Question

I’m writing a gen_server, which we’ll just call gen_server_db, with nothing too terribly special about it. There is a possibility that a library it uses (emysql) will experience a connection failure while trying to find a database server (in gen_server_db:init()). When I catch the exception and try to print something to the console, I get zip. Neither of the io:format messages, in the code sample below, ever find their way to the console. I seem to recall hearing a reason for this, long ago, but I can’t recall why.

init(Args) ->
    % I've condensed the way I actually get this stuff to one line, but if I have a
    % database online I connect properly, so I know that I'm getting the Host, User, etc.  
    {Host, User, Password, DB, PoolSize} = application:get_env(gen_server_db, config),

    init_mysql_connection(gen_server_db_pool, PoolSize, User, Password, Host, DB),

    % When the net connection to the db is down, I never get here.
    ok.

init_mysql_connection(bgo_db_pool, PoolSize, User, Password, Host, DB) ->
    try
        emysql:add_pool(bgo_db_pool, PoolSize, User, Password, Host, 3306, DB, utf8)
    catch
        exit:failed_to_connect_to_database ->
            io:format("Cannot connect to the mysql database server.  Retrying in 1 sec.~n"),
            timer:sleep(1000),
            init_mysql_connection(bgo_db_pool, PoolSize, User, Password, Host, DB);
        Error:Reason ->
            io:format("Database connection error: ~p:~p~n", [Error, Reason]),
            1/0
    end.

Answer 1

All I/O sent through io:format is sent to the current group_leader of the process being run. It is then this processes responsibility to print the message using io:fwrite.

If you just want to dump some debug info while coding you can use erlang:display/1