I use Monit 5.27.2 and MariaDB 10.5.15 on my Debian 11 servers.

My servers are set up in Multi-Source MariaDB Replication.

Sometimes, replication stops and I am not notified. I have not seen anywhere a prepared configuration for Monit to send such alerts.

The errors are visible with the following query:

SHOW SLAVE STATUS;

With those values particularly interesting:

I have not found any way to use Monit to monitor directly the result of a SQL statement. However, it seems possible to monitor the return code of a command with versions before Monit 5.29.0 (and the output of a command after).

I can probably prepare a Bash/Shell script that will check the output of the SQL statement and then a Monit config that will send me an alert in case of issue. If I do, I will post this as an answer here. I was just wondering whether there was already a known solution to this.

I expect this to work with MySQL.

I intend to test this with those SQL queries:

STOP SLAVE;
START SLAVE;

I got this to work as I expected. I did this on all servers sources of the replication.

Installing

Here is the file monit_replication.sh:

#!/bin/bash   
user="YOUR_USER"
password="YOUR_PASSWORD"

result=`echo "show slave status"|mysql -EsB --user="$user" --password="$password"`

contains_with_yes(){
        if echo "$1"|grep -i "$2"|grep -i "Yes" > /dev/null; then
                echo "$2 ok"
        else
                echo "$2 not ok"
                exit 1
        fi
}
 
contains_with_yes "$result" "Slave_IO_Running"
contains_with_yes "$result" "Slave_SQL_Running"
exit 0

Made this executable:

chmod 700 /YOUR_PATH_TO_THE_FILE/monit_replication.sh

Added to /etc/monit/conf-enabled/mysql:

 check program MySQL_replication with path "/YOUR_PATH_TO_THE_FILE/monit_replication.sh"
   every 120 cycles
   if status > 0 then alert
   group mysql 

Testing

Ran this SQL:

STOP SLAVE;

Restarted monit:

/etc/init.d/monit restart

Found in /var/log/monit.log:

[2022-07-04T10:46:55+0000] error    : 'MySQL_replication' status failed (1) -- Slave_IO_Running not ok

Received this email:

monit alert -- Status failed MySQL_replication

Status failed Service MySQL_replication

    Date:        Mon, 04 Jul 2022 10:46:55
    Action:      alert
    Host:        YOUR_HOST
    Description: status failed (1) -- Slave_IO_Running not ok

Your faithful employee,
Monit

This validates the test. Restoring replication with this SQL query:

START SLAVE;

Other solutions

If any simpler solution based on Monit can alert me of a replication failure on MariaDB, I would be happy to accept it as the solution to the question.