MySQL下的DB Link

素颜马尾好姑娘i 2021-12-15 01:35 122阅读 0赞

**前言：**

在实际工作中，我们可能会遇到需要操作其他数据库实例的部分表，但又不想系统连接多库。此时我们就需要用到数据表映射。如同Oracle中的DBlink一般，使用过Oracle DBlink数据库链接的人都知道可以跨实例来进行数据查询，同样的，Mysql自带的FEDERATED引擎完美的帮我们解决了该问题。本篇文章介绍FEDERATED引擎的开启和使用。

#### 1.开启FEDERATED引擎 ####

若需要创建FEDERATED引擎表，则目标端实例要开启FEDERATED引擎。从MySQL5.5开始FEDERATED引擎默认安装 只是没有启用，进入命令行输入`show engines;` FEDERATED行状态为NO。

mysql> show engines;
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    | Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    | PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
    | MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
    | CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
    | BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
    | MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
    | InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
    | ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
    | MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
    | FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    9 rows in set (0.00 sec)

在配置文件\[mysqld\]中加入一行：federated，然后重启数据库，FEDERATED引擎就开启了。

mysql> show engines;
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    | Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    | CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
    | MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
    | MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
    | BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
    | PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
    | MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
    | ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
    | InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
    | FEDERATED          | YES     | Federated MySQL storage engine                                 | NO           | NO   | NO         |
    +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
    9 rows in set (0.00 sec)

#### 2.使用CONNECTION创建FEDERATED表 ####

使用CONNECTION创建FEDERATED引擎表通用模型：

CREATE TABLE (......) 
    ENGINE =FEDERATED CONNECTION='mysql://username:password@hostname:port/database/tablename'

简单创建测试：

# 源端表结构及数据
    mysql> show create table test_table\G
    *************************** 1. row ***************************
           Table: test_table
    Create Table: CREATE TABLE `test_table` (
      `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主键',
      `stu_id` int(11) NOT NULL COMMENT '学号',
      `stu_name` varchar(20) DEFAULT NULL COMMENT '学生姓名',
      `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
      `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
      PRIMARY KEY (`increment_id`),
      UNIQUE KEY `uk_stu_id` (`stu_id`)
    ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 COMMENT='学生表'
    1 row in set (0.00 sec)
    
    mysql> select * from test_table;
    +--------------+--------+----------+---------------------+---------------------+
    | increment_id | stu_id | stu_name | create_time         | update_time         |
    +--------------+--------+----------+---------------------+---------------------+
    |            1 |   1001 | wang     | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            2 |   1002 | dfsfd    | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            3 |   1003 | fdgfg    | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            4 |   1004 | sdfsdf   | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            5 |   1005 | dsfsdg   | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            6 |   1006 | fgd      | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    +--------------+--------+----------+---------------------+---------------------+
    6 rows in set (0.00 sec)
    
    # 目标端建表及查询
    # 注意ENGINE=FEDERATED CONNECTION后为源端地址 避免使用带@的密码
    mysql> CREATE TABLE `test_table` (
        ->   `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主键',
        ->   `stu_id` int(11) NOT NULL COMMENT '学号',
        ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '学生姓名',
        ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
        ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
        ->   PRIMARY KEY (`increment_id`),
        ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
        -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='学生表' CONNECTION='mysql://root:root@10.50.60.212:3306/source/test_table';
    Query OK, 0 rows affected (0.01 sec)
    
    mysql> select * from test_table;
    +--------------+--------+----------+---------------------+---------------------+
    | increment_id | stu_id | stu_name | create_time         | update_time         |
    +--------------+--------+----------+---------------------+---------------------+
    |            1 |   1001 | wang     | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            2 |   1002 | dfsfd    | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            3 |   1003 | fdgfg    | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            4 |   1004 | sdfsdf   | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            5 |   1005 | dsfsdg   | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    |            6 |   1006 | fgd      | 2019-06-21 10:52:03 | 2019-06-21 10:52:03 |
    +--------------+--------+----------+---------------------+---------------------+
    6 rows in set (0.00 sec)

