[bluGill]

What is a unit?

I have never seen a formal definition. Without that we cannot have any discussion.

To some a unit is a function. To some it is a module (generally someone else's module). To some it is an entire application. To some it is the entire computer in your embedded device. To some it is the entire device... Most people have no clue what we are talking about and don't car (also should not care).

[mrkeen]

My working definition is that a unit is something which can be unit-tested. By that I mean: the code being tested has the same properties that you expect of a good unit-test. I guess fast and deterministic are good enough properties for a working definition.

writeFile(fname,"Hello, World") is only one thing, but its behavior will depend on the state of the filesystem, so it's not a unit.

parseComplicatedObject(bytes) could be a unit (even if it calls out to many other sub-parsers - as long as they are also units).

One thing I see in a lot of companies is efforts to reduce test flakiness. Devs will attempt this work in src/test. But if the code is flaky, and you change the test from flaky to reliable, then you have just decreased the realism of your test. Like I mentioned with my comment above, you reduce test flakiness by doing the hard work in src/main. The src/test changes should be easy after that.