[erlang-questions] UDP socket - ealready ERROR

[Guilherme Andrade]

Hi,

On 08/12/15 11:18, Bogdan Andu wrote:
> [...]
>
>     MpsConn = {mps_conn_fsm,{mps_conn_fsm, start_link, [Sock,
> SslRecvTimeout, false]},
>             temporary, 5000, worker, [mps_conn_fsm]},
>    
>     {ok, {{simple_one_for_one, 0, 1}, [MpsConn]}}.
[...]
>
> mps_dbg@REDACTED <mailto:mps_dbg@REDACTED>)1>
> (mps_dbg@REDACTED <mailto:mps_dbg@REDACTED>)1>
> mps_conn_sup:start_child().
> {ok,<0.62.0>}
> (mps_dbg@REDACTED <mailto:mps_dbg@REDACTED>)2>
> mps_conn_sup:start_child().
> {ok,<0.64.0>}

Here is the culprit: you're binding the socket only *once* in the
supervisor[1], which will be its controlling process, and then launching
two workers which will try both to read from the same socket (which they
cannot do because they don't control it) and then close it (which, if
execution were to even reach that point, wouldn't also be what I imagine
you most likely intend because you would end up closing the same socket
twice.)

One solution is to remove the socket from the child spec and move the
socket binding to the worker code.

In any case, if it were me, I would first try to have a single binding
process which receives the datagrams and launches workers, and avoid
overload by combining the {active, N}[2] flow with whichever
backpressure metric your system would signal; there's no particular
advantage to having multiple bindings over the same port - you won't
really end up processing multiple flows at once as if they were multiple
gen_tcp accepted sockets.

On a final note, I would also advise against executing potentially
blocking operations on OTP processes' init/1 and make it asynchronous
instead (e.g. by casting an initialisation request to itself) or you'll
risk choking up the supervision tree.


[1]: http://www.erlang.org/doc/man/supervisor.html#id242517
[2]: http://www.erlang.org/doc/man/inet.html#setopts-2

-- 
Guilherme

http://www.gandrade.net/
PGP: 0x602B2AD8 / B348 C976 CCE1 A02A 017E 4649 7A6E B621 602B 2AD8

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