MySQL MVCC principle revealed: How to deal with read and write conflicts in concurrent transactions?

MySQL MVCC principle revealed: How to deal with read and write conflicts in concurrent transactions?

Introduction:
In a database system, concurrent execution of transactions is essential. However, concurrent execution also brings a series of problems, one of which is read and write conflicts. When multiple transactions read and write the same data at the same time, inconsistencies may occur. To solve this problem, MySQL introduced the multi-version concurrency control (MVCC) mechanism. This article will reveal the principle of MVCC and analyze in detail how MySQL handles read and write conflicts in concurrent transactions.

1. MVCC Overview
   MVCC is a mechanism to implement concurrency control, which uses version numbers to isolate transactions. Each data row will have a version number, and read and write operations are judged based on the version number. Read operations can only read committed transactions, while write operations require judgment and processing of other transactions.
2. Transaction read operation
   When a transaction performs a read operation, MySQL will determine the visible data rows based on the transaction startup time and snapshot version number. The specific judgment conditions are as follows:

a) If the creation version number of the data row is greater than the transaction start time, it means that the data row was created later, then this transaction is not visible.
b) If the deleted version number of the data row is less than or equal to the transaction start time, it means that the data row has been deleted, and then this transaction is not visible.
c) If the creation version number of the data row is less than or equal to the transaction start time, and the deletion version number is greater than the transaction start time or is empty, then this transaction is visible.

Through the above rules, a transaction can read data that has been submitted before it is started, but uncommitted data and data modified by other executing transactions are invisible.

3. Transaction write operation
   When a transaction performs a write operation, MySQL will judge and process it based on the version number of the data row. The specific processing method is as follows:

a) If transaction A wants to modify the data row, but the data row has been modified by other transaction B (that is, the version number does not match), then transaction A will roll back, An error message indicates a write operation conflict.
b) If the transaction wants to delete the data row, but the data row has been modified by other transactions (that is, the version number does not match), then the transaction will create a new version of the data row and set the deletion mark to the version number of the current transaction .
c) If the data row to be modified or deleted by the transaction does not exist (that is, the version number is empty), the transaction will create a new version of the data row, and the version number is set to the version number of the current transaction.

Through the above processing methods, MySQL ensures that transaction write operations will not cause data conflicts and inconsistencies.

Sample code:
In order to better understand the principle of MySQL MVCC, a sample code is given below to demonstrate the processing process in the case of read and write conflicts in concurrent transactions.

-- 创建测试表
CREATE TABLE test (
    id INT PRIMARY KEY,
    value VARCHAR(20) NOT NULL,
    version INT NOT NULL
);

-- 插入测试数据
INSERT INTO test (id, value, version) VALUES (1, 'A', 1);

-- 事务1：读操作
START TRANSACTION;
SELECT * FROM test WHERE id = 1;
-- 结果：id=1, value='A', version=1

-- 事务2：写操作
START TRANSACTION;
-- 修改数据行，并将version+1
UPDATE test SET value = 'B', version = version + 1 WHERE id = 1;
-- 提交事务
COMMIT;

-- 事务1：再次读操作
SELECT * FROM test WHERE id = 1;
-- 结果：id=1, value='B', version=2

Through the above example code, you can see that after transaction 2 modifies the data row, when transaction 1 reads the data again, the modified data row has been read and the version value has been updated, ensuring ensure data consistency.

Conclusion:
MySQL's MVCC mechanism solves the read-write conflicts of concurrent transactions through the judgment and processing of version numbers. By comparing the transaction start time, snapshot version number and data row version number, MySQL achieves data isolation and consistency. In practical applications, rational use of the MVCC mechanism can improve the concurrency and performance of the database.

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

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