One principle to follow when writing a unit test is that a unit test should ideally not cross boundaries.


Michael Feathers takes a harder stance in saying…

A test is not a unit test if:

  • It talks to the database
  • It communicates across the network
  • It touches the file system
  • It can’t run at the same time as any of your other unit tests
  • You have to do special things to your environment (such as editing config files) to run it

Tests that do these things aren’t bad. Often they are worth writing, and they can be written in a unit test harness. However, it is important to be able to separate them from true unit tests so that we can keep a set of tests that we can run fast whenever we make our changes.

Speed isn’t the only benefit of following these rules. In order to make sure your tests don’t reach across boundaries, you have to make sure the unit under test is easily decoupled from code across its boundary, which provides benefits for the code being tested.

Suppose you have a function that pulls a list of coordinates from the database and calculated the best fit line for those coordinates. Your unit test for this method should ideally not make an actual database call, as that is reaching across a boundary and coupling your method to a specific data access layer.

Reaching across a boundary is not the only sin of this method. Data access is an orthogonal concern to calculating the best-fit line of a series of points. In The Pragmatic Programmer, Andrew Hunt and Dave Thomas tout Orthogonality as a key trait of well written code. In an interview on, Andy describes orthogonality like so:

The basic idea of orthogonality is that things that are not related conceptually should not be related in the system. Parts of the architecture that really have nothing to do with the other, such as the database and the UI, should not need to be changed together. A change to one should not cause a change to the other. Unfortunately, we’ve seen systems throughout our careers where that’s not the case.

Ideally, you would refactor the method so that the data the method needs is provided to it via some other means (another method passing the data via arguments, dependency injection, whatever). That other means, whatever it is, can perform the necessary data access: that’s not your concern at this moment. You aren’t testing that other means (right now at least, you might later), you’re focused on testing this unit.

This isolation enforced by unit test can be challenging, as it’s easy to get distracted by these other orthogonal concerns. For example, if this method doesn’t do data access, which one does? However, having the discipline to focus on the unit being tested can help shape your code so that it follows the single responsibility principle (SRP for short). If your test needs to access an external resource, it might just be violating SRP.

This provides several key benefits.

  • Your function is no longer tightly coupled to the current system. You could easily move it to another system that happened to have a different data access layer.
  • Your unit test of this function no longer needs to access the database, helping to keep execution of your unit tests extremely fast.
  • It keeps your unit test from being too fragile. Changes to the data access layer will not affect this function, and therefore the unit test of this function.

All this decoupling will provide long term benefits for the maintainability of your code.