Principles and application practices of MySQL locks

MySQL lock principle and application practice

Abstract: MySQL is a commonly used relational database management system that has powerful concurrent processing capabilities. When multiple users access the database at the same time, in order to ensure data consistency and integrity, MySQL uses a lock mechanism to control access to shared resources. This article will introduce the principles of MySQL locks, including lock levels, lock classifications and lock conflict handling methods, and combine it with specific code examples to demonstrate the application practice of MySQL locks.

1. Introduction
   With the rapid development of the Internet, the demand for concurrent access to databases is increasing. When multiple users access the database at the same time, if there is no appropriate locking mechanism, data consistency and integrity problems will occur. Therefore, MySQL provides a rich locking mechanism to ensure data integrity and reliability.
2. Principle of lock
   In MySQL, lock is a mechanism used to restrict access to shared resources. When a user or connection needs to modify a resource, it can request a lock. If the resource is already locked by another user or connection, the request will be blocked until the resource is released. MySQL supports multiple lock levels, including table-level locks and row-level locks.
3. Classification of locks
   MySQL locks can be divided into two categories: shared locks and exclusive locks. A shared lock is also called a read lock, which allows multiple users to read the resource at the same time, but does not allow modification operations to the resource. An exclusive lock is also called a write lock. It only allows one user to modify the resource. Other users must wait for the write lock to be released before they can read or modify the resource.
4. Lock levels
   MySQL provides multiple lock levels, and you can choose the appropriate lock level according to actual needs. Common lock levels include:

4.1 Table-level locks: lock the entire table, suitable for scenarios where the entire table is operated, such as backup, table structure changes, etc. .

4.2 Row-level locks: Lock a certain row or several rows in the table. It is suitable for scenarios where a single or a small amount of data is operated, such as query, update, etc.

5. Lock conflict handling methods
   MySQL has different handling methods for lock conflicts, mainly including waiting for locks and immediate return.

5.1 Waiting for locks: When a request cannot obtain the required lock, MySQL will add the request to the waiting queue and wait for the lock to be released before processing.

5.2 Return immediately: When a request cannot obtain the required lock, MySQL will return an error message immediately without entering the waiting queue.

6. Application practice example
   The following uses a specific example to demonstrate the application practice of MySQL lock.

6.1 Create a test table
First, we create a test table to simulate the actual data operation scenario.

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `age` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

6.2 Add data
Next, we add some test data to the table.

INSERT INTO `user` (`name`, `age`)
VALUES ('张三', 20), ('李四', 25), ('王五', 30);

6.3 Use exclusive lock to update data
Then, we use exclusive lock to update the data.

START TRANSACTION;
SELECT *
FROM `user`
WHERE `age` = 25
FOR UPDATE;
UPDATE `user`
SET `age` = 26
WHERE `age` = 25;
COMMIT;

6.4 Use shared locks to read data
Finally, we use shared locks to read data.

START TRANSACTION;
SELECT *
FROM `user`
WHERE `age` = 26
LOCK IN SHARE MODE;
COMMIT;

7. Conclusion
   The MySQL lock mechanism is an important tool to ensure concurrent data access and plays a key role when multiple users access the database at the same time. By understanding the principles, classifications, and application examples of MySQL locks, we can better understand and use MySQL's lock mechanism to ensure data consistency and integrity.

Reference materials:
1. "MySQL 5.7 Reference Manual"
2. "High-Performance MySQL: Optimization, Backups, and Replication" book

The above is This introduction to the principles and application practices of MySQL locks is hoped to be helpful to everyone when using MySQL for database concurrency processing.

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