### Understand the alert

This alert calculates the maximum size of the MySQL Galera cluster over a 2-minute period, starting from one minute ago. If you receive this alert, it means that there has been a significant change in the cluster size, which might affect the database's performance, stability, and data consistency.

### What is MySQL Galera Cluster?

MySQL Galera Cluster is a synchronous multi-master cluster for MySQL, built on the Galera replication plugin. It provides high-availability and improved performance for MySQL databases by synchronizing data across multiple nodes.

### What does the cluster size mean?

The cluster size refers to the number of nodes participating in a MySQL Galera Cluster. An optimal cluster size ensures that the database can handle more significant workloads, handle node failures, and perform automatic failovers.

### Troubleshoot the alert

- Determine the current cluster size

  1. Connect to any node in the cluster and run the following SQL query:

     ```
     SHOW STATUS LIKE 'wsrep_cluster_size';
     ```

  2. The query will display the current number of nodes in the cluster.

- Identify the cause of the cluster size change

  1. Check the MySQL and Galera logs on all nodes to identify any issues, such as network connectivity issues, node crashes, or hardware problems.

  2. Review the logs for events such as joining or leaving of the cluster nodes. Look for patterns that could lead to instability (e.g., frequent node join & leave events).

- Resolve the issue

  1. Fix any identified problems causing the cluster size change. This may involve monitoring and resolving any network issues, restarting failed nodes, or replacing faulty hardware.

  2. If necessary, plan and execute a controlled reconfiguration of the Galera cluster to maintain the optimal cluster size.

### Useful resources

1. [Galera Cluster Documentation](https://galeracluster.com/library/documentation/)
2. [Monitoring Galera Cluster for MySQL or MariaDB](https://severalnines.com/database-blog/monitoring-galera-cluster-mysql-or-mariadb)