Trigger a flow if a message isn’t received after a defined time

Problem

You want a flow to be triggered if a message is not received after a defined time. For example, you expect to receive a sensor reading every 5 seconds and need to know if it fails to arrive.

Solution

Use the Trigger node to detect when a message has not arrived after a defined interval.

Example

[{"id":"6ea53ad8.2362a4","type":"debug","z":"ac14500e.2c57d","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","x":450,"y":1160,"wires":[]},{"id":"3da6946e.184a5c","type":"inject","z":"ac14500e.2c57d","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":100,"y":1160,"wires":[["38caaff4.03f6d","6ea53ad8.2362a4"]]},{"id":"38caaff4.03f6d","type":"trigger","z":"ac14500e.2c57d","op1":"","op2":"timeout","op1type":"nul","op2type":"str","duration":"5","extend":true,"units":"s","reset":"","bytopic":"all","name":"Watchdog","x":270,"y":1200,"wires":[["ae477709.016088"]]},{"id":"ae477709.016088","type":"debug","z":"ac14500e.2c57d","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":450,"y":1200,"wires":[]}]

Discussion

In the example flow, the top branch represents the normal flow of the messages. They also get passed to the Trigger node on a second branch of the flow.

The Trigger node is configured to initially send nothing, then to wait for 5 seconds before sending a "timeout" message. The option to extend the delay if new messages arrive is also selected. This means as long as messages continue to arrive, the node will not do anything. Once 5 seconds passes after the last message to arrive, it will send on the "timeout" message.