Testing simple GenServer that write (async) to ETS

Questions / Help
,
1

Hello,

I have a simple GenServer that have a few functions (write, delete and update records in ETS). All functions is async (GenServer.cast). Now I am trying to find a right way to testing that module.

In my simple test i have something like

setup_all do
  MyServer.save({"a", 1})
end

test "test save item" do
  item = MyServer.lookup("a")
  assert length(item) == 1
end

test "test delete item" do
  MyServer.delete("a")  
  delItem = MyServer.lookup("a")
  assert delItem == []
end

First (save test) works fine because setup_all executed before test execution start.
But with second test I have a problem. Due to async calls, delItem = MyServer.lookup(“a”) finished before record really deleted from ETS and test fails.

Any idea how to test similar functionality will be very helpful.

Thank you!

2

Not sure if this is best approach but I’ve added a synchronous method to gensever (called handle_call(:sync, ...) and I call it from test. Because elixir process does one thing at a time and messages are ordered, I know that if such sync call finishes, the async casts before had to run already.

Interested how others approach this!

2 Likes
3

Thanks a lot! Very interesting solution and I think that I can use it. Much better than Process.sleep :slight_smile:

4

You can also wrap that function in a

if Mix.env() == :test do
     def handle_call(....), do: ....
end

If you don’t need it in other envs.

Another option that sometimes might be needed is to just have an helper function that executes a given fun and loops until X time has passed or a condition is achieved.

def wait_until(fun, timeout \\ 500, time \\ 0) 

def wait_until(fun, timeout, time) when time > timeout, do: false
def wait_until(fun, timeout , time) do
  case fun.() do
     is_ok when is_ok in [true, :ok] or is_tuple(is_ok) and elem(0, is_ok) == :ok -> true
     _ -> 
      Process.sleep(1) 
      wait_until(fun, timeout, time + 1)
   end
end

And then use it as:


assert Support.wait_until(
  fn() -> 
     case MyServer.lookup("a") do
        [] -> :ok
        _ -> false
     end
  end,
  200
)
1 Like
5

FWIW, you can use system messages for this too - for instance, :sys.get_state() which all GenServers support.

1 Like