Read mysql45 - a simple understanding of global locks and table locks

Global lock

Global lock is to lock the entire database instance ,mysql A global read lock command is provided ：Flush tables with read lock (FTWRL)
When you need to keep the entire library in a read-only state , You can use the entire command , When the entire command is executed , All write operations are blocked ：
Data update statement （ Data addition, deletion and modification ）、 Data definition statement （ Include Build table 、 Modify table structure, etc ） Commit statements for and update class transactions .

The background of global lock is that the database needs to be backed up , At this time FTWRL Command prevents other threads from writing to the database , At this time, the entire library is in a read-only state .

If the backup is not locked, the result is ：

Back up before purchasing the course , But only backup A Of account, Will be A The account of 200 Backup , Then back up after purchasing the course course,
After that, if you use the backup data to restore the database , It will lead to A Of account still 200, however coures There will be one more course ; It's not right .
If you back up course, Purchase course , Backup again account, It will lead to course still null, however account Less .

So if the backup is not locked , This will cause problems in the data obtained from the backup data . If during the backup operation , Open transaction , At the end of the backup , Resubmission complete , To ensure that the backup data is correct .

The official logic backup tool is mysqldump. When mysqldump Using parameter –single-transaction When , guide A transaction is started before the data , To ensure a consistent view . And because the MVCC Support for , The data in this process is Can be updated normally .

For the storage engine MylSAM This engine does not support transactions , You need to use it FTWRL The command ensures that the entire library is read-only during backup .
set global readonly=true s You can also make the database read-only , There are two differences ：

• , In some systems ,readonly The value of will be used to do other logic , For example, it is used to judge whether a database is the primary database or the standby database library . therefore , modify global The way the variables affect the surface is larger , Not recommended .
• 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 be updated normally . Instead, set the entire library to readonly after , If the client has an exception , Then the database will remain readonly state , This will lead to the whole The library is not writable for a long time , High risk .

Table lock

There are two kinds of table level locks ： One is watch lock ; The other is metadata lock (meta data lock,MDL).

The syntax of table lock is lock tables …read/write . And FTWRL similar , It can be used unlock tables Active release lock , It can also be released automatically when the client is disconnected . We need to pay attention to ,lock tables Syntax will restrict the reading and writing of other threads Outside , It also defines the next operation objects of this thread . for instance , If in a thread A In the implementation of lock tables t1 read, t2 write; This statement , Other threads write t1、 read Write t2 All of the statements will be blocked . meanwhile , Threads A In execution unlock tables Before , It can only be read t1、 Reading and writing t2 Gymnastics do . Linking t1 Not allowed , Naturally, you can't access other tables .

Another type of table level lock is another MDL M （metadata lock) m .MDL You don't need to explicitly use , When accessing a table, it will be Automatically add .
When modifying the table structure , Add or modify fields , When deleting fields , Will add one to the watch MDL Write lock , Do not write data at this time ;
When modifying data , Add data or update data , When deleting data , Add a to the table MDL Read the lock , The data can be read only after the operation is completed .

• Read locks are not mutually exclusive , So you can have multiple threads to add, delete, modify and query a table at the same time .
• Between read lock and write lock 、 Write locks and write locks are mutually exclusive , To ensure the security of the operation to change the structure of the table . therefore , If there are two lines The program should add fields to a table at the same time , One of them can't be executed until the other has finished .

Add fields to a table , Or modify the fields , Or the Caucasian , Need to scan the data of the whole table .

sessionA Start... First , Query data , then sessionB start-up , Query data , next sessionC start-up , Modify table structure , It was blocked at this time , It will lead to sessionD Also blocked .
sessionC The blocking operation may be due to sessionA The read operation of is not over yet . Because read locks and write locks are mutually exclusive , So in sessionA Of MDL Before the read lock is released ,sessionC No MDL Write the lock .

If the query statement of this table is very high frequency , And the client has a retry mechanism , Is to create a new one after the query timeout session Query again , This will cause the thread pool of this library to be full . So when modifying the table structure or data , If there is a long transaction , And never submitted , We need to consider first kill Drop the transaction or suspend the modification operation .