DDD - What is a complex domain?

Question

It is often said that DDD (Domain-driven Design) is better suited for complex domains instead of simpler ones.

What characterizes a complex domain? (please be more specific than "it has complex business rules");

Which are examples of complex domains?

How can I classify a domain as complex (i.e. suitable for DDD) or not?

Answer 1

From my experience 3 most important thing that makes your domain complex:

Size

Big domains tends to increase complexity. Handling and coordinate a lot of things is always hard.

Rules and invariants

Domains (even domains with just a couple of bounded context) might have a lot of domain rules and invariants and/or a lot of nuances in its use cases and process. This increase complexity. Rules that spam a lot of changes in a entity or inter-domains events are often the complex business rules.

Context

Context complexity is hard to explain wihtout a example. Let's put in the table a context complexity related to a entity named Product.

Depending of context; a entity could means different things in your domain. A Product entity doesn't means the same for Factory context, Marketing context, Sales context, PostSales support context, etc.

If the data, user cases, process, behaviour, etc related to Product entity, in every context, are very different the complexity increase a lot even when you just have a handful of context and entities. This usually means that you have many Product entities (one in every context) even if all of them are supported by the same persistence store (in case of ER store, the same table/s).