Is NDA in conflict with YAGNI?

I got a question on my previous post (The No Dependency Architecture (NDA)) if NDA is not in conflict with YAGNI, and when I was writing the response it became more or less an actual post, so there you go :)

Explaining the thought behind the No Dependency Architecture (NDA) in combination with YAGNI in one sentence would be something like this: “You anticipate change at a technical level, not at a functional level”.

I believe that the “You Ain’t Gonna Need It” (YAGNI) principle does not imply that writing software accordingly to the SOLID principles is bad practice. When you are following these principles (and there are others that are good to follow too) then you are writing software that assumes change, right? I believe that YAGNI is more targeted to functional overhead.

I.e. using a IoC container to fulfill the Dependency Inversion Principle (DIP) is not in conflict with YAGNI because that is at a technical level, but when you are writing extra code for no other reason than that you assume the object might be used in a certain other way also, then you are violating the YAGNI principle because then you are creating additional functionality that is not needed.

From Wikipedia (http://en.wikipedia.org/wiki/You_Ain't_Gonna_Need_It) YAGNI, short for 'You Ain't Gonna Need It', suggests to programmers that they should not add functionality until it is necessary

So I would recommend using the various SOLID principles when writing your software assuming things will change because that is just good practice. Change is a fact, we just don’t know what will change, but by conforming to SOLID we can handle these future changes without too much headache.

As to using an Event Driven Architecture, I am surely not saying each object should only interact using events, that would not make much sense, but most (larger) systems will be build up using multiple sub systems. Having these sub systems communicate via events is a very good way to isolate them from each other. Then it is unknown to each sub system who they are really talking to (i.e. dispatching the events to). In fact it is a 1 to n relationship where the dispatched event might have no, one or many subscribers.

To make the actual handling of subscribing and publishing easier you could use a service bus that deal with that for you. This will also make the system easier adaptable to change. You can also specify that the event can be handled by any instance of the same subscriber enabling load balancing.

“Don’t call us, we call you” (http://en.wikipedia.org/wiki/Hollywood_Principle); is the basis of an Event Driven Architecture, and this is very true, because no system is waiting on an answer from another system. They fire events and they act on events, it is merely coincidence if these events have anything to do with each other.

So I would also recommend using an Event Driven Architecture (EDA) straight from the beginning, perhaps not using a service bus from the start, but just using EDA will make refactoring to use one at a later stage a lot easier.