dblink(Database Link)数据库链接顾名思义就是数据库的链接，就像电话线一样，是一个通道，当我们要跨本地数据库，访问另外一个数据库表中的数据时，本地数据库中就必须要创建远程数据库的dblink,通过dblink本地数据库可以像访问本地数据库一样访问远程数据库表中的数据。

当整合两个位于不同服务器上数据库的内容，就遇到了远程访问数据库的问题。在oracle中可以通过dblink来实现跨本地数据库来访问另外一个数据库中的数据。MySQL中的federated插件来实现类似的功能。

实现功能：

可以在Linux系统中MySQL数据库(target端)中建立宿主机MySQL数据库(source端)中某个表的link，当在Linux中读取link表时，就相当于直接读取宿主机中的原始表内容。

实现步骤：

1. 查看target端(Linux虚拟机中)是否安装了federated插件：

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         |

| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |

| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |

| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |

| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |

| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |

+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

显示没有安装federated插件

2. 安装federated插件：

mysql>install plugin federated soname 'ha_federated.so';

ERROR 1125 (HY000): Function 'federated' already exists

说明已经安装过了，但没有启用

[root@localhost etc]# service mysql stop

[root@localhost etc]# mysqld_safe --federated &

[root@localhost etc]# 140811 01:20:21 mysqld_safe Logging to '/var/lib/mysql/localhost.localdomain.err'.

140811 01:20:22 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

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         |

| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |

| FEDERATED          |YES     | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |

| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |

| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |

| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |

+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

federated插件已经启用

3. 配置/etc/my.conf，设置federated为默认启动

[root@localhost etc]# vi /etc/my.conf

在文件中加入一行：

federated

重启mysql服务

service mysql restart

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         |

| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |

| FEDERATED          | YES      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |

| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |

| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |

| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |

+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+

已经设置为默认启动了。

4. 在source端建立测试表，我是通过Navicat建立表的，其sql文件为：

DROP TABLE IF EXISTS `e_eledata`;

CREATE TABLE `e_eledata` (

`ID` bigint(20) unsigned NOT NULL auto_increment COMMENT 'ID',

`E_ELEMETERHEAD_ID` bigint(20) default NULL COMMENT '电表表头ID',

`DAQDT` timestamp NULL default NULL COMMENT '数据采集时间',

`DLDT` timestamp NULL default NULL COMMENT '数据入库时间',

`APCURRENT` decimal(10,3) default NULL COMMENT 'A相电流。单位：A。',

`BPCURRENT` decimal(10,3) default NULL COMMENT 'B相电流。单位：A。',

`CPCURRENT` decimal(10,3) default NULL COMMENT 'C相电流。单位：A。',

`APVOLTAGE` decimal(10,3) default NULL COMMENT 'A相电压。单位：V。',

`BPVOLTAGE` decimal(10,3) default NULL COMMENT 'B相电压。单位：V。',

`CPVOLTAGE` decimal(10,3) default NULL COMMENT 'C相电压。单位：V。',

PRIMARY KEY  (`ID`)

) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='电路数据表';

5. 在target端建立link表，可以直接改写source表的sql脚本文件为：

DROP TABLE IF EXISTS `e_eledata_link`;

CREATE TABLE `e_eledata_link` (

`ID` bigint(20) unsigned NOT NULL auto_increment COMMENT 'ID',

`E_ELEMETERHEAD_ID` bigint(20) default NULL COMMENT '电表表头ID',

`DAQDT` timestamp NULL default NULL COMMENT '数据采集时间',

`DLDT` timestamp NULL default NULL COMMENT '数据入库时间',

`APCURRENT` decimal(10,3) default NULL COMMENT 'A相电流。单位：A。',

`BPCURRENT` decimal(10,3) default NULL COMMENT 'B相电流。单位：A。',

`CPCURRENT` decimal(10,3) default NULL COMMENT 'C相电流。单位：A。',

`APVOLTAGE` decimal(10,3) default NULL COMMENT 'A相电压。单位：V。',

`BPVOLTAGE` decimal(10,3) default NULL COMMENT 'B相电压。单位：V。',

`CPVOLTAGE` decimal(10,3) default NULL COMMENT 'C相电压。单位：V。',

PRIMARY KEY  (`ID`)

) ENGINE=FEDERATED AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='电路数据表'

CONNECTION='mysql://usrname:password@192.168.1.98:3306/databasename/table  //宿主机的数据库

其中：

usrname为宿主机中MySQL的用户名

password为相应的密码

(要保证该用户具有远程登陆的权限，可以通过以下命令来设置：

mysql>GRANT ALL PRIVILEGES ON *.* TO 'usrname'@'%' IDENTIFIED BY 'password' WITH GRANT OPTION;

其中*.*是指对用户开放所有数据库和表的权限，如果只开放某一个数据库的一个表为databasename.table；'%'指的是该用户可以从任意的一个ip地址来远程访问数据库，包括本地，如果要限制用户从特定的ip来访问，将其改为'ip地址')

192.168.1.98是source数据库的ip，这里为我宿主机的ip

3306为数据库的端口，默认一般为3306

database 和table分别为source端数据库的名称和表名称

将该sql脚本在target端运行

6. 实现跨本地数据库的访问

在target端通过访问e_eledata_link表来访问source端e_eledata表

mysql> select *from e_eledata_link;