Race condition when mocking concurrent API calls

Questions / Help
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1

Hello, I’m trying to test some code that fires off multiple external API calls in parallel using Task.async_stream(). I’m using Mox to mock these external api calls like you see below. They work 95% of the time, but sometimes the second item’s api call fires off first, which results in a failed test. It seems that Mox cares about the order it receives its calls. Is there a way around this?

code:

Task.async_stream(employee_ids, fn emp_id -> EmployeeClient.get(emp_id) end)

Mocks:

expect(Mock, :get, fn "/employees/1" ->
  {:ok,
   %HTTPoison.Response{
    status_code: 200
   }}
end)

expect(Mock, :get, fn "/employees/2" ->
  {:ok,
   %HTTPoison.Response{
    status_code: 200
   }}
end)

Error msg:

** (FunctionClauseError) no function clause matching in anonymous fn/1 in ----edited out file names----: anonymous fn("/employees/2") in ----edited out file names-----

Thanks!

2

Are you sure that you can mock it like that? Because I am pretty sure that the second call will override the first one, so call for Mock.get("/employees/1") will obviously fail as there is no clause matching.

3

Right as @hauleth notes, those definitions are clobbering each other. If you want to pattern match at the function level, do so at the function level:

expect(Mock, :get, fn
"/employees/1" ->
  {:ok,
   %HTTPoison.Response{
    status_code: 200
   }}

"/employees/2" ->
  {:ok,
   %HTTPoison.Response{
    status_code: 200
   }}
end)
3 Likes
4

Thanks @benwilson512. That is much cleaner than what I’ve been working with so I’m excited to refactor. I’m going to have to refactor a number of tests and run the suite 50+ times to verify for sure, but I think this solves my intermittent failure problem too. I’ll mark your answer as the solution as soon as I know.

5

I believe you can because I have 3 different routes mocked this way, and the others work without overwriting and pass 100% of the time. Its only this route that’s expecting concurrent calls that fails and its only failing about 5% of the time