The Ultimate Guide to High Availability with MariaDB
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Xpand Monitor

Xpand Monitor

Overview

The Xpand Monitor is a monitor that monitors a Xpand cluster. It is capable of detecting the cluster setup and creating corresponding server instances within MaxScale.

Required Grants

The monitor user must have the following grants:

CREATE USER 'maxscale'@'maxscalehost' IDENTIFIED BY 'maxscale-password';
GRANT SELECT ON system.membership TO 'maxscale'@'maxscalehost';
GRANT SELECT ON system.nodeinfo TO 'maxscale'@'maxscalehost';
GRANT SELECT ON system.softfailed_nodes TO 'maxscale'@'maxscalehost';

Further, if you want be able to softfail and unsoftfail a node via MaxScale, then the monitor user must have SUPER privileges:

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

Configuration

A minimal configuration for a monitor requires one server in the Xpand cluster, and a username and a password to connect to the server. Note that by default the Xpand monitor will only use that server in order to dynamically find out the configuration of the cluster; after startup it will completely rely upon information obtained at runtime. To change the default behaviour, please see the parameter dynamic_node_detection.

To ensure that the Xpand monitor will be able to start, it is adviseable to provide more than one server to cater for the case that not all nodes are always up when MaxScale starts.

Note: All services that use servers monitored by xpandmon should use the cluster parameter to define the set of servers they use. This will guarantee that the services use servers that are valid members of the XPand cluster.

[TheXpandMonitor]
type=monitor
module=xpandmon
servers=server1,server2,server3
user=myuser
password=mypwd

[MyService]
type=service
router=readconnroute
cluster=TheXpandMonitor
user=myuser
password=mypwd

Dynamic Servers

The server objects the Xpand monitor creates for each detected Xpand node will be named like

@@<name-of-xpand-monitor>:node-<id>

where <name-of-xpand-monitor> is the name of the Xpand monitor instance, as defined in the MaxScale configuration file, and <id> is the id of the Xpand node.

For instance, with the Xpand monitor defined as above and a Xpand cluster consisting of 3 nodes whose ids are 1, 2 and 3 respectively, the names of the created server objects will be:

@@TheXpandMonitor:node-1
@@TheXpandMonitor:node-2
@@TheXpandMonitor:node-3

When dynamic servers are created, the values for the configuraton settings max_routing_connections, persistmaxtime, persistpoolmax and proxy_protocol are copied from the settings of the bootstrap servers. Note that the values of these settings must be identical on every bootstrap server.

Common Monitor Parameters

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

Xpand Monitor optional parameters

These are optional parameters specific to the Xpand Monitor.

cluster_monitor_interval

Defines, in milliseconds, how often the monitor checks the state of the entire cluster. The default value is 60000 (1 minute), which should not be lowered as that may have an adverse effect on the Cluster itself.

cluster_monitor_interval=120000ms

The interval is specified as documented here. If no explicit unit is provided, the value is interpreted as milliseconds in MaxScale 2.4. In subsequent versions a value without a unit may be rejected.

health_check_threshold

Defines how many times the health check may fail before the monitor considers a particular node to be down. The default value is 2.

health_check_threshold=3

dynamic_node_detection

By default, the Xpand monitor will only use the bootstrap nodes in order to connect to the Xpand cluster and then find out the cluster configuration dynamically at runtime.

That behaviour can be turned off with this optional parameter, in which case all Xpand nodes must manually be defined as shown below.

[Node-1]
type=server
address=192.168.121.77
port=3306
...

[Node-2]
...

[Node-3]
...

[TheXpandMonitor]
type=monitor
module=xpandmon
servers=Node-1, Node-2, Node-3
dynamic_node_detection=false

The default value of dynamic_node_detection is true.

See also health_check_port.

health_check_port

With this optional parameter it can be specified what health check port to use, if dynamic_node_detection has been disabled.

health_check_port=4711

The default value is 3581.

Note that this parameter is ignored unless dynamic_node_detection is false. Note also that the port must be the same for all nodes.

Commands

The Xpand monitor supports the following module commands.

softfail

With the softfail module command, a node can be softfailed via MaxScale. The command requires as argument the name of the Xpand monitor instance (as defined in the configuration file) and the name of the node to be softfailed.

For instance, with a configuration file like

[TheXpandMonitor]
type=monitor
module=xpandmon
...

then the node whose server name is @@TheXpandMonitor:node-1 can be softfailed like

$ maxctrl call command xpandmon softfail TheXpandMonitor @@TheXpandMonitor:node-1

If the softfailing of a node is successfully initiated, then the status of the corresponding MaxScale server object will be set to Draining, which will prevent new connections from being created to the node.

When the number of connections through MaxScale to the node has dropped to 0, its state will change to Drained. Note that the state Drained only tells that there are no connections to the node, not what the state of the softfailing operation is.

unsoftfail

With the unsoftfail module command, a node can be unsoftfailed via MaxScale. The command requires as argument the name of the Xpand monitor instance (as defined in the configuration file) and the name of the node to be unsoftfailed.

With a setup similar to the softfail case, a node can be unsoftfailed like:

$ maxctrl call command xpandmon unsoftfail TheXpandMonitor @@TheXpandMonitor:node-1

If a node is successfully softfailed, then a Draining status of the corresponding MaxScale server object will be cleared.

SOFTFAILed nodes

During the cluster check, which is performed once per cluster_monitor_interval, the monitor will also check whether any nodes are being softfailed. The status of the corresponding server object of a node being softfailed will be set to Draining, which will prevent new connections from being created to that node.

When the number of connections through MaxScale to the node has dropped to 0, its state will change to Drained. Note that the state Drained only tells that there are no connections to the node, not what the state of the softfailing operation is.

If a node that was softfailed is UNSOFTFAILed then the Draining status will be cleared.

If the softfailing and unsoftfailing is initiated using the softfail and unsoftfail commands of the Xpand monitor, then there will be no delay between the softfailing or unsoftfailing being initiated and the Draining status being turned on/off.

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