The manual states this about read_only option:

"Changing this to 1 will fail if you have table locks or transactions pending, while it will wait until other locks are released or transactions are completed."

https://mariadb.com/kb/en/mariadb/server-system-variables/#read_only

I have several issues with this sentence.

1. It almost seems contradictory, but if not, then it needs some further clarification.

For instance, it says, "Changing this to 1 will fail if you have ... transactions pending, while it will wait until other ... transactions are completed."

I'm not quite sure what the difference in a pending transaction and a transaction that is not yet completed but is working on being completed (perhaps it means the COMMIT was already issued?).

2. It says, "Changing this to 1 will fail if you have... transactions pending...".

In my tests, if I have a transaction pending, and I issue the "SET @@GLOBAL.read_only=1", then the SET commands takes effect, and then if I try to COMMIT the pending transaction, it fails. Thus this is the opposite of what the manual states.

I don't know if the manual is incorrect, or if the behavior from read_only is incorrect.

3. It says, "Changing this to 1 will fail if you have table locks... pending..."

Again, I tested this, and if I issue a TABLE LOCK on a table, and then issue SET @@GLOBAL.read_only=1" in another session, the SET command waits until you UNLOCK TABLES. So it does not fail either.

Again, I don't know if the manual is incorrect, or if the behavior from read_only is incorrect.

#2 test case:

Session #1:

mysql> select * from gtest;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into gtest (id) values (5);

Query OK, 1 row affected (0.03 sec)

Session #2:

mysql> set @@global.read_only=1;

Query OK, 0 rows affected (0.00 sec)

Session #1:

mysql> commit;

ERROR 1290 (HY000): The MariaDB server is running with the --read-only option so it cannot execute this statement

mysql>

mysql> select * from gtest;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.00 sec)

#3 test case:

Session #1:

mysql> lock table gtest write;

Query OK, 0 rows affected (0.00 sec)

Session #2:

mysql> set @@global.read_only=1;

...<hangs>...

Session #1:

mysql> insert into gtest (id) values (5);

Query OK, 1 row affected (0.03 sec)

mysql> commit;

Query OK, 0 rows affected (0.00 sec)

mysql> unlock tables;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from gtest;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

|    5 |

+------+

5 rows in set (0.00 sec)

mysql> select version();

+-----------------+

| version()       |

+-----------------+

| 10.0.26-MariaDB |

+-----------------+

1 row in set (0.03 sec)

Session #2:

Query OK, 0 rows affected (35.18 sec)

(See how long it waited?)