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mysqlerlangmariadberlang-shell

Erlang failing to connect to mysql/mariadb

I cannot connect to mysql/mariadb with erlang.

The requirements according to this github-page: https://github.com/mysql-otp/mysql-otp

Requirements:

Erlang/OTP version R16B or later
MySQL database version 4.1 or later or MariaDB
GNU Make or Rebar or any other tool for building Erlang/OTP applications

Version:

14> erlang:system_info(otp_release).
"22"

I'm not sure if this requirement is needed anymore but I added this:

[mysqld] default_authentication_plugin=mysql_native_password

to my /etc/my.cnf. But this is probably irrelevant since the error is an undefined function. I can compile the code but I cannot run it. Any help to get this working is much appreciated.

The code:

-module(mydatabase).
-compile(export_all).

connect_to_database() -> 
Conn = mysql:start_link([{host, "localhost"}, {user, "user"},
                       {password, "password"}, {database, "MyDatabase"}]) ,
           
  case Conn of
      {ok, Pid} -> io:fwrite("~w~w~n", [ok,Pid]);
      {error, ConnErr}  -> io:fwrite("error : ~p ~n", [ConnErr])
    end.
              
 start() -> connect_to_database().

mariadb is running:

sudo systemctl status mariadb
[sudo] password for user: 
● mariadb.service - MariaDB 10.4.13 database server
   Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor >
   Active: active (running) since Sun 2020-06-28 15:33:50 CEST; 1h 4min ago

The error message:

12> c(mydatabase).     
mydatabase.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,mydatabase}
13> mydatabase:start().
** exception error: undefined function mysql:start_link/1
 in function  mydatabase:connect_to_database/0 (mydatabase.erl, line 1
Solution

  • You forgot about this requirement:

    GNU Make or Rebar or any other tool for building Erlang/OTP applications

    According to the mysql-otp docs:

    MySQL/OTP is a driver for connecting Erlang/OTP applications to MySQL and MariaDB databases.

    An OTP application requires a certain architecture, and the mysql driver needs to be listed as a dependency in the application. Your error is due to the fact that there is no function named mysql:start_link/1 in Erlang. Rather, that's a third party function that your code has to somehow access, hence the Usage as a Dependency section in the docs.

    The following steps can be used to create an OTP application which employs mysql/mariaDB as a dependency:

    1. Install rebar3.

    2. Create a rebar3 app:

      ~/erlang_programs/mysql1$ rebar3 new app myapp
===> Writing myapp/src/myapp_app.erl
===> Writing myapp/src/myapp_sup.erl
===> Writing myapp/src/myapp.app.src
===> Writing myapp/rebar.config
===> Writing myapp/.gitignore
===> Writing myapp/LICENSE
===> Writing myapp/README.md

    3. Add mysql-otp as a dependency in the rebar.config file:

        ~/erlang_programs/mysql1$ cd myapp
  ~/erlang_programs/mysql1/myapp$ ls
  LICENSE       rebar.config
  README.md src

    like this:

        {erl_opts, [debug_info]}.
    {deps, [

      {mysql, ".*", {
                      git, "https://github.com/mysql-otp/mysql-otp",
                      {tag, "1.6.0"}
                    }
      }

    ]}.

    {shell, [
      % {config, "config/sys.config"},
        {apps, [myapp]}
    ]}.

    1. Put your source code in the src directory:

       ~/erlang_programs/mysql1/myapp$ cd src
 ~/erlang_programs/mysql1/myapp/src$ ls
 my_mysql.erl    myapp_app.erl
 myapp.app.src   myapp_sup.erl

    my_mysql.erl:

    -module(my_mysql).
-compile(export_all).

do_mysql(Name, Info) ->
    {ok, MysqlPid} = mysql:start_link(
                  [{host, "localhost"}, 
                   {user, "root"},
                   {password, ""}, 
                   {database, "mydb"}
                  ]
                ),

     ok = mysql:query(
           MysqlPid, 
           "INSERT INTO people (name, info) VALUES (?, ?)", [Name, Info]
        ),

     {ok, ColumnNames, Rows} = mysql:query(
                MysqlPid, 
                <<"SELECT * FROM people">>),

     io:format("ColumnNames: ~p~nRows: ~p~n", [ColumnNames, Rows]).

    1. Fetch the dependencies and compile all the source code:

       ~/erlang_programs/mysql1/myapp$ rebar3 compile
 ===> Verifying dependencies...
 ===> Fetching mysql ({git,"https://github.com/mysql-otp/mysql-otp",
                                  {tag,"1.6.0"}})
 ===> Compiling mysql
 ===> Compiling myapp
 src/my_mysql.erl:2: Warning: export_all flag enabled - all functions will be exported

    2. Launch the shell and automatically construct paths to all your .beam files (in the deeply nested directories that rebar3 puts them):

       ~/erlang_programs/mysql1/myapp$ rebar3 shell
 ===> Verifying dependencies...
 ===> Compiling myapp
 Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [kernel-poll:false]

 ===> The rebar3 shell is a development tool; to deploy applications in production, consider using releases (http://www.rebar3.org/docs/releases)
 ===> Booted myapp
 Eshell V9.3  (abort with ^G)

    3. Execute your function:

       1> my_mysql:do_mysql("Jeffsy", "2.0").
 ColumnNames: [<<"id">>,<<"name">>,<<"info">>]
 Rows: [[1,<<"7stud">>,<<"abc">>],
        [2,<<"Beth">>,<<"xxx">>],
        [3,<<"Diane">>,<<"xyz">>],
        [4,<<"Kathy">>,<<"xyz">>],
        [5,<<"Kathy">>,<<"xyz">>],
        [6,<<"Dave">>,<<"efg">>],
        [7,<<"Tom">>,<<"zzz">>],
        [8,<<"David">>,<<"abc">>],
        [9,<<"Eloise">>,<<"abc">>],
        [10,<<"Jess">>,<<"xyz">>],
        [11,<<"Jeffsy">>,<<"2.0">>]]
 ok