Unit tests versus the unit tested

I recently read the great and oft-cited article about testing microservice architectures by Cindy Sridharan over on Medium. It’s broadly applicable beyond just “microservices”, so I highly recommend giving it a read. I was struck by this passage in particular:

The main thrust of my argument wasn’t that unit testing is completely obviated by end-to-end tests, but that being able to correctly identifying the “unit” under test might mean accepting that the unit test might resemble what’s traditionally perceived as “integration testing” involving communication over a network.
— Cindy Sridharan, “Testing Microservices, the Sane Way

Articulating what you’re actually trying to test is one of the most underrated skills in testing. It seems like a simple question to ask, but often a forgotten one. Asking “what information does this test give me that no other test does?” is great heuristic for determining whether a test, especially an automated one, is worth keeping. It’s also a convenient go-to when evaluating whether a tester knows what they’re doing, and when trying to understand what a test does.

What interests me about this passage is that Cindy is highlighting how “the thing being tested” gets conflated with the “unit” in “unit testing”.

When I used to train new testers at my company, I would present four levels of testing: unit, component, integration, and system. Invariably, people would ask what the difference between component and integration was. Unit tests were easy because that tended to be the first (and sometimes) only kind of testing incoming devs were familiar with. System tests were easily understood as the ones at the end with everything up and running. But when are you testing a component versus an integration? It’s not obvious, so it’s no surprise that three-tiered descriptions are much more common these days.

For us, testing a component was sometimes a single service. At other times it was testing a well defined system of a larger program (one that probably would have been a microservice if we built the system from scratch). Inputs and outputs were controlled directly, and no other services needed to be running for it to communicate with. We defined a “unit” as the smallest possible thing that could be tested (often a single function) and the “component” was running the actual program.

Is that different from an “integration” test of those units? Not really. Running the component is an integration of smaller units. But it was convenient for us to separate those tests from testing how separate services communicated with each other. You can ask the same question anywhere along the continuum from function to system. Is testing a class a unit test or a component test? Where do you draw the lines?

The confusion highlighted in the passage above, to me, is only because of different definitions of “unit”. If you want to call the thing, the service or behaviour, that you’re testing a “unit” instead of a “component”, go for it. If you want to call the communication between two services the “unit” (as this article does), great. This ambiguity should not be an obstacle to understanding the point: you should know what you’re testing and have a reason for testing it.

And, of course, that you should be testing the most important things. Which means, for example, don’t mock away the database if your service’s main responsibility is writing to the database. The problem isn’t that Cindy is saying “unit tests should be communicating over the network,” though you might read it that way if you’re dogmatic about the term “unit test”. She’s saying “communicating over the network is important, so I’m going to prioritize that communication as the subject (unit) of my tests.”

For all these terms it’s unlikely we’re ever going to setting on unambiguous definitions. Let’s just try to be clear about what we mean when we use them, and clear about what we’re testing with each test.

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