Tuesday, February 19, 2013

Turtles all the way down

A while back I had a heated passionate debate about dependency injection with a colleague. He was concerned about the lifetime of objects and which object had the responsibility of creating the other. While some of his points were valid, I took a position that a lot of that doesn’t matter if you allow the Inversion of Control container to manage this for you – simply push dependencies through the constructor. The argument, comically, sounded a bit like this:

Colleague: “But who creates these dependencies?”

Me: “That doesn’t matter. Put them in the constructor, let the container take care of that.”

Colleague: “Ok, so who creates this object?”

Me: “Again, doesn’t matter. It’s placed in the constructor of it’s parent, along with any other dependencies it needs.”

Colleague: “Arrrg! Who creates that object??”

Me: “Dude! It’s turtles all the way down.”

The reference “turtles all the way down” is to what I believed to be an Iroquois cosmological myth that the earth is flat and rests on the back of a great tortoise, which in turn rests on top of a larger tortoise that is on top of a larger tortoise, etc, etc… all the way down. I recently discovered that source of this expression is unknown and there are many different variants of this theme. The best variant is the Hindu belief that the earth sits on top of a tiger or elephant, which is on top of a tortoise, which is then, well, …uh, suppose we change the subject?

Over the years, I’ve adopted this expression to represent a fundamental lesson I was taught in object oriented programming: “objects should not have a top”. That is, objects should be open ended, allowing them to be extensible or mixed-in to other different applications and implementations. Our objects should be aware of their dependencies but not in control of their implementation or lifetime, and the framework that glues them together should be abstracted away. This idea is echoed by the Open/Closed principle and the Dependency Inversion principle, and it is demonstrated every time we write automated tests – the first consumers of our designs.

In my analogy, turtles all the way down accurately describes how I feel we should design our objects, but to my colleagues credit it doesn’t reflect that at some point there must be a reckoning point – someone must know how to assemble the parts to make a whole. I suppose Aristotle would point to an unmoved mover, responsible for the order of the universe. Douglas Adams would point to the man behind the man that probably has another man behind him. I’d point to some top-level application service that has the configuration that magically wires everything together.

It feels good to know there’s some sort of finality to the infinite regress problem: our application glues it all together.  So maybe now we can catch up on our sleep, knowing that we don’t have to realize infinity before we can close our eyes. At least until we see our application as part of a batch process in a much larger service, hosted in the cloud, resting on the back of a very large turtle.

Happy coding.