The last few weeks I’ve been thinking about the architectural pattern known as Clean, Onion, Hexagonal, or Ports’n’Adaptors. I’m curious if many people are applying it in the Django world.
The premise is for your core application entity classes and business rules to be plain old objects, with no dependencies. In particular, they are not dependent on the interfaces between your application and external systems, such as your persistence mechanism, or your web framework. Instead, external interface components depend upon your core business objects. This essentially moves the database from the ‘bottom’ layer of the old traditional ‘three layer architecture’, to form a part of the topmost layer – a sibling with the ‘UI.’
For inbound messages (e.g handling a web request) this is straightforward – Django calls your view code which calls your business layer, but keep your business layer separate from your Django code, so it is stand-alone and unit-testable. For outbound messages, such as then rendering the web page in response, it’s slightly more complicated: Your business logic must pass the result (typically a pure data structure) back to your web-aware code, but without your business logic depending on the web-aware component. This requires an inversion of control.
That way, all your business logic can easily be tested in unit tests, with no mocking required. You still need some end-to-end tests to verify integration, but you shouldn’t need to involve your UI or database in testing every detail of your business logic.
Also, you can easily switch out your external system interfaces, such as persistence, to use another RDBMS, another ORM, a NoSQL store, or an in-memory version for testing Since the core of your application doesn’t have any dependency on these components, it is oblivious to the change. The business logic, because it doesn’t depend on Django, is no longer riddled with Django’s convenient ORM database access.
Same thing goes for switching out your web framework, or calling the same logic from web UI or web API calls. And again, for switching out your UI: add a command line application, or a console UI. The core application logic is unaffected, and your new interface components contain only the code that is specific to that interface’s concerns.
Another side effect is that your web framework, if you’re using one, becomes a peripheral detail which depends upon your core application, rather than the other way round. Your Django project would become a subdirectory of your project, rather than dominating your project directory structure. Since the business logic formerly contained within it is now elsewhere (in your core business objects) the Django project is now very thin. Views, for example, are delegations to single business-layer functions. The Django project now contains just the web-oriented aspects of your project, as it should.
These ideas all seem like relatively straightforward software engineering, and I feel a bit foolish for not having been aware of them all these years. I console myself that I’m not alone.
UncleBob’s Ruby Midwest keynote “Architecture – The Lost Years” attributes one source of this idea to Ivar Jacobsen’s 1994 book Object Oriented Software Engineering : A Use Case Driven Approach (2nd-hand hardbacks cheap on Amazon.)
I see a few people applying these ideas to Rails, but are many people out there doing this in Django? I plan to refactor a small vertical slice of our monster Django app into this style, to try and prove the idea for myself.