Thursday, September 17, 2015

DCI, DDD and the concept of roles

All of the architectural approaches in software engineering aim to solve similar problems. One of the techniques that is often used is the concept of roles.

When you do OOP, you may use interfaces (especially with statically typed languages). Interfaces help us thinking in roles - in this context this is the role the object plays.

The DCI architecture is opposing the current class-oriented thinking. They aim to introduce the full OOP which is object based, not so class-based as we're used to now. To simplify things a bit, in DCI you should be able to implement an interface on an object basis, not on the class level.

For example, if you have an ebook object, you might want to temporarily treat it as "a digital product" in the Shipping context (where it's delivered via email to the customer). Not all ebooks are being delivered at the same time, so there's no point in all of them (at the class level) having the shipping abilities. For the time, when it's important to ship the digital product, the ebook has a new role. When it's used in another context (listed in a catalog?) it doesn't have the role - it doesn't have some methods.

That's the DCI conceptual way of using roles.

Now, what is the DDD way?

In DDD, we use the concept of bounded contexts. They represent the boundaries in which a different language is used. In a way, it's similar to DCI Contexts.

In DDD, we would have the Catalog context and the Shipping context as well. Similarly we will have the concept of ebook and digital products. They are not called roles, here, though. Also, it's probably more of an implementation detail, but most DDD implementations would have different objects (different identities) for the ebook (as in Catalog) and for the digital product (as in Shipping).

The ebook and the digital product represent the same thing and very likely they are aggregate roots in their bounded contexts. Keeping them as separate objects (often, with their own separate persistence) requires some synchronisation mechanism. If the ebook title changes, it may need to be changed in all contexts. In DDD, domain events are one way of ensuring the (often eventual) consistency.

As you see, different kinds of architectures need to solve similar problems. Here, both DDD and DCI use the role-oriented thinking. The implementations are different, though.

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