MySQL下的DB Link-有解無憂

前言：

在實際作業中，我們可能會遇到需要操作其他資料庫實體的部分表，但又不想系統連接多庫，此時我們就需要用到資料表映射，如同Oracle中的DBlink一般，使用過Oracle DBlink資料庫鏈接的人都知道可以跨實體來進行資料查詢，同樣的，Mysql自帶的FEDERATED引擎完美的幫我們解決了該問題，本篇文章介紹FEDERATED引擎的開啟和使用，

1.開啟FEDERATED引擎

若需要創建FEDERATED引擎表，則目標端實體要開啟FEDERATED引擎，從MySQL5.5開始FEDERATED引擎默認安裝只是沒有啟用，進入命令列輸入show engines; FEDERATED行狀態為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)

在組態檔[mysqld]中加入一行：federated，然后重啟資料庫，FEDERATED引擎就開啟了，

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創建FEDERATED表

使用CONNECTION創建FEDERATED引擎表通用模型：

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

簡單創建測驗：

# 源端表結構及資料
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 '自增主鍵',
  `stu_id` int(11) NOT NULL COMMENT '學號',
  `stu_name` varchar(20) DEFAULT NULL COMMENT '學生姓名',
  `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
  `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
  PRIMARY KEY (`increment_id`),
  UNIQUE KEY `uk_stu_id` (`stu_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 COMMENT='學生表'
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)

# 目標端建表及查詢
# 注意ENGINE=FEDERATED CONNECTION后為源端地址 避免使用帶@的密碼
mysql> CREATE TABLE `test_table` (
    ->   `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主鍵',
    ->   `stu_id` int(11) NOT NULL COMMENT '學號',
    ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '學生姓名',
    ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
    ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
    ->   PRIMARY KEY (`increment_id`),
    ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
    -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='學生表' CONNECTION='mysql://root:[email protected]: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創建FEDERATED表

如果要在同一服務器上創建多個FEDERATED表，或者想簡化創建FEDERATED表的程序，則可以使用該CREATE SERVER陳述句定義服務器連接引數，這樣多個表可以使用同一個server，

CREATE SERVER創建的格式是：

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

之后創建FEDERATED表可采用如下格式：

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

示例演示：

# 目標端創建指向源端的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)

# 目標端創建FEDERATED表
mysql> CREATE TABLE `s1` (
    ->   `increment_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '自增主鍵',
    ->   `stu_id` int(11) NOT NULL COMMENT '學號',
    ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '學生姓名',
    ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
    ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
    ->   PRIMARY KEY (`increment_id`),
    ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
    -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='學生表' 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 '自增主鍵',
    ->   `stu_id` int(11) NOT NULL COMMENT '學號',
    ->   `stu_name` varchar(20) DEFAULT NULL COMMENT '學生姓名',
    ->   `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
    ->   `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
    ->   PRIMARY KEY (`increment_id`),
    ->   UNIQUE KEY `uk_stu_id` (`stu_id`)
    -> ) ENGINE=FEDERATED DEFAULT CHARSET=utf8 COMMENT='學生表' CONNECTION='test_link/s2';
Query OK, 0 rows affected (0.01 sec)

4.FEDERATED使用總結

基于MySQL5.7.23版本，筆者在源端及目標端實驗了多種DDL及DML，現簡單總結如下，有興趣的同學可以試試看，

目標端建表結構可以與源端不一樣推薦與源端結構一致
源端DDL陳述句更改表結構目標端不會變化
源端DML陳述句目標端查詢會同步
源端drop表目標端結構還在但無法查詢
目標端不能執行DDL陳述句
目標端執行DML陳述句源端資料也會變化
目標端truncate表源端表資料也會被清空
目標端drop表對源端無影響

5.FEDERATED引擎最佳實踐

目前FEDERATED引擎使用范圍還不多，若確實有跨實體訪問的需求，建議做好規范，個人總結最佳實踐如下：

源端專門創建只讀權限的用戶來供目標端使用，
目標端建議用CREATE SERVER方式創建FEDERATED表，
FEDERATED表不宜太多，遷移時要特別注意，
目標端應該只做查詢使用，禁止在目標端更改FEDERATED表，
建議目標端表名及結構和源端保持一致，
源端表結構變更后目標端要及時洗掉重建，

參考：
https://dev.mysql.com/doc/refman/5.7/en/federated-create.html
https://dev.mysql.com/doc/refman/5.7/en/create-server.html

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