mysql只读模式下数据迁移，保证数据一致性

在MySQL数据库中，在进行数据迁移和从库只读状态设置时，都会涉及到只读状态和Master-slave的设置和关系。


经过实际测试，对于MySQL单实例数据库和master库，如果需要设置为只读状态，需要进行如下操作和设置：

一般单实例情况下将MySQL设置为只读状态的命令：

# mysql -uroot -pmysql> show global variables like "%read_only%";mysql>flush tables with read lock;mysql>set global read_only=1;mysql> show global variables like "%read_only%";

将MySQL从只读设置为读写状态的命令：

mysql> unlock tables;mysql>  set global read_only=0;

对于需要保证master-slave主从同步的salve库，如果要设置为只读状态，需要执行的命令为：
mysql> set global read_only=1; 打开只读开关


将salve库从只读状态变为读写状态，需要执行的命令是：
mysql> set global read_only=0; 打开读写开关


对于数据库读写状态，主要靠 "read_only"全局参数来设定；

默认情况下，数据库是用于读写操作的，所以read_only参数也是0或faluse状态，这时候不论是本地用户还是远程访问数据库的用户，都可以进行读写操作；

如需设置为只读状态，将该read_only参数设置为1或TRUE状态，但设置 read_only=1 状态有两个需要注意的地方：
1.read_only=1只读模式，不会影响slave同步复制的功能，所以在MySQL slave库中设定了read_only=1后，通过 show slave status\G 命令查看salve状态，可以看到salve仍然会读取master上的日志，并且在slave库中应用日志，保证主从数据库同步一致；
2.read_only=1只读模式，可以限定普通用户进行数据修改的操作，但不会限定具有super权限的用户的数据修改操作；在MySQL中设置read_only=1后，普通的应用用户进行insert、update、delete等会产生数据变化的DML操作时，都会报出数据库处于只读模式不能发生数据变化的错误，但具有super权限的用户，例如在本地或远程通过root用户登录到数据库，还是可以进行数据变化的DML操作；

为了确保所有用户，包括具有super权限的用户也不能进行读写操作，就需要执行给所有的表加读锁的命令 "flush tables with read lock;"，这样使用具有super权限的用户登录数据库，想要发生数据变化的操作时，也会提示表被锁定不能修改的报错。

这样通过 设置"read_only=1"和"flush tables with read lock;"两条命令，就可以确保数据库处于只读模式，不会发生任何数据改变，在MySQL进行数据库迁移时，限定master主库不能有任何数据变化，就可以通过这种方式来设定。

但同时由于加表锁的命令对数据库表限定非常严格，如果再slave从库上执行这个命令后，slave库可以从master读取binlog日志，但不能够应用日志，slave库不能发生数据改变，当然也不能够实现主从同步了，这时如果使用 "unlock tables;"解除全局的表读锁，slave就会应用从master读取到的binlog日志，继续保证主从库数据库一致同步。

为了保证主从同步可以一直进行，在slave库上要保证具有super权限的root等用户只能在本地登录，不会发生数据变化，其他远程连接的应用用户只按需分配为select,insert,update,delete等权限，保证没有super权限，则只需要将salve设定"read_only=1"模式，即可保证主从同步，又可以实现从库只读。


相对的，设定"read_only=1"只读模式开启的解锁命令为设定"read_only=0";设定全局锁"flush tables with read lock;"，对应的解锁模式命令为："unlock tables;".

