2b5ec8f162
When a Galera cluster loses a member, it will recalculate the wsrep_local_index values. As the index is zero-based, we can be certain that in a valid cluster there will always be a node with an index of 0. If the galeramon can't find a node with an index of 0, it means that either the cluster hasn't stabilized and there's a pending recalculation of the index or that there's no connectivity between MaxScale and the node with the index value 0. With this change and default settings, active-active MaxScale setups with Galera clusters should always choose the same node as the master.
130 lines
4.7 KiB
Markdown
130 lines
4.7 KiB
Markdown
# Galera Monitor
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## Overview
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The Galera Monitor is a monitoring module for MaxScale that monitors a Galera cluster. It detects whether nodes are a part of the cluster and if they are in sync with the rest of the cluster. It can also assign master and slave roles inside MaxScale, allowing Galera clusters to be used with modules designed for traditional master-slave clusters.
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By default, the Galera Monitor will choose the node with the lowest `wsrep_local_index`
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value as the master. This will mean that two MaxScales running on different
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servers will choose the same server as the master.
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## Configuration
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A minimal configuration for a monitor requires a set of servers for monitoring and a username and a password to connect to these servers. The user requires the REPLICATION CLIENT privilege to successfully monitor the state of the servers.
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```
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[Galera Monitor]
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type=monitor
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module=galeramon
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servers=server1,server2,server3
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user=myuser
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passwd=mypwd
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```
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## Common Monitor Parameters
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For a list of optional parameters that all monitors support, read the [Monitor Common](Monitor-Common.md) document.
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## Galera Monitor optional parameters
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These are optional parameters specific to the Galera Monitor.
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### `disable_master_failback`
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If a node marked as master inside MaxScale happens to fail and the master status is assigned to another node MaxScale will normally return the master status to the original node after it comes back up. With this option enabled, if the master status is assigned to a new node it will not be reassigned to the original node for as long as the new master node is running.
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```
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disable_master_failback=true
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```
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### `available_when_donor`
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This option only has an effect if there is a single Galera node being backed up an XtraBackup instance. This causes the initial node to go into Donor state which would normally prevent if from being marked as a valid server inside MaxScale. If this option is enabled, a single node in Donor state where the method is XtraBackup will be kept in Synced state.
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```
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available_when_donor=true
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```
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### `disable_master_role_setting`
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This disables the assignment of master and slave roles to the Galera cluster nodes. If this option is enabled, Synced is the only status assigned by this monitor.
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```
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disable_master_role_setting=true
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```
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### `use_priority`
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Enable interaction with server priorities. This will allow the monitor to deterministically pick the write node for the monitored Galera cluster and will allow for controlled node replacement.
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```
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use_priority=true
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```
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### `root_node_as_master`
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This option controls whether the write master Galera node requires a
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_wsrep_local_index_ value of 0. This option is enabled by default and was
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introduced in MaxScale 2.1.0.
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A Galera cluster will always have a node which has a _wsrep_local_index_ value
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of 0. Based on this information, multiple MaxScale instances can always pick the
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same node for writes.
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If the `root_node_as_master` option is disabled for galeramon, the node with the
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lowest index will always be chosen as the master. If it is enabled, only the
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node with a a _wsrep_local_index_ value of 0 can be chosed as the master.
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## Interaction with Server Priorities
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If the `use_priority` option is set and a server is configured with the
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`priority=<int>` parameter, galeramon will use that as the basis on which the
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master node is chosen. This requires the `disable_master_role_setting` to be
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undefined or disabled. The server with the lowest positive value in _priority_
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will be chosen as the master node when a replacement Galera node is promoted to
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a master server inside MaxScale.
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Nodes with a non-positive value (_priority_ <= 0) will never be chosen as the master. This allows
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you to mark some servers as permanent slaves by assigning a non-positive value
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into _priority_.
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Here is an example with two servers.
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```
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[node-1]
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type=server
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address=192.168.122.101
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port=3306
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priority=1
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[node-2]
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type=server
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address=192.168.122.102
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port=3306
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priority=3
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[node-3]
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type=server
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address=192.168.122.103
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port=3306
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priority=2
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[node-4]
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type=server
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address=192.168.122.104
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port=3306
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priority=0
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```
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In this example `node-1` is always used as the master if available. If `node-1`
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is not available, then the next node with the highest priority rank is used. In
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this case it would be `node-3`. If both `node-1` and `node-3` were down, then
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`node-2` would be used. Because `node-4` has a value of 0 in _priority_, it will
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never be the master. Nodes without priority are considered as having the lowest
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priority rank and will be used only if all nodes with priority ranks are not
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available.
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With priority ranks you can control the order in which MaxScale chooses the
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master node. This will allow for a controlled failure and replacement of nodes.
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