Overview of locks

MySQL lock is a commonly used mechanism when operating MySQL database. MySQL lock can ensure that multiple users can cooperate with each other when performing read and write operations at the same time, avoiding problems such as data inconsistency or read and write conflicts. This article will introduce the basic knowledge and specific application of MySQL lock in detail.
MySQL lock is a typical concurrency control mechanism in a multi-user database system, which allows multiple simultaneous operations to complete corresponding operations. When multiple users access the same series of tables at the same time, it is easy to have read and write conflicts. By using the MySQL lock mechanism, you can ensure that there will be no inconsistencies when querying the database.

Classification of locks

MySQL locks are a mechanism for controlling concurrent access. According to the characteristics and usage scenarios of locks, MySQL locks can be divided into two types: shared locks and exclusive locks.

A shared lock is a lock that allows concurrent reading of resources, also known as a read lock. Multiple users can acquire the shared lock of the same resource at the same time, but when the shared lock is held, no user can acquire the exclusive lock of the resource, that is, the write lock. Shared locks can effectively avoid data inconsistency problems caused by multiple users modifying resources at the same time.

An exclusive lock is a lock that locks a resource, also known as a write lock. When a user uses an exclusive lock to write to a resource in the database, other users cannot obtain any type of lock on the resource, including shared locks and exclusive locks. Exclusive locks are mainly used to solve the concurrency problem that multiple users write to the same resource at the same time.

In MySQL, different lock levels can be used to control transactions and concurrent access, including read uncommitted, read committed, repeatable read, and serialization levels. According to the needs of the scene, different lock levels can be set to avoid data conflicts and performance problems caused by multiple accesses to the same data. Among them, when the default repeatable read transaction isolation level is used, MySQL will automatically add shared locks for read operations and exclusive locks for write operations.

Application scenarios of locks

Database transaction management

In MySQL, by using transaction mechanism and lock mechanism, concurrency problems caused when multiple users access the same database resource can be avoided. Using the transaction isolation level to manage transactions can control the access mode of users to operate the database. At the same time, the MySQL lock mechanism can be used to lock specific resources in the database, thereby avoiding data access conflicts and data inconsistencies.

Multi-threaded program development

In the development of multi-threaded programs, in order to ensure the consistency of data operations and the security of threads, using the MySQL lock mechanism can effectively avoid the problem that multiple threads access the same database object at the same time. By using shared locks and exclusive locks to ensure the integrity of the database, multiple threads can work better together.

Database backup and recovery

During the backup and recovery process of the database, the MySQL lock mechanism can be used to lock the read and write operations of the database table to ensure the integrity of the database during backup and recovery. By setting the lock level, you can avoid data inconsistency caused by concurrent operations and ensure data integrity.

For high-concurrency application scenarios such as online games

In high-concurrency application scenarios such as online games, multiple players may access the database at the same time. Coordinated operations are an important part of ensuring the stability of the game. Using the MySQL lock mechanism can prevent multiple players from accessing the same resource at the same time. Issues such as access violations and data inconsistencies.

How to use the lock

According to the type and application scenarios of MySQL locks, shared locks or exclusive locks can be used for database access control. Shared locks allow multiple users to read a common resource, while exclusive locks control write operations to the database. In MySQL, different lock levels can be used to control transactions and concurrent access, including uncommitted read, read committed, repeatable read, and serialization. According to different scenarios, different lock levels can be set to avoid data conflicts and performance problems caused by multiple accesses to the same data. Among them, when using the default repeatable read transaction isolation level, MySQL will automatically add shared locks for read operations and exclusive locks for write operations. In addition, the following methods can also be used to use MySQL locks:

Lock the table
In MySQL, use the LOCK TABLES and UNLOCK TABLES statements to lock and unlock the entire table. In particular, during data backup and recovery operations, in order to avoid data changes, EXCLUSIVE locks can be used to lock required data during backup and recovery.
Lock the row
In MySQL, the specified row can be locked by the SELECT...FOR UPDATE statement. SELECT... FOR UPDATE will add exclusive locks to all rows in the query result set to achieve its query purpose.

Application examples of locks

In the MySQL database, if multiple users read the same data item at the same time, data conflicts or inconsistencies are prone to occur, and shared locks are needed to ensure the correctness of the data. The following is an application example of a shared lock:

Suppose there is an order table order, and multiple users execute query operations at the same time, the code is as follows:

SELECT * FROM order WHERE status = 1;

In the above code, if multiple users perform query operations at the same time without any data modification operations, using shared locks can ensure that each user only reads correct data during the query period.

Shared or exclusive locks can be implemented by using the FOR SHARE or FOR UPDATE clause in the query statement. As follows:

SELECT * FROM order WHERE status = 1 FOR SHARE;

This statement will add a shared lock to the result set obtained from the query, so that other users can share access to each data item in the result set, avoiding data conflicts or data inconsistencies.

Application examples of exclusive locks
Exclusive locks in MySQL are often used to deal with read-write resource conflicts. Common scenarios include operations such as data modification, data deletion, and data insertion. The following is an application example of an exclusive lock:
Suppose there is a user table user, and multiple users need to perform concurrent storage operations on the table, the code is as follows:

UPDATE user SET balance = balance + 100 WHERE id = 1;

In the above example, if multiple users modify the balance of the same user, data conflicts will arise. An exclusive lock can be used to avoid this problem. Locking data items can be achieved by using the FOR UPDATE clause in the query statement, examples are as follows:

START TRANSACTION; 
SELECT * FROM user WHERE id = 1 FOR UPDATE;
UPDATE user SET balance = balance + 100 WHERE id = 1;
COMMIT;

In the above code, the SELECT statement uses the FOR UPDATE clause, which can obtain an exclusive row-level lock to prevent other users from modifying the same row at the same time.

The MySQL lock mechanism has an important application in database development, which can avoid abnormal operation and wrong modification of data, thereby ensuring the correctness and consistency of the database. In actual programming, it is necessary to select an appropriate MySQL lock mechanism based on specific business needs and performance issues, and use lock levels and lock types reasonably to ensure the normal operation of the system.

Summarize

The MySQL locking mechanism is an important part of the MySQL database. This article introduces the basic knowledge and application scenarios of MySQL locks, and gives two specific application examples. When actually writing code, business needs and performance issues need to be considered comprehensively, and MySQL locks need to be used cautiously and reasonably.

In-depth understanding of MySQL lock mechanism and application scenarios