We have not been able to find a way to determine if a MQTT gateway or if a sensor is offline in Ignition. Are there any “keep alive” settings or “last will and testament” setting available for determining that a device connection is lost? Simply looking at the timestamps does not appear to be reliable because if the value has not changed from the device, the device does not publish an update.
The MQTT Gateway maintains a TCP socket connection for the MQTT data to the MQTT broker at all time. The status of that TCP socket will tell you if the Gateway is connected or not.
The sensors on the other hand wake up, transmit, then go back to sleep. There is no way to determine if they are still functioning other than to look at the expected delay interval of the sensor, then on the server side see if that interval duration has expired. If the interval has expired then it should be assumed the device is offline.
If there is no change in value from the sensor, I am not seeing an updated timestamp on the MQTT client.