Well, IMHO, the primary intent of such architecture style (Hexagoanl, Ports&Adapters, Onion …) is that it allows you to focus on your domain, how you will deliver value instead of focusing first on UI, frameworks or storage issues. It allows you to defer such decisions.
As Jeffrey says, the ability to swap out "infrastructure" elements is a nice side effect of such architecture style. Even if you will not switch from one RDBMS to another every 6 months, it’s quite reassuring knowing that it would be possible doing it without pain, though.
Rather than thinking about changing your storage mechanism on a regular basis or as you said “swapping out of key infrastructure elements”, just think about third parties services that you’d plug to your system. Those are eager to change on a regular basis; you would also switch from one provider to another. This is quite a more common scenario we are used to face with on a more regular basis. In this particular case, the domain behavior won’t change, the interfaces will stay the same, you won’t have to change a single line of code into your core domain layer. Only the implementation made somewhere in your infrastructure layer might have to change. That’s a another noteworthy benefit from that kind of architecture!
Please read this nice Uncle Bob article about Clean Architecture where he explains why the ability to defer critical infrastructure decision is really cool!
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Could you provide an example of where you have swapped out a third party service?
We have tons of examples where we switched from one provider to another (from payment providers to live feeds providers or whatever provider). The business stays the same, the domain behaviors are still the same. Changing a provider should not have any kind of impact on your business. You don’t have to change the way your business work, where the value really is, just because you change from one provider to another, it makes no sense. Isolating your domain behaviors in an independent core layer, with no dependencies on any third parties libraries, frameworks or provider services, definitely help you to deal with changes.
I have the feeling that you’re trying to convince yourself whether to go with Onion. You might be on the wrong track only thinking about migrating to new infrastructure related stuff (db, third parties stuff...). Focus on your domain instead. Ask yourself if your domain is complex enough to require such an architecture style. Don’t use a bazooka to kill a fly. As Simon Brown says: "Principles are good, but make sure they’re realistic and don’t have a negative impact"!
If your application is quite small, with no complex business domain, go for classic n-tiers architecture, that’s ok; don’t change things just for the sake of it or just because of any buzzword. But also keep in mind that an isolated core business layer without dependencies, as in Onion architecture, might be very easy to unit test!
Now for your additional questions:
Did the implementation of this architecture + rework to migrate to new infrastructure significantly decrease the total effort required, if compared to a traditional, coupled approach?
It depends! :-) In tightly coupled applications as soon as there’s a new infrastructure element to be migrated, there is little doubt that you’ll surely have to modify code in every layers (including the business layer). But if this application is small, quite straightforward, well organized with a descent test code coverage, this shouldn’t be a big deal. Now, if it’s quite big, with a more complex business domain, it might be a good idea to isolate that layer in a totally separate layer with no dependencies at all, ensuring that infrastructure changes won’t cause any business regression.
Do developers find coupled, hard-referenced code easier to write and debug than loosely coupled, indirectly referenced code, but the eventual payoff for infrastructure changes makes this worth it?
Well, ask your teammates! Are they used to work with IOC? Remember that architecture design and choices must be a team decision. This must be something shared by the whole team.
