Unit test execution speed (how many tests per second?)

Question

What kind of execution rate do you aim for with your unit tests (# test per second)? How long is too long for an individual unit test?

I\'d be interested in knowing if

Answer 1

All unit tests should run in under a second (that is all unit tests combined should run in 1 second). Now I'm sure this has practical limits, but I've had a project with a 1000 tests that run this fast on a laptop. You'll really want this speed so your developers don't dread refactoring some core part of the model (i.e., Lemme go get some coffee while I run these tests...10 minutes later he comes back).

This requirement also forces you to design your application correctly. It means that your domain model is pure and contains zero references to any type of persistance (File I/O, Database, etc). Unit tests are all about testing those business relatonships.

Now that doesn't mean you ignore testing your database or persistence. But these issues are now isolated behind repositories that can be separately tested with integration tests that is located in a separate project. You run your unit tests constantly when writing domain code and then run your integration tests once on check in.