mysql只读模式的设置方法与实验

      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：
      将MySQL设置为只读状态的命令：
# mysql -uroot -p
mysql> 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;


     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;


     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：

(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;<span style="color:#ff0000;">ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement</span>(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：

mysql> use data03;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> 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: 0mysql> 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);<span style="color:#ff0000;">ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock</span>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 lockmysql> mysql> delete from t01 where id1=5;ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lockmysql> 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。

mysql> mysql> show databases;+--------------------+| Database           |+--------------------+| information_schema || bitvc              || data03             || ga                 || jiradb             || meibi              || meibi02            || mysql              || performance_schema || sbtest             |+--------------------+10 rows in set (0.00 sec)mysql> mysql> 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> show slave status\G*************************** 1. row ***************************               Slave_IO_State: Waiting for master to send event                  Master_Host: 172.32.1.200                  Master_User: repl                  Master_Port: 3307                Connect_Retry: 60              Master_Log_File: mysql-bin.000009          Read_Master_Log_Pos: 5853               Relay_Log_File: huobiDBtest-relay-bin.000002                Relay_Log_Pos: 6016        Relay_Master_Log_File: mysql-bin.000009             Slave_IO_Running: Yes            Slave_SQL_Running: Yes              Replicate_Do_DB:           Replicate_Ignore_DB:            Replicate_Do_Table:        Replicate_Ignore_Table:       Replicate_Wild_Do_Table:   Replicate_Wild_Ignore_Table:                    Last_Errno: 0                   Last_Error:                  Skip_Counter: 0          Exec_Master_Log_Pos: 5853              Relay_Log_Space: 6195              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: 0Master_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: 01 row in set (0.00 sec)mysql> mysql> flush tables with read lock;Query OK, 0 rows affected (0.00 sec)mysql> show slave status\G*************************** 1. row ***************************               Slave_IO_State: Waiting for master to send event                  Master_Host: 172.32.1.200                  Master_User: repl                  Master_Port: 3307                Connect_Retry: 60       <span style="color:#ff0000;">       Master_Log_File: mysql-bin.000009          Read_Master_Log_Pos: 6531</span>               Relay_Log_File: huobiDBtest-relay-bin.000002                Relay_Log_Pos: 6016        Relay_Master_Log_File: mysql-bin.000009             Slave_IO_Running: Yes            Slave_SQL_Running: Yes              Replicate_Do_DB:           Replicate_Ignore_DB:            Replicate_Do_Table:        Replicate_Ignore_Table:       Replicate_Wild_Do_Table:   Replicate_Wild_Ignore_Table:                    Last_Errno: 0                   Last_Error:                  Skip_Counter: 0        <span style="color:#ff0000;">  Exec_Master_Log_Pos: 5853</span>              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: 120Master_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: Waiting for global read lock           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: 01 row in set (0.00 sec)mysql> mysql> mysql> select * from data03.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> unlock tables;Query OK, 0 rows affected (0.00 sec)mysql> show slave status\G*************************** 1. row ***************************               Slave_IO_State: Waiting for master to send event                  Master_Host: 172.32.1.200                  Master_User: repl                  Master_Port: 3307                Connect_Retry: 60<span style="color:#ff0000;">              Master_Log_File: mysql-bin.000009          Read_Master_Log_Pos: 6531</span>               Relay_Log_File: huobiDBtest-relay-bin.000002                Relay_Log_Pos: 6694        Relay_Master_Log_File: mysql-bin.000009             Slave_IO_Running: Yes            Slave_SQL_Running: Yes              Replicate_Do_DB:           Replicate_Ignore_DB:            Replicate_Do_Table:        Replicate_Ignore_Table:       Replicate_Wild_Do_Table:   Replicate_Wild_Ignore_Table:                    Last_Errno: 0                   Last_Error:                  Skip_Counter: 0         <span style="color:#ff0000;"> Exec_Master_Log_Pos: 6531</span>              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: 0Master_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: 01 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)