### Understand the alert

This alert is triggered when the replication status of a MySQL server is indicating a problem or failure. Replication is important for redundancy, data backup, or load balancing. Issues with replication threads can lead to data inconsistencies or potential loss of data.

### Troubleshoot the alert

1. Identify the failing thread:

   As mentioned above, use the appropriate command for your MySQL or MariaDB version to check the status of replication threads and determine which of them (I/O or SQL) is not running.

   For MySQL and MariaDB before v10.2.0, use:

   ```
   SHOW SLAVE STATUS\G
   ```

   For MariaDB v10.2.0+, use:

   ```
   SHOW ALL SLAVES STATUS\G
   ```

2. Inspect the MySQL error log:

   The MySQL error log can provide valuable information about the possible cause of the replication issues. Check the log for any replication-related error messages:

   ```
   tail -f /path/to/mysql/error.log
   ```

   Replace `/path/to/mysql/error.log` with the correct path to the MySQL error log file.

3. Check the source MySQL server:

   Replication issues can also originate from the source MySQL server. Make sure that the source server is properly configured and running, and that the binary logs are being written and flushed correctly.

   Refer to the [MySQL documentation](https://dev.mysql.com/doc/refman/5.7/en/replication-howto.html) for more information on configuring replication.

4. Restart the replication threads:

   After identifying and resolving any issues found in the previous steps, you can try restarting the replication threads:

   ```
   STOP SLAVE;
   START SLAVE;
   ```

   For MariaDB v10.2.0+ with multi-source replication, you may need to specify the connection name:

   ```
   STOP ALL SLAVES;
   START ALL SLAVES;
   ```

5. Verify the replication status:

   After restarting the replication threads, use the appropriate command from step 1 to verify that the threads are running, and that the replication is working as expected.

### Useful resources

1. [How To Set Up Replication in MySQL](https://www.digitalocean.com/community/tutorials/how-to-set-up-replication-in-mysql)
2. [MySQL Replication Administration and Status](https://dev.mysql.com/doc/refman/5.7/en/replication-administration-status.html)
3. [Replication Replica I/O Thread States](https://dev.mysql.com/doc/refman/5.7/en/replica-io-thread-states.html)
4. [Replication Replica SQL Thread States](https://dev.mysql.com/doc/refman/5.7/en/replica-sql-thread-states.html)