MySQL global lock and table lock

MYSQL The lock inside can be divided into ： Global lock , Table lock , Row-level locks

1 Global lock

Lock the whole database instance
MYSQL Provide global lock reading methods :Flush tables with read lock(FTWRL)
This command makes the entire library read-only , After using this command , Data update statement 、 Data definition statements and commit statements of update class transactions are blocked

1.1 Use scenarios

Global logical backup
risk ：

1. If the main database is backed up , Cannot update during backup , Business suspension
2. If backing up from the Library , Cannot perform primary database synchronization during backup binlog, Cause master-slave delay
   The official logic backup tool mysqlddump, When mysqldump Using parameter –single-transaction When , Will start a transaction , Make sure you get a consistent view . because MVCC Support for , The data in this process can be updated normally

Consistent reading is good , But only if the engine supports this isolation level
If you want the full library to be read-only , Why not use set global readonly=true The way ？？？
1 In some systems ,readonly The value of will be used for other logic , For example, judge the primary and standby databases . So modify global The way variables affect too much
2 There are differences in exception handling mechanisms , If you execute FTWRL After the command, due to the abnormal disconnection of the client , that MYSQL This global lock will be released automatically , The whole library can return to the normal update state . Instead, set the entire library to readonly after , If the client has an exception , Then the database will remain readonly state , This will cause the entire library to be in a non writable state for a long time , High risk

2 Table lock

MYSQL There are two kinds of meter level locks inside , One is watch lock , One is metadata lock (meta data lock, MDL)

2.1 Table locks

Syntax of table lock : lock tables…read/write
have access to unlock tables Active release lock , It can also be released automatically when the client is disconnected .lock tables The syntax will restrict the reading and writing of other threads , It also defines the next operation objects of this thread
about InnoDB This kind of engine supports row lock , commonly Don't use lock tables Command to control concurrency , After all, locking the whole table still has a great impact .

2.2 MDL

You don't need to explicitly use , It will be added automatically when accessing a table
effect ： Ensure the correctness of reading and writing
1 When adding, deleting, modifying and querying a table , Add MDL Read the lock
2 When changing the table structure , Add MDL Write lock
3 Read locks are not mutually exclusive
4 Read-write lock , Write locks are mutually exclusive , It is used to ensure the security of the schema operation of the change table
5 MDL Will not be released until the transaction is committed , When making table structure changes , Be careful not to lock online queries and updates