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.
By default, the Galera Monitor will choose the node with the lowest
wsrep_local_index value as the master. This will mean that two MaxScales
running on different servers will choose the same server as the master.
Galera clusters and slaves replicating from it
MaxScale 2.4.0 added support for slaves 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.
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.
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.
This option allows Galera nodes to be used normally when they are donors in an
SST operation when the SST method is non-blocking
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
mariabackup. Read the
documentation for more details.
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.
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.
This option controls whether the write master 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.
root_node_as_master option is disabled for galeramon, the node with the
lowest index will always be chosen as the master. If it is enabled, only the
node with a a wsrep_local_index value of 0 can be chosen as the master.
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 slave servers, automatically sorted, with possible master 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 slave 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
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
master 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 master node when a replacement Galera node is promoted to
a master 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 master.
Nodes with a non-positive value (priority <= 0) will never be chosen as the master. This allows you to mark some servers as permanent slaves by assigning a non-positive value into priority.
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=0
In this example
node-1 is always used as the master if available. If
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-3 were down, then
node-2 would be used. Because
node-4 has a value of 0 in priority, it will
never be the master. Nodes without priority parameter are considered as
having the lowest priority rank and will be used only if all nodes
with priority parameter are not available.
With priority ranks you can control the order in which MaxScale chooses the master node. This will allow for a controlled failure and replacement of nodes.