Translator between bounded contexts in DDD (and some other questions)

Question

I've been reading Eric Evans' DDD: Tackling Complexity in the Heart of the Software and in the section in context maps, Evans cited an example of 2 bounded contexts (Booking context and Network Traversal Service) using a translator to integrate them.

If I understand correctly, when we create a model, we put everything into bounded contexts creating conceptual boundaries for the domain. My questions are:

1. If everything should be in a bounded context, where should the translator be located? In Evans' sample diagram, the translator is outside (in between) both bounded contexts.

2. Say we have a single team working on a ERP. Should the ERP be put in several bounded contexts or a single one only. Based on Evans' sample, the bounded contexts were devised so that multiple teams could work on their own model. But since this is a single team, wouldn't they benefit on a single model so integration wouldn't be an issue? Or did I understand this wrong?

3. In question 2's case, if multiple bounded contexts, what if in implementation, we need a class from Accounting to be used in Payroll? I don't think we need a translator here but I'm not sure how to share the required class. Will just referencing the needed class from another bounded context be fine?

4. And lastly, how is a module in DDD related to a bounded context?

Answer 1

1. In an Infrastructure layer (outside the Domain), it's a technical detail.

2. A bounded context(BC) emerges from the Domain, that's why we identify them and not defining them. Once identified, if there are many BCs and there are developers available then the workload can be split so that the app can be developed faster. A single model is not advisable, you want a single domain model per BC and also a simplified read model for querying (CQRS).

3. I don't know but for me Payroll is part of Accounting, there aren't really 2 BC. Account is a BC, however other BC might need a Payroll but that definition is specific to the BC. And probably the definition is just really some data (a read model). Payroll behaviour should stay in Accounting. So you need to have clearly defined what each BC understands by "Payroll". Usually you'll have a domain service who will use concepts from both BC and which will use the 'translator'.

4. It isn't. A module just groups things together from a technical point of view. A BC is pretty virtual, you might choose to have a project or a module corresponding to one BC but that's your decision how to organize things.

Answer 2

1. If everything should be in a bounded context, where should the translator be located? In Evans' sample diagram, the translator is outside (in between) both bounded contexts.

I think this question has no sense. Each BC is a boundary in wich a domain model exist, and the boundary is determined by the UL, each BC has its own UL. If you just create one model, you don't need translation. Instead of spliting a big model into several smaller ones, you are joining them.

2. Say we have a single team working on a ERP. Should the ERP be put in several bounded contexts or a single one only. Based on Evans' sample, the bounded contexts were devised so that multiple teams could work on their own model. But since this is a single team, wouldn't they benefit on a single model so integration wouldn't be an issue? Or did I understand this wrong?

I think you understood wrong. You split a single model into several BCs according to the UL, not to the available teams. Then if you don't have enough teams for the number of BCs created, a team will have to develop more than one BC. By the way, the opposite is not desirable, i.e., a BC shouldn't be developped by more than one team.

3. In question 2's case, if multiple bounded contexts, what if in implementation, we need a class from Accounting to be used in Payroll? I don't think we need a translator here but I'm not sure how to share the required class. Will just referencing the needed class from another bounded context be fine?

You shouldn't reference a domain object of another BC. That BC should protect its domain model, that's the application layer for. The application layer expose DTOs, plain objects, it shouldn't expose the domain model to the outside world. What you are asking for is a shared kernel (a model shared by other BC).

4. And lastly, how is a module in DDD related to a bounded context?

A module is just a java package, useful for keeping together things that a related to each other. So you split the source code of a BC in modules. Do it according to the UL, not to technical aspects. For instance, a module for each aggregate, not a module for entities, another one for repositories, etc.