#### 3.使用CREATE SERVER创建FEDERATED表 ####

如果要在同一服务器上创建多个FEDERATED表，或者想简化创建FEDERATED表的过程，则可以使用该CREATE SERVER语句定义服务器连接参数，这样多个表可以使用同一个server。

CREATE SERVER创建的格式是：

CREATE SERVER fedlink
    FOREIGN DATA WRAPPER mysql
    OPTIONS (USER 'fed_user', PASSWORD '123456', HOST 'remote_host', PORT 3306, DATABASE 'federated');

之后创建FEDERATED表可采用如下格式：

CREATE TABLE (......) 
    ENGINE =FEDERATED CONNECTION='test_link/tablename'

示例演示：

# 目标端创建指向源端的server
    mysql> CREATE SERVER test_link 
        ->   FOREIGN DATA WRAPPER mysql 
        ->   OPTIONS (USER 'root', PASSWORD 'root',HOST '10.50.60.212',PORT 3306,DATABASE 'source');
    Query OK, 1 row affected (0.00 sec)
    
    mysql> select * from mysql.servers\G
    *************************** 1. row ***************************
    Server_name: test_link
           Host: 10.50.60.212
             Db: source
       Username: root
       Password: root
           Port: 3306
         Socket: 
        Wrapper: mysql
          Owner: 
    1 row in set (0.00 sec)
    
    # 目标端创建FEDERATED表
    mysql> CREATE TABLE `s1` (
        ->   `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主键',
        ->   `stu_id` int(11) NOT NULL COMMENT '学号',
        ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '学生姓名',
        ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
        ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
        ->   PRIMARY KEY (`increment_id`),
        ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
        -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='学生表' CONNECTION='test_link/s1';
    Query OK, 0 rows affected (0.01 sec)
    
    mysql> CREATE TABLE `s2` (
        ->   `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主键',
        ->   `stu_id` int(11) NOT NULL COMMENT '学号',
        ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '学生姓名',
        ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
        ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
        ->   PRIMARY KEY (`increment_id`),
        ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
        -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='学生表' CONNECTION='test_link/s2';
    Query OK, 0 rows affected (0.01 sec)

#### 4.FEDERATED使用总结 ####

基于MySQL5.7.23版本，笔者在源端及目标端实验了多种DDL及DML，现简单总结如下，有兴趣的同学可以试试看。

*  目标端建表结构可以与源端不一样 推荐与源端结构一致
 *  源端DDL语句更改表结构 目标端不会变化
 *  源端DML语句目标端查询会同步
 *  源端drop表 目标端结构还在但无法查询
 *  目标端不能执行DDL语句
 *  目标端执行DML语句 源端数据也会变化
 *  目标端truncate表 源端表数据也会被清空
 *  目标端drop表对源端无影响

#### 5.FEDERATED引擎最佳实践 ####

目前FEDERATED引擎使用范围还不多，若确实有跨实例访问的需求，建议做好规范，个人总结最佳实践如下：

1.  源端专门创建只读权限的用户来供目标端使用。
2.  目标端建议用CREATE SERVER方式创建FEDERATED表。
3.  FEDERATED表不宜太多，迁移时要特别注意。
4.  目标端应该只做查询使用，禁止在目标端更改FEDERATED表。
5.  建议目标端表名及结构和源端保持一致。
6.  源端表结构变更后 目标端要及时删除重建。

> 参考：  
> [https://dev.mysql.com/doc/refman/5.7/en/federated-create.html][https_dev.mysql.com_doc_refman_5.7_en_federated-create.html]  
> [https://dev.mysql.com/doc/refman/5.7/en/create-server.html][https_dev.mysql.com_doc_refman_5.7_en_create-server.html]

![公众号.jpg][.jpg]

[https_dev.mysql.com_doc_refman_5.7_en_federated-create.html]: https://dev.mysql.com/doc/refman/5.7/en/federated-create.html
[https_dev.mysql.com_doc_refman_5.7_en_create-server.html]: https://dev.mysql.com/doc/refman/5.7/en/create-server.html
[.jpg]: /images/20211214/31898f0b28e44ad4963323a66dc46ece.png