating relations. When the connections to be made between different contexts, they tend to bleed into each other.

Identify why each sub model in the project and define its distinct contexts. Name each bounded context and include the name in the ubiquitous language. Describe points of contact between models and their communications. Preferably, the communication is through service-classes and not directly into other defined context.

The context map is often visualized as a simple map which splits the domain into subdomain, and arrows showing which part talks to which part. In the model these often are candidates for submodels, and packages in languages supporting such and even separate micro-services in a large system.