当然设定了read_only=1后，所有的select查询操作都是可以正常进行的。




实验一：设置了read_only=1后，远程业务用户进行数据库修改会提示ERROR 1290错误：

 copy(test01@172.32.1.200) [data03]> show tables;  +------------------+  | Tables_in_data03 |  +------------------+  | t01              |  | t02              |  | user             |  +------------------+  3 rows in set (0.00 sec)    (test01@172.32.1.200) [data03]>   (test01@172.32.1.200) [data03]>   (test01@172.32.1.200) [data03]> show global variables like "%read_only%";  +------------------+-------+  | Variable_name    | Value |  +------------------+-------+  | innodb_read_only | OFF   |  | read_only        | ON    |  | tx_read_only     | OFF   |  +------------------+-------+  3 rows in set (0.00 sec)    (test01@172.32.1.200) [data03]>   (test01@172.32.1.200) [data03]>   (test01@172.32.1.200) [data03]> delete from t01 where id1=3;  ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement  (test01@172.32.1.200) [data03]> update t01 set id1=id1+30 where id1=3;  ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement  (test01@172.32.1.200) [data03]> insert into t01(id1,a1,b1) values(9,9,9);  ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement  (test01@172.32.1.200) [data03]>   (test01@172.32.1.200) [data03]> select * from t01;  +-----+------+------+  | id1 | a1   | b1   |  +-----+------+------+  |   1 |    1 |    1 |  |   2 |    2 |    2 |  |   4 |    4 |    4 |  |   5 |    5 |    5 |  |   6 |    6 |    6 |  +-----+------+------+  5 rows in set (0.00 sec)    (test01@172.32.1.200) [data03]>

实验二：设定了全局读写后，具有super权限的用户进行数据修改后，也会提示错误ERROR 1223：

view plain copymysql> use data03;  Reading table information for completion of table and column names  You can turn off this feature to get a quicker startup with -A    Database changed  mysql> show tables;  +------------------+  | Tables_in_data03 |  +------------------+  | t01              |  | t02              |  | user             |  +------------------+  3 rows in set (0.00 sec)    mysql> select * from t01;  +-----+------+------+  | id1 | a1   | b1   |  +-----+------+------+  |   1 |    1 |    1 |  |   2 |    2 |    2 |  |   4 |    4 |    4 |  |   5 |    5 |    5 |  |   6 |    6 |    6 |  +-----+------+------+  5 rows in set (0.00 sec)    mysql>   mysql>  show global variables like "%read_only%";  +------------------+-------+  | Variable_name    | Value |  +------------------+-------+  | innodb_read_only | OFF   |  | read_only        | ON    |  | tx_read_only     | OFF   |  +------------------+-------+  3 rows in set (0.00 sec)    mysql>   mysql>   mysql> insert into t01(id1,a1,b1) values(8,8,8);         Query OK, 1 row affected (0.00 sec)    mysql> update t01 set a1=a1+10 where id1=2;  Query OK, 1 row affected (0.00 sec)  Rows matched: 1  Changed: 1  Warnings: 0    mysql> delete from t01 where id1=4;  Query OK, 1 row affected (0.00 sec)    mysql> select * from t01;  +-----+------+------+  | id1 | a1   | b1   |  +-----+------+------+  |   1 |    1 |    1 |  |   2 |   12 |    2 |  |   5 |    5 |    5 |  |   6 |    6 |    6 |  |   8 |    8 |    8 |  +-----+------+------+  5 rows in set (0.00 sec)    mysql>   mysql> flush tables with read lock;  Query OK, 0 rows affected (0.00 sec)    mysql>   mysql> insert into t01(id1,a1,b1) values(9,9,9);  ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock  mysql>   mysql> update t01 set a1=a1+10 where id1=5;  ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock  mysql>   mysql> delete from t01 where id1=5;  ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock  mysql>   mysql>

实验三：MySQL从库设定read_only=1后主从同步正常，设定表读锁后，不能同步，解除读锁后，主从同步恢复。

copy

1. mysql>

2. mysql> show databases;

3. +--------------------+

4. | Database |

5. +--------------------+

6. | information_schema |

7. | bitvc |

8. | data03 |

9. | ga |

10. | jiradb |

11. | meibi |

12. | meibi02 |

13. | mysql |

14. | performance_schema |

15. | sbtest |

16. +--------------------+

17. 10 rows in set (0.00 sec)

18. mysql>

19. mysql>

20. mysql>

21. mysql> show global variables like "%read_only%";

22. +------------------+-------+

23. | Variable_name | Value |

24. +------------------+-------+

25. | innodb_read_only | OFF |

26. | read_only | ON |

27. | tx_read_only | OFF |

28. +------------------+-------+

29. 3 rows in set (0.00 sec)

30. mysql>

31. mysql> show slave status\G

32. *************************** 1. row ***************************

33. Slave_IO_State: Waiting for master to send event

34. Master_Host: 172.32.1.200

35. Master_User: repl

36. Master_Port: 3307

37. Connect_Retry: 60

38. Master_Log_File: mysql-bin.000009

39. Read_Master_Log_Pos: 5853

40. Relay_Log_File: huobiDBtest-relay-bin.000002

41. Relay_Log_Pos: 6016

42. Relay_Master_Log_File: mysql-bin.000009

43. Slave_IO_Running: Yes

44. Slave_SQL_Running: Yes

45. Replicate_Do_DB:

46. Replicate_Ignore_DB:

47. Replicate_Do_Table:

48. Replicate_Ignore_Table:

49. Replicate_Wild_Do_Table:

    1. Replicate_Wild_Ignore_Table:

    1. 
       

