1. It makes your code a mess.
2. You probably wrote the wrong thing anyway, because how do you know what the customer wants if you don’t even know what they do?
DDD argues you should develop software by:
1. Talking to customers about the domain until you have a shared language with specific terminology.
2. Write that down as a model.
3. Write that model as software.
I think one of the biggest misses in software engineering (and something I’ve seen cost $100Ms) is software developers not talking to customers about what they do with the software. Some of the worst bugs happen because domain experts mean something by a term of art that software developers misunderstand.
DDD overlaps heavily with the “applied category theory” perspective on software, so DDD tends to be popular with functional programming types.
Domain driven design is about understanding the business requirements enough to know “this is fundamental to the domain” versus “this is going to change next month”.
In the accounting example, when you talk to the accountants and develop an ubiquitous language, you’ll notice that long term concepts are “assets”, “transactions”, “taxes”, etc. while the transient things are exact tax rates, how transactions are structured (eg, net 30), etc.
So you’d build your system to match:
- A foundation of transacting assets/currency/inventory
- A system for defining transaction flows
- A system for reporting transaction aggregates (eg, for tax filings)
Domain driven design doesn’t say which of those to hardcode versus make user configurable — it says to talk to your users until you understand the domain, then build software that reflects that.
Like most good ideas, people get way too formulaic about it sometimes.