Update MariaDBMonitor documenation
Adds table of contents, a mention of semisynchronous replication and clarifies disk space parameters a bit.
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Esa Korhonen
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# MariaDB Monitor
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Up until MariaDB MaxScale 2.2.0, this monitor was called _MySQL Monitor_.
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Table of Contents
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=================
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* [Overview](#overview)
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* [Master selection](#master-selection)
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* [Configuration](#configuration)
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* [Common Monitor Parameters](#common-monitor-parameters)
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* [MariaDB Monitor optional parameters](#mariadb-monitor-optional-parameters)
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* [detect_replication_lag](#detect_replication_lag)
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* [detect_stale_master](#detect_stale_master)
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* [detect_stale_slave](#detect_stale_slave)
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* [mysql51_replication](#mysql51_replication)
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* [multimaster](#multimaster)
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* [ignore_external_masters](#ignore_external_masters)
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* [detect_standalone_master](#detect_standalone_master)
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* [failcount](#failcount)
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* [allow_cluster_recovery](#allow_cluster_recovery)
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* [enforce_read_only_slaves](#enforce_read_only_slaves)
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* [maintenance_on_low_disk_space](#maintenance_on_low_disk_space)
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* [Cluster manipulation operations](#cluster-manipulation-operations)
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* [Operation details](#operation-details)
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* [Manual activation](#manual-activation)
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* [Automatic activation](#automatic-activation)
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* [Limitations and requirements](#limitations-and-requirements)
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* [External master support](#external-master-support)
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* [Configuration parameters](#configuration-parameters)
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* [auto_failover](#auto_failover)
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* [auto_rejoin](#auto_rejoin)
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* [switchover_on_low_disk_space](#switchover_on_low_disk_space)
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* [replication_user and replication_password](#replication_user-and-replication_password)
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* [failover_timeout and switchover_timeout](#failover_timeout-and-switchover_timeout)
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* [verify_master_failure and master_failure_timeout](#verify_master_failure-and-master_failure_timeout)
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* [servers_no_promotion](#servers_no_promotion)
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* [promotion_sql_file and demotion_sql_file](#promotion_sql_file-and-demotion_sql_file)
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* [handle_server_events](#handle_server_events)
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* [Troubleshooting](#troubleshooting)
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* [Using the MariaDB Monitor With Binlogrouter](#using-the-mariadb-monitor-with-binlogrouter)
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* [Example 1 - Monitor script](#example-1---monitor-script)
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## Overview
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@ -11,6 +48,8 @@ the replication cluster by performing failover, switchover and rejoin. Backend
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server versions older than MariaDB/MySQL 5.5 are not supported. Failover and
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other similar operations require MariaDB 10.0.2 or later.
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Up until MariaDB MaxScale 2.2.0, this monitor was called _MySQL Monitor_.
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## Master selection
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Only one backend can be master at any given time. A master must be running
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@ -201,10 +240,21 @@ disable `read_only`. Otherwise the monitor would quickly re-enable it.
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### `maintenance_on_low_disk_space`
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This feature is enabled by default. If a running server that is not the master
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or a relay master is out of disk space (as defined by the general monitor
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setting `disk_space_threshold`) the server is set to maintenance mode. Such
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servers are not used for router sessions and are ignored when performing a
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failover or other cluster modification operation.
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or a relay master is out of disk space the server is set to maintenance mode.
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Such servers are not used for router sessions and are ignored when performing a
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failover or other cluster modification operation. See the general monitor
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parameters [disk_space_threshold](./Monitor-Common.md#disk_space_threshold) and
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[disk_space_check_interval](./Monitor-Common.md#disk_space_check_interval)
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on how to enable disk space monitoring.
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Once a server has been put to maintenance mode, the disk space situation
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of that server is no longer updated. The server will not be taken out of
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maintanance mode even if more disk space becomes available. The maintenance
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flag must be removed manually:
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```
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maxadmin clear server server2 Maint
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maxctrl clear server server2 Maint
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```
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## Cluster manipulation operations
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@ -440,6 +490,13 @@ The backends must all use GTID-based replication, and the domain id should not
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change during a switchover or failover. Master and slaves must have
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well-behaving GTIDs with no extra events on slave servers.
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Failover may lose events. If a master goes down before sending new events to at
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least one slave, those events are lost when a new master is chosen. If the old
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master comes back online, the other servers have likely moved on with a
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diverging history and the old master can no longer join the replication cluster.
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To minimize the chance for this happening, use
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[semisynchronous replication](https://mariadb.com/kb/en/library/semisynchronous-replication/).
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Switchover requires that the cluster is "frozen" for the duration of the
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operation. This means that no data modifying statements such as INSERT or UPDATE
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are executed and the GTID position of the master server is stable. When
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@ -520,22 +577,20 @@ redirected to Slave B, the current master.
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#### `switchover_on_low_disk_space`
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This feature is disabled by default. If set to `on`, when the disk space of a
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server is exhausted, it will cause the server to be put in maintenance mode.
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If the server is the current master, then a switchover will also be triggered.
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This feature is disabled by default. If enabled, the monitor will attempt to
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switchover a master server low on disk space with a slave. The switch is only
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done if a slave without disk space issues is found. If
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`maintenance_on_low_disk_space` is also enabled, the old master (now a slave)
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will be put to maintenance during the next monitor iteration.
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In order for this parameter to have any effect, `disk_space_threshold` must
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have been specified for the
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[server](../Getting-Started/Configuration-Guide.md#disk_space_threshold)
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or the [monitor](./Monitor-Common.md#disk_space_threshold), and
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[disk_space_check_interval](./Monitor-Common.md#disk_space_check_interval)
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for the monitor.
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For this parameter to have any effect, `disk_space_threshold` must be specified
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for the [server](../Getting-Started/Configuration-Guide.md#disk_space_threshold)
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or the [monitor](./Monitor-Common.md#disk_space_threshold).
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Also, [disk_space_check_interval](./Monitor-Common.md#disk_space_check_interval)
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must be defined for the monitor.
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```
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switchover_on_low_disk_space=true
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```
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Note that once the server has been put in maintenance mode, the disk space
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situation will no longer be monitored and the server will thus not automatically
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be taken out of maintanance mode even if disk space again would become available.
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#### `replication_user` and `replication_password`
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