CODEWITHSUNDEEP
elixir
bschaeffer
bschaeffer

Reputation: 2904

Elixir Processes Monitoring: :EXIT vs :DOWN

I usually see examples of process monitoring where the code for handling a monitored process exit is the following:

handle_info({:DOWN, ref, :process, pid}, state)

But I have also seen examples where they are matching an :EXIT instead of a :DOWN message.

So far, I have only been able to trigger :DOWN messages in my own examples, which includes standard Process.exit and GenServer.stop messages, as well as raising an exception in the monitored process.

When will I receive an :EXIT message?

Upvotes: 3

Views: 3209

Answers (1)

Dogbert
Dogbert

Reputation: 222128

:EXIT is sent to the process that another process tried to exit using Process.exit (with a reason other than :kill) but the process was trapping exits. :DOWN is sent to a process that is monitoring another process and the monitored process exits for any reason.

Here's an example of both:

pid = spawn(fn ->
  Process.flag(:trap_exit, true)
  receive do
    x -> IO.inspect {:child, x}
  end
end)
Process.monitor(pid)
Process.sleep(500)
Process.exit(pid, :normal)
Process.sleep(500)
# A process cannot trap `:kill`; it _has_ to exit.
Process.exit(pid, :kill)
receive do
  x -> IO.inspect {:parent, x}
end

Output:

{:child, {:EXIT, #PID<0.70.0>, :normal}}
{:parent, {:DOWN, #Reference<0.0.8.223>, :process, #PID<0.73.0>, :normal}}

Upvotes: 15

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