Implementing partial synchronous replication in SQL Galera Cluster for improved performance

26 Sep 2023

Galera
MySQL

In a SQL Galera Cluster, replication plays a crucial role in ensuring data consistency and high availability. By default, Galera Cluster uses synchronous replication, where every transaction must be replicated on all nodes before it is considered committed. While this ensures data integrity, it can sometimes impact the overall performance of the cluster.

To address this issue, Galera Cluster offers the option to configure partial synchronous replication, which allows for improved performance while still maintaining strong consistency guarantees. In this article, we will explore how to implement partial synchronous replication in SQL Galera Cluster.

What is Partial Synchronous Replication?

Partial synchronous replication is a feature that allows you to control the level of synchronicity between nodes in a Galera Cluster. Instead of requiring every node to confirm the replication of each transaction, you can define a subset of nodes that must confirm the replication while allowing other nodes to lag behind. This approach is particularly useful in scenarios where there are differences in the hardware capacities of the nodes or network latencies.

Configuring Partial Synchronous Replication

To implement partial synchronous replication in SQL Galera Cluster, follow these steps:

Open the configuration file (my.cnf or my.ini) on each node of the cluster.
Locate the [mysqld] section in the configuration file.
Add the following line to enable partial synchronous replication:
```
wsrep_sync_wait = 3
```
The wsrep_sync_wait parameter defines the number of acknowledgments required for the transaction to be considered committed. In this example, we set it to 3, meaning that at least 3 nodes must acknowledge the replication before a transaction is considered committed.
Save the configuration file and restart the MySQL service on each node.
Verify the configuration by connecting to one of the nodes and executing the following SQL query:
```
SHOW VARIABLES LIKE 'wsrep_sync_wait';
```
The output should show the configured value of wsrep_sync_wait.

Monitoring Partial Synchronous Replication

Once partial synchronous replication is enabled, it’s important to monitor the cluster to ensure it is working as expected. Some key metrics to monitor include:

Flow Control: Flow control is the mechanism used by Galera Cluster to slow down the replication process to prevent overloaded nodes from falling too far behind. Monitoring flow control metrics helps identify potential performance bottlenecks and fine-tune the cluster configuration accordingly.
Certification Failures: Certification failures occur when a transaction cannot be certified for replication due to conflicts with concurrent transactions. Monitoring certification failures helps identify transactional conflicts and optimize application logic or database schema to avoid or minimize them.
Replication Lag: Replication lag refers to the delay between the completion of a transaction on one node and its replication on other nodes. Monitoring replication lag helps ensure that the lag is within an acceptable range and doesn’t impact the overall performance.

Conclusion

Implementing partial synchronous replication in SQL Galera Cluster can significantly improve performance while still maintaining strong data consistency guarantees. By configuring partial synchronous replication, you can strike a balance between performance and data integrity based on your specific requirements and cluster configuration.

To fully leverage the benefits of partial synchronous replication, it’s essential to monitor the cluster’s performance and address any issues proactively. With careful configuration and monitoring, you can achieve enhanced performance and availability in your SQL Galera Cluster.

#Galera #MySQL #Database #Replication