How to understand MySQL's lock and concurrency control technology?

How to understand MySQL's lock and concurrency control technology?

MySQL is a commonly used relational database management system. It supports concurrent access and operation of data, and also provides some lock and concurrency control technologies to ensure data consistency and concurrency. This article will introduce MySQL's lock and concurrency control technology in detail, and use code examples to deepen understanding.

1. MySQL’s concurrent access

Before understanding MySQL’s lock and concurrency control technology, let’s first understand how MySQL supports concurrent access. MySQL handles concurrent operation requests through multiple threads, including the main thread, query thread and update thread.

The main thread is responsible for receiving and processing requests from clients and forwarding them to the corresponding thread for processing. The query thread is responsible for processing query operations and returning query results to the client. The update thread is responsible for handling operations such as inserts, updates, and deletes and reflecting them into the storage engine.

2. MySQL’s locking mechanism

MySQL’s locking mechanism is an important means to ensure data consistency and concurrency. According to the granularity of the lock, MySQL locks can be divided into table-level locks and row-level locks.

1. Table-level lock

Table-level lock locks the entire table, which can limit other transactions' read and write operations on the table. Table-level locks are divided into two modes: shared locks (also called read locks) and exclusive locks (also called write locks).

Shared locks (S locks) can be acquired by multiple transactions at the same time to ensure concurrent access to shared resources. Multiple transactions can hold multiple shared locks at the same time, but when a transaction holds a shared lock, other transactions cannot obtain an exclusive lock.

Exclusive lock (X lock) is a lock acquired when writing to a table. It only allows one transaction to acquire it exclusively and is used to ensure data consistency. When a transaction holds an exclusive lock, other transactions cannot obtain shared locks or exclusive locks.

2. Row-level lock

Row-level lock locks a row in the table, which can restrict other transactions from reading and writing the row. Row-level locks can refine the lock granularity and improve concurrency performance. MySQL's row-level locks are mainly divided into shared locks and exclusive locks.

Shared locks (S locks) are used to ensure concurrent read operations of the same row by multiple transactions. Multiple transactions can acquire shared locks at the same time, but when a transaction holds a shared lock, other transactions cannot acquire an exclusive lock.

Exclusive lock (X lock) is used to ensure the consistency of concurrent write operations on the same row. When a transaction holds an exclusive lock, other transactions cannot obtain shared locks or exclusive locks.

3. MySQL’s concurrency control technology

MySQL’s concurrency control technology mainly includes optimistic concurrency control (Optimistic Concurrency Control) and pessimistic concurrency control (Pessimistic Concurrency Control).

1. Optimistic concurrency control

Optimistic concurrency control is a version number-based mechanism that assumes that conflicts between transactions rarely occur. Optimistic concurrency control in MySQL is mainly achieved by using version numbers and CAS (Compare and Swap) operations.

The sample code is as follows:

-- 创建表
CREATE TABLE `books` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) NOT NULL,
  `author` VARCHAR(100) NOT NULL,
  `version` INT(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

-- 插入数据
INSERT INTO `books` (`name`, `author`) VALUES ('book1', 'author1');
INSERT INTO `books` (`name`, `author`) VALUES ('book2', 'author2');

-- 更新数据
UPDATE `books` SET `author` = 'new author' WHERE `id` = 1 AND `version` = 0;

In the above code, each row of the books table has a version number (version field), update The version numbers need to be compared during the operation. If the version numbers match, the update operation is performed; otherwise, it means that the row data has been modified by other transactions and the update fails.

2. Pessimistic Concurrency Control

Pessimistic concurrency control is a locking-based mechanism that assumes that conflicts between transactions occur frequently. Pessimistic concurrency control in MySQL is mainly achieved through the use of locks.

The sample code is as follows:

-- 开启事务
START TRANSACTION;

-- 查询数据并上锁
SELECT * FROM `books` WHERE `id` = 1 FOR UPDATE;

-- 更新数据
UPDATE `books` SET `author` = 'new author' WHERE `id` = 1;

-- 提交事务
COMMIT;

In the above code, when the query operation is performed using the FOR UPDATE statement, an exclusive lock will be added to the queried data row. The lock operation ensures that other transactions cannot read or modify the row of data.

4. Summary

MySQL's lock and concurrency control technology are important means to ensure data consistency and concurrency. Through the flexible use of table-level locks and row-level locks, as well as optimistic concurrency control and pessimistic concurrency control, the concurrency performance of the system and data consistency can be effectively improved. In actual applications, it is necessary to choose an appropriate concurrency control strategy based on business needs and system characteristics to obtain better performance and user experience.

The above is a detailed introduction on how to understand MySQL's lock and concurrency control technology. Through the demonstration of sample code, I hope readers can better understand and apply MySQL's lock and concurrency control technology.

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