A bounded context is a full application?

Question

I've been reading about DDD and bounded contexts and I think I'm getting the idea wrong. At first, I liked the idea of subdomains and bounded contexts, I understood it like that: there's a software to be developed, but attacking all at once is too much, so we break it into logical pieces and develop each at once. Another problem we solve is ambiguities on the ubiquitous language.

This led me to think about bounded contexts as basically just folders where I group and bound code related to some specific piece of the application. This code I believed to be made up from things like

• The domain model of that bounded context, including abstractions for repositories and services
• Infrastructure layer for that bounded context, implementations of repositories and so on

Of course, being the domain model and infrastructure properly separeted within the bounded context.

Reading further, it seems, however, that each bounded context is an entire application on it's own right. It seems, sometimes, that each bounded context has it's own application layer, for instance.

This made me confused, because sometimes I don't want to end up developing tons of applications, I just one to develop one. The bounded context division of the application was supposed to build one app, not many apps to be integrated.

I've seem this question where @MikeSW says both approaches presented by the OP are valid. What I'm asking is about a third structure:

<bc 1>
 |_ domain
 |_ infrastructure
<bc 2>
 |_ domain
 |_ infrastructure
|_ application
|_ presentation

At least for all the applications I've seem this makes much more sense. I want one app, not several apps with several presentations, but I still want to be able to break the domain and benefit of things like "bounding the ubiquitous language".

So, is a bounded context a full aplication? Or can a bounded context be used like I understood and felt more useful? There are any problems with my approach?

Answer 1

The domain layer is usually the most complex part of your program, and can also change often due to business requirements and refactoring. So you generally don't want to expose it directly to your presentation layer or other bounded contexts. If you feel that you can expose it, it might be the case that your application logic or use case methods are mixed into your domain layer, or that your program is not large or complex enough to require multiple BC's to begin with. Otherwise, I would go with including the application layer in each BC to protect the domain model's integrity and expose only the commands that need to be called from a use case perspective.

I want one app, not several apps with several presentations, but I still want to be able to break the domain and benefit of things like "bounding the ubiquitous language".

You can have a thin application layer for each bounded context, and still have a single presentation layer. This is sometimes called a "composite UI", which should be considered a separate BC in itself. If you need to handle common logic such as authentication, create another application service or facade in the composite UI and have it handle the authentication before in turn calling the application service of an outside BC.

I think most of the examples you see in books and on the web are over-simplified in that they have 1 BC per physical running application (and perform some kind of network communication between them), whereas in the real world you might have a complex application that you need to split into separate logical units, but not run them as separate processes unless the need arrives.

Answer 2

At the end of the day the answer is both. The important thing to take away from bounded context is not how you structure your app, but that you have different spaces where you model specific behavior relating to some context. How you define the boundaries between these contexts is dependent on the problem you need to solve.

There is nothing wrong using namespaces(folders) to define bounded contexts. Like you said most of the time you are simply writing one application. You can also define your bounded context by having separate projects for each context. In this case your presentation layer will reference the project it needs.

There are many right ways to code DDD. You should ask yourself "Am I following the core principle by doing it this way"

Answer 3

The bounded context describes a subset of the complete solution and everything within that context serves that context. So, imo, each context has it's own domain so it could be a separate application or just a subsystem of the same project. The point of the "context" is that the ubiquitous language applies directly TO that context. For example, a User in the Account context might mean something completely different than a User in the Sales context. Each "User" will have different capabilities and follow different rules in each context. Each context needs to be isolated from any other context and are not allowed to share references (unless it's via a 'Shared' context); any communication should be mediated through a service that sits on top of that context. A context doesn't even have to follow DDD to be "DDD compliant" since each context can follow it's own approach (e.g. domain driven, data driven, etc.). Contexts are simply silos that outline a logical section of the business.

Whatever you need to do to prevent direct references across contexts is fine whether that means different namespaces, different assemblies within a solution, or different projects altogether.