why is the socket closed Erlang - sockets

Question

I'm new in Erlang.
My Problem is, that when I start the client for the 1st time everything seems okay, I get the sorted list: <<1,5,72,97,108,108,111>>.
But by the 2nd time it won't receive the sorted list, because I think the socket is closed. The output from the Client is "Connection closed".
Here is my code:
Client
-module(client).
-export([client/0]).
client() ->
case gen_tcp:connect("localhost", 6000, [{mode, binary}]) of
{ok, Sock} ->
Data = [1, "Hallo", 5],
gen_tcp:send(Sock, Data),
receive
{tcp, _, Bin} ->
io:fwrite("Received sorted list from server: ~w~n", [Bin]);
{tcp_closed, _} ->
io:fwrite("Connection closed"),
gen_tcp:close(Sock)
end;
{error,_} ->
io:fwrite("Connection error! Quitting...~n")
end.
Server
-module(server).
-export([server/0]).
-import(mergeSort,[do_recv/1]).
%creates a tcp socket on Port 6000
server() ->
{ok, Listen} = gen_tcp:listen(6000, [{keepalive, true}, %send keepalive packets
{reuseaddr, true}, %reuse address
{active, once}, %socket is active once
{mode, list}]), %binary traffic
spawn(fun() -> parallel_connection(Listen) end).
%server is listening
%accepts the connection
%starts MergeSort
parallel_connection(Listen) ->
io:fwrite("Listening connections..~n"),
{ok, Socket} = gen_tcp:accept(Listen),
io:fwrite("Connection accepted from ~w~n", [Socket]),
spawn(fun() -> parallel_connection(Listen) end),
do_recv(Socket).
MergeSort
-module(mergeSort).
-export([do_recv/1]).
merge_sort(List) -> m(List, erlang:system_info(schedulers)).
%break condition
m([L],_) ->
[L];
%for more than one scheduler
m(L, N) when N > 1 ->
{L1,L2} = lists:split(length(L) div 2, L),
%self () returns Pid, make_ref() returns almost unique reference
{Parent, Ref} = {self(), make_ref()},
%starts a new process for each half of the list
%and sends Message to Parent
spawn(fun()-> Parent ! {l1, Ref, m(L1, N-2)} end),
spawn(fun()-> Parent ! {l2, Ref, m(L2, N-2)} end),
{L1R, L2R} = receive_results(Ref, undefined, undefined),
lists:merge(L1R, L2R);
m(L, _) ->
{L1,L2} = lists:split(length(L) div 2, L),
lists:merge(m(L1, 0), m(L2, 0)).
receive_results(Ref, L1, L2) ->
receive
{l1, Ref, L1R} when L2 == undefined -> receive_results(Ref, L1R, L2);
{l2, Ref, L2R} when L1 == undefined -> receive_results(Ref, L1, L2R);
{l1, Ref, L1R} -> {L1R, L2};
{l2, Ref, L2R} -> {L1, L2R}
after 5000 -> receive_results(Ref, L1, L2)
end.
do_recv(Socket) ->
%{ok, {Address, _}} = inet:peername(Socket),
receive
{tcp, Socket, List} ->
try
Data = merge_sort(List),
gen_tcp:send(Socket, Data),
io:fwrite("Sent sorted list to ~w | Job was done! Goodbye :)~n", [Socket]),
gen_tcp:close(Socket)
catch
_:_ ->
io:fwrite("Something went wrong with ~w | Worker terminated and connection closed!~n", [Socket]),
gen_tcp:close(Socket)
end;
{tcp_closed, _} ->
io:fwrite("Connection closed ~n");
{error, _} ->
io:fwrite("Connection error from ~w | Worker terminated and connection closed!~n", [Socket]),
gen_tcp:close(Socket)
end.
Can anyone help me?

Answer 1

When you call client:client/0, it creates a connection, sends its data, receives the response, then returns. Meanwhile, the server closes the socket. When you call client:client/0 again, it again creates a connection and sends data, but then it receives the tcp_closed message for the previous socket, and then it returns.
You can fix this by specifying the client socket in your receive patterns:
receive
{tcp, Sock, Bin} ->
io:fwrite("Received sorted list from server: ~w~n", [Bin]);
{tcp_closed, Sock} ->
io:fwrite("Connection closed"),
gen_tcp:close(Sock)
end;
In this code, the variable Sock replaces both the underscores in the original code, in the {tcp, _, Bin} and {tcp_closed, _} tuples. This forces the messages to match only for the specified socket.