Best strategy to test GenServers which use timers (sends_after)

Instead of sleep/assert in the test monitor the supervisor of the unit and be notified when it stops. When the supervisor stoped everything should also be gone if properly linked, but you could also check that afterwards.

That is a good idea. In general, in can just check the root DynamicSupervisor for all its children to check if the Supervisor specific to the unit is gone. That would be step 5 of my provided example.

So, I should Monitor the unit Supervisor and use assert_receive instead of Process.sleep(), right? Is assert_receive blocking the process as well? If so, there is not a huge difference to sleep/assert and it exposes a bit more of the underlaying mechanics (since I assert for the Supervisor to shutdown instead of asking the system for the list of running units).

From the implementation I’d say it’s way different from the fact that you need to wait it’s not. But given that you want to test a timeout I’m not sure how you want to prevent waiting for things to happen. Also the monitor will send the test a message as soon as the supervisor exists, so no need to guess a sleep() duration and no need for sleeping longer than needed.

Ok, that’s a good point. I will go that route then.

I have been in this situation as well, and what I did was to:

• Allow timers to be configured in some way (like e.g. using Specify. This was actually one of the reasons why I created that library).
• In my tests, reduce the timeouts from the hours, minutes or seconds range to a couple of milliseconds
  • Keep in mind that the timeout should be long enough such that race conditions should not occur. In essence, it should be an order of magnitude larger than the time it takes to do the actual non-timeout work. But making it more than this one order of magnitude means that your tests will be slower than necessary.
• Now in your tests, let your test listener sleep for a duration slightly longer than the configured timeout (or e.g. use assert_receive), and it will be able to properly test that the GenServer(s) time out as expected.