MySQL MVCC Principle Analysis and Application Guide

MySQL MVCC Principle Analysis and Application Guide

Abstract:
MySQL is a very popular relational database management system with good concurrency performance. This is due to MySQL's multi-version concurrency control (MVCC) technology. This article will delve into the principles of MySQL MVCC and provide some guidance on practical application scenarios.

1. Introduction
   MVCC is a technology used to control concurrent access to the database. MySQL uses an MVCC-based storage engine, such as InnoDB, which performs well in transaction concurrency control.
2. MVCC Principle
   MVCC achieves concurrency control by creating an independent snapshot for each transaction. Each transaction gets a system version number at the beginning, which determines the range of data it can see. Each data row has a creation version and an expired version. A transaction can only see data rows whose creation version is less than or equal to its version number and whose expired version is greater than its version number. In this way, read and write operations between different transactions will not interfere with each other.
3. Code Example
   In order to better understand how MVCC works, a simple code example is given below. Suppose there is a table named "students" that contains two fields: id and name.

-- 创建表
CREATE TABLE students (
  id INT PRIMARY KEY,
  name VARCHAR(100)
) ENGINE=InnoDB;

-- 添加数据
INSERT INTO students (id, name) VALUES (1, 'Alice');
INSERT INTO students (id, name) VALUES (2, 'Bob');
INSERT INTO students (id, name) VALUES (3, 'Charlie');

Now, let’s start two transactions to read and modify the data respectively.

-- 事务1
START TRANSACTION;
SELECT * FROM students;

-- 事务2
START TRANSACTION;
SELECT * FROM students;

During the execution of transaction 1, before transaction 2 starts, we modify the data.

-- 事务1
UPDATE students SET name = 'Eve' WHERE id = 1;

At this time, in transaction 1, we can see that the record with id 1 has been modified, and in transaction 2, we can also see the original data. This is due to MVCC's snapshot mechanism.

-- 事务1
COMMIT;

-- 事务2
SELECT * FROM students;

After transaction 1 is submitted, the modified data can also be seen in transaction 2.

4. Application Guide
   MVCC technology has a wide range of uses in practical applications. The following are some practical guidelines for using MVCC.

4.1. Read-write separation
Due to the existence of MVCC technology, we can use the read-write separation architectural pattern in MySQL. Multiple read-only instances can read data from the main database, which can increase the reading performance of the system.

4.2. Concurrency Tuning
MVCC can effectively improve the concurrency of the database, especially in read-intensive scenarios. System performance can be better optimized by properly setting transaction isolation levels and adjusting parameters such as the number of database connections.

4.3. Avoid lock contention
Using MVCC can avoid contention problems caused by traditional row-level locks. Multiple transactions can read data concurrently without blocking each other, improving the concurrency performance of the system.

Conclusion:
MySQL's MVCC technology is one of the important means to achieve high concurrency performance. This article introduces the principles of MVCC and how to apply MVCC in practical applications. By understanding and mastering the working principle of MVCC, the performance of the database system can be better optimized.

Reference materials:

1. https://dev.mysql.com/doc/refman/8.0/en/innodb-multi-versioning.html

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