在MySQL數據庫中，在進行數據遷移和從庫只讀狀態設置時，都會涉及到只讀狀態和Master-slave的設置和關系。
經過實際測試，對于MySQL單實例數據庫和master庫，如果需要設置為只讀狀態，需要進行如下操作和設置：
一般單實例情況下將MySQL設置為只讀狀態的命令：
# 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%";
將MySQL從只讀設置為讀寫狀態的命令：
mysql> unlock tables; mysql> set global read_only=0;
對于需要保證master-slave主從同步的salve庫，如果要設置為只讀狀態，需要執行的命令為：
mysql> set global read_only=1; 打開只讀開關
將salve庫從只讀狀態變為讀寫狀態，需要執行的命令是：
mysql> set global read_only=0; 打開讀寫開關
對于數據庫讀寫狀態，主要靠 “read_only”全局參數來設定；
默認情況下，數據庫是用于讀寫操作的，所以read_only參數也是0或faluse狀態，這時候不論是本地用戶還是遠程訪問數據庫的用戶，都可以進行讀寫操作；
如需設置為只讀狀態，將該read_only參數設置為1或TRUE狀態，但設置 read_only=1 狀態有兩個需要注意的地方：
1.read_only=1只讀模式，不會影響slave同步復制的功能，所以在MySQL slave庫中設定了read_only=1后，通過 show slave status\G 命令查看salve狀態，可以看到salve仍然會讀取master上的日志，并且在slave庫中應用日志，保證主從數據庫同步一致；
2.read_only=1只讀模式，可以限定普通用戶進行數據修改的操作，但不會限定具有super權限的用戶的數據修改操作；在MySQL中設置read_only=1后，普通的應用用戶進行insert、update、delete等會產生數據變化的DML操作時，都會報出數據庫處于只讀模式不能發生數據變化的錯誤，但具有super權限的用戶，例如在本地或遠程通過root用戶登錄到數據庫，還是可以進行數據變化的DML操作；
為了確保所有用戶，包括具有super權限的用戶也不能進行讀寫操作，就需要執行給所有的表加讀鎖的命令 “flush tables with read lock;”，這樣使用具有super權限的用戶登錄數據庫，想要發生數據變化的操作時，也會提示表被鎖定不能修改的報錯。
這樣通過 設置“read_only=1”和“flush tables with read lock;”兩條命令，就可以確保數據庫處于只讀模式，不會發生任何數據改變，在MySQL進行數據庫遷移時，限定master主庫不能有任何數據變化，就可以通過這種方式來設定。
但同時由于加表鎖的命令對數據庫表限定非常嚴格，如果再slave從庫上執行這個命令后，slave庫可以從master讀取binlog日志，但不能夠應用日志，slave庫不能發生數據改變，當然也不能夠實現主從同步了，這時如果使用 “unlock tables;”解除全局的表讀鎖，slave就會應用從master讀取到的binlog日志，繼續保證主從庫數據庫一致同步。
為了保證主從同步可以一直進行，在slave庫上要保證具有super權限的root等用戶只能在本地登錄，不會發生數據變化，其他遠程連接的應用用戶只按需分配為select,insert,update,delete等權限，保證沒有super權限，則只需要將salve設定“read_only=1”模式，即可保證主從同步，又可以實現從庫只讀。
相對的，設定“read_only=1”只讀模式開啟的解鎖命令為設定“read_only=0”;設定全局鎖“flush tables with read lock;”，對應的解鎖模式命令為：“unlock tables;”.
當然設定了read_only=1后，所有的select查詢操作都是可以正常進行的。
實驗一：設置了read_only=1后，遠程業務用戶進行數據庫修改會提示ERROR 1290錯誤：
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; <span >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]>
實驗二：設定了全局讀寫后，具有super權限的用戶進行數據修改后，也會提示錯誤ERROR 1223：
view plain copy mysql> 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); <span >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 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>
實驗三：MySQL從庫設定read_only=1后主從同步正常，設定表讀鎖后，不能同步，解除讀鎖后，主從同步恢復。
copy
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: 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>
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
Master_Log_File: mysql-bin.000009
Read_Master_Log_Pos: 6531
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: 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: 120
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: 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: 0
1 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
Master_Log_File: mysql-bin.000009
Read_Master_Log_Pos: 6531
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 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
這個命令是全局讀鎖定，執行了命令之后所有庫所有表都被鎖定只讀。一般都是用在數據庫聯機備份，這個時候數據庫的寫操作將被阻塞，讀操作順利進行。
解鎖的語句也是unlock tables。
2.LOCK TABLES tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}
這個命令是表級別的鎖定，可以定制鎖定某一個表。例如： lock tables test read; 不影響其他表的寫操作。
解鎖語句也是unlock tables。
重點：
這兩個語句在執行的時候都需要注意個特點，就是 隱式提交的語句。在退出MySQL終端的時候都會隱式的執行unlock tables。也就是如果要讓表鎖定生效就必須一直保持對話。
P.S. MYSQL的read lock和wirte lock
read-lock: 允許其他并發的讀請求，但阻塞寫請求，即可以同時讀，但不允許任何寫。也叫共享鎖
write-lock: 不允許其他并發的讀和寫請求，是排他的(exclusive)。也叫獨占鎖
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