Comparison of multi-version concurrency control (MVCC) between MySQL and TiDB

MySQL and TiDB are two commonly used relational databases. They use different multi-version concurrency control (MVCC) mechanisms to implement concurrent access control. In this article, we will compare the MVCC mechanisms of these two databases and give code examples to better understand how they work.

The MVCC mechanism is a technology used to handle concurrent access, which allows multiple transactions to access different versions of the database at the same time. This mechanism can improve the concurrency performance of the database, avoid conflicts between transactions, and ensure data consistency.

First, let’s take a look at MySQL’s MVCC mechanism. In MySQL, each database transaction has a unique transaction ID (TxnID). Whenever a transaction is executed in the database, it creates a new version to store the modified data. This new version will contain a copy of the original data and store relevant transaction information (such as transaction ID and transaction status) in the version. This way, other transactions can continue to access versions of the original data without being affected by the executing transaction. When a transaction commits or rolls back, the corresponding version will be cleared or marked as invalid.

The following is a MySQL MVCC sample code:

-- 创建一个表 CREATE TABLE students ( id INT PRIMARY KEY, name VARCHAR(50), age INT ); -- 启动一个事务 START TRANSACTION; -- 插入一条记录 INSERT INTO students (id, name, age) VALUES (1, 'Tom', 20); -- 更新记录 UPDATE students SET age = 21 WHERE id = 1; -- 提交事务 COMMIT; -- 查看最新版本的数据 SELECT * FROM students; -- 启动另一个事务 START TRANSACTION; -- 查询数据（读取旧版本） SELECT * FROM students; -- 提交事务 COMMIT;

Next, let’s take a look at TiDB’s MVCC mechanism. TiDB is a distributed relational database system based on the design ideas of Google Spanner paper and uses an MVCC algorithm called Snapshot Isolation. In TiDB, each transaction has a globally unique transaction ID (StartTS). Whenever a transaction is executed, it creates a new timestamp (TS) as the start time of the transaction. In TiDB, each data row has a start time and end time range, indicating the validity period of the data version. A transaction can only read data versions committed before its start time, which avoids reading dirty data and non-repeatable reads.

The following is a TiDB MVCC sample code:

-- 创建一个表 CREATE TABLE students ( id INT PRIMARY KEY, name VARCHAR(50), age INT ); -- 启动一个事务 BEGIN; -- 插入一条记录 INSERT INTO students (id, name, age) VALUES (1, 'Tom', 20); -- 更新记录 UPDATE students SET age = 21 WHERE id = 1; -- 提交事务 COMMIT; -- 查看最新版本的数据 SELECT * FROM students; -- 启动另一个事务 BEGIN; -- 查询数据（读取旧版本） SELECT * FROM students; -- 提交事务 COMMIT;

As can be seen from the above code example, there are some differences in the implementation of MVCC between MySQL and TiDB. MySQL uses transaction IDs to manage data versions, while TiDB uses timestamps to manage data versions. In addition, TiDB's MVCC mechanism also uses the Snapshot Isolation algorithm, which can provide a higher transaction isolation level.

In short, both MySQL and TiDB use the MVCC mechanism when handling concurrent access. By using MVCC, these two databases can improve concurrency performance and ensure data consistency. I hope that through the introduction and code examples of this article, readers can better understand the MVCC mechanism and its application of MySQL and TiDB.

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