Made of Bugs

Design for Testability

When designing a new software project, one is often faced with a glut of choices about how to structure it. What should the core abstractions be? How should they interact with each other?

In this post, I want to argue for a design heuristic that I've found to be a useful guide to answering or influencing many of these questions:

Optimize your code for testability

Specifically, this means that when you write new code, as you design it and design its relationships with the rest of the system, ask yourself this question: "How will I test this code? How will I write automated tests that verify the correctness of this code, with minimal irrelevant assumptions about the environment or the rest of the system?" And if you don't have a good answer to that question, redesign your abstractions or interfaces until you do.

I've found this heuristic valuable in two different ways, and I'll discuss both here.

You get good tests

This one is, perhaps, obvious: If you set out with a goal of having good tests, you will likely end up with good tests.

However, I think it's worth saying a bit more about it.

First, I want to emphasize how wonderful it is to work in a code base with good tests. The process of verifying changes is simple: just run the tests. No need to set up complicated development environments and poke at the system manually or interactively; just make test, and get, if not a guarantee, high confidence that your code does what you think it does and hasn't broken anything important.

In large and mature software systems, the hardest part of making changes is not the change itself, but making the change in a way that doesn't regress any other important behavior. Good, fast, tests that can provide basic assurances quickly are therefore an enormous productivity improvement.

Second, in order to really realize the benefits of good tests, you need to be able to run tests – or, at a mininum, the subset of tests covering your changes – quickly, in order to keep a fast development cycle.

In order to preserve this property as a system scales, you need to have good unit tests. There are a lot of religious arguments out there about the boundaries between "unit", "functional", "integration", and other sorts of tests; By "unit" tests here I mean tests that exercise some specific module or component of the code base with minimal dependencies on or invocation of other code in the application.

You will always also need some number of integration tests, which exercise the application end-to-end, in order to shake out the inevitable subtle interactions betwen components. But relying primarily on unit tests brings significant advantages:

  • Unit tests are fast: By exercising a limited amount of code, they run more quickly than tests that must invoke the entire application.
  • More importantly, unit tests scale: In a codebase with mostly unit tests, test speed should scale linearly with application size. With more functional end-to-end tests, you risk scaling closer to quadratically, as each component requires tests that in turn incidentally exercise nearly every other component.
  • Because unit tests are associated with clear pieces of code, it's easy to run only the tests corresponding to a specific change, providing an even faster feedback loop to development.

Good unit tests don't happen by accident, however. It's very hard to have good unit tests without having good "units": Portions of code with narrow and well-defined interfaces, which can be tested at those interfaces.

The best way to have such logical units, which then permit excellent tests, which in turn permit rapid feedback cycles while maintaining a mature project, is just to do so directly: Write code with this end in mind.

You get better code

The second reason to write code with the tests in mind is that it actually ends up with better code!

Let's consider at some of the characteristics that easy-to-test code should have:

A preference for pure functions over immutable data

Pure functions over immutable data structures are delightfully easy to test: You can just create a table of (example input, expected output) pairs. They're generally also easy to fuzz with tools like QuickCheck, since the input is easy to describe.

Small modules with well-defined interfaces

If a chunk of code has a small, well-defined interface, we can write black-box tests in terms of that interface, which test the described interface contracts, without caring excessively about either the internals of the module, or about the rest of our system.

A separation of IO and computation

IO is, in general, harder to test than pure code. Testable systems therefore isolate it from the pure logic of the code, to allow them to be tested separately.

Explicit declaration of dependencies

Code that implicitly plucks a database name out of the global environment is much harder to test than code that accepts a handle as an argument – you can call the latter with different handles over the course of your test, to test against a clean database each time, to test in multiple threads at once, or whatever else you need.

In general, testable code accepts dependencies as explicit arguments at some point, instead of assuming their implicit availability.

If I look at these characteristics, I find that they're also the characteristics of good, well-structured code in general! By architecting a system with testing in mind, we generally get a better-factored system than we would have, otherwise.

Furthermore, by approaching these properties indirectly – through the lense of testing – we make them more concrete. It's easy to debate indefinitely what the right modules, interfaces and structures are for a software system; But faced with the concrete frame of "What makes it easy to adequately test this code?", it's easier to evaluate our choices and to decide when we've gone far enough.


There are few things more important for a complex software system than the ability to make changes with confidence, and high-quality automated tests are one of the most important tools we have to this end. Good tests don't happen by accident, or even by brute-force effort: they happen by design, because the application was written to enable them.

As you write software, always be asking yourself "How will I test this software's correctness?" and be willing to design to that goal. In return, you'll get a system you can be – and stay – much more confident of, and one that's better-structured otherwise.