Lock mechanism in MySQL

MySQL Common storage engine lock mechanism
MyISAM and MEMORY： Table lock (table-level locking)
BDB： Page level lock (page-level locking) Or watch lock , The default is page level lock
InnoDB： Row-level locks (row-level locking) And watch level locks , The default is row level lock
One 、 Classification of locks
1、 According to the granularity of the lock ：

Table lock ：
characteristic ：
Lock the whole table of current operation . Locked fast , Less resource consumption , A deadlock will not occur . Maximum locking granularity , The probability of triggering lock conflict is the highest , Lowest degree of concurrency .MyISAM and InnoDB The engine supports table level locks .

lock table  Table name  read;
--  Can only read , It can't be modified , But another user can use , Only the locked watch can be operated , Other tables cannot be accessed 
unlock table;
 --  Unlock 

Page level lock ：
characteristic ：
The locking granularity is between row level lock and table level lock , The watch lock is fast , But there are many conflicts , Less line level conflict , But the speed is slow. . Page level compromises , Lock an adjacent set of records at a time . Cost and lock time are between table lock and row lock , A deadlock occurs . The concurrency is average .BDB Support page level lock .

Row-level locks ：
characteristic ： Lock only the row of the current operation . Locking granularity minimum , High concurrency , It can reduce database operation conflicts . But locking costs a lot , Slow speed , A deadlock occurs .InnoDB Row level locking is supported .

InnoDB Supported row level lock types ：
1、Record Lock: Lock index entries , Lock qualified rows . Other transactions cannot modify or delete locked items ;
2、Gap Lock: Between index items “ The gap ” Lock , Lock the range of records （ Lock the gap before the first record or the gap after the last record ）, Does not contain the index entry itself . Other transactions cannot insert data within the scope of the lock , This prevents other transactions from adding phantom lines .
3、Next-key Lock： Lock the index entry itself and the index range . namely Record Lock and Gap Lock The combination of . It can solve the problem of unreal reading （ It's also MySQL The default isolation level is read repeatedly ）.
2、 According to the function of lock

Shared lock ：
characteristic ： Shared lock （Share Locks, Short for S） Also known as read lock , If business T For data objects A add S lock , The transaction T Only read A, Other affairs can only be right again A Add S lock , Instead of X lock , until T Release A Upper S lock . This ensures that other transactions can be read A, But in T Release A Upper S You can't do it before you lock it A Make any changes . Other users can read data concurrently , But no transaction can acquire an exclusive lock on the data , Until all shared locks have been released .
usage ：

SELECT …LOCK IN SHARE MODE;

Exclusive lock ：
characteristic ： Exclusive lock （Exclusive lock, Short for X lock ） Also known as write lock , If business T For data objects A add X lock , Only T Read and modify A, Nothing else can be done to A Add any type of lock , until T Release A The lock on the . It prevents any other transaction from acquiring a lock on the resource , Until the original lock on the resource is released at the end of the transaction . In the update operation (INSERT、UPDATE or DELETE) Always apply an exclusive lock in the process .
usage ：

SELECT …FOR UPDATE;

3、 Intent locks

effect ：
When a transaction needs to acquire a resource lock , If you encounter the resource that you need has been occupied by exclusive lock , The transaction may need to lock the table of rows and add an appropriate intent lock on it . Intentional locks can coexist .

Intention sharing lock （IS）：
characteristic ： Indicates that the transaction is ready to record the shared lock for the data row , Before a transaction applies a shared lock to a data row, it must first obtain the data of the table IS lock .

Intention exclusive lock （IX）：
characteristic ： Indicates that the transaction is ready to add an exclusive lock to the data row , The transaction must obtain the data of the table before adding an exclusive lock to a data row IX lock .

Be careful ：IX,IS Is the table level lock , Not at the row level X,S Lock conflict , It'll only work at the table level X,S conflict .
Two 、 Deadlock and deadlock avoidance
There is a deadlock situation ：

InnoDB Row level locking is implemented based on index , If the query statement is to hit any index , that InnoDB Can use watch level lock . If the table does not have any indexes defined , that InnoDB A hidden cluster index will be created and used to lock records .

MyISAM Always get all the locks you need at once ,InnoDB The lock is acquired step by step , When both transactions need to obtain the lock held by the other party , Causing both sides to wait , This creates a deadlock . After a deadlock occurs ,InnoDB It's generally detectable , And causes a transaction to release the lock back , The other can acquire a lock to complete the transaction .
How to avoid deadlock ：

1、 Reduce the probability of deadlocks by using table-level locks ;
2、 Multiple programs try to agree to access tables in the same order ;
3、 The same transaction locks as many resources as possible at once .