A single domain can grow large when applying Domain Driven Design. It can become very hard to contain a single model when using ubiquitous language to model the domain. Classic examples prevalent in many domains would be
User models. A bounded context allows you to break down a large domain into smaller, independent contexts.
In different contexts a customer may be something completely different, depending on who you ask and how you use the model. For example, take three bounded contexts within a typical domain that allows customer administration, customer notifications and general reporting.
A customer is their account id, social media accounts, email and any marketing preferences. Anything that would be required to uniquely identify a customer, and send a notification.
+ Id + Email + Marketing Preferences + Social
When reporting customers are nothing more than statistics. A unique customer ID is more than enough just for aggregation and statistic collection.
Allowing the customer to administer their account would require anything personally related to the customer to be modelled.
+ Id + First Name + Last Name + Address + Email
Despite the common elements such as Id and email, the other elements are specific to the context in which the customer is used. One of the biggest mistakes I've made by ignoring a bounded context is to see a common model and try to apply this everywhere. This leads to less code, but increases coupling. A single small change in one context can cause a rippling effect. In fact the best solution is to have a customer model per context.
The result of this approach is you will end up with at least three models using the example above. While structural duplication increases, coupling decreases. Each context can change and evolve at its own pace. This is a good thing. No business logic here is being duplicated, only the model. As each context operates in its own speciality, there should never be a case where this is problematic.
- Structural duplication outside of bounded context is not a bad thing.
- Resist the urge to use a base class for common attributes. This is especially true if you use an ORM or anything that will couple you further when these models are used.
- Ending up with multiple models per bounded context is likely going to happen, embrace it.