Erlang 无法连接到 mysql/mariadb

Question

我无法使用 erlang 连接到 mysql/mariadb。

根据这个 github-page 的要求： https://github.com/mysql-otp/mysql-otp

要求：

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

版本：

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

我不确定是否需要这个要求，但我添加了这个：

[mysqld] default_authentication_plugin=mysql_native_password

到我的 /etc/my.cnf。但这可能无关紧要，因为错误是未定义的函数。 我可以编译代码，但我无法运行它。非常感谢您为这项工作提供任何帮助。

代码：

-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 正在运行：

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

错误信息：

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

你忘记了这个要求：

GNU Make 或 Rebar 或任何其他用于构建 Erlang/OTP 应用程序的工具

根据mysql-otp docs：

MySQL/OTP 是将 Erlang/OTP 应用程序连接到 MySQL 的驱动程序 和 MariaDB 数据库。

OTP application需要一定的架构，mysql驱动需要在应用中列为依赖。您的错误是由于 Erlang 中没有名为 mysql:start_link/1 的函数。相反，这是您的代码必须以某种方式访问​​的第三方函数，因此文档中的 Usage as a Dependency 部分。

以下步骤可用于创建使用 mysql/mariaDB 作为依赖项的OTP application：

1. Install rebar3.

2. 创建钢筋3 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. 在 rebar.config 文件中添加 mysql-otp 作为依赖项：

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

像这样：

    {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]}
    ]}.

4. 把你的源代码放在src目录下：

    ~/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]).

5. 获取依赖并编译所有源代码：

    ~/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
   

6. 启动 shell 并自动构建所有 .beam 文件的路径（在 rebar3 放置它们的深层嵌套目录中）：

    ~/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)
   

7. 执行你的函数：

    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