50. Last_Errno: 0

51. Last_Error:

52. Skip_Counter: 0

53. Exec_Master_Log_Pos: 5853

54. Relay_Log_Space: 6195

55. Until_Condition: None

56. Until_Log_File:

57. Until_Log_Pos: 0

58. Master_SSL_Allowed: No

59. Master_SSL_CA_File:

60. Master_SSL_CA_Path:

61. Master_SSL_Cert:

62. Master_SSL_Cipher:

63. Master_SSL_Key:

64. Seconds_Behind_Master: 0

65. Master_SSL_Verify_Server_Cert: No

66. Last_IO_Errno: 0

67. Last_IO_Error:

68. Last_SQL_Errno: 0

69. Last_SQL_Error:

70. Replicate_Ignore_Server_Ids:

71. Master_Server_Id: 2003307

72. Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d

73. Master_Info_File: /data/mysqldata/3308/data/master.info

74. SQL_Delay: 0

75. SQL_Remaining_Delay: NULL

76. Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it

77. Master_Retry_Count: 86400

78. Master_Bind:

79. Last_IO_Error_Timestamp:

80. Last_SQL_Error_Timestamp:

81. Master_SSL_Crl:

82. Master_SSL_Crlpath:

83. Retrieved_Gtid_Set:

84. Executed_Gtid_Set:

85. Auto_Position: 0

86. 1 row in set (0.00 sec)

87. mysql>

88. mysql> flush tables with read lock;

89. Query OK, 0 rows affected (0.00 sec)

90. mysql> show slave status\G

91. *************************** 1. row ***************************

92. Slave_IO_State: Waiting for master to send event

93. Master_Host: 172.32.1.200

94. Master_User: repl

95. Master_Port: 3307

96. Connect_Retry: 60

97. Master_Log_File: mysql-bin.000009

98. Read_Master_Log_Pos: 6531

99. Relay_Log_File: huobiDBtest-relay-bin.000002

100. Relay_Log_Pos: 6016

101. Relay_Master_Log_File: mysql-bin.000009

102. Slave_IO_Running: Yes

103. Slave_SQL_Running: Yes

104. Replicate_Do_DB:

105. Replicate_Ignore_DB:

106. Replicate_Do_Table:

107. Replicate_Ignore_Table:

108. Replicate_Wild_Do_Table:

109. Replicate_Wild_Ignore_Table:

110. Last_Errno: 0

111. Last_Error:

     1. 
        

     1. Skip_Counter: 0

     1. 
        

     1. 
        

112. Exec_Master_Log_Pos: 5853

113. Relay_Log_Space: 6873

114. Until_Condition: None

115. Until_Log_File:

116. Until_Log_Pos: 0

117. Master_SSL_Allowed: No

118. Master_SSL_CA_File:

119. Master_SSL_CA_Path:

120. Master_SSL_Cert:

121. Master_SSL_Cipher:

122. Master_SSL_Key:

123. Seconds_Behind_Master: 120

124. Master_SSL_Verify_Server_Cert: No

125. Last_IO_Errno: 0

126. Last_IO_Error:

127. Last_SQL_Errno: 0

128. Last_SQL_Error:

129. Replicate_Ignore_Server_Ids:

130. Master_Server_Id: 2003307

     1. Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d

     1. 
        

