Bounded Contexts

I regularly fall into the same trap when building applications: Trying to find the one model that unifies all the use-cases and allows to solve every problem. From discussions with other developers I know not to be the only one making this mistake.

It is propably a human trait trying to find the one line that connects every dot. However in software design there is a case to be made for the separation of contexts to tackle complexity. Eric Evans described this in detail with the “Bounded Context” pattern in his “Domain-Driven Design” book. According to an interview with Evans I read somewhere, he considers it one of the most important patterns in the whole book.

The essence of this strategic pattern is to embrance the separation of different parts of an application and develop different models as different domain contexts. Consistency and constraints are enforced within one context, but don’t necessarily hold in another context.

The goal is simplication and freeing us from the burden of finding that one model that fits all use cases. It is much less awkward to have some parts of the model reimplemented for different purposes than creating God objects that try to unify everything, and failing at this task.

Evans goes even further and introduces a significant amount of patterns that describe the relationship between different bounded contexts.

An Experiment

At the SoCraTes conference in August last year, I participated in a DDD architecture game by Cyrille Martraire that focused on distilling bounded contexts. We were given a very simple domain concept “Customer” and assigned different roles in the business.

Everyone described their perfect customer very differently, imagining the Customer object that fits all these requirements was quite an eye opener. Even if there is no such single software that operates a business, going to the extreme and defining 10 different angles for a single concept made me clear, that sometimes different overlapping implementations are not such a bad thing.

Problems and Solutions

One problem when applying bounded contexts is data synchronization. Synchronization between contexts can be reached in many different ways, three of which are:

  1. The Domain Event pattern that integrates nicelly into Domain-Driven Design and simplifies the synchronization with other parts of an application.
  2. Correlation Ids of the same entity into the different contexts and a clean seperation of the data (no shared data necessary).
  3. Another way are immutable read models (value objects) of the data managed in another context (entity).

In any way relying on Eventual Consistency helps solving the synchronization problem.

Depending on how your bounded contexts seperate from each other, duplication of classes is necessary as well. This is something that has to be accounted for and you should make sure that only one context is responsible for changing the same data. Different contexts can have different behaviors on the same data. This is actually a point where Domain-Driven Design and Data, Context and Interaction (DCI) intersect.

Conclusion

Applying bounded contexts to an application or a cluster of applications helps developers and domain experts to focus on problems in their specific context. Sometimes each context even has its own domain-expert. The benefit for developers is a simplified abstraction of different parts of the modelled problem.

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