Nsryjdtyk

While trying to understand how to work with servers and hot code loading I stumbled over a problem which I have stripped down to the following code:

server.erl

-module(server).

-export([start/0, connect/1]).



start() ->

    {ok, Listen} = gen_tcp:listen(8080, [binary, {packet, raw}, {active, true}]),

    spawn(?MODULE, connect, [Listen]).



connect(Listen) ->

    {ok, Socket} = gen_tcp:accept(Listen),

    spawn(?MODULE, connect, [Listen]),

    loop(Socket).



loop(Socket) ->

    receive

        {tcp, Socket, Data} ->

            io:format("1st version received ~p~n", [Data]),

            loop(Socket);

        {tcp_closed, Socket} ->

            io:format("socket closed~n")

    end.

client.erl

-module(client).

-export([request/0]).



request() ->

    {ok, Socket} = gen_tcp:connect("localhost", 8080, [{packet, raw}, binary]),

    gen_tcp:send(Socket, <<"Hello">>).

Starting the server and sending a request creates the expected output.

1> server:start().

<0.62.0>

2> client:request().

ok

1st version received <<"Hello">>

After changing the format statement to "2nd version", compiling and loading the code and executing two requests (because the connect/1 process currently waiting for connections was spawned before the change) the result is still as expected.

3> c(server).       

{ok,server}

4> client:request().

ok

1st version received <<"Hello">>

5> client:request().

ok

2nd version received <<"Hello">>

However, after compiling and loading the code twice in a row, there is no output printed in the terminal any more, although the server is obviously still running, since gen_tcp:connect returns a socket.

6> c(server).

{ok,server}

7> c(server).

{ok,server}

8> client:request().

ok

I suspect that this behaviour has something to do with erlang killing all processes with code older than two versions but I fail to really understand what is happening here.
Since this is educational I am more interested in knowing why this exact code doesn't work rather than an actual solution to the problem.

Thanks

I am pretty certain this is the two-versions limit.

If you want to confirm that, replace the calls from spawn(...) to spawn_link(...) -- if the processes die, your shell will crash as well and you'll know they have been killed.

Another way to test it is whether you can replace the following:

6> c(server).

{ok,server}

7> c(server).

{ok,server}

8> client:request().

ok

By:

6> c(server).

{ok,server}

7> client:request().

ok

8> c(server).

{ok,server}

9> client:request().

ok

If this works fine, the difference is the message in the middle that allows the code to update to a newer code version of the fully-qualified function call (Module:Fun(Args)), preventing the crash.