131. Master_Info_File: /data/mysqldata/3308/data/master.info

132. SQL_Delay: 0

133. SQL_Remaining_Delay: NULL

134. Slave_SQL_Running_State: Waiting for global read lock

135. Master_Retry_Count: 86400

136. Master_Bind:

137. Last_IO_Error_Timestamp:

138. Last_SQL_Error_Timestamp:

139. Master_SSL_Crl:

140. Master_SSL_Crlpath:

141. Retrieved_Gtid_Set:

142. Executed_Gtid_Set:

143. Auto_Position: 0

144. 1 row in set (0.00 sec)

145. mysql>

146. mysql>

147. mysql> select * from data03.t01;

148. +-----+------+------+

149. | id1 | a1 | b1 |

150. +-----+------+------+

151. | 1 | 1 | 1 |

152. | 2 | 2 | 2 |

153. | 4 | 4 | 4 |

154. | 5 | 5 | 5 |

155. | 6 | 6 | 6 |

156. +-----+------+------+

157. 5 rows in set (0.00 sec)

158. mysql>

159. mysql> unlock tables;

160. Query OK, 0 rows affected (0.00 sec)

161. mysql> show slave status\G

162. *************************** 1. row ***************************

163. Slave_IO_State: Waiting for master to send event

164. Master_Host: 172.32.1.200

165. Master_User: repl

166. Master_Port: 3307

167. Connect_Retry: 60

168. Master_Log_File: mysql-bin.000009

169. Read_Master_Log_Pos: 6531

170. Relay_Log_File: huobiDBtest-relay-bin.000002

171. Relay_Log_Pos: 6694

172. Relay_Master_Log_File: mysql-bin.000009

173. Slave_IO_Running: Yes

174. Slave_SQL_Running: Yes

175. Replicate_Do_DB:

176. Replicate_Ignore_DB:

177. Replicate_Do_Table:

178. Replicate_Ignore_Table:

179. Replicate_Wild_Do_Table:

180. Replicate_Wild_Ignore_Table:

181. Last_Errno: 0

        Last_Error:                    Skip_Counter: 0        Exec_Master_Log_Pos: 6531                 Relay_Log_Space: 6873                Until_Condition: None                 Until_Log_File:                   Until_Log_Pos: 0             Master_SSL_Allowed: No             Master_SSL_CA_File:              Master_SSL_CA_Path:                 Master_SSL_Cert:               Master_SSL_Cipher:                  Master_SSL_Key:           Seconds_Behind_Master: 0  Master_SSL_Verify_Server_Cert: No                  Last_IO_Errno: 0                  Last_IO_Error:                  Last_SQL_Errno: 0                 Last_SQL_Error:     Replicate_Ignore_Server_Ids:              Master_Server_Id: 2003307             Master_UUID: 6f68eea7-76e9-11e4-8f99-00221904cd5d              Master_Info_File: /data/mysqldata/3308/data/master.info               SQL_Delay: 0            SQL_Remaining_Delay: NULL        Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it             Master_Retry_Count: 86400          Master_Bind:         Last_IO_Error_Timestamp:        Last_SQL_Error_Timestamp:                  Master_SSL_Crl:              Master_SSL_Crlpath:              Retrieved_Gtid_Set:               Executed_Gtid_Set:               Auto_Position: 0  1 row in set (0.00 sec)    mysql> select * from data03.t01;  +-----+------+------+  | id1 | a1   | b1   |  +-----+------+------+  |   1 |    1 |    1 |  |   2 |   12 |    2 |  |   5 |    5 |    5 |  |   6 |    6 |    6 |  |   8 |    8 |    8 |  +-----+------+------+  5 rows in set (0.00 sec)

##########################################

.FLUSH TABLES WITH READ LOCK

这个命令是全局读锁定，执行了命令之后所有库所有表都被锁定只读。一般都是用在数据库联机备份，这个时候数据库的写操作将被阻塞，读操作顺利进行。

解锁的语句也是unlock tables。

2.LOCK TABLES tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}

这个命令是表级别的锁定，可以定制锁定某一个表。例如： lock tables test read; 不影响其他表的写操作。

解锁语句也是unlock tables。

重点：

这两个语句在执行的时候都需要注意个特点，就是 隐式提交的语句。在退出MySQL终端的时候都会隐式的执行unlock tables。也就是如果要让表锁定生效就必须一直保持对话。

P.S. MYSQL的read lock和wirte lock

read-lock: 允许其他并发的读请求，但阻塞写请求，即可以同时读，但不允许任何写。也叫共享锁

write-lock: 不允许其他并发的读和写请求，是排他的(exclusive)。也叫独占锁