How to understand and apply MySQL MVCC principles

How to understand and apply MySQL MVCC principle

Introduction:

MySQL is a commonly used relational database management system, which uses MVCC ( Multi-Version Concurrency Control) principle to ensure data consistency and concurrency. MVCC is a transaction concurrency control method that implements read and write operations on data based on version management.

This article will introduce the basic concepts of MVCC principle and how to apply MVCC in MySQL.

1. Overview of MVCC Principle

MVCC is a mechanism introduced by MySQL to improve concurrency. In traditional concurrency control methods, such as lock mechanisms, when a transaction modifies certain data, other transactions need to wait for the transaction to release the lock before modifying the data. This approach will result in a large number of transaction waits, thus affecting concurrency performance.

In contrast, MVCC adopts a multi-version synchronization control method. Each transaction can see a snapshot of the database, which is the data status at a certain point in time in the database.

In MVCC, each database record has a version number (or timestamp). When a transaction modifies a record, it actually creates a new version of the record. When other transactions read the record, they can only read the version before the transaction started, and the modification operation of the record by the transaction will not affect the read operation of the record by other transactions.

2. MVCC implementation in MySQL

In order to implement MVCC, MySQL stores multiple versions of each database row record. In the InnoDB storage engine, each row of records contains three hidden columns, namely transaction ID (Transaction ID), creation time and expiration time. The transaction ID is used to indicate under which transaction the update operation occurred, the creation time is used to indicate the creation time of the version, and the expiration time is used to indicate the effective time range of the version.

In MySQL, the operations of reading data can be divided into two types:

1. Consistent Read: When reading data, only the current transaction can be read The data version before starting. This ensures that the data read within a transaction is consistent.
2. Snapshot Read: When reading data, you can read the latest data version. This reading method is suitable for scenarios where there is no concurrent writing and can improve reading efficiency.

3. Application of MVCC principle

The MVCC principle is widely used and can be used to solve the problem of read and write conflicts of multiple concurrent transactions on the same data.

The following is a simple Python code example that demonstrates how to use MVCC to implement concurrency control in MySQL.

import threading
import time
import pymysql

# 创建多个线程并发执行事务
def execute_transactions():
    conn = pymysql.connect(host='localhost', user='root', password='password', db='test', charset='utf8mb4')
    cursor = conn.cursor()
    
    try:
        cursor.execute('BEGIN')
        cursor.execute('SELECT balance FROM accounts WHERE id = 1')
        balance = cursor.fetchone()[0]
        time.sleep(1)  # 模拟其他事务操作
        cursor.execute('UPDATE accounts SET balance = %s WHERE id = 1', (balance - 100,))
        cursor.execute('COMMIT')
    except Exception as e:
        cursor.execute('ROLLBACK')
        print(repr(e))
    finally:
        cursor.close()
        conn.close()

# 创建多个线程并发执行事务
threads = []
for i in range(10):
    t = threading.Thread(target=execute_transactions)
    threads.append(t)

# 启动线程
for t in threads:
    t.start()

# 等待所有线程执行结束
for t in threads:
    t.join()

In the above code example, we created multiple threads to execute transactions concurrently. Each transaction reads the account balance from the database and deducts 100 yuan. Due to the use of the MVCC mechanism, each transaction can only read the data version before the transaction starts, and the modification operations on the account balance will not affect each other. In this way, data consistency and concurrency are guaranteed.

Summary:

This article introduces the principle of MVCC and its application in MySQL. MVCC implements concurrent access control to data through the version management mechanism, improving the concurrency performance of the database. In practical applications, we can use MVCC to solve the problem of read and write conflicts between multiple concurrent transactions on the same data.

The above is the detailed content of How to understand and apply MySQL MVCC principles. For more information, please follow other related articles on the PHP Chinese website!