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Summary:
Implements per-database scope read-only setting. The read-only can be
turned on in `alter database` statement. Privilege to set read-only
follows alter-database ACL.
ALTER DATABASE db_name [SUPER_READ_ONLY | READ_ONLY = FALSE | TRUE]]
`SUPER_READ_ONLY = TRUE` will prevent all write transactions from
committing for any user (including super users).
`READ_ONLY = TRUE` will prevent any write transaction from committing
for regular users, while super users can still write to the database.
`SUPER_READ_ONLY = FALSE` will turn off super_read_only on the database,
and the database remains on read_only (for regular users),
`READ_ONLY = FALSE` will turn off read_only on the database,
The READ_ONLY database status can be shown by `show create database`.
mysql> alter database test read_only = true;
Query OK, 1 row affected (0.00 sec)
mysql> show create database test;
+----------+---------------------------------------------------------------------------+
| Database | Create Database |
+----------+---------------------------------------------------------------------------+
| test | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 READ_ONLY */ |
+----------+---------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> alter database test super_read_only = true;
Query OK, 1 row affected (0.00 sec)
mysql> show create database test;
+----------+---------------------------------------------------------------------------------+
| Database | Create Database |
+----------+---------------------------------------------------------------------------------+
| test | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 SUPER_READ_ONLY */ |
+----------+---------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> alter database test super_read_only = false;
Query OK, 1 row affected (0.00 sec)
mysql> show create database test;
+----------+---------------------------------------------------------------------------+
| Database | Create Database |
+----------+---------------------------------------------------------------------------+
| test | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 READ_ONLY */ |
+----------+---------------------------------------------------------------------------+
1 row in set (0.00 sec)
Details:
* READ_ONLY flag is persisted in db.ops and survives restarts.
* Alter database will only return when no write transaction is
still committing. Uncommitted write transactions will fail after
alter database succeeds.
* DB options are stored in each thread (using hash map), so checking
the read_only options doesn't grab a global lock. Only db_read_only
that is turned on is stored in the local hash map.
* The first time thead local hash map is initialized (once), shared db
option map will be locked and accessed. Since the db options are not
automatically loaded into the shared hash map (cache) at the
beginning, we have to explicitly load the db options during the
first time any thread initializes its local hash map.
* create/alter/delete database DDLs will iterate through thread array
to update local db opt (if needed), similar to how show processlist
works.
* write transactions are checked against local hashmap at commit time
(either explicit commit or auto-commit). This will make the explicit
commit and auto-commits consistent, and avoid problems such that
explicit commit may become a "read-only" transaction if we block
write-statements at the parsing time.
* The list of databases accessed in a write transaction is obtained
through the metadata locks (MDL) that the transaction holds. As I
noted in the code, this may rollback some false positives, such as
the read-only DB in the list may not actually be modified in the
transaction (for cross-db transactions). We do not differentiate
such cases.
Reference patch:
facebook@8194409
facebook@c971129
facebook@e26e9b2
facebook@a9f6f3d
facebook@b1ff9a6
facebook@548c7ca
Differential Revision: D9379904- Loading branch information
