Ups, it’s probably catch that you need.
amnu3387
benonymus
That doesn’t work either 
I tried to exact same with catch instead of rescue and the result is the same error:
** (exit) exited in: GenServer.call(Checksum.Checksum, :calculate, 15)
** (EXIT) time out
0
lud
You need to catch the exit properly: https://elixir-lang.org/getting-started/try-catch-and-rescue.html#exits
4
benonymus
lud
Now the problem is that your calling process will still receive a message with the response, because the GenServer will send it, as it will have received the call.
So what you can do is wrap your call in a task:
def calculate_checksum() do
Task.async(fn ->
try do
GenServer.call(__MODULE__, :calculate, 15)
catch
:exit, {:timeout, {GenServer, :call, _}} -> {:error, :timeout}
end
end)
|> Task.await()
end
Note that Task.await also has a timeout, and like GenServer.call/3 the default is 5000, You don’t care as you are targetting 15ms here. But you can set it to :infinity it that makes sense.
The task process is linked to the caller, so if you have an error/exit that is not the timeout, it will still propagate to your calling process, no risk of missing errors.
The task will return with the checksum or {:error, :timeout}. If there is a timeout, as I have said the GenServer will still send the response. But it will send it to the task process, which will by dead by then. And sending a message to a non-existent process is fine in Elixir. That is the way to safely ignore the response.
Finally, as you handle a short timeout and ignore the response, keep in mind that the GenServer will still have to calculate the checksum, it will still do the requested work to send the response which will be ignored.
1
benonymus
In practice it does not seem like a problem that the calling process receives the response later, because I already sent the response by the time the result would arrive.
Do you think it is still worth it to wrap it in a Task?
0
lud
I really depends. What do you mean by “I already sent the response”? What is the context of your calling process?
0
benonymus
I call the calculate_checksum from a with where I expect it to be {:ok, result}, and I have an else in the with where I send a different response if I got the timeout error.
Like this
with {:ok, checksum} <- Checksum.calculate_checksum(String.to_integer(params["timeout"])) do
conn
|> put_status(200)
|> put_view(ChecksumView)
|> render("checksum.json", checksum: checksum, message: "Checksum calculated successfully!")
else
_ ->
conn
|> put_status(408)
|> put_view(ChecksumView)
|> render("408.json")
end
0
lud
So the context is a HTTP request handler / Phoenix controller. Then I guess you don’t care, indeed, though I am not sure.
1
paulanthonywilson
The timeout thing is confusing; I can understand the mixup with the other kind of GenServer timeout..
This is an old post about calls timing out GenServer call time-outs
tl;dr maybe you might want is something along the lines of
try do
{:ok, checksum} <- Checksum.calculate_checksum(String.to_integer(params["timeout"]))
conn
|> put_status(200)
|> put_view(ChecksumView)
|> render("checksum.json", checksum: checksum, message: "Checksum calculated successfully!")
catch
:exit, _value ->
conn
|> put_status(408)
|> put_view(ChecksumView)
|> render("408.json")
end
Bear in mind that the GenServer process and on completion “the reply message will be sent to the client and if unhandled will clutter the [client] mailbox”.
edit missed Genserver.call with timeout doesn't end up with :timeout message - #16 by lud on scanning through the thread - sorry pretty much what I said.
1