Explain MySQL semi-synchronous replication.

MySQL semi-synchronous replication balances data consistency and performance by waiting for at least one slave library to confirm before the master library returns to the client. 1) Enable semi-synchronous replication on the main library: SET GLOBAL rpl_semi_sync_master_enabled = 1; 2) Enable semi-synchronous replication on the slave library: SET GLOBAL rpl_semi_sync_slave_enabled = 1; This method not only improves data consistency, but does not seriously affect performance like synchronous replication.

introduction

In the database world, MySQL replication technology has always been the key to ensuring high data availability and high reliability. Today, we will dive into MySQL semi-synchronous replication (Semi-Synchronous Replication). Semi-synchronous replication is an improvement of MySQL based on traditional asynchronous replication, which finds a balance between data consistency and performance. Through this article, you will understand the basic principles, implementation methods, and precautions for semi-synchronous replication.

Review of basic knowledge

MySQL replication technology is mainly divided into asynchronous replication and synchronous replication. In asynchronous replication, the master library writes the transaction to the binary log and returns it to the client immediately without waiting for confirmation from the slave library (Slave). In contrast, synchronous replication requires that the master library must ensure that all slave libraries have received and applied transactions before returning to the client. Although this method ensures data consistency, it has a great impact on performance.

Semi-synchronous replication is a compromise between asynchronous and synchronous replication. It requires that at least one slave has received the transaction before returning to the client, but does not force all slaves to be acknowledged. This approach not only improves data consistency without severely affecting performance like synchronous replication.

Core concept or function analysis

Definition and function of semi-synchronous replication

Semi-synchronous replication is designed to improve data consistency while minimizing the impact on performance as much as possible. Its main function is to ensure that at least one slave has the latest data when the master library fails, thereby reducing the risk of data loss.

Simply put, the workflow of semi-synchronous replication is as follows:

 -- Enable semi-sync replication on the main library SET GLOBAL rpl_semi_sync_master_enabled = 1;
-- Enable semi-sync replication on the slave library SET GLOBAL rpl_semi_sync_slave_enabled = 1;

How it works

The implementation of semi-synchronous replication depends on the plug-in mechanism of MySQL. The master library sends transactions to the slave library through the rpl_semi_sync_master plug-in and waits for at least one ACK (acknowledgement) signal of the slave library. If no ACK is received within the specified time, the main library will fall back to asynchronous replication mode to ensure that the transaction is not blocked indefinitely.

The slave library receives transactions through the rpl_semi_sync_slave plug-in and sends an ACK signal to the master library after the transaction is applied. The entire process involves network communication and transaction confirmation, which may have a certain impact on system performance.

The implementation principle of semi-synchronous replication also involves some technical details, such as:

• Time complexity : Semi-synchronous replication increases the time when the master library waits for the slave library to confirm, but this time is usually controllable and can be adjusted by configuring the parameter rpl_semi_sync_master_timeout .
• Memory Management : Because transactions waiting for confirmation need to be cached in memory, it may have an impact on the memory usage of the main library.

Example of usage

Basic usage

It is very simple to enable semi-synchronous replication, just set the corresponding parameters on the master and slave libraries:

 -- Enable semi-sync replication on the main library SET GLOBAL rpl_semi_sync_master_enabled = 1;
SET GLOBAL rpl_semi_sync_master_timeout = 1000; -- Set the timeout to 1 second -- Enable semi-sync replication from the library SET GLOBAL rpl_semi_sync_slave_enabled = 1;

These commands will take effect immediately, but for persistent settings, it is recommended to configure them in the configuration file.

Advanced Usage

In practical applications, it may be necessary to configure semi-synchronous replication more carefully according to specific needs. For example, you can control whether to fall back to asynchronous replication when the slave library is not available by adjusting rpl_semi_sync_master_wait_no_slave parameter:

 -- Set not to fall back to asynchronous replication when no slave library is available SET GLOBAL rpl_semi_sync_master_wait_no_slave = 1;

This configuration is suitable for scenarios where data consistency is extremely high, but it is important to note that it may cause main library transactions to block.

Common Errors and Debugging Tips

Common problems in semi-synchronous replication include:

• Timeout error : If the network latency is high, it may cause the master to wait for the slave to confirm the timeout. At this time, it can be solved by increasing the value of rpl_semi_sync_master_timeout .
• Slave Library Failure : If the slave library fails, the master library may fall back to asynchronous replication mode. To avoid this, multiple slave libraries can be configured and the number of slave libraries that need to wait for confirmation can be set through the rpl_semi_sync_master_wait_for_slave_count parameter.

When debugging these issues, you can get the details by viewing the MySQL error log:

 -- View error log SHOW GLOBAL VARIABLES LIKE 'log_error';

Performance optimization and best practices

When using semi-synchronous replication, the following points can help you optimize performance and improve code quality:

• Performance comparison : Semi-synchronous replication increases the waiting time of the main library, but this impact can be controlled by configuration parameters. For example, the balance between performance and consistency can be found by tuning rpl_semi_sync_master_timeout .

   -- Adjust timeout to optimize performance SET GLOBAL rpl_semi_sync_master_timeout = 500; -- Reduce waiting time

• Best Practice : When configuring semi-synchronous replication, it is recommended:

  • Multi-slave library configuration : Configure multiple slave libraries to improve system fault tolerance and availability.
  • Monitoring and Alarming : Set up monitoring and alarm mechanisms to promptly discover and deal with problems in semi-synchronous replication.
  • Code readability : Add detailed comments to the configuration file to ensure that other team members can understand and maintain the configuration.

Through these practices, you can better utilize semi-synchronous replication to improve the reliability and performance of MySQL databases.

In practical applications, I have encountered a case: on an e-commerce platform, due to high network latency, semi-synchronous replication often timed out, causing the main library to fall back to asynchronous replication mode. We finally solved this problem by increasing the value of rpl_semi_sync_master_timeout and configuring multiple slave libraries. This experience tells me that the configuration of semi-synchronous replication needs to be adjusted according to the specific application scenario in order to maximize its effectiveness.

I hope this article can help you better understand and apply MySQL semi-synchronous replication technology. If you have any questions or experience sharing, please leave a message in the comment area to discuss.

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