MySQL MVCC principle analysis: Why is it the best choice for concurrency control?

MySQL MVCC principle analysis: Why is it the best choice for concurrency control?

In a relational database, data consistency and concurrency control are crucial. As one of the most popular relational database management systems, MySQL uses the MVCC (Multi-Version Concurrency Control, multi-version concurrency control) mechanism to achieve concurrency control. This article will provide an in-depth analysis of the MySQL MVCC principle and discuss why it is the best choice for concurrency control.

1. The basic concept of MVCC
   The MVCC mechanism is an optimistic concurrency control strategy that allows multiple transactions to read and modify database data concurrently without interfering with each other. Each transaction creates a visibility view at the beginning, which determines the version of the data that the transaction can see.

In MVCC, each data row will have multiple versions, and each version has a timestamp mark. When a transaction starts, it creates a visibility view based on the timestamp of the transaction start time. In this view, a transaction can only see row versions that have been committed before its start time.

2. The implementation principle of MVCC
   MySQL uses multiple data structures to support the implementation of MVCC, the most important of which are undo log and Read View.

a. Undo log
Undo log is an operation record used to roll back a transaction. When a transaction starts, MySQL will create an undo log for it to record the modification operations of the transaction on the database. When a transaction is rolled back, the undo log can be used to restore the data to its pre-modification state.

b. Read View
Read View is a logical timestamp, which determines the data version that a transaction can see when reading data. Each transaction has its own Read View, which determines visibility based on the timestamp of the start of the transaction.

The implementation of Read View relies on the version chain (Version Chain) mechanism in the InnoDB storage engine. When a transaction starts, it finds the data versions that comply with the visibility rules from the version chain and records them in the Read View. In this way, during transaction execution, it can only see those data versions recorded in Read View.

3. Advantages of Concurrency Control
   The MVCC mechanism provides many advantages for concurrency control, making it the best choice:

a. High concurrency
MVCC allows multiple transactions to read and modify database data concurrently, improving the concurrency performance of the system. Since each transaction has its own visibility view, transactions do not interfere with each other. In this way, the system can execute multiple transactions at the same time, reducing lock competition and improving concurrency performance.

b. High isolation
MVCC provides a higher level of isolation. Read operations are not blocked by write operations, and write operations are not blocked by read operations. In this way, read operations and write operations can be executed concurrently, improving the concurrency of the system.

c. Improved the flexibility of concurrency control
MVCC allows transactions to read and modify data without any locks. This optimistic concurrency control strategy reduces the use of locks and reduces the performance loss caused by lock conflicts. At the same time, MVCC enables transactions to be executed in a non-blocking manner, improving the response performance of the system.

4. Code Example
   The following is a simple MySQL MVCC example that demonstrates concurrent execution of transactions and the use of visibility views:

-- Create a test table
CREATE TABLE test (
id INT PRIMARY KEY,
name VARCHAR(50),
age INT
) ENGINE=InnoDB;

--Open transaction 1
START TRANSACTION;

--Transaction 1 inserts a piece of data
INSERT INTO test (id, name, age) VALUES (1, 'Alice', 20);

--Open Transaction 2, start timestamp is t2
START TRANSACTION;

--Transaction 2 reads data
SELECT * FROM test WHERE id = 1;

--Transaction 2 modifies Data
UPDATE test SET age = 25 WHERE id = 1;

-- Submit transaction 2
COMMIT;

-- Transaction 1 read data
SELECT * FROM test WHERE id = 1;

-- Commit transaction 1
COMMIT;

In the above example, transaction 1 and transaction 2 are executed concurrently. The time point (t2) when transaction 2 reads data is before transaction 1 commits. Therefore, transaction 2 can only see the data version before transaction 1 commits. After transaction 1 is submitted, transaction 2 can see the modification results of transaction 1.

With this example, we can clearly see how MVCC’s visibility view works and why it is the best choice for concurrency control.

Summary:
The MySQL MVCC mechanism is an optimistic concurrency control strategy that allows multiple transactions to read and modify database data concurrently. MVCC implements visibility view management by using undo logs and Read View. MVCC has the advantages of high concurrency, high isolation, and flexibility, making it the best choice for concurrency control. By in-depth understanding of MVCC principles, developers can better design and optimize database systems and improve the performance and efficiency of concurrency control.

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