Erlang failing to connect to mysql/mariadb - mysql

Question

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

Answer 1

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:
Install rebar3.
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
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]}
]}.
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]).
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
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)
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