Code needs boundaries, but not too many

Let’s talk about boundaries in programs. I need them, otherwise programs grow increasingly inscrutable and impossible to change. A lack of boundaries is nearly as bad as spaghetti code; i.e. it’s really bad.

But, too many boundaries can also make a program inscrutable. To the absurd, a program composed entirely of black boxes each of a single narrow function and behavior is all indirection. Indirection is a cost I pay when I introduce boundaries, e.g. “Your princess is in another castle.” I want to have just the right number of boundaries; not too few, not too many.

Further, I want to avoid establishing the wrong boundaries if I can. Boundaries are hard to move around; creating them is an implicit act of making some kinds of changes more difficult. Awkward boundaries make it difficult to write correct code; hurried developers will yield to the temptation to circumvent the boundary. If you do manage to identify an awkward boundary and correct it, you’ll have some temporary churn in your program while you rejigger the boundary and the code on both sides of it.

On the other hand, the right boundaries are wonderful. They create leverage for the developers working on both sides of the boundaries. They get more done, only needing to know about the boundary and not what lies on the other side. Establishing good boundaries is the first step towards encapsulation and abstraction.

We used to make boundaries from packages and libraries. Now we have added out-of-process services, message passing, and infrastructure as boundaries. This will probably turn out as a net benefit, but right now we’re chasing novelty, blog posts, and conference talks at the expense of increased complexity. We aren’t really “engineering” our boundaries.

Creating boundaries too eagerly increases the odds of imposing the wrong boundaries and churning on said boundaries. Creating boundaries too lazily imposes a high cost of change to create those boundaries once you discover them and accept the implementation challenge.

My favorite kind of boundary is a bounded context. Its a wonderful epiphany that we don’t all have to agree on the precise definition of words and responsibilities if we can agree where the fences (boundaries) go.

Gary Bernhardt has nice things to say about boundaries. If you like this, you’ll love his ideas.