[erlang-questions] gen_server question

Howard Yeh hayeah@REDACTED
Wed Jan 14 01:58:47 CET 2009

I read the gen_server stuff recursively today.

i liked the middleman trick when multi_calling remote nodes.

very nice stuff. proc_lib and sys are cool too.

On Tue, Jan 13, 2009 at 6:05 AM, Hynek Vychodil
<vychodil.hynek@REDACTED> wrote:
> It's very well written. Anyway for one who can't found it in code, answer
> is: In case when server doesn't exists gen_server:call function exits with
> {noproc, {gen_server, call, [Name, Request, Timeout]}}.
> On Tue, Jan 13, 2009 at 2:44 PM, Christian <chsu79@REDACTED> wrote:
>> > correct me if i am wrong:
>> >
>> > messages are dropped if the receiving process is dead.
>> >
>> > with gen_server, if you use synchronized call, you can specify a timeout
>> > value
>> >
>> > gen_server:call(ServerRef, Request, Timeout)
>> >
>> > "The call may fail for several reasons, including timeout and the
>> > called gen_server dying before or during the call."
>> >
>> > I think you get either a badarg (pid doesn't exist (as it's dead)) or
>> > timeout (if the process died after message had been sent)
>> Every time I want to remind myself on how this works i look at the
>> do_call/4 function in
>> lib/stdlib/src/gen.erl, it is the one that implement the
>> gen_server:call currently.
>> (Disclaimer, there can of course be undocumented features in the code
>> that one should not rely on. But it is so much easier to understand
>> code than english that describes code. :)
>> What you learn is that gen_server:call will create a monitor on the
>> server for the duration of the call. So it will detect immediately if
>> the server has crashed, or if it crashes while it is waiting on a
>> reply. Another fun trivia is that the monitor reference is used as a
>> unique id to identify the correct request/response from the
>> gen_server.
>> Timeouts are really just relevant for gen_servers that are so bogged
>> down that they cant get around to process your request and reply to it
>> in time. (Also some code there for if its a node that doesnt support
>> monitoring of processes). The gen_server will not notice that the
>> caller have timed out, so it will process the request when it gets to
>> it, unless it has crashed for other reasons by then.
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> --
> --Hynek (Pichi) Vychodil
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