# Xpand Monitor **NOTE** The Xpand monitor is intended for use with a native Xpand cluster, not with the Xpand storage engine. ## 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. ## 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](#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. ``` [TheXpandMonitor] type=monitor module=xpandmon servers=server1,server2,server3 user=myuser password=mypwd ``` ## Dynamic Servers The server objects the Xpand monitor creates for each detected Xpand node will be named like ``` @@:node- ``` where `` is the name of the Xpand monitor instance, as defined in the MaxScale configuration file, and `` 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 ``` ### Grants Note that the monitor user _must_ have `SELECT` grant on the following tables: * `system.nodeinfo` * `system.membership` * `system.softfailed_nodes` You can give the necessary grants using the following commands: ``` GRANT SELECT ON system.membership TO 'myuser'@'%'; GRANT SELECT ON system.nodeinfo TO 'myuser'@'%'; GRANT SELECT ON system.softfailed_nodes TO 'myuser'@'%'; ``` Further, if you want be able to _softfail_ and _unsoftfail_ a node via MaxScale, then the monitor user must have `SUPER` privileges, which can be granted like: ``` GRANT SUPER ON *.* TO 'myuser'@'%'; ``` The user name must be changed to the one actually being used. ## Common Monitor Parameters For a list of optional parameters that all monitors support, read the [Monitor Common](Monitor-Common.md) 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](../Getting-Started/Configuration-Guide.md#durations). 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). ### `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 initated and the `Draining` status being turned on/off.