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### Understand the alert
The `mysql_galera_cluster_state_crit` alert is triggered when the Galera node state is either `Undefined`, `Joining`, or `Error`. This indicates that there is an issue with a Galera node in your MySQL Galera Cluster.
### What is a MySQL Galera Cluster?
MySQL Galera Cluster is a synchronous, multi-master database cluster that provides high availability, no data loss, and scalability for your MySQL databases. It uses Galera replication library and MySQL server to achieve these goals.
### Troubleshoot the alert
To troubleshoot the MySQL Galera Cluster State Critical alert, follow these steps:
1. Inspect the MariaDB error log
Check the MariaDB error log for any relevant error messages that can help identify the issue.
```
sudo tail -f /var/log/mysql/error.log
```
2. Check the Galera node's status
Connect to the problematic MySQL node and check the Galera node status by running the following query:
```
SHOW STATUS LIKE 'wsrep_%';
```
Take note of the value of `wsrep_local_state` and `wsrep_local_state_comment`.
3. Diagnose the issue
- If `wsrep_local_state` is 0 (`Undefined`), it means the node is not part of any cluster.
- If `wsrep_local_state` is 1 (`Joining`), it means the node is trying to connect or reconnect to the cluster.
- If `wsrep_local_state` is 5 (`Error`), it means the node has encountered a consistency error.
4. Resolve the issue
- For an `Undefined` state, check and fix the wsrep configuration settings and restart the node.
- For a `Joining` state, ensure that the node can communicate with the other nodes in the cluster and make sure that the cluster's state is healthy. Then, retry joining the node to the cluster.
- For an `Error` state, the node may need to be resynchronized with the cluster. Restart the mysqld process on the affected node, or you may need to perform a full state transfer to recover.
5. Monitor the cluster
After resolving the issue, monitor the cluster to ensure that all nodes are healthy and remain in-sync.
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