minhajuddin
Catching stray messages which may fill up your message inbox is considered a good practice in erlang. Do we need to catch them even in a GenServer?
defmodule Ticker do
use GenServer
def handle_info(:tick, state) do
schedule_tick(state)# this makes sure that we catch up if a work is delayed
work(state)
{:noreply, state}
end
# to catch stray messages?
require Logger
def handle_info(oops, state) do
Logger.warn("UNEXPECTED_EVENT_IN_TICKER: #{inspect oops}")
{:noreply, state}
end
end
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minhajuddin
I just looked at the code elixir/lib/elixir/lib/gen_server.ex at main · elixir-lang/elixir · GitHub and there is a default implementation which should catch all messages, However as soon as we define a
handle_infothe default function should be overridden. So, I guess this is needed to catch stray messages.hubertlepicki
Yes you should define it. And your hunch is good, for exactly the reason you mention.
Elixir’s intro docs on GenServer make it clear in this section:
swelham
@minhajuddin yes you’re right. If you override
handle_infoyou will need to have a catch all implementation as well, otherwise as you stated the messages will not be removed from the inbox.Qqwy
I just looked through the Elixir documentation on GenServer.handle_info, and it is not mentioned there yet. I think this would be a great clarification. I’ll open a Pull Request
.
Qqwy
I made a PR and @josevalim was very quick to reply:
So it seems that we arrived at an incorrect conclusion before.
swelham
Thats interesting, I didn’t realise that. Has this always been the case do you know? I based my answer from what I read in Elixir in Action and wonder if it’s just where the book as fallen out of date.
edit: actually scrap that. I just flicked back through the book to double check. It appears my memory is awful
it clearly states the gen server would crash if you don’t supply the match all override
minhajuddin
Thanks for verifying it
hubertlepicki
I do think the conclusion was correct (for the most part). You probably don’t want your GenServers to crash at random.
The fact that it will crash sooner than later is other story. But the result is the same, crashing and unstable GenServer. You most likely want to do catch all.
Qqwy
I would not consider a crash when an unexpected message is received a ‘random’ crash (in contrast to an ‘inbox is full’ crash): It is a fail-fast indication that you are passing messages around that cannot be understood by the receiver. As José astutely pointed out, this is useful in certain situations.
Then again, most messages we use with GenServers are of course cast/calls, which do not use handle_info at all.
gregvaughn
If you can’t explain, for your particular application, why you want to crash (and restart into a known state) when an unexpected message is received, then IMHO, you should have that catch all
handle_infofunction that at least logs what it got. When I was first learning OTP, I got confused at messages I did and did not receive until it all clicked, and it’s only made worse if the GenServer is restarting in the middle of it all.I think you should have the catch all
handle_infountil you can explain why you do not want it.