[erlang-questions] TCP flow control problem

Alain O'Dea <>
Mon Jan 31 18:27:20 CET 2011


Bengt is right.  I suggest using a gen_server to collect the data and look for message boundaries.  As it collects data it would prepend it to an iodata() in its own State structure.  When it encounters a message boundary it would cast the data in its State plus the data up to and including the message boundary onto a delegate gen_server for processing.  The data in the packet after the message boundary would then become the new partial message data in it's State.

Below is pseudo-code for this:

-record(state, {partial_message :: iodata(), delegate :: pid()}).

handle_info({tcp, {data, Data}}, _From, #state{partial_message = Partial} = State) ->
    NextPartial = case boundary_split(Data) of
        {MsgEnd, NextMsg} ->
            tcp_message_handler:handle_message(State#state.delegate, [MsgEnd | Partial]),
            NextMsg;
        Data ->
            [Data | Partial]
    end,
    {noreply, State#state{partial_message = NextPartial}.


On 2011-01-31, at 4:19, Bengt Kleberg <> wrote:

> Greetings,
> 
> TCP is a stream protocol. It does not preserve message boundaries. You
> have to handle that yourself. Sleeping one second is one way. When you
> do not sleep, how do you separate the messages?
> 
> 
> bengt
> 
> On Sun, 2011-01-30 at 23:38 +0100, Nav wrote:
>> I have implemented two TCP servers using gen_fsm, and gen_server, respectively.
>> I am using {active, once} option, after getting each message.
>> 
>> When I try to send data from one side to another in a loop, the data
>> gets corrupted (first 1 or 2 message arrive ok, but following gets
>> corrupted) when it arrives at other side.
>> 
>> However, if I put a timer:sleep(1000) before sending each message,
>> then everything seems to go fast.
>> 
>> What seems to go wrong by sending data too fast? Do I need to set some
>> more socket options?
>> 
>> Thanks, Nav
>> 
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