[erlang-questions] UDP concurrent server

[Bogdan Andu]

Hi Bengt,

Yes I am aware that udp sockets are working different than tcp sockets,
but I was wondering if there is an idiom for udp as it is for tcp to make
things concurrent.

Having in mind the limitations of udp it seems that the best option
is to implement the version with a controller process that receives udp
packets
and in spawning processes to actually handle that packet based some info,
and immediately
fetch the next message from message box.
My only concern here is the bottleneck. The message box can easily be
overloaded
and the response of the server exponentially delayed.
Is there any best practices to handle such situations avoiding message box
overloading
while handling say 1 million of concurrent (udp) connections ?

Thank you,

/Bogdan




On Wed, Dec 9, 2015 at 12:35 PM, Bengt Johansson E <
bengt.e.johansson@REDACTED> wrote:

> Hi Bogdan!
>
>
>
> Your questions make me a bit confused. I wonder **why** you want two
> processes waiting for packets from the same socket and **what** you
> expect to happen?
>
>
>
> Generally one usually only speaks about concurrent servers regarding TCP
> where you bind to a socket waiting for incoming connections and once a
> connection is established, you spawn a process (or thread depending on
> language) and process the data coming over the connection concurrently.
>
> Note that the main loop of the server that waits for responses is always
> sequential. The new process handling the connection gets a new socket with
> a free port used only for that particular connection.
>
>
>
> But! UDP lacks support for connections mainly since it is a message based
> protocol and hence is devoid of any connection abstraction J
>
>
>
> Still the question remains what is a concurrent UDP server? I guess what
> you want to achieve is some kind of distribution of incoming packets to
> several processes to handle them. In that case you should either go for the
> solution of TCP to set up new socket for each communication or write a
> simple process that classifies the incoming packets and distributes them to
> the correct process based on some information in the packet, fi. Some
> identifier you have chosen to identify the connection. Basically  you have
> to implement an application specific load balancer – or rather load
> distributor – hoping that the time taken to actually process the packets
> greatly overshadows the time it takes to distribute them. J
>
>
>
> Anyway, there is no way the underlying UDP stack and/or erts can make the
> decision for you. To which process to send the packet that is.
>
>
>
> Hope that helps!
>
> BR, BengtJ
>
>
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>
> *From:* erlang-questions-bounces@REDACTED [mailto:
> erlang-questions-bounces@REDACTED] *On Behalf Of *Bogdan Andu
> *Sent:* den 9 december 2015 10:15
> *To:* Erlang
> *Subject:* [erlang-questions] UDP concurrent server
>
>
>
> following the thread
> https://groups.google.com/forum/?hl=en#!topic/erlang-programming/6Q3cLtJdwIU
>
> as it seems that POSt to topic does not work
>
>
> After more tests the basic questions that remains ..
>
> Is there a way to have more than one process be blocked
> in gen_udp:recv/2 call as this seems to not be possible,
> probably because the way udp sockets work.
>
> Sequentially works as expected, but when when I try to spawn another
> process
> that makes and attempt to execute gen_udp:recv/2 while the first process
> already does
> gen_udp:recv/2 , the second process gives elready error. This means that 2
> process
> cannot concurrently do gen_udp:recv/2 .
>
> In scenario with socket {active, once} or {active, true} there is only one
> process that can
> receive the message from socket (the one that does gen_udp:open/2 ),
> and for multi-threaded applications this quickly can become a bottleneck.
> In this case, however, elready error disappears of course.
> .
> I tried both variants and both have disavantages.
>
> Is there an idiom for designing a udp concurrent server in Erlang?
> So far, it seems impossible.
>
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