# 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`

A truth value which controls if replication lag between the master and the
slaves is monitored. This allows the routers to route read queries to only
slaves that are up to date. Default value for this parameter is false.

To detect the replication lag, MaxScale uses the _maxscale_schema.replication_heartbeat_
table. This table is created on the master server and it is updated at every heartbeat
with the current timestamp. The updates are then replicated to the slave servers
and when the replicated timestamp is read from the slave servers, the lag between
the slave and the master can be calculated.

The monitor user requires INSERT, UPDATE, DELETE and SELECT permissions on the
maxscale_schema.replication_heartbeat table and CREATE permissions on the
maxscale_schema database. The monitor user will always try to create the database
and the table if they do not exist.

### `detect_stale_master`

Allow previous master to be available even in case of stopped or misconfigured
replication.

Starting from MaxScale 2.0.0 this feature is enabled by default. It is disabled
by default in MaxScale 1.4.3 and below.

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
```

### `detect_stale_slave`

Treat running slaves servers without a master server as valid slave servers.

This feature is enabled by default.

If a slave server loses its master server, the replication is considered broken.
With this parameter, slaves that have lost their master but have been slaves of
a master server can retain their slave status even without a master. This means
that when a slave loses its master, it can still be used for reads.

If this feature is disabled, a server is considered a valid slave if and only if
it has a running master server monitored by this monitor.

```
detect_stale_slave=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
```

### `multimaster`

Detect multi-master replication topologies. This feature is disabled by default.

When enabled, the multi-master detection looks for the root master servers in
the replication clusters. These masters can be found by detecting cycles in the
graph created by the servers. When a cycle is detected, it is assigned a master
group ID. Every master in a master group will receive the Master status. The
special group ID 0 is assigned to all servers which are not a part of a
multi-master replication cycle.

If one or more masters in a group has the `@@read_only` system variable set to
`ON`, those servers will receive the Slave status even though they are in the
multi-master group. Slave servers with `@@read_only` disabled will never receive
the master status.

By setting the servers into read-only mode, the user can control which
server receive the master status. To do this:

- Enable `@@read_only` on all servers (preferrably through the configuration file)
- Manually disable `@@read_only` on the server which should be the master

This functionality is similar to the [Multi-Master Monitor](MM-Monitor.md)
functionality. The only difference is that the MySQL monitor will also detect
traditional Master-Slave topologies.

### `failover`

Failover mode. This feature takes a boolean parameter is disabled by default.

This parameter is intended to be used with simple, two node master-slave pairs
where the failure of the master can be resolved by "promoting" the slave as the
new master. Normally this is done by using an external agent of some sort
(possibly triggered by MaxScale's monitor scripts), like
[MariaDB Replication Manager](https://github.com/tanji/replication-manager)
or [MHA](https://code.google.com/p/mysql-master-ha/).

The failover mode in mysqlmon is completely passive in the sense that it does
not modify the cluster or any servers in it. It labels a slave server as a
master server when there is only one running server. Before a failover can be
initiated, the following conditions must have been met:

- The monitor has repeatedly failed to connect to the failed servers
- There is only one running server among the monitored servers
- @@read_only is not enabled on the last running server

When these conditions are met, the monitor assigns the last remaining server the
master status and puts all other servers into maintenance mode. This is done to
prevent accidental use of the failed servers if they came back online.

When the failed servers come back up, the maintenance mode needs to be manually
cleared once replication has been set up.

**Note**: A failover will cause permanent changes in the data of the promoted
  server. Only use this feature if you know that the slave servers are capable
  of acting as master servers.

### `failcount`

Number of failures that must occur on all failed servers before a failover is
initiated. The default value is 5 failures.

The monitor will attemt to contact all servers once per monitoring cycle. When
_failover_ mode is enabled, all of the failed servers must fail _failcount_
number of connection attemps before a failover is initiated.

The formula for calculating the actual number of milliseconds before failover
can start is `monitor_interval * failcount`. This means that to trigger a
failover after 10 seconds of master failure with a _monitor_interval_ of 1000
milliseconds, the value of _failcount_ must be 10.

### `failover_recovery`

Allow recovery after failover. This feature takes a boolean parameter is
disabled by default.

Normally if a failover has been triggered and the last remaining server is
chosen as the master, the monitor will set all of the failed servers into
maintenance mode. When this option is enabled, the failed servers are allowed to
rejoin the cluster.

This option should be enabled when failover in MaxScale is used in conjunction
with an external agent that resets the slave status for new master servers. One
of these agents is the _replication-manager_ which clears the slave
configuration for each new master and removes the read-only mode.

## 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.
