MySQL MVCC principle analysis: How to solve data concurrency problems?

MySQL MVCC principle analysis: How to solve data concurrency problems?

In database systems, data concurrency issues are a very important and common challenge. When multiple users perform read and write operations on the database at the same time, problems such as data inconsistency and lost updates may occur. In order to solve these problems, MySQL introduced the MVCC (Multiple Version Concurrency Control) mechanism.

MVCC is a concurrency control technology that achieves isolation between different transactions by creating a database snapshot for each transaction. While one transaction is reading data, other transactions can continue to modify the data without interfering with each other. This mechanism greatly improves the concurrency and performance of the database system.

Let’s analyze the principles of MySQL MVCC and demonstrate it through code examples.

1. Transaction and version number

In MySQL, each transaction has a unique transaction ID (transaction ID), which is used to identify the transaction. At the same time, each data row also has a version number (or creation timestamp), which is used to identify the version of the data row.

2. Data Snapshot

When a transaction starts, MySQL will create a database snapshot for the transaction. This snapshot is an image based on the current database state, reflecting the version numbers of all data rows at that time.

3. Read operation

When a transaction performs a read operation, it determines visibility based on its own transaction ID and the version number of the data row. If the version number of a data row is later than the start time of the transaction, then the transaction cannot see the update of the data row. This can avoid "dirty read" and "non-repeatable read" problems.

4. Write operation

When a transaction performs a write operation, MySQL will create a new version number for it and insert the new version of the data row into the database . At the same time, the original data rows remain unchanged to ensure that other transactions can continue to read the old version of the data.

5. Transaction Commit and Rollback

When a transaction commits, it updates the version numbers of all modified data rows to the version number of the current transaction. In this way, other transactions can see the modifications of this transaction. If a transaction is rolled back, its modifications are undone and the row version numbers are restored to the state they were in before the transaction began.

Now, let us demonstrate the application of MySQL MVCC through a simple sample code.

Suppose we have a table "employee" which contains employee information, including name and salary. Our goal is to achieve concurrent read and write operations while ensuring the correctness of the data.

First, we create a table and insert several pieces of data:

CREATE TABLE employee (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(50),
salary INT
);

INSERT INTO employee (name, salary) VALUES ('Alice', 5000);
INSERT INTO employee (name, salary) VALUES ('Bob', 6000);
INSERT INTO employee (name, salary) VALUES ('Charlie', 7000);

Next, we use two different transactions to modify and query the salary at the same time:

-- Transaction 1:
START TRANSACTION;

UPDATE employee SET salary = 5500 WHERE name = 'Alice';

-- Transaction 2:
START TRANSACTION;

SELECT * FROM employee WHERE name = 'Alice';

Transaction 1 will update Alice's salary to 5500, while transaction 2 will read Alice's salary. According to the principle of MVCC, transaction 2 will not see the updates of transaction 1 because transaction 2 is started earlier than the modification of transaction 1.

Finally, we submit these two transactions and query the results:

-- Transaction 1:
COMMIT;

-- Transaction 2:
SELECT * FROM employee WHERE name = 'Alice';

The result returned by transaction 2 is still the original salary of 5000, not the modified 5500 by transaction 1.

To sum up, MySQL MVCC is an effective concurrency control technology. It achieves the isolation and isolation of concurrent operations by creating a snapshot for each transaction and determining the visibility of data based on the version number. consistency. Developers can make full use of MySQL's concurrency capabilities when writing applications based on the principles of MVCC to improve system performance and reliability.

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