Galera Monitor

Overview

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 primary and replica roles inside MaxScale, allowing Galera clusters to be used with modules designed for traditional primary-replica clusters.

By default, the Galera Monitor will choose the node with the lowest wsrep_local_index value as the primary. This will mean that two MaxScales running on different servers will choose the same server as the primary.

WSREP Variables and Their Effects

The following WSREP variables are inspected by galeramon to see whether a node is usable. If the node is not usable, it loses the Master and Slave labels and will be in the Running state.

• If wsrep_ready=0, the WSREP system is not yet ready and the Galera node cannot accept queries.

• If wsrep_desync=1 is set, the node is desynced and is not participating in the Galera replication.

• If wsrep_reject_queries=[ALL|ALL_KILL] is set, queries are refused and the node is unusable.

• With wsrep_sst_donor_rejects_queries=1, donor nodes reject queries. Galeramon treats this the same as if wsrep_reject_queries=ALL was set.

• If wsrep_local_state is not 4 (or 2 with available_when_donor=true), the node is not in the correct state and is not used.

Galera clusters and replicas replicating from it

MaxScale 2.4.0 added support for replicas replicating off of Galera nodes. If a non-Galera server monitored by galeramon is replicating from a Galera node also monitored by galeramon, it will be assigned the Slave, Running status as long as the replication works. This allows read-scaleout with Galera servers without increasing the size of the Galera cluster.

Required Grants

The Galera Monitor requires the REPLICATION CLIENT grant to work:

CREATE USER 'maxscale'@'maxscalehost' IDENTIFIED BY 'maxscale-password';
GRANT REPLICATION CLIENT ON *.* TO 'maxscale-user'@'maxscalehost';

if set_donor_nodes is configured, the SUPER grant is required:

GRANT SUPER ON *.* TO 'maxscale'@'maxscalehost';

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

[Galera-Monitor]
type=monitor
module=galeramon
servers=server1,server2,server3
user=myuser
password=mypwd

Common Monitor Parameters

For a list of optional parameters that all monitors support, read the Monitor Common document.

Galera Monitor optional parameters

These are optional parameters specific to the Galera Monitor.

disable_master_failback

• Type: boolean
• Default: false
• Dynamic: Yes

If a node marked as primary inside MaxScale happens to fail and the primary status is assigned to another node MaxScale will normally return the primary status to the original node after it comes back up. With this option enabled, if the primary status is assigned to a new node it will not be reassigned to the original node for as long as the new primary node is running. In this case the Master Stickiness status bit is set which will be visible in the maxctrl list servers output.

available_when_donor

• Type: boolean
• Default: false
• Dynamic: Yes

This option allows Galera nodes to be used normally when they are donors in an SST operation when the SST method is non-blocking (e.g. wsrep_sst_method=mariabackup).

Normally when an SST is performed, both participating nodes lose their Synced, Master or Slave statuses. When this option is enabled, the donor is treated as if it was a normal member of the cluster (i.e. wsrep_local_state = 4). This is especially useful if the cluster drops down to one node and an SST is required to increase the cluster size.

The current list of non-blocking SST methods are xtrabackup, xtrabackup-v2 and mariabackup. Read the wsrep_sst_method documentation for more details.

disable_master_role_setting

• Type: boolean
• Default: false
• Dynamic: Yes

This disables the assignment of primary and replica roles to the Galera cluster nodes. If this option is enabled, Synced is the only status assigned by this monitor.

use_priority

• Type: boolean
• Default: false
• Dynamic: Yes

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.

root_node_as_master

• Type: boolean
• Default: false
• Dynamic: Yes

This option controls whether the write primary Galera node requires a wsrep_local_index value of 0. This option was introduced in MaxScale 2.1.0 and it is disabled by default in versions 2.1.5 and newer. In versions 2.1.4 and older, the option was enabled by default.

A Galera cluster will always have a node which has a wsrep_local_index value of 0. Based on this information, multiple MaxScale instances can always pick the same node for writes.

If the root_node_as_master option is disabled for galeramon, the node with the lowest index will always be chosen as the primary. If it is enabled, only the node with a a wsrep_local_index value of 0 can be chosen as the primary.

This parameter can work with disable_master_failback but using them together is not advisable: the intention of root_node_as_master is to make sure that all MaxScale instances that are configured to use the same Galera cluster will send writes to the same node. If disable_master_failback is enabled, this is no longer true if the Galera cluster reorganizes itself in a way that a different node gets the node index 0, writes would still be going to the old node that previously had the node index 0. A restart of one of the MaxScales or a new MaxScale joining the cluster will cause writes to be sent to the wrong node, thus resulting in an increasing the rate of deadlock errors and sub-optimal performance.

set_donor_nodes

• Type: boolean
• Default: false
• Dynamic: Yes

This option controls whether the global variable wsrep_sst_donor should be set in each cluster node with slave' status. The variable contains a list of replica servers, automatically sorted, with possible primary candidates at its end.

The sorting is based either on wsrep_local_index or node server priority depending on the value of use_priority option. If no server has priority defined the sorting switches to wsrep_local_index. Node names are collected by fetching the result of the variable wsrep_node_name.

Example of variable being set in all replica nodes, assuming three nodes:

SET GLOBAL wsrep_sst_donor = "galera001,galera000"

Note: in order to set the global variable wsrep_sst_donor, proper privileges are required for the monitor user that connects to cluster nodes. This option is disabled by default and was introduced in MaxScale 2.1.0.

Interaction with Server Priorities

If the use_priority option is set and a server is configured with the priority=<int> parameter, galeramon will use that as the basis on which the primary node is chosen. This requires the disable_master_role_setting to be undefined or disabled. The server with the lowest positive value of priority will be chosen as the primary node when a replacement Galera node is promoted to a primary server inside MaxScale. If all candidate servers have the same priority, the order of the servers in the servers parameter dictates which is chosen as the primary.

Nodes with a negative value (priority < 0) will never be chosen as the primary. This allows you to mark some servers as permanent replicas by assigning a non-positive value into priority. Nodes with the default priority of 0 are only selected if no nodes with higher priority are present and the normal node selection rules apply to them (i.e. selection is based on wsrep_local_index).

Here is an example.

[node-1]
type=server
address=192.168.122.101
port=3306
priority=1

[node-2]
type=server
address=192.168.122.102
port=3306
priority=3

[node-3]
type=server
address=192.168.122.103
port=3306
priority=2

[node-4]
type=server
address=192.168.122.104
port=3306
priority=-1

In this example node-1 is always used as the primary if available. If node-1 is not available, then the next node with the highest priority rank is used. In this case it would be node-3. If both node-1 and node-3 were down, then node-2 would be used. Because node-4 has a value of -1 in priority, it will never be the primary. Nodes without priority parameter are considered as having a priority of 0 and will be used only if all nodes with a positive priority value are not available.

With priority ranks you can control the order in which MaxScale chooses the primary node. This will allow for a controlled failure and replacement of nodes.