D4no0
Reliable way to test Process.send_after/4
I have a few genservers that need to fetch some data from an external resource every 15 minutes. The solution I’m using is just plain Process.send_after/4.
The problem is when it comes to testing.
The way I did it in the past was to make the interval configurable and set it to something like 100ms and check for updates with Process.sleep/1 in tests. This works, however this seems more like a hack to me and slows down tests considerably if you have a lot of tests like this one, not to mention that it makes tests flaky in case the genserver is doing a lot of work.
Is there a better way to do this kind of testing?
Marked As Solved
mudasobwa
I use mox for that.
defmodule Recurrent do
@callback send_after(pid(), any(), non_neg_integer()) :: :ok
end
defmodule Recurrent.Impl do
@behaviour Recurrent
@impl Recurrent
def send_after(pid, message, milliseconds),
do: Process.send_after(pid, message, milliseconds)
end
That way you already have a single place to do whatever else while sending a message, like telemetrying it, or writing a log, or something. The testing becomes as easy as defining a mock for it.
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LostKobrakai
Testing things that happen in isolation within an external process can either be polled (sleep) or you make the process inform the test (it sending a message when it did the thing). Your option here really is the latter.
I lately have sometimes gone the way of doing the latter through telemetry. I add a telemetry emit to the thing I’m interested in and in the test add a handler, which sends a message to the test process. I like this one for it being very not a home grown system, not specific to the tested process and also generally being a useful guidance on where telemetry might be useful given I also want to test it.
ausimian
I don’t think your ‘configurable timer’ approach is too bad - why not set it to 1ms?
Other options include calling the genserver callback functions directly from your test process (you have to construct the state parameter yourself), but it rather depends on exactly what you want your tests to demonstrate.
mudasobwa
In finitomata I mock my own internals to provide a fancy testing framework for the library via Finitomata. ExUnit. In a nutshell, it implements a listener which sends a message back to the test process, making it possible to simply assert_receive/2 in it.
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