#Node Red Roster Node
This node keeps a record of all unique messages to it and outputs an array of them. 'Unique' is defined as messages which contain a unique value of the index specified by the configuration parameter 'indexProperty'
The node keeps a record of the last time that each unique value was received. If the last message was less than 'timeout' seconds ago, the entry is marked as active otherwise it is marked as inactive.
Typically, this node is used to keep track of active devices. Each device sends a periodic message with say, a unique deviceID to a single MQTT topic, which forwards the messages to the roster node which uses deviceID as its indexProperty.
The roster array can then be used to show all known devices and their active state.