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