Foreword:
By default, our MySQL instance is read-write. But in some cases, we can set the entire instance as read-only, such as when doing migration maintenance or setting the slave library as read-only. In this article, let's take a look at the knowledge about MySQL settings read-only.
1. About the read_only parameter
In the MySQL system, there are two read-only parameters, read_only and super_read_only, refer to the official documentation, here are the functions of these two parameters:
The read_only parameter is not enabled by default, and it will prevent users without super privileges from performing database modification operations. After it is enabled, a --read-only error will be prompted when a normal-privileged user performs operations such as insert, update, and delete. However, users with super privileges can still perform alter operations.
The super_read_only parameter is also disabled by default. When enabled, it will not only prevent ordinary users, but also prevent users with super privileges from making changes to the database.
read_only and super_read_only are related, and the relationship between them is as follows:
- Setting super_read_only=on also implicitly sets read_only=on.
- Setting read_only=off implicitly sets super_read_only=off.
- You can turn on read_only without turning on super_read_only.
However, turning on read_only from the library does not affect the master-slave synchronization, that is, the slave side will still read the logs on the master, and apply the logs in the slave instance to ensure that the master-slave databases are synchronized consistently. (After testing, turning on super_read_only from the library side still does not affect the master-slave synchronization.)
Let's take a look at the usage of the read_only parameter:
# 查看 read_only 参数
mysql> show global variables like '%read_only%';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| innodb_read_only | OFF |
| read_only | OFF |
| super_read_only | OFF |
| transaction_read_only | OFF |
| tx_read_only | OFF |
+-----------------------+-------+
# 动态修改 read_only 参数 （若想重启生效 则需将 read_only = 1 加入配置文件中）
mysql> set global read_only = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like 'read_only';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| read_only | ON |
+---------------+-------+
# read_only 开启的情况下 操作数据
# 使用超级权限用户
mysql> create table tb_a (a int);
Query OK, 0 rows affected (0.05 sec)
# 使用普通权限用户
mysql> create table tb_b (b int);
ERROR 1290 (HY000): The MySQL server is running with the --read-only option so it cannot execute this statement
# 开启 super_read_only，再次使用超级权限用户来操作数据
mysql> set global super_read_only = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like 'super_read_only';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| super_read_only | ON |
+-----------------+-------+
mysql> create table tb_c (c int);
ERROR 1290 (HY000): The MySQL server is running with the --super-read-only option so it cannot execute this statement
# 关闭 read_only 参数
mysql> set global read_only = 0;
Query OK, 0 rows affected (0.00 sec)
2.flush tables with read lock 设置
In addition to the read_only parameter, executing flush tables with read lock can also set the database to a read-only state, so what is the difference between the two? Let's first understand the role of flush tables with read lock.
Executing this command will add a global read lock to the database, making the database in a read-only state, and the following statements will be blocked: data update statements (addition, deletion, modification), data definition statements (creating tables, modifying table structures, etc.) and submission of update transactions statement. Let's do a specific experiment:
# 执行FTWRL
mysql> flush tables with read lock;
Query OK, 0 rows affected (0.02 sec)
# 进行数据变更操作
mysql> insert into tb_a values (1);
ERROR 1223 (HY000): Can't execute the query because you have a conflicting read lock
# 解锁
mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into tb_a values (1);
Query OK, 1 row affected (0.01 sec)
It is worth noting that executing flush tables with read lock from the database will cause the SQL thread to be stuck and delay the master and backup. The difference from enabling the read_only parameter is that after executing flush tables with read lock, other clients will continue to wait for data change operations instead of reporting an error immediately, which is extremely easy to cause the database to hang. Be careful when executing this command.
In terms of personal database operation and maintenance experience, generally only the slave database needs to be set to read-only status. It is recommended to enable read_only or super_read_only on the slave database side to avoid manual writing. flush tables with read lock is suitable for data migration, which can ensure that no data changes occur in the database, but pay attention to unlocking in time.
Summarize:
This article mainly introduces the knowledge related to MySQL read-only status. In fact, except for the slave database, other instances rarely set the global read-only status. It is only necessary to set the database to read-only status when encountering certain requirements. The purpose of writing this article The purpose is also to have a reference when encountering such needs.