MySQL 45 lecture learning notes (VI) global lock

One . Global lock : Lock the entire database instance

Mysql It provides a way to add global read lock The order is

Flush tables with read lock（FTWRL） 

When you need to make the whole library Read only status , You can use this command , Statements from other threads will Blocked

data to update sentence ( Data Additions and deletions ),

data Definition sentence （ Include Build table , Modify table structure etc. ）

Update transaction Submit sentence

Typical use scenario of global lock -- Make a full library logical backup

That is the The whole database, every table select Come out and save it as text During the backup process The entire library is read-only , But let The whole library is read-only, which is dangerous

• Main database backup , Update cannot be performed during backup , Business basically stopped
• Backup from the library , During the backup, the slave database cannot synchronize with the master database binlog, It can lead to Master-slave delay
• No locks Words , The library that the backup system backs up is not a logical point in time , This view is logically inconsistent .

The official logic backup tool is mysqldump

When mysqldump, Using parameter -single-transaction When , A transaction will be started before importing data , To ensure a consistent view

because MVCC This data can be updated normally .

Table lock MySql There are two types of lock at the inner table level ：

• One is watch lock ,
• One 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 grammar In addition to restricting the reading and writing of other threads Also limited The next operation object of this thread .

• For example , If in a thread A perform lock tables t1 t2 write This statement , Other threads write t1, Reading and writing t2 All of the statements will be blocked
• meanwhile , Threads A In execution unlock tables Previously, you could only read t1, Reading and writing t2 The operation of . Linking t1 Not allowed , Naturally, you can't access other tables .

When there is no finer grained lock , Table locking is the most common way to handle concurrency .

Two . The watch lock is MDL(metadata lock)

• MDL You don't need to explicitly use , When accessing a table, it will be automatically added
• MDL The role of is , Ensure the correctness of reading and writing
• If a query is traversing data in a table , Another thread changes the table structure during execution , Delete a list , Then the result obtained by the query thread does not match the table structure , Definitely not .
• therefore , stay MySQL5.5 Version to introduce MDL, When a table is adding, deleting, modifying, and querying , add MDL Read the lock , When it comes to watch When making structural changes , add MDL Write lock

• Read locks are not mutually exclusive , So you can have multiple threads to add, delete, modify and query a table at the same time
• Read and write locks are mutually exclusive , Write locks are mutually exclusive , To ensure the security of the operation to change the structure of the table . therefore , If two threads want to add fields to a table at the same time , One of them has to wait for the other to finish executing .

Add fields to a small table , Cause the whole library to hang

• In the transaction MDL lock . Apply at the beginning of statement execution , But it will not be released immediately after the statement ends , Instead, it will be released after the whole transaction is committed .
• First of all, we have to deal with long affairs , The transaction does not commit , Will always occupy MDL lock . stay MySQL Of information_schema Library innoDB_trx In the table , You can find the current transaction in progress 、
• You have to DDL The changed table happens to have a long transaction running , Consider suspending DDL perhaps kill Long Affairs

Special scenes

If the table you want to change is a hotspot table , Although the amount of data is small , But the requests are frequent , And you have to add a field , What should you do

• stay alert table Statement to set the waiting time , If the specified waiting time can be obtained MDL It's better to write lock , Don't block the following business statements if you can't get them , Give up first , Then the developer or DBA Repeat the process by retrying the command .