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 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 functionality. The only difference is that the MySQL monitor will also detect traditional Master-Slave topologies.

detect_standalone_master

Detect standalone master servers. This feature takes a boolean parameter and is disabled by default. In MaxScale 2.1.0, this parameter was called failover.

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 or MHA.

When the number of running servers in the cluster drops down to one, MaxScale cannot be absolutely certain whether the last remaining server is a master or a slave. At this point, MaxScale will try to deduce the type of the server by looking at the system variables of the server in question.

By default, MaxScale will only attempt to deduce if the server can be used as a slave server (controlled by the detect_stale_slave parameter). When the detect_standalone_master mode is enabled, MaxScale will also attempt to deduce whether the server can be used as a master server. This is done by checking that the server is not in read-only mode and that it is not configured as a slave.

This mode in mysqlmon is completely passive in the sense that it does not modify the cluster or any of the servers in it. It only labels the last remaining server in a cluster as the master server.

Before a server is labeled as a standalone master, the following conditions must have been met:

  • Previous attempts to connect to other servers in the cluster have failed, controlled by the failcount parameter

  • There is only one running server among the monitored servers

  • The value of the @@read_only system variable is set to OFF

In 2.1.1, the following additional condition was added:

  • The last running server is not configured as a slave

If the value of the allow_cluster_recovery parameter is set to false, the monitor sets all other servers into maintenance mode. This is done to prevent accidental use of the failed servers if they came back online. If 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 standalone server is labeled as a master. The default value is 5 failures.

The monitor will attempt to contact all servers once per monitoring cycle. When detect_standalone_master is enabled, all of the failed servers must fail failcount number of connection attempts before the last server is labeled as the master.

The formula for calculating the actual number of milliseconds before the server is labeled as the master is monitor_interval * failcount.

allow_cluster_recovery

Allow recovery after the cluster has dropped down to one server. This feature takes a boolean parameter is enabled by default. This parameter requires that detect_standalone_master is set to true. In MaxScale 2.1.0, this parameter was called failover_recovery.

When this parameter is disabled, if the last remaining server is labeled 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 only when MaxScale is used in conjunction with an external agent that automatically reintegrates failed servers into the cluster. One of these agents is the replication-manager which automatically configures the failed servers as new slaves of the current master.

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.