# MySQL Monitor

## Overview

The MySQL Monitor is a monitoring module for MaxScale that monitors a Master-Slave replication cluster. It assigns master and slave roles inside MaxScale according to the actual replication tree in the cluster.

## Configuration

A minimal configuration for a  monitor requires a set of servers for monitoring and a username and a password to connect to these servers.

```
[MySQL Monitor]
type=monitor
module=mysqlmon
servers=server1,server2,server3
user=myuser
passwd=mypwd

```

The user requires the REPLICATION CLIENT privilege to successfully monitor the state of the servers.

```
MariaDB [(none)]> grant replication client on *.* to 'maxscale'@'maxscalehost';
Query OK, 0 rows affected (0.00 sec)
```

## Common Monitor Parameters

For a list of optional parameters that all monitors support, read the [Monitor Common](Monitor-Common.md) document.

## MySQL Monitor optional parameters

These are optional parameters specific to the MySQL Monitor.

### `detect_replication_lag`

Detect replication lag between the master and the slaves. This allows the routers to route read queries to only slaves that are up to date.

```
detect_replication_lag=true
```

### `detect_stale_master`

Allow previous master to be available even in case of stopped or misconfigured 
replication. This allows services that depend on master and slave roles to continue functioning as long as the master server is available.

This is a situation which can happen if all slave servers are unreachable or the replication breaks for some reason.

```
detect_stale_master=true
```

### `mysql51_replication`

Enable support for MySQL 5.1 replication monitoring. This is needed if a MySQL server older than 5.5 is used as a slave in replication.

```
mysql51_replication=true
```

## Example 1 - Monitor script

Here is an example shell script which sends an email to an admin when a server goes down.

```
#!/usr/bin/env bash

#This script assumes that the local mail server is configured properly
#The second argument is the event type
event=${$2/.*=/}
server=${$3/.*=/}
message="A server has gone down at `date`."
echo $message|mail -s "The event was $event for server $server." admin@my.org

```

Here is a monitor configuration that only triggers the script when a master or a slave server goes down.

```
[Database Monitor]
type=monitor
module=mysqlmon
servers=server1,server2
script=mail_to_admin.sh
events=master_down,slave_down
```

When a master or a slave server goes down, the script is executed, a mail is sent and the administrator will be immediately notified of any possible problems.
This is just a simple example showing what you can do with MaxScale and monitor scripts.