Comparison of data consistency and asynchronous replication between MySQL and TiDB

Introduction:
In distributed systems, data consistency has always been an important issue. MySQL is a traditional relational database management system that uses asynchronous replication to achieve data replication and high availability. The emerging distributed database system TiDB uses the Raft consistency algorithm to ensure data consistency and availability. This article will compare the data consistency and asynchronous replication mechanisms of MySQL and TiDB, and demonstrate their different characteristics through code examples.

1. MySQL data consistency and asynchronous replication

1. Data consistency: MySQL uses the master-slave replication mechanism, that is, one master database synchronizes data to multiple slaves database for high availability and load balancing. Data update operations on the master database will be recorded through the binary log (binlog) and asynchronously propagated to the slave database for execution. This means that there may be a certain delay in the data between the master database and the slave database, and there will be certain data inconsistencies.
2. Asynchronous replication: MySQL's asynchronous replication mechanism means that the master database writes the data change operation to the binary log and returns it to the client immediately without waiting for confirmation from the slave database. This can improve performance and throughput, but can lead to data inconsistency between the master and slave databases. For example, when the primary database fails, the data that has been written to the primary database may not have been fully synchronized to the secondary database.

2. TiDB’s data consistency and Raft consistency algorithm

1. Data consistency: TiDB uses the Raft consistency algorithm to ensure data consistency and availability. The Raft consensus algorithm divides all nodes into three roles: Leader, Follower and Candidate. The Leader is responsible for receiving client requests and synchronizing data change operations to other nodes through a series of elections and replication mechanisms to ensure data consistency. Therefore, TiDB can provide strongly consistent data access.
2. Asynchronous replication: TiDB's Raft consistency algorithm is a synchronous replication mechanism, that is, when the Leader receives a write request, it must wait until the write is successful before it can be returned to the client. This ensures that data replication is synchronous and avoids data inconsistency issues. Although synchronous replication may impact performance and throughput, the system's concurrent processing capabilities can be increased by increasing the number of nodes.

3. Comparison of code examples
Take a simple transfer scenario as an example to compare the data consistency and asynchronous replication mechanisms of MySQL and TiDB.

MySQL code example:

-- 創(chuàng)建轉(zhuǎn)賬記錄表
CREATE TABLE transfer (
  id INT PRIMARY KEY AUTO_INCREMENT,
  from_user INT,
  to_user INT,
  amount DECIMAL(10, 2)
);
-- 執(zhí)行轉(zhuǎn)賬操作（示例）
INSERT INTO transfer (from_user, to_user, amount) VALUES (10, 20, 100);
UPDATE account SET balance = balance - 100 WHERE id = 10; -- 扣除轉(zhuǎn)出賬戶的金額
UPDATE account SET balance = balance + 100 WHERE id = 20; -- 增加轉(zhuǎn)入賬戶的金額

TiDB code example:

-- 創(chuàng)建轉(zhuǎn)賬記錄表
CREATE TABLE transfer (
  id INT PRIMARY KEY AUTO_INCREMENT,
  from_user INT,
  to_user INT,
  amount DECIMAL(10, 2)
);
-- 執(zhí)行轉(zhuǎn)賬操作（示例）
BEGIN;
INSERT INTO transfer (from_user, to_user, amount) VALUES (10, 20, 100);
UPDATE account SET balance = balance - 100 WHERE id = 10;
UPDATE account SET balance = balance + 100 WHERE id = 20;
COMMIT;

As can be seen from the above code example, MySQL's data update operation is asynchronous, that is, when the update statement is executed Then it is returned to the client without waiting for confirmation from the database. TiDB ensures data consistency by using the Raft algorithm, that is, when performing an update operation, it must wait for confirmation from the Leader node before it can be returned to the client.

Conclusion:
There are obvious differences in the data consistency and asynchronous replication mechanisms of MySQL and TiDB. MySQL's data consistency is based on master-slave replication. Through the asynchronous replication mechanism, while ensuring high throughput, data inconsistency may occur. TiDB ensures strong consistency of data through the Raft consistency algorithm, but this may have a certain impact on performance. Therefore, when choosing a database system, you need to weigh the trade-off between data consistency and performance based on business scenarios and requirements.

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